Pump RPM to achieve flow 1

[nucleareng78]

I am a fairly new engineer and have a pump replacement project. The pumps we are using are Hayward Gordon screw centrifugal pumps and I need some help on the pump curve.

After reading the pump curve (matching the design flow and head), I found that it should operate at 750 RPM. After getting a quote from the vendor, they set me up with a standard 900 RPM motor...

Question, do we need a VFD then in order to get the desired flow rate and head needed for the design? We are not wanting to add margin to the system with additional flow or head. I'm so confused about this and need help!

Below is the link to the pump curve. We are wanting to operate at 3750 GPM and 20 ft WC. The current pumps we are replacing say "Variable RPM" on their pump curve so I'm assuming they also used a VFD???

http://www.haywardgordon.com/documents/variableXCS12-C.pdf

Thanks for the help! I'm still trying to learn being a new engineer...

 

[Artisi]

Change pumps type, size , supplier.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

Select a pump that such that your desired operating conditions (3750 GPM and 20 ft WC) will be at the BEP of the pump. Otherwise, you are wasting your time. Design your pump for the application.

 

[nucleareng78]

That's the problem. There are VERY few pump vendors that supply a screw centrifugal pump.

-We cannot use a vortex pump (not capable of high flow, efficiency drops way off)
-We cannot use a generic centrifugal pump due to solids pumping
-We can only use a low shear pump (impeller designed to that)

We may be able to handle the additional flow/head. There is a 45 degree elbow directly in front of the pump's suction. A good rule of thumb is to have a suction piping length of at least 5 times the diameter of the pipe. This would make us push the pump back 5 feet from where it is pumping the balls from.

This additional 5 feet would require additional flow/head from my understanding....so maybe it was meant to be!!

 

[Artisi]

How was this application handled before the introduction of screw centrifugal pumps?

Look at ITT-Goulds "shearpeller" - may not be as efficient hydraulically but if it hits the duty you need then it's very efficient in terms of doing the job and ease of installation.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[nucleareng78]

I really liked the shearpeller, but with our flow rate the pressure is too high. We could make some modifications to make it work but due to the way the suction is in relation to the discharge our piping won't work.

We are not considering using a V-belt to get the flow/head we want. The design flow rate for the current pumps is 4200 GPM and with a 900 RPM motor we get 4500-4600 GPM...is this really that much of an increase that it could affect the whole system? I really don't like V-belts since they have more maintenance issues.

May have to end up using a VFD....