Why does my system head curve change depending on the status of other, parallel pumps?Why does the static head point on a system head curve change depending on whether the other pumps in the station are on or off?Problem ID#: 90596
The y-intercept is only equal to 'static head' in the simple case where there is one pump in the system. This case also happens to be the one that most engineers learned about in school because it was easy to solve by hand, so this causes a lot of confusion. If you have more than one pump the results are slightly different. Take for example two pumps in parallel, PMP-1 and PMP-2. When looking at the system head curve for PMP-1, the value where Q=0 is the minimum head that a pump must provide before it can deliver any water into the pipeline. This is dependent on what the second pump in your system is doing.Try looking at it this way:Q. When PMP-1 and PMP-2 are both off (and assuming there are no demands in the system) what is the HGL elevation at the common downstream node?A. The level of water in the downstream reservoir.Q. What is the minimum head that pump PMP-1 must provide before it can deliver any water?A. The difference between the HGL on the discharge side of the pump and the suction side of the pump. That's what we know as 'static head'. So far so good.Q. If PMP-2 is turned on and PMP-1 is still off, what's the HGL elevation at the common downstream node?A. You need to do a calculation here, but clearly it will be something higher than when both pumps are off, since PMP-2 is now adding head.Q. So what's the minimum head that PMP-1 must provide before it can deliver any water?A. The difference between the HGL on the discharge side and the HGL on the suction side. This explains the head value on the system head curve when flow is zero. You could say that pump PMP-1 must overcome the static head PLUS the dynamic head of PMP-2 before it can deliver any water.If you are designing PMP-1, you must pick a pump that works well when PMP-2 is either on or off, so you need to look as system head curves for both cases.
Note that if you have the latest version of WaterCAD or WaterGEMS, you can use the Combination Pump Curve tool, which always shows the system head curve with all others pumps turned off.
System Head Curves in WaterGEMS and WaterCAD
Pump Station and Pump Combination Curves