An entire pumping system is a network of fluid handling components such as pumps, valves, pipes and tanks. The purpose of the pumping system is to provide a certain flow and pressure to the fluid needed by the process unit.
Pump Types and Concepts
There are many different pump types, but two basic categories are 1) kinetic and 2) positive displacement. Positive displacement pumps are more suitable for constant flow applications. Where there is flow variation, then kinetic pumps like the centrifugal (also known as rotodynamic) and axial flow (also known as turbine type) pumps discussed in this section, are more suitable. See the References section for sources of information on other pump types, applications and standards.
Figure 2.2 - Centrifugal pump system and internal design, ABB Drives, 2005
In a centrifugal pump, the fluid enters the pump at the impeller eye, the center of the pump. This is the ‘suction’ part of the pump. The rotating impeller imparts velocity (kinetic energy) to the fluid as it is accelerated along the impeller blades and squeezed through the narrow volute, at right angle to its line of entry. As the volute cross-section area increases, the fluid velocity decreases and a higher pressure (head) develops towards the discharge end of the pump.
In an axial pump, the fluid is accelerated by the action of a propeller and moves parallel to the axis or shaft of the pump. In mixed flow pumps, the impeller uses both radial and axial motion to accelerate the fluid along a path that exits the pump at an angle with the pump axis or shaft.
Pump Characteristics
The performance of the pump is given by pump curves, which show how much pressure (Head) can be developed at various flow rates (capacities) at a given speed. Note that Head (H) has units of length (ft.) but is actually a measure of a fluid’s energy per unit weight. The distance refers to the height that a column of the fluid could support under gravity. A useful approximate conversion formula between Head and psi is:
H (ft) = ( H (psi) x 2.31 ) / Sp Sp =1 (for water, approximately) H (m) = (H (kp) x 9.8) / Sp
The head of a fluid has four components:
− Static head, which is independent of flow, increases with vertical height above the pump centerline and is negative for heights below the pump.
− Pressure head, which is due to other forces such as backpressure in a closed tank, and is also independent of flow.
− Friction head, which is due to resistance in elements such as pipes, valves &
fittings, and increases with flow.
− Dynamic head, which is a function of the velocity of the fluid, and is given by : Hv = V2 / 2g.
Where:
Hv = velocity head (ft or m) V = fluid velocity (ft/s or m/s)
g = acceleration due to gravity (32.2 ft/s2 or 9.8 m/s2)
Note that the dynamic head is usually rather small and may be ignored in most pumping applications.
Axial-flow pump performance curves are steep, meaning that they can deliver nearly constant flow over a wide range of pressure. Centrifugal pump curves are much flatter, meaning that they can provide a range of flows over a relatively narrow pressure range.
Power Plant Pump Applications:
In a power plant, the circulating pump supplies cooling water at large flow rate and low head to the condenser for heat exchange. Condensate (also known as hotwell) pumps are medium flow, medium head pumps which move the condensed steam from the condenser, through feedwater heat exchangers, to the aerator tank. The boiler feedwater booster pump moves water from the de-aerator to the man boiler feedwater pump, which supplies water at high pressure to the steam generator (boiler).
Circulating water pumps:
Circulating pumps are usually mixed flow pumps, with axial designs being used for very low head applications. Note that although dynamic (velocity) head is typically ignored in most pump applications, it is a significant factor in low head, high capacity systems such as the plant cooling water-circulating system. Typical flow rates for circulating water pumps are between 0.35gpm and 0.75gpm per generated kW. (GE Utility Division 1983)
Condensate pumps:
Condensate pumps are usually 2-stage pumps that operate with a high vacuum on the inlet. The discharge pressure must be sufficient to overcome the friction in the piping and low-pressure feed-water heaters, and any static lift required to pump the condensate to the level of the low-pressure feed-water heaters. (Homer M. Rustebakke, GE Utility Division, 1983).Typical power required by condensate pumps is on the order of 3hp per MW of generating capacity
Boiler feedwater (BFW) pumps:
Boiler feedwater (BFW) pumps are usually multi-stage, volute type centrifugal pumps. Typical boiler feed pump power requirements are about 10,000hp (7500kW) for a 300MW plant, then increase by 3300hp (2500kW) for every 100MW (Homer M. Rustebakke, GE Utility Division, 1983). The head of boiler feed pump depends upon the boiler load and thus steam pressure, which varies with plant loading. On large units, there is still usually a feed water control valve for filling and very low load, with pump speed taking over as load rises past some threshold (ISA 2005). Feed-water pumps are characterized by high reliability requirements and fairly high dynamics during plant load changes.
