Modern Pump Selection Part 5: How the System Curve Moves
/By Chad Edmondson
Effective and efficient pump selection demands a thorough understanding of the system curve and how it reflects what happens in the actual system. This is especially true in today’s variable speed systems. It’s so important that even though we’ve touched on it many times before, we are going to “hit pause” in this series to focus specifically on the movement of the system curve in a variable speed pumping system.
Figure 1
The system curve pivots up or down with each and every flow change that occurs out in a live system (Figure 1). Generally speaking, as 2-way valves open to increase flow, the system curve pivots down. As 2-way valves close to reduce flow, increasing system resistance, the system curve pivots up. Consequently, the operating point of a variable speed pump (where the system curve intersects with the pump curve) will also move either to the right and down (valves opening), or to the left and up (valves closing). Every point in between represents the control range of the pump in a given system.
While the pump curve never changes, the speed at which the pump operates will. Therefore, we must consider not just the 100% speed curve, but the entire operating range of the pump that is represented by all the parallel pump curve lines on a single pump curve. If we were to put a dot at every corresponding operational point that occurs in a system over its entire operating range, there would be a concentration of dots in the areas where the system operates most frequently. Throw a lasso around all those dots and you’ve captured the control range for your pump. That is basically what we have done here in Figure 2.
Figure 2
Notice the lower blue line with the triangles. This curve represents the operation of a multi-zone system when all of the outer valves (those furthest from the pump) are closed. The blue line with the squares reflects what is happening in the same system when the inner valves (those closest to the pump) are closed. Why the distinction between the two? The reason is that (in a direct return system) under part load conditions, it takes more pump head to drive flow to zones that are furthest from the pump than it does to drive flow through zones closest to the pump.
So where do we want to pick our pump? As we stated earlier in this series, we want to pick a pump whose areas of highest efficiency are mostly contained within the operating range of our system. Assuming we have done a detailed system head loss calculation and we know that the system will be installed as designed, we can pick a pump accordingly, maximizing its operating efficiency in our system.
Figure 3
This method of selection places our design point just to the right of BEP, or the best efficiency point for our pump. (Figure 3) That’s fine because we know in a typical variable volume system the design conditions only occur about 1% of the time. Notice that most of the operating range of the system keeps the pump operating at or above 75% efficiency.
However, ASHRAE recommends that we select a pump so that the design point of our system falls just to the left of BEP, which typically isn’t quite as efficient. Why is that?
Figure 4
It goes back to what we discussed in Part 4. If we are bold in our quest for maximum efficiency and pick the pump so that our design point is just to the right of BEP, then we run the risk of unstable operation. Worst case scenario, our pump falls off its curve and cavitates. For that reason, ASHRAE recommends building in a little bit of a safety factor by picking a pump to operate slightly to the left of BEP. 9 (Figure 4) Specifically ASHRAE states:
“Where possible, pumps should be chosen to operate to the left of the BEP because the pressure in the actual system may be less than design due to overstated data for pipe friction and for other equipment. Otherwise, the pump operates at a higher flow and possibly in the turbulent region.”
-- 2012 ASHRAE Systems and Equipment Handbook, p 44.11
Ultimately it is up to the engineer to decide how confident he or she is of the accuracy of the system head loss calculations. In either case we want as much of the system operating range to fall within the efficiency islands of the pump as possible.
