Condensate and Flash Steam Recovery

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Condensate and

flash steam recovery

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Introduction

Steam is usually generated for one of two reasons :

to produce power, as in power stations and co-generation plants. to carry energy for heating and process systems.

When a kg of steam condenses, a kg of condensate at the same pressure and temperature is formed. An efficient steam distribution system will make good use of this condensate. Failure to do so makes no financial, technical or environmental sense.

Steam, used for heating, gives up its latent heat, which is a large proportion of its total heat. The remainder is held by the condensed water. As well as having heat content, the condensate is also a distilled form of water, which is ideal for use as boiler feedwater. An efficient installation will collect condensate and either return it to the deaerator, boiler feedtank, or use it in another process. Only when there is a real risk of contamination should condensate not be returned to the boiler. But then it may be possible to collect the condensate and use it as hot process water or pass it through a heat exchanger where its heat content can be recovered before discharging to drain.

Condensate is discharged through traps from a higher to a lower pressure. As a result of this drop in pressure, some of the condensate will then re-evaporate into 'flash steam'. The proportion that will 'flash off' is determined by the pressure difference between the steam and condensate sides of the system, and a figure of 10 % to 15 % by mass is typical. However, the percentage volumetric change can be considerably more. Condensate at 7 bar g will lose about 13 % of its mass when flashing to atmospheric pressure, but the steam produced will require a space some 200 times larger than the condensate from which it was formed. This can have the effect of choking undersized trap discharge lines, and should be taken into account when sizing these lines.

The flash steam generated can contain up to half of the total energy of the condensate, hence flash steam recovery is an essential part of an energy efficient system. Condensate and flash steam discharged to waste means replacement feedwater, more fuel, and increased running costs.

This technical reference guide will look at two essential areas -condensate management and flash steam recovery. Some of the apparent problem areas will be outlined and solutions offered. Illustrations, together with tables and charts to which reference is made, are included in the text. Basic steam tables can be found at the end of this guide.

Note: the term 'trap' is used to denote a steam trapping device which could be a steam trap, a pumping trap, or a pump-trap combination. The ability of any steam trap to pass condensate relies upon the pressure difference across it, whereas a pumping trap or a pump-trap combination is able to remove condensate irrespective of pressure differences across it.

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Condensate return

Monetary value. Condensate is a valuable resource and even

the recovery of small quantities is often economically justifiable. The discharge from a single steam trap is often worth recovering. Unrecovered condensate is replaced by cold make-up water with additional costs of water treatment and fuel to heat the water from a lower temperature.

Water charges. Any condensate which is not returned needs to be replaced by make-up water, incurring further water charges from the local water supplier.

An effective condensate recovery system, collecting the hot condensate from the steam using equipment and returning it to the boiler feed system, can pay for itself in a remarkably short period of time. Fig. 1 shows a typical steam and condensate circuit, where condensate is returned to the boiler feedtank.

Why return condensate and reuse it?

Fig. 1 A typical steam and condensate circuit

Feedtank Make-up water Feedpump Steam Boiler Space heating system Process vessel Condensate Vat Vat Pan Pan Condensate Steam Steam

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 200 400 600 800 1 000 1 200 1 400 1 600 1 800 2 000 2 200 2 400 2 600 2 800100 120 134 144 152 159 165 170 175 180 184 188 192 195 198

Figure 2 shows the relative amounts of energy in steam and condensate at various pressures.

Maximising boiler output. Colder boiler feedwater will reduce

the steaming rate of the boiler. The lower the feedwater temperature, the more heat,and thus fuel needed to raise steam.

Boiler feedwater quality. Condensate is a distilled water which contains almost no dissolved solids (TDS). Blowdown is used to reduce the concentration of dissolved solids in the boiler. More condensate returned to the feedtank reduces the need for blowdown and thus reduces the energy lost from the boiler.

Summary of reasons for condensate recovery.

Water charges are reduced.

Effluent charges and possible cooling costs are reduced. Fuel costs are reduced.

Boiler blowdown is reduced - less energy lost from boiler. Chemical treatment is reduced.

kJ/kg

Fig. 2 Heat content of steam and condensate

Pressure bar g Saturated steam temperature °C

Heat available for flash steam release to atmospheric pressure

Latent heat (enthalpy of evaporation) Total heat of steam

Heat in condensate at steam temperature

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The following example demonstrates how savings are possible by returning condensate to the boiler feedtank. Savings will obviously depend on the cost of fuel and water, and this example gives typical costs in the UK at the time of writing. The fuel used in this example is a heavy fuel oil with a gross calorific value of 42 MJ/litre.

Fuel savings based on the following average temperatures

Condensate return temperature = 90°C Make-up water temperature = 10°C Temperature difference = 80°C

Each kg of condensate not returned must be replaced by 1 kg of cold make-up water that will need heating to the same temperature. Heat required to raise 1 kg of cold make-up water by 80°C:

1 kg x 80°C x 4.19 kJ/kg °C = 335 kJ/kg

Basing the calculations on an average of 10 000 kg/h evaporation rate, and where none of the condensate is presently returned, 24 hours a day, 7 days a week, 50 weeks of the year (8 400 h/year), the nett energy required to replace the heat in the make-up water is:

10 000 kg/h x 335 kJ/kg x 8 400 h/year = 28 140 GJ / per year If the average boiler efficiency is 85 %, gross energy needed to heat the make-up water

2 8140 GJ / year

= 33106 GJ/year 0.85

With a calorific value of 42 MJ / litre, potential savings on fuel 33106 GJ / year

= 788 000 litres / year 42 MJ / litre

With fuel at £0.15 / litre, cost savings = £ 788 000 x 0.15

Therefore, potential annual fuel savings = £ 118 200

Condensate recovery cost saving example

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Effluent savings. The condensate that was not recovered would

have to be discharged to waste which may also be charged by the water authority.

Total amount of water to waste in one year also equals 84 000 m³ If effluent costs £0.45 per m³ = £37 800

Therefore, potential annual effluent savings = £37 800

Total potential savings. The total annual potential savings for

10 000 kg/h evaporated based on none of the condensate presently being returned are :

fuel savings = £ 118 200 water savings = £ 51 240 effluent savings = £ 37 800 total savings = £ 207 240

It follows that for each 1% of condensate returned per 10 000 kg/h evaporated in the above example, a saving of 1% of each of the above values would be possible.

To calculate relative savings based on the same reasoning, use the formulae on the next page by putting figures in the blank boxes.

Fuel savings (on 80°C increase = £ in feedwater)

Water savings = £

Effluent savings = £

Total = £

This sample calculation does not include a value for savings due to correct TDS control and reduced blowdown which will further reduce water loss and boiler chemical costs. These can vary substantially from location to location, but should always be considered in the final analysis. Consult Spirax Sarco for advice regarding any specific installation.

Further information on how to calculate savings by automatic TDS control is available in the Spirax Sarco Technical Reference Guide TR-GCM-01, 'Water treatment, storage and blowdown for steam boilers'.

Clearly, when assessing condensate management for a specific project, such savings should be determined and included.

