The PIPE-FLO® Overtime module is an excellent tool for understanding how your piping system will behave as conditions change over a period of time, but it also can be used for determining optimum set points and goal seeking. In this article we'll take a look at how to determine a VFD set point to maintain an end of line pressure.
The system consists of three pumps in parallel with a fourth auxiliary pump isolated from the system. The pumps supply water to a header that supports nine processing units. The processing units attached to the header need a minimum of 15 psi of supply water to operate properly. When we look at the design case where all units require 50 gpm and the motors are running at 1750 rpm, we can see that the pressure at the end of the line is only 13.64 psi (dark blue is higher pressure). An iterative approach can be used adjust the pumps’ speed until the 15 psi is achieved, but depending on the complexity of the system this could take some time and there may be a temptation to overspeed just to get it over with.
With the PIPE-FLO® Overtime Module, we can use Rules to adjust the speeds of the three pumps until we achieve the desired End-of-Line pressure. We’ll take a two-step approach using coarse and then fine adjustments to zero in on the correct speed set-point.
When we set-up for the simulation “End of Line to 15 psi”, we create four rules.
When End of Line Pressure is:
Multiply Pump Speed By:
Too Low Coarse
>14 psi and <15 psi
Too Low Fine
>15 psi and <16 psi
Too High Fine
Too High Coarse
This is what the “Too High Fine” rule looks like in Overtime:
Since we’re only seeking the end goal and not looking at response times in this case, the time settings aren’t really a factor here. When we run the simulation, we can watch the speed adjust until it hovers around a value within the range of our adjustments, which in this case is 1795 rpm.
Looking back at the model we see that the pressure at end of line is 15 psi.
Using this method emulates a pressure controlled VFD, and allows the engineer to quickly evaluate different set points for different loads. Now let’s assume the actual demands for each unit were not all at 50 gpm as specified in the original design. At this point we can see the pumps are now running faster than they need to and are wasting energy.
We can run the same simulation on the new lineup and here is what happens to the pump speed. Overtime uses the coarse adjustment and then the fine adjustment to identify the ideal speed of 1670 RPM.
This can be done for a wide variety of expected conditions. If we replaced six of the nine units that required 60 gpm, then we would get the following, which tells us to set the pumps at 1905 RPM:
So you can see that Overtime, aside from its system control logic simulation capabilities, can also provide rapid results when goal seeking for operational parameters.
Of course, if we did have fluctuating loads and were interested in the system’s behavior over time, Overtime can simulate the actual VFD behavior. We’ll cover that in another article.