Pump selection

[IMNO]

Hello,
I am designing a water transmission main from a source to a community. The community contain different Blocks, each one has underground storage tank. The idea is to install the pump the source and fill each tank one by one using the same set of pumps. As the blocks have different length from the source and different elevation difference as compare to each other and source, therefore I calculated the pump head considering the worst head loss required from source to a blocks.
Now using that pump head, I am getting very high terminal head/residual pressure for the remaining blocks. To utilize the available head I reduce the diameters of pipes and limit the velocity in pipe to 3.4m/sec. (we are using HDPE pipe)
Now I have the following questions
1- Is it possible to use centrifugal pump with vfd so that it can supply constant flow rate but vary pump head as per requirement?
2- Is it better to loss the head using valves?
3- Is it safe to take velocity up to 3.4m/sec . May result in water hammer?

 

[jraef]

This is actually a very good application for a VFD driven pump, it will eliminate the water hammer issues and probably save you money in energy costs during times of low flow requirements. But given that you are asking this question, I think you should find and get help from a local EE with direct experience in applying VFDs to centrifugal pumps. There are numerous issues and pitfalls involved, too many for an Engineering Tips website. What you will need to provide them will be the absolute maximum motor power (HP or kW) required at the pump shaft for the worst case scenario, then assume either a flow or pressure transmitter in the pipe. Pressure is probably a better way to do this because your demand will be highly variable.

In addition, you will need what is called "PID loop control" capability either in the drive or in another controller. PID (Proportional, Integral, Derivative) control allows the pump speed command to follow a variable signal, i.e. a pressure transducer, to maintain a set point without overshoot and hunting of the speed. Most VFDs are now capable of PID control on their own, which saves money, but if anything happens to the VFD you will need to replicate that programming in replacement units, limiting you to the exact same unit as a replacement, which may be complicated in the future. If you want SCADA capability (remote monitoring and control at a central location), then do the PID in the SCADA system so that the VFD can be replaced in an emergency with any VFD readily available because it will only need basic setup.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

With variable speed centrifugal pumps you should be aware of reflected pressure waves and critical frequencies.

Bill
--------------------
"Why not the best?"
Jimmy Carter