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Savings in currency used in 'D' =

£ 335 x A x B x C x D E x F

where:

A = average evaporation rate in tonnes/h B = hours per year

C = percentage increase in condensate return D = cost per unit of fuel ( £ / litre; £ / therm; £ / kg)

E = calorific value of fuel per same unit ( MJ / litre; MJ / therm; MJ / kg) F = boiler efficiency

eg, consider the previous example, if a 30 % increase in condensate return is to be made, annual cost savings on fuel:

£ 335 x 10 x 8 400 x 30 x 0.15 42 x 85

fuel savings = £ 35 470

Savings in currency used in 'C' =

A x B x C x D 100

where:

A = average evaporation rate in tonnes/h B = hours per year

C = cost per m³ of water

D = percentage increase in condensate return

eg, consider the previous example, if a 30 % increase in condensate return is to be made, annual cost savings on water:

£ 10 x 8 400 x 0.61 x 30 100

water savings = £ 15 372

Savings in currency used in 'C' =

A x B x C x D 100

where:

A = average evaporation rate in tonnes / h B = hours per year

C = cost per m³ of effluent Fuel savings

Water savings

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Condensate return lines

The subject of condensate piping will divide naturally into four basic sections where the requirements and considerations of each will differ. They are:

Type of line Pipe sized to carry Drain lines to traps condensate

Discharge lines from traps flash steam Common return lines flash steam

Pumped return lines pumped condensate

The condensate must flow from the steam space outlet to the trap. The steam space and the body of the trap upstream of its orifice will usually be at the same pressure, and flow usually occurs due to the force of gravity. As there is no significant pressure drop between the process and the trap, no flash steam is present in the pipe, and it can be sized to carry condensate only.

It should never be assumed that the plant outlet connection indicates the correct size for the trap or condensate pipe, especially in the case of temperature controlled processes where low differentials in pressure can occur across the trap under part-load conditions. Each process will have its own system conditions, and should be treated with these in mind. Refer to the later section 'Stall and the stall point' for further details. Stall is also discussed in Reference Guides:

Steam trapping and air venting - TR-GCM-11

Condensate removal from heat exchangers - TR-GCM-23

Long drain lines from plant can fill with steam and prevent condensate getting to the trap. The effect is generally termed 'steam locking'. To minimise this risk, drain lines should be kept short (Fig. 3), first falling vertically wherever possible before any horizontal run, to ensure the trap is below the plant outlet. This also encourages gravitational flow between the outlet and the trap. Float traps are also available with steam lock release devices to alleviate the problem.

Drain lines to traps

Fig. 3 Keep drain lines short

✗

✔

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These carry condensate, incondensable gases, and flash steam from the trap to the condensate return system (Fig. 4). Flash steam is formed due to the pressure drop across the trap orifice, caused by the difference in pressure between the steam and condensate systems.

During start-up of a steam system, condensate will be cool with little or no flash steam, but the condensing rate will be maximum, and air will have to pass with the condensate. Soon, as the system heats up, full steam load may occur, the pressure in the steam space will be at its highest, and the amounts of flash steam released in the discharge line immediately after the trap will be at their greatest. Trap discharge lines are sized on full load conditions because of this. In so doing, the pipe will be adequately sized for start-up loads, including the efficient purging of non-condensable gases.

Discharging traps into flooded return mains is best avoided, especially from blast action traps draining steam pipelines at saturation temperature. Pumped and rising condensate lines often follow the same route as steam lines, and it is tempting to simply connect drain trap discharge lines into them. The high volume of flash steam released into long flooded lines will violently push the water along the pipe, causing waterhammer, noise, and in the extreme, mechanical failure of the pipe. The solution is to avoid discharging into flooded lines by returning condensate and

Fig. 4 Trap disharge lines pass condensate, flash, and incondensables

Discharge lines from traps

Discharging into flooded return lines

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Common return lines

Fig. 5 A swept tee connection

Steam main

Condensate main

Where condensate from more than one trap flows to the same collecting point such as a vented receiver, it is feasible to run a common line into which the individual lines can discharge, as long as certain conditions are met, and the pipework is adequately sized. When connecting to the common line, swept tees will help to reduce mechanical stress and erosion at the joint (Fig. 5).

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If this is not possible, use a float trap to discharge into the flooded line (Fig. 6). The energy dissipated from the relatively small continuous flow from the float trap can usually be absorbed by the flooded line, especially when fitted with a diffuser such as the DF2.

Fig. 6 Float trap with diffuser into a flooded line

Steam Condensate

Diffuser

Another alternative is to use a thermostatic trap which holds back condensate until it cools below the steam saturation temperature thus reducing the amount of flash steam formed (Fig. 7). To avoid waterlogging the steam main, the use of a generous collecting pocket on the main, plus a cooling leg of 2 to 3 m of unlagged pipe to the trap is essential. The cooling leg gives storage for condensate while it is cooling to the discharge temperature. If there is any danger of waterlogging the steam main, do not use this method. Always consult expert advice from Spirax Sarco if in any doubt.

Diffuser Condensate

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Temperature controlled plant with steam traps draining into flooded lines

Take care if condensate from steam traps on temperature controlled plant is discharged into flooded lines. The back pressure could have a derogatory effect on the performance of the trap and the efficiency of the process (Fig. 8).

Heat exchanger

Steam trap Pumping trap

Heat exchanger

Flooded common line

Fig. 8 Discharge from steam traps into non-flooded lines if possible.

Non - flooded common line

Discharge lines at different pressures

However, condensate from more than one temperature controlled process may join a common line as long as this line is:

a) designed to slope in the direction of flow to a collection point b) sized to cater for the cumulative effects of any flash steam from each of the branch lines at full load.

The concept of connecting the discharges from traps at different pressures is sometimes misunderstood.

If the branch lines and the common line are correctly sized, the pressures downstream of each trap should be virtually the same. However, if these lines are undersized, the flow of condensate and flash steam will be restricted due to a build up of back pressure caused by the increased friction along the pipe. Condensate flow from traps operating at lower pressures will tend to be restricted first.

Each part of the discharge piping system should be sized to carry any flash steam present at acceptable velocities. The discharge from a high pressure trap will not interfere with that from a low pressure trap if the discharge lines and common line are properly sized and sloped in the direction of flow. A later section "Sizing of condensate lines" gives further details.

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Discharge lines from vented pumps

Sizing condensate lines

Fig. 9 Condensate recovery from a vented receiver

Plant Plant Plant

Receiver

Pump Vent

pumped condensate

As mentioned previously, the four main situations for sizing condensate lines are:

Type of line Pipe sized to carry Drain lines to traps condensate

Discharge lines from traps flash steam Common return lines flash steam

Pumped return lines pumped condensate

Flash steam may ultimately be separated from the condensate and used in a recovery system, or vented to atmosphere from a suitable receiver (Fig. 9). The residual hot condensate from the latter can be pumped on to a suitable collecting tank such as a boiler feedtank. When the pump is served from a vented receiver, the return line will be fully flooded with condensate having little or no tendency to create flash steam.

Flow in a pumped return line is intermittent as the pump starts and stops according to needs. The pump discharge rate will be higher than the rate at which condensate enters the pump. It is the pump discharge rate that determines the size of the discharge line.

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Sizing drain lines to traps

From steam mains

Mains diameter - D Pocket diameter - d1 Pocket depth - d2

Up to 100 mm nb d1 = D Minimum d2 = 100 mm

125 - 200 mm nb d1 = 100 mm Minimum d2 = 150 mm

250 mm and above d1 = D / 2 Minimum d2 = D

d2 d1

Steam main

D

Condensate return

A simple rule is to make the line to the trap the same size as

the trap connections. This presupposes, however, that the trap

itself has been sized on sound technical reasoning. A brief synopsis follows:

Steam traps basically fall into two distinct areas of application, steam mains or process applications.

The condensate load per trap is affected by various factors such as the size of the pipe, pressure, degree of insulation, ambient temperature, number of traps used along a defined length, position and situation of the pipe. The Technical Reference Guide 'Steam Distribution' (TR-GCM-03), gives information for condensate loads with different sized pipes at various pressures.

