Constant Pressure Well Systems — Cycle Stop Valve vs Axial Flow Pumps and Variable Frequency Drives
I disagree with the following statements regarding Submersible Well Pumps.
“They start them into a closed valve and then slow open them.”
“Also they do that because they are often starting into pipes with high pressure and you don’t want to try and fight back flow as the motor tries to get upto speed.”
I disagree doing this with an axial flow pump.
The above quoted statements are true for Radial Flow pumps ONLY.
Submersible Pumps are based on axial flow due to the constraint of the well bore.
An AxialFlow Pump does not “unload” when the discharge port is closed or significantly reduced, just the opposite of a Radial Flow Pump. An Axial Flow Pump sees the greatest load on startup.
Introducing a Back Pressure increasing device will cause the power required to increase while reducing the flow.
Yes I can force my SubDrive 75 to only produce 2.5 GPM, for my GEO Heatpump, if I add apporximately 130 pound increase of back pressure. The pump will require all 1-1/2 HP to operate under those conditions. But the pump will produce low flows if the frequency is reduced via a VFD, and do so at a greatly reduced power load.
But the SubDrive 75 has its Achilles Heel when operated at low flow for extended periods such as the GEO presents. The sensor switch and the pump checkvalve both suffer from repeated excursions. The switch is an easily replaced item; having to pull the pump for the check valve is costly. And the failed check valve causes “backspin” on shutdown; if the controller tries to restart the flow during backspin, significant damaging operational conditions can occur and destroy the motor or the controller. Spoken from experience!
There exist lower cost VFD’s which allow complete control of the pump operation. They are computer controlled with a user interface for selecting a myriad of parameters. I am currently switching my SubDrive 75 VFD with one of these. I will still use the SubDrive 75 1-1/2 Hp 3 Phase pump. It will utilize a couple of appropriately configured “60/40” switches connected to the VFD variable frequency input terminals.