I have been designing different size centrifugal pumps for subsea applications powered by either an M200 or possibly a stripped down T500. One question I have that I am hoping someone can answer is the following. It is likely anybody that wasn’t expelled 3 times from kindergarden knows the answer to this.
If say for example at x depth I have a x psi and my pump outputs 12 psi, because of the water at both the inlet and outlet side of the pump being equal psi when the pump is off, is the water coming out when switched on then at at x psi+12 psi? In other words, do I have the same efficiency at that depth as I would have at the surface in terms of water volume coming out of the pump?
A pump can be thought of as a device that increases pressure differential. So if you operate it at the surface, or at a depth that corresponds to 12psi (8.5m in saltwater), the pump is going to take the inlet pressure, and boost it the same amount as it would if it were shallow or deep. If the inlet and outlet depth of your pump are at the same depth, that’s all there is to it!
In terms of efficiency, as water is negligibly compressible there is not likely to be any variation with depth. If you’re pumping things to the surface, then the amount of pressure required certainly scales with depth (both from overcoming gravity, and the friction in a hose or pipe as length increases.) In a traditional pump using a shaft seal, the efficiency may decrease slightly as more friction from the higher pressure on the shaft seal reduces power available for moving water.
A peristatic pump would probably rule itself out for the intended usage I have at this time because of the pulsing of the water flow and they appear to take up more size than a centrifugal pump of similar flow. Having said that, it is exactly that pulsing effect that makes that type of pump a candidate for another usage I have so thank you for the tip on them.