It is sufficient to consider a condensate load for each drain trap based on 1% of the steam capacity of the main and traps placed every 50 m if insulated, and 5% and 25 m if not. Whatever the size of the main and traps, it is important they are served by an adequately sized drain pocket. As a guide, see below (Fig. 10):

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From process applications

Fig. 11 Typical constant pressure application

Air vent

Jacketed pan Reducing valve

Trap set

The drain line off-take should be at least 25 to 30 mm from the bottom of the pocket for mains up to 100 mm, and roughly a third to centre of the pocket for larger mains. This allows a space below the outlet for dirt and scale to collect, and the bottom may be fitted with a blowdown valve for cleaning purposes.

On most drain points, by sizing the trap to pass approx twice the rated design load at the working pressure (minus any back pressure) will allow it to cope with both start-up and running loads.

The method of selecting and sizing the trap depends on whether the process is temperature controlled or not, but in either case the pipe should be sized as below on the worst condition.

i) Applications on constant steam pressure

Some applications work on a constant pressure supply, such as presses, ironers, ovens, unit heaters, radiant panels, boiling pans etc. When an adequate steam supply is provided, the working pressure tends to remain fairly constant even under varying load conditions. The worst condition will apply at start-up when the steam pressure will tend to drop and the condensation rate is at its highest due to the large difference in temperature between the steam and cold metal.

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ii) Applications with temperature control

If the process is temperature controlled, the system operating parameters and layout need to be considered in greater detail as the heat load may change during normal operation. The steam pressure and condensate load in the heat exchanger will alter as the steam control valve modulates to meet this change, and as the steam pressure reduces, so does the trap's capacity.

Take the case of an air heater battery which is designed to heat air from -5o_{C to 25}o_{C using steam at 3.2 bar (145}o_{C). If the incoming air} temperature rises to 5o_{C, the}DT and heat load will be reduced by 30%. The steam temperature will reduce by ratio, and once established, its pressure can be established from steam tables. Steam temp. at full load = 145 o_{C (a)}

Steam temp. at no load = 25 o_{C (b)} ie, steam temperature range = 120 o_{C (a - b)}

30% of range = 40 o_{C (c) = ( [a - b] >< 0.3)} steam temp. at 30 % reduction = 105 o_{C (a - c)}

steam pressure at 105 o_{C = 0.2 bar g (from steam tables)} The pressure in the heat exchanger has reduced from 3.2 bar g to 0.2 bar g, and will reduce the trap's capacity. If the trapping device were a float trap and sized on the full load at 3.2 bar g, then it is possible that its capacity may be below that needed at the lower pressure. It is for this reason that it is important to size the float trap on the minimum heat load rather than the full load. Should the steam space pressure reduce enough to approach the condensate pressure, stall will occur and the trapping device is selected and sized on the load at stall point.

Not all temperature controlled applications will stall. Stall will not occur if the steam space pressure at the minimum heat load is higher than the condensate back pressure.

Whether the trapping device is a float trap, a pumping trap, or a mechanical pump and float trap in combination, will depend on the system operating requirements and the piping infrastructure. The drain line can usually be the same size as the trap especially on shorter lines, but on lines over 5 m, should be checked on the table in Fig. 37 on page 48, against a pressure drop of up to 160 Pa / m.

The size of the trap discharge line needs to be determined by a different set of rules, and this is considered next.

Stall and its implications on trap sizing is discussed in further detail in a later chapter, and in the Technical Reference Guide "Condensate Removal from Heat Exchangers" (TR-GCM-23)

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The section of pipeline downstream of the trap will carry both condensate and flash steam at the same pressure and temperature. This complex situation is called "two phase flow", where the mixture of fluids will have the characteristics of both steam and water in proportion to how much of each component is present. Consider this by example where 10% of condensate forms flash steam : Sizing discharge

lines from traps

As each kg of condensate at 4 bar g passes through the trap, 0.1 kg will become steam at 100°C, and 0.9 kg will become water at 100°C. However, the respective volumes will depend on the specific volume of each at the pressure in the line (0 bar g). 0.9 kg of condensate will have a volume of = 0.0009 m³ 0.1 kg of flash steam will have a volume of

0.1 kg >< 1.673 m³/kg (spec. vol.at 0 bar g) = 0.1673 m³ Total volume of 1 kg of the mixture = 0.1682 m³ Therefore, 0.0009 >< 100 = 0.5% is volume of water in the line 0.1682

and, 0.1673 >< 100 = 99.5% is the volume of flash steam Fig. 12 Quantity of flash

steam from condensate

13 12 11 10 9 8 7 6 5 4 3 2 1 0

Pressure on traps bar g

0 0.02 0.06 0.10 0.14 0.18 0.22

Flash steam pressures

Example

kg Flash per kg condensate 2.5 bar g 2.0 bar g 0 bar g 0.5 bar g 1.0 bar g 1.5 bar g

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Steam lines are sized with attention to maximum velocities. Dry saturated steam can safely travel up to 40 m/s. Wet steam needs to travel somewhat slower (15 to 25 m/s) as it carries moisture which can have an erosive and damaging effect on fittings and valves if travelling too fast. Similarly, trap discharge lines can be regarded as steam lines carrying very wet steam, and should be sized on similar velocities.

Condensate discharge lines from traps are notoriously more difficult to size than steam lines due to the two phase flow characteristic. In practice, it is impossible to determine what is going on inside the pipe with any certainty.

Although the amount of flash steam produced is related to the pressure difference across the trap, there are other factors that will have some bearing on what is happening inside the pipe. For example,

If, for some reason, the condensate on the upstream side of the trap is cooler than the saturation temperature, the amount of flash formed after the trap is reduced. This can reduce the size of the line needed.

If the line slopes down from the trap to its termination, the degree of slope will have an effect on the flow of condensate, but to what magnitude, and how can this be quantified?

On longer lines, radiation losses from the line may condense some of the flash, its volume will decrease along with its velocity, and there may be a case for reducing the line size. But at what point should it be reduced and by how much?

If the discharge line lifts up to an overhead return line, there will be times when the lifting line will be full of cool condensate, and times when flash steam from the trap may evaporate some or all of this condensate. Should the line be sized on flash steam velocity or the quantity of condensate?

Most processes operate some way below their full load condition for most of their running cycle, which reduces the amount of flash produced for most of the time. Should the designer size on the full load condition when it may not be warranted due to the frequency and small amount of time it occurs?

On temperature controlled plant, the pressure differential across the trap will itself change depending on the heat load. This will affect the amount of flash steam produced in the line.

Due to the conflicting nature of all the above, an exact calculation of line size would be complex and probably inaccurate. In practice, experience has shown that if trap discharge lines are sized on comfortable flash steam velocities and certain recommendations are adhered to, few problems will arise.

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Correctly sized trap discharge lines that slope in the direction of flow and are open-ended are non-flooded and allow flash steam to pass unhindered over the condensate, (Fig. 13). A minimum slope of 1 in 70 (150 mm drop every 10 m) is recommmended. A simple visual check will usually confirm if the line is sloping - if no slope is apparent it is not sloping enough! Recommendations

on trap discharge lines

If unavoidable, non-pumped rising lines (Fig. 14) should be kept as short as possible and fitted with a non-return valve to stop condensate falling back down to the trap. They should discharge into the top of overhead return lines to allow easy passage of flash steam into them. It is sensible to consider slightly larger pipes having lower flash steam velocities to reduce the risk of waterhammer and noise from the steam trying to find passage through the liquid condensate in the rising line.

