Pump RPM to achieve flow
Pump RPM to achieve flow
(OP)
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/variableXCS...
Thanks for the help! I'm still trying to learn being a new engineer...
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/variableXCS...
Thanks for the help! I'm still trying to learn being a new engineer...





RE: Pump RPM to achieve flow
The curve you have posted, is this the curve the pump vendor supplied with their quote, or is it the curve for the installed unit?
Are you operating at 50 or 60 Hz.
The curve you supplied indicates 750 not 900RPM for the duty specified.
At first look I would say the pump is oversized and you should be looking at a smaller unit running at a higher speed, unless of course the appliccation dictates a slow running pump.
Why are you asking in here, surely the pump vendor would be the one to ask as to WHY.
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.)
RE: Pump RPM to achieve flow
Why would the pump curve indicate "Variable" then? I much rather ask here first before making myself look dumb in front of the vendor...
RE: Pump RPM to achieve flow
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.)
RE: Pump RPM to achieve flow
RE: Pump RPM to achieve flow
Why are you looking at centrifugal screw pumps - any good reason?
Why can't you tell if the original pumps were VFD?
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.)
RE: Pump RPM to achieve flow
RE: Pump RPM to achieve flow
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.)
RE: Pump RPM to achieve flow
The pump is directly driven due to space constraints. I figured we'd need a VFD, but wanted to get others opinions to make sure (thanks TenPenny).
The spec they sent me said 750 RPM but in the motor description it says 900 RPM....thus I would need a seperate VFD. It's just that most of the time for such a small motor it isn't worth the cost of having a VFD. The VFD isn't located on the pump but on the breaker, which is why I'm having a hard time confirming where it's at.
On the other hand we could also adjust a control valve to get the hydrualics we're trying to acheive. Operating at 900 RPM may give us too much flow and head.
RE: Pump RPM to achieve flow
60Hz 10 pole is only 720RPM with probably a lesser speed in operation - so duty is not achievable.
60Hz 8 pole is 900RPM also a little less in operation.
As a mind reader / psychic - possibly they are offering 60Hz 8pole speed with reduced inpeller -- again, ask the pump 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.)
RE: Pump RPM to achieve flow
If you're trying to insult my intelligence, stop. I post on here to learn and get advice and not to be made fun of.
RE: Pump RPM to achieve flow
You either use VFD, belt drive or gear box to get the RPM to meet the performance you need if the required RPM can not be met with the standard motor speed.
For your required flow of 3750 GPM at 20 ft, you need to run the pump at 750 RPM.
For 60 Hz supply, a 10 pole motor has a synchronized speed of 720 RPM only. Therefore you have to use a 8 pole motor which has a synchronised speed of 900 RPM. Actual RPM will be less to to slip.
At 900 RPM the pump will give about 35 ft head at 3750 GPM or about 5700 GPM at 20ft head.
If space is not a constrain you can use a higher RPM motor and reduce to the pump speed by belt drive of a speed reduction gear box.
If you can not perform discharge valve throttling and also can not accept higher flow rate, you will have to use VFD.
To check what will the flow you will be getting without valve throtlling, you need to plot the system curve on the pump curve.
RE: Pump RPM to achieve flow
RE: Pump RPM to achieve flow
Once the pump is operating it will not be using the entire 40 HP. The design flow requirements is 3740 gpm but the pump must be able to produce 4200 gpm if the system is "dirty". We're pumping small sponge balls here.
RE: Pump RPM to achieve flow
RE: Pump RPM to achieve flow
The pump's impeller cannot be trimmed or modified as it is a screw impeller (huge). I'm thinking we can either have the control valve adjusted or run a hydraulic calculation to confirm that the additional flow/head is not a huge deal.
RE: Pump RPM to achieve flow
It doesn't seem that impractical to me to add a VFD. Many times the VFD drives are installed in the MCC lineup close to the motor starter circuitry (see if you can find out from your electrical folks). There are many units where the VFD and start-stop "bucket" are located in the same enclosure.
I don't think anyone was picking on you about contacting the vendor. Asking questions about the vendor's bid is a good way to address any of your concerns. Most vendors a more than happy to answer your technical questions. Maybe you have a policy where yoou need to involve the purchasing folks in the discussion.
Good luck!
RE: Pump RPM to achieve flow
The vendor just told me "if you plan on using a VFD, it will have to be supplied by others." I just don't get why they would point to me on the pump curve where I want it to operate at but then give me a 900 RPM pump, leaving me out to dry. The purchasing people at the plant will not be too found of a VFD due to their cost and additional engineering cost to evaluate and spec one out. One thing would be to ask the vendor if VFDs had been used on these motors before, because some motors overheat when hooked up to a VFD.
thanks for all the help so far.
RE: Pump RPM to achieve flow
Has it been determined that this pump must not be operated at rated motor speed?
Is cleaning a temporary situation, or is cleaning a full time operation?
Can it not be operated at a higher head and flow for short periods of time while cleaning, then turned off?
Will it product too much head, or too much flow, or both?
Why can you not clean at a higher head or flow?
Why do you need a valve in the main discharge line? Is it possible to use a bar-tee and valve to recycle back to suction to reduce head and flow while keeping the main discharge line clear.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Pump RPM to achieve flow
Our system runs full time but if it were to shutdown it would not be a big deal. These balls clean the condenser tubes.
We're also adding additional piping to the suction line to prevent aviation and flashing that is occurring. A 5 ft run will help prevent this, which having additional flow may be somewhat necessary.
RE: Pump RPM to achieve flow
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.)
RE: Pump RPM to achieve flow
RE: Pump RPM to achieve flow
-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!!
RE: Pump RPM to achieve flow
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.)
RE: Pump RPM to achieve flow
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....