Sizing Pumps in Series
by Engineered Software, Inc.
Some system designs require pumps to be operated in series, in which the discharge of one pump eventually leads to the suction of another. This is often done when pipeline and component losses in the system are substantial and it is not feasible (or at least not economical) to achieve the required total head with a single pump. Additionally, series operated pumps may be used to vary the flow through a pipeline, accommodate a wide variation in pumped fluid properties, or compensate for variable downstream conditions. Some examples of systems that utilize pumps in series operation include long pipelines, steam-condensate loops, and primary-secondary chilled water systems. However, one may see boosters pumps installed in a variety of situations.
Series pump operation can be easily modeled in PIPE-FLO or Flow of Fluids and any model can be set-up to facilitate the sizing of pumps in series. In this article, we will be using PIPE-FLO but Flow of Fluids can also be used to perform this pump sizing calculation. While modeling pumps in series is simply a matter of specifying the pump curves, sizing pumps for series operation requires a few additional steps. To illustrate the procedure the PIPE-FLO model shown in the figure below will be used.
The system has two pumps in series and a design capacity of 1000 gpm. Water is pumped from a reservoir to a storage tank, traversing 8 miles and an elevation gain of 500 ft. Pump 1 is located near the reservoir at sea level and Pump 2 is located 4 miles downstream at an elevation of 300 ft.
The tendency, particularly for those new to PIPE-FLO, is to enter set flow rates for both pumps in series, using the “Size pump for” lineup setting. If a “Size pump for” value of 1000 gpm is specified for both Pump 1 and 2, PIPE-FLO will return an over-controlled error when calculation is attempted. This is because the system has been over specified. Two flow rates in series have been set, essentially isolating the portion of the system between the two pumps.
To work around the over-controlled error, users must focus on sizing one pump at a time. Start with the first pump in series, Pump 1, and provide a boundary condition at a suitable place downstream of the pump. In this example, the location of Pump 2 is already specified and a good starting point would be to estimate the pressure needed at the suction of Pump 2. A quick survey of the pump models under consideration reveals that a suction pressure of 0 psi g at Pump 2 should more than meet the required NPSH at the design flow for any of the pump models.
To ensure that Pump 1 is sufficiently sized to provide the necessary pressure at the Pump 2 suction, a set boundary pressure is used for the sizing procedure. A short run of pipe, so as not to affect the head loss in the system, is added prior to the Pump 2 suction and is connected to a boundary pressure set to 0 psi g. These additional components are solely to help us size our pumps and are not actual system components. Calculating the system gives a total head of 400 ft for Pump 1. This is the total head necessary to achieve a suction pressure of 0 psi g at the inlet of Pump 2 and meet the design capacity of 1000 gpm. The next step is to go ahead and size Pump 1 for the calculated design point.
After selecting Pump 1, the curve data is imported into the PIPE-FLO model and the pump is set to the “Fixed speed pump” lineup setting. Now the design point for Pump 2 can be determined. Since we no longer need the boundary pressure used for sizing Pump 1 it can be isolated or deleted from the model. Pump 2 is set to “Size pump for” 1000 gpm and the system is calculated. The over-controlled error is no longer an issue since only a single flow is set in the pipeline. Pump 1 is now running on its curve and its flow rate is determined by the system resistance. The calculated design point for Pump 2 is about 318 ft of head at 1000 gpm. To complete the model Pump 2 is selected and the curve data is imported into PIPE-FLO.
This procedure will work for sizing any series pumping arrangement. It is a matter of sizing the pumps one at a time, starting with the furthest upstream pump and proceeding downstream using a boundary condition to determine the total head for sizing each successive pump. Perhaps the most difficult aspect is coming up with the boundary condition to use for sizing each pump. This requires the designer to determine the location of the pumps, the contribution of each pump to the total head needed to achieve the desired capacity, and accounting for various system requirements such as pump NPSHr, equipment design pressures, and capital costs.