Important: A rising line should only be used where the lowest

steam pressure in the process is guaranteed to be higher than the total condensate back pressure. If not, the process will waterlog unless a pumping trap or pump/trap combination is used to provide proper drainage against the back pressure. Fig. 13 Discharge line sloping 1 : 70 in the direction of flow

1 : 70 slope = 150 mm per 10 m run Process

easy passage for condensate easy passage for flash steam

vented receiver and pump vent

pumped condensate

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Process

Fig. 14 Keep rising lines short and connect to the top of return lines

1 : 70 slope = 150 mm per 10 m run

vented receiver and pump vent

pumped condensate

Return lines themselves should also slope down and be non-flooded (Fig. 14). To avoid flash steam occurring in non-flooded return lines, hot condensate from trap discharge lines should drain into vented receivers (or flash vessels where appropriate), from where it can be pumped on to its final destination via a flooded line at a lower temperature.

The Condensate pipe sizing chart (Fig. 15).

The condensate pipe sizing chart can be used to size any type of condensate line.

Lines containing two-phase flow, such as trap discharge lines, are selected according to the pressures either side of the trap. The chart works around acceptable flash steam velocities according to the pipe size and percentage flash steam formed.

The chart can be entered on lower temperatures than the steam saturation temperature, such as may be the case when using thermostatic steam traps for condensate discharge.

Flash steam has to pass through the condensate

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Pipe sizes can be estimated for pumped lines containing condensate below 100°C, as shown by example 5. Also, short drain lines to traps (less than 5 m) can be determined in a similar way. Note: in the case of pumped lines, the pressure drop and velocity must always be checked by referring the condensate flowrate to the pipe size against the table provided in Fig.37 (pages 48 and 49).

The chart is used to size trap discharge lines on full load conditions. It is not necessary to consider any oversizing factors for start-up load or the removal of non condensable gases. On the lower chart, establish the point where the steam and condensate pressures meet. Move vertically up to the upper chart to choose the selected condensate rate. If the discharge line is falling (non-flooded) and the selection is on or between lines, choose the lower line size. If the discharge line is rising (flooded), choose the upper line size, (Fig. 15).

Some examples for sizing trap discharge lines follow.

Note: The reasoning behind sizing a trap and a discharge line is

different, and it is perfectly normal for a trap discharge line to be a different size than the trap it is serving. However, the normal ancillary equipment associated with the steam trap set, such as the isolation valves, strainer, trap testing chamber, and check valve can be the same size as the trapping device whatever the discharge line size.

A condensate line sizing chart is provided for photocopying in Appendix 1 (page 81).

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Fig. 15 Condensate line sizing chart 100,000 50,000 20,000 10,000 5,000 2,000 1,000 500 200 100 50 20 10 Condensate rate kg/h

Condensate system pressure bar g

Condensate line size mm

150 100 80 65 40 32 25 20 15 10 6 30 10 0 0.5 1 2 3 4 5 50 5 2 1 3 4 6 500 400 350 300 250 200 20 6 1 2 4 3 250 100 120 140 160 180 200

Steam temperature °C Steam system pressure bar g

30 10 5 0 0.5 1 2 20 15 50

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Example 1

Fig. 16 Example 1 - non-flooded pressurised trap discharge line

H.P. steam

6 bar

Float trap set

Shell and tube heat exchanger

L. P. steam 1.7 bar 25 Ø

Flash vessel

A steam trap passing a full load of 1 000 kg/h at 6 bar g saturated steam pressure through a sloping discharge line down to a flash vessel at 1.7 bar g.

As the discharge line is non-flooded, the lower figure of 25 mm is selected from the chart.

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Example 2

Fig. 17 Example 2 - flooded trap discharge line

Add the 0.5 bar static pressure (5 m head) to the 3.5 bar condensate pressure to give 4 bar g back pressure.

As the discharge line is rising and thus flooded, the upper figure of 32 mm is selected from the chart.

H.P. steam 18 bar 3.5 bar 32 Ø Air vent Float trap SA control valve acting

as air vent and condensate drain on start up

5 m

A steam trap passing a full load of 1000 kg/h at 18 bar g saturated steam pressure through a discharge line rising 5 m up to a pressurised condensate return line at 3.5 bar g.

Discharge line being sized

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Example 3

Fig. 18 Example 3 - non-flooded vented trap discharge line

A steam trap passing a full load of 200 kg/h at 2 bar g saturated steam pressure through a sloping discharge line falling down to a vented condensate receiver at atmospheric pressure.

As the line is non-flooded, the lower figure of 20 mm is selected from the chart.

H.P. steam

Plate heat exchanger

20 Ø

Vent

25 Ø To high level condensate return line

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Example 4 A pumping trap passing a full load of 200 kg/h at 4 bar g saturated steam space pressure through a discharge line rising 5 m up to a non-flooded condensate return line at atmospheric pressure.

Fig. 19 Example 4 - flooded trap discharge line

The 5 m static pressure contributes the total back pressure of 0.5 bar g.

As the trap discharge line is rising, the upper figure of 25 mm is selected from the chart.

H.P. steam 4 bar Air flow 5 m Discharge line being sized 25 Ø

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The automatic condensate pump shown in example 3 can also have its discharge line sized by the chart. The pump discharge rate is sized on 6 times the maximum expected inlet rate, in this case 6 >< 200 kg / h = 1 200 kg / h.

Example 5

Example 6

Fig. 20 Example 5 - pumped discharge line

Because the condensate will have lost its flash steam content to atmosphere via the receiver vent, the pump will only be pumping liquid condensate. In this instance, it is only necessary to use the top graph as shown in the example. As the line from the pump is rising, the upper figure of 25 mm is chosen.

A useful tip for lines of 100 m or less is to choose the discharge pipe the same size as the pump. Also refer to the later section on condensate pumping for further details.

A balanced pressure thermostatic steam trap draining a hot table operating on a constant steam pressure of 2.6 bar g discharges condensate at 20°C below saturation temperature from a 2 metre cooling leg up to an overhead non-flooded condensate line 2 metres above the trap. The full load is 100 kg/h.

The saturation temperature of steam at 2.6 bar g is 140°C, so the discharge temperature from the trap will be around 120°C. The chart is then entered on the temperature scale at 120°C rather

Condensate in Vent 25 Ø Pumped condensate out

Sloping non-flooded return line

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Example Sizing common

return lines

It is sometimes required to connect several trap discharge lines from separate processes into a common return line. Problems will not occur if the following considerations are met:

a) the common line is not flooded and slopes in the direction of flow to an open end or a vented receiver, or a flash vessel if the conditions allow.

b) the diameter of the common line is sized on the cumulative sizes of the branch lines.

The common line size downstream of two connected trap discharge lines is the root of the sum of the squares of the connected lines.

The example shows three heat exchangers, each separately controlled and each operated at the same time. Loads shown are full condensate loads and occur at 3 bar g in the steam space. The common line slopes down to the flash vessel at 1.5 bar g situated in the same plant room. Condensate in the flash vessel falls via a float trap down to a vented receiver from where it is pumped direct to the boiler house.

The trap discharge lines are sized on full load with steam pressure at 3 bar g and condensate pressure of 1.5 bar g, and as each is not flooded, the lower line sizes are picked from the graph. Line 1 picked as 20 mm, 2 picked as 20 mm, 3 picked as 15 mm Common line for 1+2 = Ö 20²+20² = 28 mm

Common line for (1+2)+3 = Ö 28²+15² = 32 mm

Fig. 21 Calculating the common line size from the discharge lines

3 bar g HE 1 Full load 750 kg/h 1" FT14HC 20 Ø 3 bar g HE 2 Full load 750 kg/h 1" FT14HC 28 Ø 3 bar g HE 3 Full load 375 kg/h 1" FT14 20 Ø 15 Ø 32 Ø Flash steam 1.5 bar g To receiver and pump

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Line Size (mm) Commercial size (mm) A 40 40 B 15 15 C 402₊₁₅2 _{= 42.7} ₄₀ D 15 15 E 152_+42.72 _{= 45.2} ₄₀ F 15 15 G 152_+45.22 _{= 47.6} ₅₀ H 15 15 J 152_+47.62 _{= 49.9} ₅₀

Fig. 22 Trap discharge lines connecting to a common line Example ? 32 Ø 15 Ø 15 Ø 15 Ø 40 Ø ? A B C D E F ? ? H G K J L ? 15 Ø

Ö

The theoretical dimension of 28 mm for the common line 1+2 does not exist as a nominal bore in commercial pipe sizes. The internal diameters of pipes can be larger or smaller than the nominal bore depending on the pipe schedule. Eg. for a DIN 2448 steel pipe, the internal diameter for a 25 mm nb is about 28.5 mm, while that for a 25 mm nb Schedule 40 pipe is about 26.6 mm.

