Pumping Over a High Point
by Engineered Software, Inc.
When placing a pump into a piping system with a high point, the following items must be determined:
- The total head needed to overcome the pressure drop in the pipeline from the source and destination tank.
- The static head needed to pump the liquid up to the high point
- The back pressure required in the downhill leg to prevent flashing at the high point.
Figure 1 shows a sample piping system in which fluid is pumped from the supply tank through the pump, through the uphill pipeline, through the downhill pipeline, and into the discharge tank (the discharge tank is represented as a pressure source set to 0 psig and no level). The suction tank and discharge tank are both at 0 feet elevation, and the high point is located 100 ft above the datum elevation. The high point tank and connecting pipeline is supplied so we can perform additional calculations.
Now we will view the calculated system.
Notice that the pump needs a total developed head of 33.49 ft to accommodate the headloss in the pipelines. In looking at the pressure at the High point (elevation 100 ft) we can see the pressure is –36.08 psig, which is well below the vapor pressure of water. As a matter of fact, the pressure at the High point is well below the 0 psi absolute which is the lowest possible achievable pressure. As a result we know that this system will not operate properly.
Another concern is if the pump is sized strictly for the headloss in the pipelines (33.49 feet of head), the pump will most likely not be able to pump up to an elevation of 100 ft. As a result we need to perform another calculation to determine the head needed to pump fluid to the High point.
To properly size the pump we must take into account the change in elevation from the Suction tank to the High Point along with the headloss in the Uphill pipeline when passing the design flow rate. To do this we will isolate the downhill pipe and open the pipeline to the High Point tank.
The pump will need to develop 117.1 ft TDH to pass 400 gpm up to the High Point tank. From these results we know the pump will be able to pump up to the high point.
Next we will need to select a pump and then evaluate its operation in the piping system. A 3x4-13 ANSI pump running at 1780 rpm was selected from the sample PUMP60 pump catalog (supplied with PIPE-FLO), and inserted into the piping system.
With the pump running at a fixed speed, the program calculates that 679.8 US gpm going through the pump, and a –23.14 psig at the High point. Once again the pressure at the high point is well below the vapor pressure of Water at 100°F, as a result flashing occurs in the downhill pipeline and stable flow cannot be achieved.
Flashing is occurring in the downhill pipeline because the resistance in the pipeline is not sufficient to prevent the fluid from flashing. As the fluid flows down the pipeline, the pressure at the high point decreases until it vaporizes, causing an unstable flow. PIPE-FLO is unable to calculate the unstable flow, and you probably do not want to have the problems associated with this type of flow in your pipeline. The solution is to put a restriction at the end of the downhill pipeline to limit the flow rate out of the pipeline to 400 gpm.
This can be done by placing a throttle valve and setting the flow rate to 400 gpm into the discharge tank.
The pressure at the High Point node is slightly positive, and notice the differential pressure across the throttle valve to limit the flow rate to 400 gpm is 36.28 psi. As you can see, by providing additional pressure drop at the outlet of the Downhill pipe the pressure at the high point will remain positive, preventing flashing in the downhill pipeline.
You can reduce the differential pressure across the throttle valve by reducing the pipe diameter on the downhill pipeline. This will cause the fluid velocity in the pipeline to increase, causing the headloss to increase as well. This in turn reduces the pressure drop needed across the back pressure valve.
It is also possible to have a negative pressure at the high point and still have stable flow. The only requirement is that the pressure at the high point must not be less than the vapor pressure of the fluid. For example when you place a pipeline (or hose) between two fluid sources at different elevations once the pipeline is full the liquid will flow. This is called a siphon. The flow will stop when you break the siphon (by allowing air into the pipeline at the high point) or when the difference in elevation causes the pressure at the high point to be lower than the vapor pressure of the fluid.