Computer aided pump selection for dense and

Section 1.5

Liquids

1.5.1 Viscous liquids

There is no doubt about it; water is the most common liquid that pumps handle. However, in a number of applications, pumps have to handle other types of liquids, e.g. oil, propylene glycol, gasoline. Compared to water, these types of liquids have different density and viscosity. Viscosity is a measure of the thickness of the liquid. The higher the viscosity, the thicker the liquid. Propylene glycol and motor oil are examples of thick or high viscous liquids. Gasoline and water are examples of thin, low viscous liquids.

Two kinds of viscosity exist:

• The dynamic viscosity (µ), which is normally measured in Pa⋅s or Poise. (1 Poise = 0.1 Pa⋅s)

• The kinematic viscosity (ν), which is normally measured in centiStokes or m2_{/s (1 cSt = 10}-6 _m2_/s)

The relation between the dynamic viscosity (µ) and the kinematic viscosity (ν) is shown in the formula on your right hand side.

On the following pages, we will only focus on kinematic viscosity (ν).

The viscosity of a liquid changes considerably with the change in temperature; hot oil is thinner than cold oil. As you can tell from figure 1.5.1, a 50% propylene glycol liquid increases its viscosity 10 times when the temperature changes from +20 to –20 o_C.

For more information concerning liquid viscosity, go to appendix L.

ν =

µ_ρ

ρ = density of liquid

Kinematic viscosity ν [cSt] Density ρ [kg/m3_] Liquid temperature t [˚C] Liquid Water 20 998 1.004 Gasoline 20 733 0.75 Olive oil 20 900 93 50% Propylene glycol 20 1043 6.4 50% Propylene glycol -20 1061 68.7

Fig. 1.5.1: Comparison of viscosity values for water and a few other liquids. Density values and temperatures are also shown

liquids does change when agitated. This calls for a few examples:

• Dilatant liquids like cream – the viscosity increases when agitated

• Plastic fluids like catsup – have a yield value, which has to be exceeded before flow starts. From that point on, the viscosity decreases with an increase in agitation • Thixotrophic liquids like non-drip paint - exhibit a decreasing viscosity with an increase in agitation The non-Newtonian liquids are not covered by the viscosity formula described earlier in this section.

1.5.3 The impact of viscous liquids on the

performance of a centrifugal pump

Viscous liquids, that is liquids with higher viscosity and/ or higher density than water, affect the performance of centrifugal pumps in different ways:

• Power consumption increases, i.e. a larger motor may be required to perform the same task

• Head, flow rate and pump efficiency are reduced

Let us have a look at an example. A pump is used for pumping a liquid in a cooling system with a liquid temperature below 0o_{C. To avoid that the liquid freezes,}

an antifreeze agent like propylene glycol is added to the water. When glycol or a similar antifreeze agent is added to the pumped liquid, the liquid obtains properties, different from those of water. The liquid will have:

These properties have to be kept in mind when designing a system and selecting pumps. As mentioned earlier, the higher density requires increased motor power and the higher viscosity reduces pump head, flow rate and efficiency resulting in a need for increased motor power, see figure 1.5.2. Q H, P, η H P η

Fig. 1.5.2: Changed head, efficiency and power input for liquid with higher viscosity

1.5.4 Selecting the right pump for a

liquid with antifreeze

Pump characteristics are usually based on water at around 20°C, i.e. a kinematic viscosity of approximately 1 cSt and a density of approximately 1,000 kg/m³.

When pumps are used for liquids containing antifreeze below 0°C, it is necessary to examine whether the pump can supply the required performance or whether a larger motor is required. The following section presents a simplified method used to determine pump curve corrections for pumps in systems that have to handle a viscosity between 5 - 100 cSt and a density of maximum 1,300 kg/m³. Please notice that this method is not as precise as the computer aided method described later in this section.

Pump curve corrections for pumps handling high viscous liquid

Based on knowledge about required duty point, Q_S, H_S, and kinematic viscosity of the pumped liquid, the correction factors of H and P₂ can be found, see figure 1.5.3.