For most practical purposes, a 25 mm nb pipe may be comfortably selected. If in doubt, seek expert advice.

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Flash steam, separated from the condensate, will be used in a flash steam recovery system or simply vented to atmosphere. The remaining hot condensate should be pumped to the boiler house where its energy content and purity can be used to good effect. The pumped return line will only carry condensate but at lower velocities (typically 1 - 2 m/s) than those experienced in the trap discharge and common lines. As seen in example 5, the pump discharge line can be selected from the condensate line sizing chart, or often simply sized the same size as the pump outlet. Refer to the following section "Condensate Pumping" for more detail.

It is important to remember that the flow in a pumped line is intermittent, as the pump usually cycles. The instantaneous flowrate while the pump discharges is higher than that which enters the pump. It is the instantaneous discharge figure that has to be considered on discharge lines.

Water in lines longer than 100 m will develop larger forces of inertia due to the larger mass of water that is moved during the pumping stroke. It is advisable to add the effects of inertia to pressure drop calculations on sizing these longer lines when mechanical pumps are used. Refer to the section 'longer delivery lines' at a later stage in this document for further details. As a general rule, the pipe should be at least one size larger than the pump outlet check valve.

Sizing pumped return lines

Pumped lines longer than 100 m

Fig. 23 An additional check valve 1 pipe length from the pump body to reduce the effect of backflow

Mechanical pump

Line over 100 m

Additional check valve 1 pipe length from pump

At the end of the pumping stroke, the condensate will tend to keep moving and can often cause a vacuum to be created downstream of the pump outlet check valve. As the momentum of the condensate falls, the vacuum creates a sudden backflow onto the check valve which can, in extreme cases, cause severe waterhammer and noise. An additional check valve fitted 1 pipe length after the pump outlet check valve tends to dampen the effect and protect the pump check valve from damage (Fig. 23).

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Mechanical pump

fall Vacuum breaker

Fig. 24 best choice - lift after the pump

Automatic air vent

fall due to obstruction

Mechanical pump

Tank

Should the falling line have to fall anywhere along its length to overcome an obstruction, then an automatic air vent fitted at the highest point will assist flow around the obstruction (Fig. 24). If there is any choice, it is always best to lift immediately after the pump to a height allowing a gravity fall to the end of the line (Fig. 24). If the fall is enough to overcome the frictional resistance of the pipe (Fig. 26), then the only back pressure onto the pump is that formed by the initial lift. A vacuum breaker can be installed at the top of the lift not only to assist the flow along the falling line but also to prevent any tendency for backflow at the end of the stroke.

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Alternatively, any question of back pressure caused by the horizontal run can be entirely eliminated by an arrangement as in Fig. 25 in which the pump simply lifts into a breaktank. The pipe from the tank should fall in accordance with the table in Fig. 26.

Fig. 26 Pipe fall to overcome frictional losses

Discharge lines from pumps vented to atmosphere are sized on the discharge rate of the pump. Condensate passing through pumping traps and pump/trap combinations in closed loop applications will often be at higher pressures and temperatures and flash steam will be formed in the discharge line.

Because of this, discharge lines from pumping traps (such as the APT14), and pump/trap combinations (such as an MFP14 and FT float trap) are sized on the trapping condition at full load and not the pumping condition, as the line has to be sized to cater for flash steam. Sizing on flash steam will ensure the line is also able to cope with the pumping condition.

Vented pumps, pumping traps and pump-trap installations

Pipefall need Pipe size (DN mm)

to overcome 15 20 25 32 40 50 65 80 100 125 150 pipe friction Litres of water per hour

25 mm in 15 m 48 140 303 580 907 1 950 3 538 5 806 12 610 22 906 37 284 25 mm in 10 m 59 177 381 694 1 134 2 449 4 445 7 257 15 680 28 576 46 492 25 mm in 8 m 69 204 442 800 1 310 2 834 5 148 8 391 18 159 33 089 53 862 25 mm in 6 m 79 231 503 907 1 487 3 220 5 851 9 525 20 638 37 602 61 223 25 mm in 5 m 86 256 553 1 007 1 642 3 551 6 441 10 568 22 770 41 821 67 538 25 mm in 4 m 93 279 598 1 093 1 778 3 878 7 030 11 521 24 811 45 994 73 571 25 mm in 3 m 113 338 730 1 329 2 168 4 672 8 527 13 925 30 073 54 073 89 356 25 mm in 2 m 140 419 907 1 655 2 694 5 851 10 614 17 327 37 421 68 039 111 128 25 mm in 1.75 m* 152 454 984 1 793 2 923 6 327 11 498 18 756 40 573 73 708 120 426 25 mm in 1.5 m 165 490 1 061 1 932 3 152 6 804 12 383 20 185 43 726 79 378 129 725 25 mm in 1 m 206 612 1 324 2 404 3 923 8 482 15 422 25 174 54 431 99 019 161 476 *(1:70)

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In nearly all steam using plants, as much condensate as possible should be returned to the boiler house to use again. Even if gravity drainage can be used from the plant to the boiler house, often the condensate must be lifted into a boiler feedtank.

Before looking at the types of pump available for condensate pumping, it may be helpful to discuss some basic pumping terminology.

Vapour pressure. This term is used to define the pressure

corresponding to the temperature at which conversion of a liquid into vapour takes place. In other words, it is the pressure at which a liquid will boil i.e.

At atmospheric pressure, water will boil at 100°C At a pressure of 7 bar g, water will boil at 170.5°C At a pressure of 0.75 bar abs, water will boil at 92°C

The vapour pressure is a very important consideration when pumping condensate. Condensate is usually close to its boiling point, which may cause difficulties where a centrifugal pump is concerned. This is because as the condensate is drawn into the pump impeller, it is accelerated and so experiences a drop in pressure. If this drop in pressure takes the condensate below the vapour pressure or saturation pressure for its temperature, the condensate will boil and some of the condensate will be released as flash steam bubbles.

As the bubbles are carried along within the water, they reach a region of increased pressure, as they leave the pump impeller. This increased pressure brings the steam bubbles back above the saturation pressure causing the bubble to implode rapidly. If this occurs while next to a solid surface, the forces exerted by the liquid rushing in to fill the spaces creates very high localised pressures. This is known as cavitation and is capable of doing a great deal of damage to a pump impeller and housing within a short period of time. It also creates noise, similar to that of gravel rotating in the pump.

It is often recommended that electrical pumps are not used to pump condensate at temperatures above 100°C. Some will have limits as low as 94°C or 96°C, depending on the design of the

Condensate pumping from

vented receivers

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Pressure head (h_p). Pressure head is simply the fluid pressure

at the point in question. e.g. A pump is required to discharge against a pressure head of 3 bar g. The pump fills from a pressure head of 0.1 bar g. Where water is the fluid, a 1 bar pressure head is equivalent to approximately 10 m of static head.

