Pumps won't make shut-off

[SteveWag]

I started up two Grundfos CR 5-7 pumps yesterday. The pumps are marked with a shut-off head of 225.6 ft. I have a curve that shows the same, as near as I can read, not sure of the .6. The curve indicates a 3501 RPM speed. These pumps are driven with 2 HP 3450 motors, and I wired them across the line (design driver includes a VFD) for startup. Suction is taken from a tank three feet away overflowing with 10 feet of clear, cold potable water. After venting air from the pumps, they were individually started and both made 172 feet of shut-off pressure. Allowances were made for suction pressure (10 feet) and gauge location (5 feet above floor level, and yes the tank sits on the floor). Why ??? I hade a new mechanical gauge and a new, very expensive, instrumentation pressure transmitter. Both gave the same readings.
Steve Wagner

 

[gepman]

Use a tachometer to measure the actual speed. At 3450 rpm the pumps should achieve (3450/3501)**2= 97% shutoff head or 219 ft. Nameplate speed only needs to be within 5% if I remember correctly so you could actually be at 3277.5 rpm. At this speed you would only put out about 197.7 ft.

Also I don't know if there is a possible impeller trim which of course would affect your head.

 

[Valvecrazy]

I am sure you have checked the obvious. That kind of pump will build quite a bit of head when running backwards. Just not as much as it should.

 

[Artisi]

Are the pumps running in the right direction?
Check the inlet side pressure (by gauge), maybe you have a blockage.

 

[gepman]

I have rechecked the NEMA MG-1 standards and the actual motor speed must not vary more than 20% of the slip (in your case 3600-3450=150) so the minimum speed would be 3420 rpm, not slow enough to make as much effect as you see (unless of course you were running at 50Hz power). You could have very low voltage which could cause great slip.

Most likely it is something suggested above by Valvecrazy or Artisi especially now that I see it is one of those multistage centrifugals.

 

[SteveWag]

Thanks guys!! I didn’t have a tach., but voltage and current were in the ball park and I was running directly from a 208 volt, 3-phase, 60 cycle supply. I have visually inspected the suction piping and it’s open. The pump takes its suction from the bottom of an 8-inch manifold 5-foot long, with about 3-feed of 2-inch pipe and a ball valve and 90 degree ell. I don’t think a minor restriction would matter at zero flow. The impellers were not trimmed; I don’t think this pump is designed for trimming. There is a prominent arrow and I interpreted it as direction of rotation, but it is pointing in the direction of FLOW. I’ll be there Monday and see if there is another indication of rotation.

Thanks!!!
Steve

 

[Valvecrazy]

The arrows up close to the shaft shows the direction of rotation. The arrow at the bottom shows the direction of flow. Can be confusing.

 

[Artisi]

Where are the valves and the 90's in relation to the inlet - it is possible that you are suffering from an entry problem which can reflect in a drop-off in performance.

 

[Valvecrazy]

Even a restricted suction has very little effect when checking shut off pressure. With flow, a suction restriction becomes important. At zero flow, the suction would need to be basically completely blocked to affect shut off pressure. Still bet, running backwards.

 

[pumpking]

It sounds highly likely a direction of rotation issue. Normally these Impellers dont get trimmed, and Grundfos are very unlikely to do it for you, all they do is change the number of Impellers to vary the pressure generated and best match the pump to the duty.

If it isnt this, then the speed check is next step in case you have dropped a phase in the motor.

Let us know how you get on...



Ash Fenn

http://www.cdrpumps.co.uk

 

[SteveWag]

The pumps were running in reverse. I guess the money spent for the factory start-up guy was wasted. Monday I reversed rotation and pumps are working as specified. Thanks to all!!!!
Steve

 

[gepman]

I think that I would ask for my money back on the factory startup. Tell them that you don't pay for on the job training.

Was he actually out of the factory or from a representative? I am surprised because the factory in Fresno is very close by and has a good reputation (although I have never used a Grundfos because they don't make the ANSI end suction industrial type pumps that I typically use).

 

[Valvecrazy]

I have been using Grundfos since 1974. I was pretty sure yours were running backwards. All the old guys I knew at Grundfos 30 years ago are now gone, and so is their expertise. Younger guys with their degrees mean well but, they will never understand pumps like someone who grew up in the pump business. Pretty obvious when the factory guy can't even check for rotation.

BTW, Grundfos now owns Peerless. So I think they now make ANSI end suction type pumps.

 

[crazytexacan]

MY MONEY was on WRONG rotation TOO.... It does happen... There are phase meters from amprobe that allow you to hook up LARGE motors right the first time.