They normally make the biggest contribution to the plant’s in-house energy consumption. Another estimate for boiler-feedwater pump consumption is around 15 MWe on a 600 MWe plant, or around 2.5% of gross power (Goodall, 1981).
Total Developed Head
From the perspective of the pump, the process unit is a load that is a combination of pressure, static and friction heads. These latter two exist on the inlet and discharge sides of the pump. On the suction (inlet) side, the pump must ‘lift’ the fluid if it is coming from a lower elevation and overcome any friction pressure drops through the inlet, together called the Total Suction Head. On the discharge side, the pump must ‘push’ the fluid against system’s static head, pressure and friction pressure drops, together called the Total Discharge Head. The pump(s) must develop enough pressure (head) to meet the combined effect of these energies when delivering the required flow. An important design criterion for any pump is therefore the Total Developed Head (TDH):
TDH = Total Discharge Head – Total Suction Head
Pump Net Positive Suction Head
On the suction side, the system must also provide sufficient suction head so that the pump does not cavitate. Cavitation is a low-pressure phenomenon that reduces performance and may lead to pump damage. Sufficient suction head is called NPSHA (Net Positive Suction Head Available) and designers must compare this with the pumps characteristic curve for NPSHR (Net Positive Suction Head Required) provided by the supplier.
In power plants, the typical margins over this published NPSHR values may run from 50% to100% for a complex boiler feedwater (BFW) system with transient suction condition operations. Particular attention to NPSH margin should be given to boiler feedwater system where load rejection and system transients are expected, (Black & Veatch, 1996)
The suction requirements of BFW pumps are sometimes met by specifying a lower-head booster pump upstream of the main BFW pump.
Pump System Load Curves
The typical load curve shows a system with fixed static and pressure head as well as friction that increases exponentially with flow. Systems without any static or pressure
head have curves starting at the zero point. Systems with high friction have steeper curves. Systems with low friction but high flow requirement will have flatter curves.
The figure below shows the pump curve for a typical centrifugal pump as well as the system load curve for the head and flow it must deliver. The pump will operate at the values of head and flow where the two curves intersect, known as the operating point, where supply meets demand.
− If two pumps operate in parallel, their pump curves are added horizontally, giving a single equivalent pump curve with higher capacity, but with heads the same as one of the pumps operating singly.
− If two pumps operate in series, then their characteristic curves are summed vertically, to give a single equivalent pump curve with higher heads, but capacities the same as one of the pumps operating singly.
Figure 2.3 – Pump head vs. flow performance and system curves Pump curve
Static
& pressure head System curve
Operating point
Friction losses
Q (flow)
H (Head)
Pump Power and Energy Efficiency
Pump output power (water horsepower: WHP) to the fluid is given by:
WHP = (Q x H x Sp) / 3960 kW = (Q x H x Sp) / 6128
Where:
Q = capacity: in GPM (Gallons Per Minute) (l/min)
H = total developed head (ft) (m)
Sp = specific gravity (density) of the pumped fluid (water = 1)
Pump input power required (also known as Brake Horsepower: (BHP) is the water horsepower divided by the pump efficiency: Eff
BHP = WHP / Eff Motor Power = kW/η (η = EFF)
For estimation of efficiency (using SI units) in centrifugal pumps with developed head (H) between 15 to 100m water and flowrate (Q) between 20 and 300 cu.m/hr one can use the following pump model (Rajan 2003) which has a standard error of about 1%:
Eff (%) = 65.08 x H -0.12446 x Q0.094734
Affinity Laws
Pumps of similar design but different impeller sizes can be compared by using the pump affinity laws. These affinity laws (see below) below describe the relation between the rotational speeds of the pump (n), flow rate (Q), head generated (H) and power absorbed (P) by the fluid:
Figure 2.4 – Pump affinity laws, (ABB Oy. Drives, 2006)
For a given speed, a pump curve has an operating point where efficiency is
maximum, referred to as the Best Efficiency Point (BEP). The region of high efficiency close to the BEP is called the ’bullseye’. Designers strive to make this the actual operating range for the system, as this will reduce both energy and maintenance costs. When the operating point moves away from the BEP , then relatively more brake horsepower (input power) is required per unit water horsepower output.