Static head (h_s). Static head is the equivalent vertical height of fluid

above the point in question. The following example best explains the measure of static head. The pump inlet in Fig. 28 is subjected to a static head (known as the suction or filling head) of 1 m, and discharges against a static head (known as the static delivery head) of 30 m. Note that in this case, the water in the bottom of the header tank is above the pump inlet (this situation is called a flooded suction), With an electrical pump the suction head is subtracted from the static delivery head, to give the net static head against which the pump has to work. With a mechanical displacement pump (Fig.29), the filling head simply provides the energy to fill the pump, and has no effect on the head against which the pump has to operate.

Net static head 29 m

0.1 bar g 3 bar g

Fig. 27 Pressure head

Fig. 28 Net static head for an electrical pump

Collecting tank Pump inlet Pump inlet Header tank Static delivery head 30 m Static suction head

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Fig. 29 Net static head for a mechanical pump hs = net static head Condensate flow flooded suction head Collecting tank Pump receiver

Friction head (h_f). The friction head is more accurately defined

as the pressure loss due to friction, and is the head required to actually move the liquid along the pipeline, and, in simple terms, increases proportionately to the square of the velocity.

Pressure loss can be found from tables showing the liquid flowrate, the pipe diameter and the pipe length. To be precise, the resistance to flow encountered by the various pipe line fittings must also be taken into account. Tables are available to calculate the equivalent length of straight pipe for various pipe fittings.

This extra 'equivalent length' for pipe fittings is then added to the actual pipe length to give a 'total equivalent length'. However, in practice, if the pipe is correctly sized, it is unusual for the pipe fittings to represent more than an additional 10 % of the actual pipe length.

A general rule which can be applied is:

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Total delivery head (hd). The total delivery head hd against

which the pump needs to operate is the sum of :

Pressure required to raise the water to the desired level h_s Pressure required to move the water through the pipes h_f Pressure in the condensate system h_p

ie

Total delivery head, hd = hs + h_f+ h_p

Pump operation. Centrifugal pumps utilise centrifugal force, which imparts a high velocity to the liquid (condensate) being pumped. Pressure energy is obtained by the rotation of an impeller fitted within a casing.

Liquid enters the pump and is directed to the centre of the rotating impeller vanes. As the impeller rotates, the liquid is passed along the impeller vanes and increases in velocity.

Pump application. The electrical pump is well suited to applications where large volumes of liquid need to be moved. Electrical pumps are usually built into a unit, often referred to as a condensate recovery unit (CRU). A CRU will usually include:

A receiver.

A control system operated by probes or floats. One or two pumps.

The instantaneous flow from the CRU can be up to 1.5 times greater than the rate at which condensate returns to the receiver. It is this pumping rate that must be considered when calculating the friction loss in the discharge line.

On twin pump units, a 'cascade' control system may also be employed which allows either pump to be selected as the 'lead' pump and the other as a 'stand-by' pump to provide back up if the condensate returning to the unit is greater than one pump can handle. This control arrangement also provides back up in the case of the one pump failing to operate; the condensate level in the tank will increase and bring the second pump into operation. Cascade type units usually pump at a rate of 1.1 times the return rate to the receiver, allowing a smaller discharge line to be considered.

It is very important that the manufacturer's literature is read regarding the discharge pumping rate. Failure to do so could result in undersizing the pump discharge pipe work.

Electrical centrifugal condensate pumps

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Fig. 30 A typical electrical condensate recovery unit (CRU) Condensate in Condensate out Condensate in Receiver Electric pump Vent Level sensor Overflow with "U" seal

To size an electric condensate recovery unit, it is necessary to know:

The amount of condensate reaching the receiver in kg/h at running load.

The temperature of the condensate. This must be below the manufacturer's specified ratings to avoid cavitation, however, manufacturers usually have different impellers to suit different temperature ranges, eg. 90°C, 94°C and 98°C.

The total discharge head required. (Will need to be calculated Sizing an electrical

condensate recovery unit

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Sizing the discharge pipework for an electric condensate recovery unit Temperature of condensate = 94°C Condensate to be handled = 1 800 kg/h Static lift ( h_s) = 30 m Length of pipe work = 150 m

Condensate back pressure = friction losses only ( h_f)

Using the data below, an initial selection of a condensate recovery unit can be made from the manufacturer's sizing chart, such as the one in Figure 31. From the chart, CRU 1 should be the initial choice subject to frictional losses in the delivery pipework. Example

Fig. 31 Typical CRU sizing chart 35 30 25 20 15 10 5 2 000 1 000 500 300 400 200 100 Pump delivery head in metres Condensate to be handled at 94°C kg/h CRU 3 CRU 2 CRU 1 1800

From the chart in Fig. 31, it can be seen that CRU 1 is actually rated to handle 2 000 kg/h of condensate.

Reading the manufacturer's data shows that the CRU will actually pump 1.5 times the maximum return rate shown on the sizing chart. i.e.:

1.5 x 2 000 kg/h = 3 000 kg/h

This ensures start-up loads can be handled without overflowing, and this is what the discharge pipe work must be sized on. As in the earlier example it is now possible to determine the optimum size for the return line.

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Fig. 32 Section of typical friction loss table for fully flooded pipelines (flowrates in L/h) Actual length of pipe work = 150 m

Equivalent length of pipe work = 150 m + 10 % = 165 m

From the pressure drop table above, using a 40 mm nb. pipe will allow a flowrate of 3 000 kg/h (L/h) and incur a pressure drop of between 120 and 140 Pa per metre. For this example 128 Pa/m is about right. Therefore the head loss to friction can be calculated;

Headloss to friction = 128 Pa / m x 165 m = 21 kPa

= approx 2.1 metres The total delivery head required by the pump is:

30 m (h_s) + 2.1 m (h_f) = 32.1 metres

The figure of 32.1 metres needs to be checked against the manufacturer's sizing chart for the CRU to confirm that there is sufficient head available - there is in this case, but had the allowable head been exceeded, then the options are to re-calculate using a larger pipe or to select a CRU with a greater lift capacity. Alternatively, it can be seen that the selected CRU1 can pump against a total head (h_d) of 35 m. With an actual static head (h_s) of 30 m, 5 m are "available" for pipe friction loss (h_f). It may be possible to install a smaller pipe and take up a larger friction loss. Reference to the pipe sizing table on page 42 will show that, if the next lower sized pipe is used (in this case 32 mm), the unit friction loss (h_f) to pass 3000 kg/h (or 3000 L/h) is 300 Pa/m, and the

Pressure drop Pipe size (mm)

Pa/m mbar/m 15 20 25 32 40 50 65 80 100 95 0.95 176 414 767 1 678 2 560 4 860 9 900 15 372 31 104 97.5 0.975 180 421 778 1 699 2 596 4 932 10 044 15 552 31 500 100 1.00 184 425 788 1 724 2 632 5 004 10 152 15 768 31 932 120 1.20 202 472 871 1 897 2 898 5 508 11 196 17 352 35 100 140 1.40 220 511 943 2 059 3 143 5 976 12 132 18 792 38 160

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Pump operation. Mechanical pumps consist of a body, into

which condensate flows by gravity, containing a float and an automatic mechanism, operating a set of changeover valves. Condensate is allowed to build up inside the body, which raises a float. When the float reaches a certain level, it triggers a vent valve to close and an inlet valve to open to allow steam to enter and pressurise the body to push out the condensate. The condensate level and the float both fall. The steam inlet valve then shuts and the vent valve opens allowing the pump body to refill. Check valves are fitted to the condensate ports to ensure correct directional flow.

It should be noted that a receiver is needed when using a pump (Fig. 33), due to its cyclical action. When the pump is discharging it is not filling, so there is a need to store the condensate which is being produced between pumping cycles.