Fig. 1.5.3: It is possible to determine the correction factor for head and power consumption at different flow, head and viscosity values

10 20 30 40 50 60 70 80 90 100 110 120 130 140 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.00 1.05 1.20 1.15 1.10 1.25 1.30 1.35 0 H = 6 m H = 10 m H = 20 m H = 40 m H = 60 m 10 cS t 20 cS t 40 cS t 60 cSt 100 cSt 5 cSt 10 cSt 20 cSt 40 cSt 60 cSt 100 cSt 5 cSt KH KP2 Q [m3/h]

Section 1.5

Liquids

where

H_W : is the equivalent head of the pump if the pumped liquid is “clean” water

P_2W : is the shaft power at the duty point (Q_S,H_W) when the pumped liquid is water

H_S : is the desired head of the pumped liquid (with agents)

P_2S : is the shaft power at the duty point (Q_s,H_s) when the pumped liquid is water (with agents)

ρ_s : is the density of the pumped liquid ρ_w : is the density of water = 998 kg/m3

The pump selection is based on the normal data sheets/ curves applying to water. The pump should cover the duty point Q,H = Q_S,H_W, and the motor should be powerful enough to handle P_2S on the shaft.

Figure 1.5.4 shows how to proceed when selecting a pump and testing whether the motor is within the power range allowed. ρ P_2S = K_P2. P_2w.

( )

s w Water Mixture Mixture 1 P_2s P P_2w Q_s Q Q 5 3 4 ρ

Fig. 1.5.4: Pump curve correction when choosing the right pump for the system

The pump and motor selecting procedure contains the following steps:

• Calculate the corrected head H_w

(based on H_S and k_H), see figure 1.5.4 1-2

• Choose a pump capable of providing performance according to the corrected duty point (Q_S, H_W) • Read the power input P_2W in the duty point (Q_S,H_w), see figure 1.5.4 3-4

• Based on P_2W , k_P2 , ρ_W , and ρ_S calculate the

corrected required shaft power P_2S , see figure 1.5.4 4-5

• Check if P_2S < P_{2 MAX} of the motor. If that is the case the motor can be used. Otherwise select a more powerful motor

ρ

_s

ρ

_w

1.5.5 Calculation example

A circulator pump in a refrigeration system is to pump a 40% (weight) propylene glycol liquid at –10°C. The desired flow is Q_S = 60 m3_{/h, and the desired head is H}

S = 12 m.

Knowing the required duty point, it is possible to find the QH- characteristic for water and choose a pump, which is able to cover the duty point. Once we have determined the needed pump type and size we can check if the pump is fitted with a motor, which can handle the specific pump load.

The liquid has kinematic viscosity of 20 cSt and a density of 1049 kg/m3_{. With Q}

S = 60 m3/h, HS = 12 m and ν = 20 cSt,

the correction factors can be found in figure 1.5.3.

k_H = 1.03

k_P2 = 1.15

H_W = k_H · H_S = 1.03 · 12 = 12.4 m

Q_S = 60 m3_/h

The pump has to be able to cover a duty point equivalent to Q,H = 60 m3/h, 12.4m. Once the necessary pump size is determined, the P₂ value for the duty point is found, which in this case is P_2W = 2.9 kW. Now it is possible to calculate the required motor power for propylene glycol mixture:

The calculation shows, that the pump has to be fitted with a 4 kW motor, which is the smallest motor size able to cover the calculated P_2S = 3.5 kW.

1.5.6 Computer aided pump selection for

dense and viscous liquids

Some computer aided pump selection tools include a feature that compensates for the pump performance curves based on input of the liquid density and viscosity. Figure 1.5.5 shows the pump performance curves from the example we just went through.

The figure shows both the performance curves for the pump when it handles viscous liquid (the full lines) and the performance curves when it handles water (the broken lines). As indicated head, flow and efficiency are reduced, resulting in an increase in power consumption.

The value of P₂ is 3.4 kW, which corresponds to the result we got in the calculation example in section 1.5.4.

H [m] η_[%] 01 2 3 4 0 2 4 6 8 10 12 14 0 10 20 0 10 20 30 40 50 60 70 30 40 50 60 70 80 Q [m3_/h] Q [m3_/h] P2 [kW]

Fig. 1.5.5: Pump performance curves

ρ_S ρ_w P_2S = k_P2 . P_2w . P_2S = 1.15 . 2.9 . 1049 998 = 3.5 kW

Section 1.5

Liquids

In document Pump Handbook Complete (Page 58-64)