Losses and Efficiency
The main losses within a centrifugal pump are due to: circulation within the casing (decreases with flow), shock losses, and friction loss (Rajan 2003). The friction loss varies with the square of flowrate and is the largest loss component when the pump is operating at higher loads. The effect of these losses is revealed in the efficiency vs. flow curve supplied by the pump manufacturer. Older pumps may suffer from impeller wear, internal leakage and bearing losses as well, and should be continuously monitored for performance degradation.
The difference between input and output power is mostly lost as heat energy to the fluid. In the minority of applications which call for heating of the fluid anyway, engineers should resist the temptation to include this effect to justify less efficient pumps or designs. Pumps operating far from their BEP will experience greater bearing and other mechanical stresses which lead to increased maintenance costs and downtime. In such applications, it is better to let heat exchangers do the job for which they were designed.
The affinity laws explain the shape and distribution of the family of pump curves which show pump head vs. flow at different pump speeds, where each curve shows pump performance at a given speed. These curves are shown in the figure below, together with the lines of constant efficiency, also determined by the Affinity Laws for pumps.
0 200 400 600 800 1000 1200
80
m: meter m3/h: cubic meters per hour kW: kilowatt r/min: revolutions per minute
150
Figure 2.5 – Variable speed pump performance curves, (US DoE Sourcebook, 2006)
Due to the similarity of the efficiency and system curves, there is little loss of efficiency when a pump operates at lower rotational speed. The new operating point is still in the pump efficiency bullseye, its ideal operating region for that speed.
Pump efficiency curves at the BEP do not change significantly with pump speed
within the family of pump performance curves. When speed is reduced by half of the rated speed, pump efficiency at the new speed’s BEP may only be reduced by 3%, for example. Pump efficiency is relatively unchanged, but the decrease in power consumption at the lower pump speed is substantial.
Figure 2.6 – Pump power and efficiency curves
BHP BEP Pump curve
Q (flow) Efficiency
H (Head)
Recall that pumps and fans are ‘cube-law’ applications because their power consumption varies with the cube of the speed of the pump or fan.
− This relationship yields large power savings even from modest speed reductions, and favors designs that seek to reduce speed, such as using Variable Speed Drives (VSDs). See section on Pump Flow Control Methods.
− The cube law also strongly favors selecting the right size of pump for the operating conditions. See section on Pump System Design & Engineering for guidelines on correct pump sizing.
Designers can also benefit from selecting high efficiency pumps, which are generally 4% to 7% more efficient and may cost little or no more than standard efficiency pumps. Using a lifecycle approach, super-efficient pumps are even more cost effective. Efficiency varies widely with pump type and flow range; according to the Hydraulic Institute, the pump efficiency of ‘large’ single-stage end-suction pumps operating at their BEP and at flow rates in the 100 cu.m/h range is 81%.
Pump Flow Control Methods
Some of this section is adapted from the application guide: ‘Using Variable Speed Drives in Pump Applications’ (ABB Oy. Drives, 2006)
The process system will usually require variable flow rates from the pump. There are four common methods to control the output of a pumping system: Throttling,
Bypassing, On-Off, and Variable speed control, as shown in the figure below.
Variable speed control may be achieved by motor frequency modulation using a Variable Frequency Drive, or by using gears and hydraulic couplings.
Figure 2.7 - Common flow control methods, (ABB Oy) Drives, 2006)
The relative power consumption of the different control methods can be estimated from the area between the x and y-axes and the operating point on a pressure-flow curve as shown in the figure below. This comparison is based on the formula for pump power (P): flow x head (P=Q x H). In the example shown, the relative power consumption for an average flow rate of 70% (of full flow) is calculated using typical values for the different flow control methods.
Throttling
Flow control can be achieved by modulating a valve immediately downstream from the pump. Throttling effectively changes the process system curve seen by the pump: the valve introduces friction into the system; this makes the system curve steeper so that it intersects the pump curve at the lower, desired flow rate, as shown in the top left chart in the figure below. In this example, the operating point is moved from (Q = 10, H = 10) to (Q = 7, H = 12.7). The relative power consumption can be calculated by P = 7 x 12.7 = 89.
This method has low capital costs, but throttled systems waste energy in two main ways: pressure drop across the valve, and because at reduced capacity the pump performs below its optimum efficiency point.