Mechanical condensate pumps

Fig. 33 A typical mechanical condensate pump

Pumped condensate Steam supply to pump

Condensate in Vent

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Pump application. Generally, mechanical pumps handle smaller amounts of condensate than electrical pumps. They are however, particularly valuable in situations where:

Condensate temperature causes cavitation. Condensate is in vacuum.

Space is at a premium.

Low maintenance is required.

The environment is hazardous, humid or wet.

Electrical supplies are not at hand (operated by steam, air or any inert gas).

Condensate has to be removed from individual items of temperature controlled equipment which may be subjected to stall conditions.

As with electrically driven pumps, they are sometimes, but not always, specified as packaged condensate recovery units. A mechanical condensate recovery unit will comprise a condensate receiver and the pump unit. No additional control system is required as the pump is fully automatic and only operates when needed. This means that the pump is self regulating.

Mechanical pumps are, however, a little more involved to size because the flow in the return line is intermittent. The pump cycles as the receiver fills and empties. The instantaneous flowrate while the pump is discharging can often be up to six times the filling rate and it is this instantaneous flowrate which must be used to calculate the size of the discharge pipe. Always refer to the pump manufacturer for data on sizing the pump and discharge line.

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To size a mechanical condensate pump, the following information is required:

The maximum condensate flowrate reaching the receiver. The motive pressure of steam or air available. The selection of steam or air depends on the application and site circumstances. The filling head available.

The total back pressure of the condensate system.

The sizing of mechanical pumps varies from manufacturer to manufacturer, and is usually based on empirical data, which are translated into factors and nomographs. The following is a typical example on how to size a mechanical pump. (The pipe length is less than 100 m and friction loss is taken as being negligible):

Condensate load = 2 200 kg/h Steam pressure available for operating pump = 5.2 bar g

Vertical lift from pump to return piping = 9.2 m Pressure in the return piping (piping friction negligible) = 1.7 bar g

Available filling head on the pump = 0.3 m Sizing a mechanical

condensate pump

Example

Total plant condensate 2 200 kg/h

1.7 bar g return main pressure

5.2 bar g operating pressure

Fig. 34 Mechanical pump sizing example

Filling head 0.3 m Reservoir _{9.2 m lift} Pump Vent Condensate manifold

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Calculate the total back pressure (delivery head h_d), against which the condensate must be pumped:

Total back pressure (h_d) = lift (h_s)+ condensate pressure (h_p) (friction loss neglected as line is shorter than 100 m)

lift (h_s) = 9.2 m

cond. pressure (h_p) = 1.7 bar g = 17 m head

Total = 9.2 + 17 m

= 26 m

Reference to Fig. 35 below shows that a DN50 pump at 5.2 bar g motive pressure will pump 2600 kg/h against a 26 m head, and will thus be the correct choice for this example.

Note: the pump is sized on the filling rate.

14 13 12 11 10 9 8 7 6 5.2 5 4 3 2

80 m lift 50 m lift 40 m lift 30 m lift 20 m lift 10 m lift 4 m lift 32 m 27 m 26 m 14 13 12 11 10 9 8 7 6 5.2 5 4 3

80 m lift 50 m lift 40 m lift 30 m lift 20 m lift 10 m lift

4 m lift

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Longer delivery lines Sizing the discharge

pipework for a mechanical condensate pump

Below 100 m long, the discharge pipe from a mechanical pump can usually be taken as the same size as the pump body. The frictional resistance of the pipe is relatively small compared to the back pressure caused by the lift and condensate return pressure, and can usually be disregarded. Above 100 m, a general rule would be to select one pipe size larger than the pump outlet check valve.

On delivery lines over 100 m, and/or where the condensate flow is near to the pump maximum, it is advisable to check the pipe size to ensure that the total friction loss (including inertia loss) does not increase above that which effects the pump's capability (or installation costs).

With 5.2 bar g motive steam and 26 m delivery head, from Fig. 35, for a DN50 pump,

Maximum pump capacity = 2 600 kg/h

Actual condensate flowrate into pump = 2 200 kg/h again, from Fig. 35, for a DN50 pump,

Max. back pressure permissible at 2 200 kg/h = 32 m therefore, max. frictional resistance allowable = 32 - 26 m

= 6m (60 kPa)

Inertia loss

On lines over 100 m, a considerable volume of liquid will be held within the pipe. The sudden acceleration of this mass of liquid at the start of the pump discharge can absorb some part of the pump energy, and this needs to be considered within the friction loss calculation by reducing the allowable friction loss by 50%, thus, Total allowable friction loss = 50 % × 60 kPa

= 30 kPa Consider delivery pipe length to be 250 m

+ 10% for additional fittings = 275 m then, max. frictional resistance allowable / m = 30 kPa

275 m approx. = 109 Pa/m Taking delivery flowrate as 6 times filling rate = 6 × 2 200

= 13 200 kg/h

Referring to Fig. 37 (the table), a frictional resistance of 109 Pa/m reveals that an 80 mm pipe is required to give an acceptable flowrate of 13 200 kg/h. In fact, the table shows that this size pipe will pass about 16 500 kg/h with this frictional resistance.

By rising up the '80 mm column', it can be seen that, by interpolation, the flowrate of 13 200 kg/h actually induces a frictional loss of about 72 Pa/m in an 80 mm pipe.

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Fully loaded pumps and longer lines

Should the condensate filling rate have been near the maximum 2 600 kg/h for the above example, say 2 500 kg/h, then less head is available for friction loss, and progressively less so for longer lines. Sizing on a filling rate of 2 500 kg/h, and a 250 m (+10%) line, referring to Fig. 35, for the DN50 pump, it can be seen that a condensate filling rate of 2 500 kg/h equates to a max. back pressure of about 27 m, hence in this instance,

available head left for friction losses = 27 - 26 m = 1 m (10 kPa) frictional resistance allowable = 10 kPa

275 m = 36 Pa / m minus allowance of 50% for inertia loss = 50 % × 36 Pa/m therefore, max. frictional resistance allowable = 18 Pa / m

As before, the discharge pipework has to be sized on the instantaneous flowrate from the pump outlet, which is taken as 6 × the filling rate. In this instance, the pipe would have been sized on 6 × 2 500 kg/h = 15 000 kg/h with a friction loss of 18 Pa/m. Fig. 37 (the table) reveals that this would require a pipe larger than 100 mm to allow the pump to operate within its capability.

Although the system would certainly work with this arrangement, it may be more economical to consider a larger pump with smaller pipework.

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Consideration of a larger pump and smaller pipeline

Fig. 36 reveals that a DN 80 pump under the same conditions of 5.2 bar g motive steam and 26 m back pressure would allow the following friction losses:

Back pressure = 26 m

At a filling rate of 2 500 kg/h, max. allowed = 35 m head available for friction loss = 35 - 26 m

= 9 m (90 kPa) 90 kPa over 250 m and inc. inertia loss = 50 % × 90

250 max. frictional resistance allowable = 180 Pa/m

Fig. 37 (the table) shows that an 80 mm pipe will accommodate 21420 kg/h with a friction loss of 180 Pa/m. Hence, in this instance, the larger pump will comfortably allow a pipe two sizes smaller than that for the smaller pump. Always check that velocity is within recommendations. The 80 mm pipe will handle the above condition at just under 1 m/s, and is therefore suitable.

The DN80 pump would cost about 10% more than the DN50 pump, but these costs could well be recovered with the difference in installation costs on longer delivery lines between an 80 mm and 100+ mm pipe plus fittings and insulation etc.

