Pump vs. System Curve Methods
When plotting pump and system curves with two or more parallel pumps, AFT Fathom offers two methods: Enhanced and Traditional.
It should be understood by the engineer that it is not possible to create an unambiguous composite pump and system curve for parallel pump systems. The reasons why are discussed here.
First, let's discuss the head curve. A composite pump curve shows a single head curve with values at particular flow rates. However, when there are parallel pumps with unequal head curves or even equal head curves with unequal flow distribution, it is not clear what flow rate to use in the composite head curve. Further, with unequal flow distribution, different heads for each parallel pump can result. But the composite head curve shows only a single head for any given flow. What head is this? The maximum, minimum, arithmetic average, flow rate weighted average, or some other value? A composite curve has uncertainty over what is meant by the composite flow rate and composite head.
Composite system curves have similar issues to composite head curves. The entire system performance is boiled down to a single head vs. flow rate curve. What flow rate is being referred to and what head is being referred to which goes with a given flow rate?
AFT Fathom attempts to create as meaningful and helpful composite pump and system curves as possible, but the fact remains that the curves AFT Fathom creates are ambiguous because of the nature of composite head vs.system curves.
The “enhanced” method was introduced in AFT Fathom 7.
For composite head curves, it assumes that the parallel pump head curves are added to the right with respect to flow rate. No attempt is made to apportion flow splits among the pump curves. With the head curves added to the right, there is a unique head value for every flow rate. In reality, the parallel pumps may not work this way, depending on the flow split among the pumps and whether or not the head curves are identical and whether there are check valves at each pump discharge. This is, however, a reasonable approximation and represents the historical way that parallel pump head curves are added together.
No matter what method is used for creating a composite system curve, composite system curves are created by fixing some flow rate to each pump and then determining the required head from the pump to deliver the assumed flow rate.
The enhanced system curve takes the flow split at the operating point and assumes that the flow split remains the same at all flow rates. It collects the required head for each pump at each flow rate and then averages these heads together arithmetically.
If the pump head curves are identical and the flow split among pumps is proportional, this method will give the same results as the Traditional method.
The “traditional” method was used in AFT Fathom 5.0 and 6.0. The traditional method assumes that the flow splits equally among all pumps at all flow rates, regardless of whether pump curves are identical or not.
The composite head curve is created by taking the composite flow rate, splitting it equally among each pump, obtaining the head for each pump at the split flow rate, and then taking the arithmetic average of the pump heads.
The composite system curve is obtained by taking the composite flow rate, splitting it equally among each pump, running the system model with these flow rates, and then taking the arithmetic average of the required heads for each pump.