VFD High Service Water Pumps

[TurtleboyTom]

I'm considering upgrading our pumps to VFDs. Current pumps are rated 465 gpm @ 245 ft of TDH. Supplied from a 40 ft tank. The distribution system is poorly designed (not a closed loop), HDPE that has frequent leaks usually at the pipe seems. Pressure is pretty high throughout the system (~90 psi). One of the options I'm considering is going to VFD pumps in order to avoid hammers when swapping pumps and to lower system pressure. I'm a new engineer and have never spec'd out a VFD. Any recommendations and considerations would be welcome. I've heard you can run into cavitation issues, etc.. if not done right. Thanks.

 

[GeoEnvGuy]

I'm a new engineer and have never spec'd out a VFD.
You are dealing with expensive pumps at that flow, ask the senior engineer or lookup what pump brands were used before and who supplied and installed them.

Any recommendations and considerations would be welcome.
Call the pump suppliers in your area that would actually supply it, they usually have an application to find the right size in there product line.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[LittleInch]

Well VFDs can be good, but they are not the answer to all issues.

first you need to work out your actual system demand and requirements and then see what you need the pumps to try and do. Also what is it you're going to control on - pressure at certain points of your system or flow? or both?

There is much chatter about this in the pumps forum so maybe go take a look there or do some searches.

VFDs for pumps usually go to about 50% of the rated speed with flow proportional to speed but head proportional to the square of the speed.

Depending on the motors, some are suitable for VFD use, some are not - you need to know as this is quite a significant expense.

You do need to get someone involved who understands the system hydraulics and what VFDs can and can't do for you.



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[TurtleboyTom]

Thanks for the advice! Another thought I had was to have the impeller trimmed and install a soft start. Cost is not really a big deciding factor as all options are being considered, even redoing the entire distribution system piping, but I'd like to do the smart and right thing.

 

[LittleInch]

Well trimming will reduce your discharge head permanently.

A soft start isn't much different from a VFD but without the variability.

Don't forget with VFDs you lose between 3-10% of your power in heat losses etc in the VFD unit (which you then need to think about how to get that heat out of your switchroom).

A lot depends on your flow demand cycle and how best to serve it with either multiple fixed speed pumps or less bigger pumps with VFDs.

Before doing anything you need to really think about what your system needs to do from min to max and what the flow profile looks like during the day and from one day to the next. Only then can you design the most cost effective system to supply the water. Any flow demand that varies by more than about 3 or 4 to 1 is not easy to design for.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

In addition to the comments of LittleInch (Petroleum):

A VFD is not the only method to eliminate pipe surges. Valve manufacturers such as cla-val and singer make valves that will do the same thing and are more suited for smaller systems like yours.

You need to determine the grade line for your system. If most of the pump head requirement is due to static head, VFD's will not be more efficient.

Find a knowledgeable pump person to guide you through the process.

 

[TurtleboyTom]

thanks, everyone.

 

[nondimensional]

Besides all the hydraulic analysis, make sure your pump motors are capable of accepting a PWM signal from the VFDs. If they are not designed for inverter duty, then they may take a serious hit on their life. Windings may get over heated. If VFD is required for these pumps and if you cannot replace the motors, consider a dVdt filter on the output side of the VFD.