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kg/h 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm Pa/m mbar/m <0.15 m/s 0.15 m/s 0.3 m/s 10 0.1 50 119 223 490 756 1447 2966 4644 9432 12.5 0.125 58 133 252 554 853 1634 3348 5220 10656 15 0.15 65 151 277 616 943 1807 3708 5760 11736 17.5 0.175 68 162 302 670 1026 1966 4032 6264 12744 20 0.2 76 176 328 720 1105 2113 4320 6732 13680 22.5 0.225 79 187 349 770 1177 2254 4608 7164 14580 0.5 25 0.25 83 198 371 814 1249 2387 4860 7596 15408 m/s 27.5 0.275 90 209 389 857 1314 2513 5112 7992 16200 30 0.3 94 220 410 900 1379 2632 5364 8352 16956 32.5 0.325 97 230 428 940 1440 2747 5616 8712 17712 35 0.35 101 241 446 979 1498 2858 5832 9072 18432 37.5 0.375 104 248 464 1015 1555 2966 6048 9396 19116 40 0.4 112 259 479 1051 1609 3071 6264 9720 19764 42.5 0.425 115 266 497 1087 1663 3175 6480 10044 20412 45 0.45 119 277 511 1123 1717 3272 6660 10368 21024 47.5 0.475 122 284 526 1156 1768 3370 6876 10656 21636 50 0.5 126 292 540 1188 1814 3463 7056 10944 22212 52.5 0.525 130 299 558 1220 1865 3553 7236 11232 22788 55 0.55 130 306 572 1249 1912 3636 7416 11520 23364 57.5 0.575 133 317 583 1282 1958 3744 7596 11808 23904 60 0.6 137 324 598 1310 2002 3816 7776 12060 24444 62.5 0.625 140 331 612 1339 2048 3888 7920 12312 24984 65 0.65 144 338 626 1368 2092 3996 8100 12600 25488 67.5 0.675 148 346 637 1397 2131 4068 8280 12852 25992 70 0.7 151 353 652 1422 2174 4140 8424 13068 26496 72.5 0.725 151 356 662 1451 2218 4212 8568 13320 27000 75 0.75 155 364 677 1476 2257 4284 8748 13572 27468 77.5 0.775 158 371 688 1505 2297 4356 8892 13788 27972 80 0.8 162 378 698 1530 2336 4464 9036 14040 28440 1 82.5 0.825 166 385 709 1555 2372 4536 9180 14256 28872 m/s 85 0.85 166 389 724 1580 2412 4608 9324 14472 29340 87.5 0.875 169 396 734 1606 2448 4680 9468 14724 29772 90 0.9 173 403 745 1627 2488 4716 9612 14940 30240 92.5 0.925 176 407 756 1652 2524 4788 9756 15156 30672 95 0.95 176 414 767 1678 2560 4860 9900 15372 31104 97.5 0.975 180 421 778 1699 2596 4932 10044 15552 31500 100 1 184 425 788 1724 2632 5004 10152 15768 31932 120 1.2 202 472 871 1897 2898 5508 11196 17352 35100 140 1.4 220 511 943 2059 3143 5976 12132 18792 38160 160 1.6 234 547 1015 2210 3373 6408 12996 20160 40680 180 1.8 252 583 1080 2354 3589 6804 13824 21420 43200 1.5 200 2 266 619 1141 2488 3780 7200 14580 22644 45720 m/s 220 2.2 281 652 1202 2617 3996 7560 15336 23760 47880 240 2.4 288 680 1256 2740 4176 7920 16056 24876 50400

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kg/h 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm Pa/m mbar/m <1 m/s 1 m/s 2 m/s 440 4.4 407 940 1732 3744 5724 10836 21924 33984 68400 460 4.6 414 961 1771 3852 5868 11088 22464 34776 70200 480 4.8 425 983 1811 3924 5976 11340 22932 35532 71640 500 5 432 1004 1850 4032 6084 11592 23436 36360 73080 520 5.2 443 1026 1886 4104 6228 11808 23904 37080 74520 540 5.4 450 1048 1926 4176 6372 12060 24372 37800 75960 560 5.6 461 1066 1962 4212 6480 12276 24840 38520 77400 580 5.8 468 1087 1998 4356 6588 12492 25272 39240 78840 600 6 479 1105 2034 4428 6732 12708 25740 39960 80280 620 6.2 486 1123 2070 4500 6840 12924 26172 40680 81720 640 6.4 493 1145 2102 4572 6948 13140 26604 41040 83160 660 6.6 500 1163 2138 4644 7056 13356 27000 41760 84240 680 6.8 511 1181 2171 4716 7164 13572 27432 42480 85680 3 700 7 518 1199 2203 4788 7272 13788 27828 43200 86760 m/s 720 7.2 526 1217 2236 4860 7380 13968 28260 43920 88200 740 7.4 533 1235 2268 4932 7488 14184 28656 44280 89280 760 7.6 540 1249 2300 5004 7560 14364 29052 44640 90360 780 7.8 547 1267 2333 5076 7704 14544 29412 45360 91800 800 8 554 1285 2362 5112 7812 14760 29808 46080 92880 820 8.2 562 1303 2394 5184 7884 14940 30204 46440 94320 840 8.4 569 1318 2423 5256 7992 15120 30564 47160 95400 860 8.6 576 1336 2452 5328 8100 15300 30924 47880 96480 880 8.8 583 1350 2480 5400 8172 15480 31284 48600 97560 900 9 590 1364 2513 5436 8280 15660 31680 48960 98640 920 9.2 598 1382 2542 5508 8388 15840 32004 49680 99720 940 9.4 605 1397 2567 5580 8460 16020 32364 50040 100800 960 9.6 612 1411 2596 5616 8568 16200 32724 50760 101880 980 9.8 619 1429 2624 5688 8640 16380 33084 51120 102960 1000 10 623 1444 2653 5760 8748 16524 33408 52560 104040 1100 11 655 1516 2786 6048 9180 17352 35064 54360 109440 1200 12 688 1588 2912 6300 9612 18144 36720 56880 114120 4 1300 13 716 1652 2894 6588 10008 18900 38160 59040 m/s 1400 14 745 1717 3154 6840 10404 19656 39600 61200 1500 15 770 1782 3269 7128 10764 20340 41040 63360 1600 16 799 1840 3380 7308 11124 21024 42480 65520 1700 17 824 1901 3485 7560 11484 21672 43920 67680 1800 18 850 1955 3589 7776 11808 45000 1900 19 871 2012 3708 7992 12132 46440 2000 20 896 2066 3780 8208 12456 47520

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Lifting condensate from

steam mains drain traps

A frequent requirement is to lift condensate from a mains drain trap to a higher level return line (Fig. 38), using the steam pressure within the trap.

Pressure can be related to the increase in lift by using the following conversion;

1 m increase in pipework lift = 0.1 bar g back pressure If a head of 5 m produces a back pressure of 0.5 bar, then this reduces the differential pressure available to push water through the trap, although under running conditions the reduction in trap capacity is likely to be significant only where low upstream pressures are being used. It is recommended that a check valve be fitted after any steam trap where there is the case of condensate being lifted. This will prevent condensate from falling back into the trap.

At start-up, the steam pressures are likely to be very low for a while, and it is common to find water backing up before the trap. This can lead to waterhammer in the space being drained, if a means of removing the condensate is not provided until sufficient steam pressure is present to overcome the back pressure. The liquid expansion thermostatic trap can often be used, discharging cold condensate to waste but closing to hot condensate, which is then forced through the trap to the return line.

High level condensate return

Steam main

Trap

Fig. 38 Use of liquid expansion trap

Liquid expansion trap

Condensate and Flash Steam Recovery

Condensate and

flash steam recovery

Contents

Contents

Introduction

Condensate return

Condensate return lines

✗

✔

Ö

Ö

Ö

Ö

Condensate pumping from

vented receivers

Lifting condensate from

steam mains drain traps