Figure 2.8 - Energy efficiency impact of various flow control methods, (ABB Oy. Drives, 2006)
In power plants, excess pump and throttling energy is not entirely wasted in applications that call for heating of the fluid anyway; as is the case in boiler feedwater regenerative heating. There are three reasons to avoid using such indirect heating to justify continued use of throttling control:
− Higher maintenance costs of the pump operating far from its BEP
− A highly-oversized valve will operate in a nearly closed state, which is not stable. This situation arises in constant speed pumping systems where each layer of engineering bureaucracy adds its own safety margin in calculating pressure drops through pipes, heat exchangers etc. and then, finally, in selecting the pumps. The control valve ends up with all these safety margins, as added pressure drops. (Liptak, 2005)
− Most importantly, for high pressure systems such as power plant boiler feedwater, is the severe wear on valve components from throttling at 2,400 psig (16.5 MPa) (Black & Veatch, 1996)
Bypassing
Although not commonly used for flow control, bypassing (also known as
“recirculation”) is applied mainly to circulation duty pumps. The flow output to the system is reduced by bypassing part of the pump discharge flow to the pump suction. Flow through the bypass system is controlled by valves. This means that the total flow increases (from 10 to 12.4), but the head decreases (from 10 to 6.6). The relative power consumption is P = 12.4 x 6.6 = 82.
A ‘minimum flow’ recirculation system is sometimes necessary if the pump operates for extended periods at low flow rates, when the pressure is too low to induce adequate flow into the system. Recirculating flow wastes energy and should therefore be kept to a minimum. Automatic modulating control of the recirculating flow system will ensure that that the recirculation flow control valve only opens when measured pump flow drops below a certain minimum threshold. Recirculating systems are sometimes required on systems equipped with variable speed flow control to guard against low flow conditions. The drive logic of a VFD speed controller may also be programmed to warn operators and to shut off the pump in these cases.
In power plants, this type of pump control may be used in boiler feed and condensate pumping systems to provide flow control and to prevent overheating of the pump at low flow rates. (Black & Veatch, 1996)
Another reason why designers may add a recirculating flow loop is to keep the pump at a higher state of readiness for increased demand. An improved design may make use of any existing, smaller and more responsive startup pump instead of a recirculation loop.
On-Off Control
On-off control is often used where stepless control is not necessary, such as
keeping the pressure in a tank between preset limits; in these applications, the pump is either running or stopped. The average flow is the relationship between the ‘on’
time and the ‘total’ time, on plus off. The relative power consumption can then be easily calculated by P = 0.7 x 100 = 70.
Variable Speed Control
The Best Efficiency Points (BEPs), when plotted for each pump curve, follow a quadratic shape that is in accordance with the affinity laws; pressure ratios are proportional to the square of flow ratios. In low static head, mostly friction systems, the system curve has approximately the same quadratic shape as the pump’s best efficiency curve. By varying the speed, therefore, the operating point can be made to follow the unchanged system curve along a line which is close to the BEP line. If the pump impeller speed is reduced, the pump curve moves downwards. If the speed is increased, then it moves upwards. This relationship ensures that the pumping capacity is exactly matched to the process requirements.
According to the earlier example, both flow rate (from 10 to 7) and head (from 10 to 6.4) are reduced at the new operating point. The relative power consumption can be calculated by P = 7 x 6.4 = 45. This simple example shows that the variable speed control method is the most energy efficient for pumping applications.
The examples were calculated for one flow rate only (70%), but the relative power consumption with different control methods depends on the flow rate. This relationship is shown in the figure below. In these curves, the pump, motor and drive efficiencies are also taken into account and for that reason the results differ somewhat to those in the previous similar figure.
Figure 2.9 - Energy efficiency impact of various flow control methods, (ABB Oy. Drives, 2006)
In power plants, boiler feedwater (BFW) pumps are driven by one or more, or combinations of: squirrel cage induction motor, synchronous motor, mechanical drive steam turbines, or gear drive from main shaft of turbine. Traditionally, pump drivers have been selected focusing on reliability at the expense of efficiency due to their critical role in the steam cycle. Ref ABB Drives. There
have been several studies (Black & Veatch 1996, EPRI 1995, ABB 2008) to compare the different options for pump drivers (prime movers) on base-loaded pulverized coal steam power plans, and the effect on net plant heat rate.
In all studies the option with a constant speed motor-driven BFW pump using
In all studies the option with a constant speed motor-driven BFW pump using