I am trying to model a 'changeover' in operation for a transfer system with a number of booster pumps in series (Pump Stations A, B, and C) to determine any transient impacts.
Initially, both PSA and PSC are operating at a given flow rate. The client's intended procedure for starting up PSB is as follows
- ramp down PSA and PSC to 'idle' speed (~50% speed)
- turn off PSC
- turn on PSB and ramp up to idle speed
- turn on PSC and ramp up to idle speed
- ramp up all pumps to full speed (at a higher flow rate than the initial conditions with only PSA and PSC operating).
I have all of the relevant speed-change patterns. My problem is that if I try to use the variable speed/torque pump type, although the pumps all turn on at the correct time, I don't get any "use" out of PSB, as both PSA and PSC go back to their original flow/head conditions - I can't figure out how to increase the flow and head to the new desired conditions.
As a workaround, I tried to run this scenario in two parts, first assessing the shutdown to idle speed (no problems) and then looking at a startup, but then again ran into problems with PSA when I tried to start it at an initial speed factor of less than 1.0 and ramp it up to a new flow condition alongside the others. So the THIRD thing I tried was to run PSA at full speed, ramp it down to idle in an initial (dummy) time period, and then start up the other two pump stations and start recording my transient conditions. Again, I could not get the full three-pump flow.
Finally I have tried to do a complete pump startup, with all three pumps off to begin with, ramping up PSA first to create the desired initial condition before starting up the other pump. But to do this I have had to artificially adjust the system with a downstream PSV to keep the pipe full and prevent the occurrence of vapour pressures across the highpoints, so that although I can make all the pumps start up in accordance with the patterns, the overall HGL is artificially inflated.
I hope this all makes sense - essentially I'm wondering whether what I'm trying to do (changing target flow rate of a speed-controlled VSD midway through a simulation) is actually possible?
Thanks, Liz