# Centrifugal Pumps

Booster Affinity Law Corrected: Arises when design input for pump speed or impeller diameter has been changed. FluidFlow allows users to model the effect of changes in pump speed and impeller diameter in a given piping system (provided you have defined a Min. and Max. speed and impeller size in the database). If we model this scenario, the pump performance data is adjusted according to the affinity laws. It is important to note that the affinity laws only hold accuracy over a range of maybe +/- 20% to the original curve parameters. The software will enunciate a warning when pump speed or impeller size has been changed to make the engineer aware of this operating condition. It is then down to engineering judgment to accept that you are operating close enough to the original curve. If the system requires higher levels of accuracy, it is recommended that the user approaches the pump manufacturer and obtains a new capacity curve based on the exact speed and impeller size you wish to model. Pump is Viscosity Law Corrected: Performance curves for centrifugal pumps are generally based on testing with clean water. The pump performance therefore needs to be corrected if the fluid being pumped has a viscosity of 4.3 cSt or higher. Note, the value of 4.3 cSt is taken from the pump performance correction chart data detailed in the Hydraulic Institute s. The reason for this correction is the pressure loss in the impeller and diffuser channels, the impeller friction and internal leakage losses depend to a large extent on the viscosity of the pumped liquid. Consequently, characteristics ascertained for water lose their validity when pumping liquid of higher viscosities, such as oil. In general, the higher the viscosity of the liquid compared to water, the greater the loss of delivery capacity and head for a given power input. In general, the resulting de-rating of the pump head and flowrate may be relatively modest though the de-rating of the efficiency can be significant with consequences for motor size, switchgear and cabling if not taken into account. FluidFlow will perform viscosity correction for the pump based on the Hydraulic Institute Standards. Note, the correction applies only to centrifugal pumps delivering Newtonian fluids and the software will only apply the correction when a "Check-Mark" has been placed in the Global Settings Option titled "Include Viscosity Correction Factor for Centrifugal Pumps" (Figure 1) AND the flowing fluid viscosity exceeds 4.3 cSt. Note, the Hydraulic Institute Standard correction methodology is valid between the viscosity range of 4.3 - 3300 cst which is the criteria which is implemented in FluidFlow. This automatic correction will only occur if the option has been enabled from the Global Settings tab which can be accessed by selecting: Options | Calculation | Global Settings or alternatively, select "F2" & Global Settings (See Figure 1). Figure 1: Global Settings Dialog. Unexpected Pump Results: If we have not defined the capacity curve for a centrifugal pump across the full quadrant and the calculated duty point for the pump is beyond the curve data available, we may achieve unexpected operating results for the pump. Figure 2 shows the capacity curve for a sample centrifugal pump. As we can see, the capacity curve is not defined across the full quadrant, i.e., the maximum curve coordinate is 2816.8 usgpm at 96.14 m fluid. If we were to model this pump in a system and the duty point was beyond this maximum coordinate value (e.g. 3100 usgpm), we may get unexpected results. This is simply due to the fact that the software needs to extrapolate the curve relationship data and provide an estimated value. Figure 2 – Pump Capacity Curve (Not Defined Across the Full Quadrant). End suction centrifugal pump curves supplied by manufacturers rarely extend far to the right hand side of the quadrant. It is therefore recommended that users make an estimate for the final curve coordinate. The estimate you make can be guessed (this is OK because we will never operate the pump anywhere near to this condition). When you make the guess at head of zero, ensure that you do not "upset" the shape of the curve for the valid data points by selecting a value that lies on a smooth curve. If you do not make this guess you run the risk of using the pump in a system that is difficult to solve (converge). Figure 3 shows how an additional row has been added where we have made an estimate of the flow at 0 head. As you can see, the curve still intersects the coordinates and is an accurate representation of the manufacturer's data. The Max Limit is retained as 2816.8 usgpm. If we solve this pump in a system and the duty point is beyond the Max Limit of 2816.8 usgpm, we will get sensible results as the curve is fully defined, however, we will also receive a warning message to that effect. This informs us that we are operating outside the range for the pump based on available manufacturer's data. We would therefore need to investigate the pump performance further with the vendor or select an alternative pump model. Further information on defining pump curves in the Database is available at: Databases | Centrifugal Pumps. Figure 3 – Pump Capacity Curve Defined Across Full Quadrant. Booster is operating in reverse flow direction: This warning arises if the pressure downstream of a pump is greater than the maximum pressure the pump can deliver. Select a larger pump or alternatively, reduce the downstream pressure, increase the pump speed or fit a larger impeller in an attempt to increase the pump capacity. Negative Static Head: The starting point of the system resistance curve is dependent on the characteristics of the system and although you may not expect it, the system curve can start at a negative value on the head axis. This can occur in systems whereby the design inlet/outlet pressure are such that flow would occur through the system without the presence of a pump. It can also occur in systems whereby the gravity effects alone would develop flow without a pump. Obviously, the flow rates achieved in these circumstances are most likely lower than you would require from the system and as such, a pump is generally included in the system. When a piping system exhibits the above conditions, the system resistance curve is plotted exactly as for any other case except that the static head is now negative. The system curve therefore begins at a negative value and also shows the flow rate which would be developed in the system by gravity/differential pressure alone. This is the point where the plotted system curve intercepts the flow axis. To demonstrate this condition further, let's consider the model in Figure 4. Figure 4: Negative Static Head. The model on the left hand side exhibits flow through the system without the use of a pump. The flow rate achieved from gravity alone is 16 m3/h. The model on the right hand side is identical with the exception that it now includes a manufacturer's pump and the flow rate developed in the system is now 39 m3/h. The supply tank in this design case has an elevation of 14.0 M with a liquid level of 1.0 M and a surface pressure of 1 ATM. The fluid pressure is therefore 14.0 + 1.0 = 15.0 M fluid. The outlet point has an elevation of 10.0 M. Therefore, the static head = 10 - 15 = - 5.0 m fluid which represents the starting point on the Head axis (see Figure 5). Figure 5: Negative Static Head (- 5.0 M). Figure 4 illustrates that the flow occurring in the system through gravity effects is 16 m3/h. Adding the specific manufacturer's pump increased the flow rate to 39 m3/h. Figure 5 shows that the system resistance curve intersects the flow axis at 16 m3/h and 0 m head, i.e., the flow rate achieved through gravitational effects alone. Pump Static Head: On the subject of static head, this refers to the difference in elevation thus, the total static head of a system is the difference in elevation between the liquid levels at the discharge and suction points of the pump. The static discharge head is the difference in elevation between the discharge liquid level and the pump centerline. The static suction head on the other hand is the difference in elevation between the suction liquid level and the pump centerline. If the static suction head has a negative value because the suction liquid level is below the pump centerline, it is often referred to as static suction lift. If either the suction or discharge liquid level is under a pressure other than atmospheric, this pressure can be considered as part of the static head or separately as an addition to the static head. Modelling Submersible Pumps: When modelling a submersible pump which of course has no suction piping, it is necessary to insert a short section of pipework between the model inlet boundary and the pump suction connection. This is necessary to complete the model connectivity. Figure 7 provides an illustration of such a design case where just 0.1m of "suction pipework" is included in the system. The pressure loss across this "suction pipework" is almost negligible relative to the losses across the other pipes and fittings in the system. Figure 7: Submersible Pump Modelling. If any of the above is unclear or you require any further information, please contact us at: support@fluidflowinfo.com.