Loss of Prime on HSC Pump 1

[Munequip]

Trying to resolve a field issue regarding a double suction split case pump with packed stuffing boxes, vacuum priming system, and long HDPE suction line.

Single suction line feeds two HSC pumps. Suction line connects to tee to feed Pump 2 and to an elbow to feed Pump 1. Pump sits ~ 14' above water level.

During initial start up, both pumps lost prime while being brought up to full speed.

At a subsequent test, Pump 1 would lose prime at ~ 1000 GPM to 1200 GPM while operating at ~ 40 hz. Pump 2 was started, and it did not lose prime this time and was brought to full speed operation. Pump 2 was stopped, then Pump 1 was started, and it successfully primed.

There was quite a bit of noise in the suction elbows feeding the pumps, and we could physically see the vacuum assist piping occasionally shake. After both pumps were operated in parallel, the noise in the suction elbow subsided and the shaking of the vacuum pipes was reduced to the point that it did not occur.

We were able to operate at full speed, and operate on the curve. (This is based on the readings of the suction and discharge pressure transmitters, mag meters, and amp draw at the VFD.)

Both the single pump operations test and the parallel pump operations test were on the pump curve.
Based on the pump tests, a single pump will deliver 4000 GPM into the system, and when operated in parallel, each pump will deliver 3000 GPM into the system.

Any thoughts on why the pumps would not prime initially, but over time, they are priming consistently?

Also looking for thoughts on why Pump 1 will lose prime around 1000 GPM to 1200 GPM while it is being brought up to full speed, but operate without loss of prime at full speed while delivering 2000 GPM and 3800 GPM. We changed the ramp time on the start up of the pumps to a roughly 1 hertz per second, and bring the pump up to full speed, and then allow the flow rate from the mag meter to adjust the pump speed.

The pumps have been in operation for a couple of weeks without a loss of prime, but Pump 1 still exhibits a “groan or grunt” around 1000 GPM, almost like it is about to lose prime, but does not and keeps going. When this "groan or grunt" occurs, the flow at the flow meter will be reduced by 50 to 100 GPM before it recovers, and then it continues on has one would expect. Any thoughts on this phenomenon.

 

[DubMac]

The groaning,grunting noises surely are cavitation and are caused by absolute pressure dropping below fluid vapor pressure at that point, i.e. insufficient NPSH.

The reason it is happening at 1000gpm point only, and only on pump#1 would be next to impossible to truly determine without much more information on your setup (a diagram, true pressure readings and NPSHA figures).

Going out on a limb here to guess it may have something to do with your vacuum assist device not kicking in immediately, or some delay in providing enough NPSH to the system.

I guess this is more of a conjectural problem since the pumps are running fine once they get to speed. The few seconds of cavitation will have no long term effects unless they start lasting longer and resultant vibration knocks something loose.

 

[bimr]

A picture or diagram of the installation would be helpful.

 

[Munequip]

I'll see if I can run down a sketch of the system.

Of note, the station was taken off line for a couple of days, and an attempt to start Pump 1 failed as it lost promeat the same point. However, last week, the pump operate wihtout a loss of prime.

 

[Munequip]

We have attached a drawing of the station piping. Note that the suction piping is 24" diameter, starts at elevation 578, and runs ~ 700 feet to elevation ~592.

As noted, the pumps were operated today, and Pump 1 initially lost prime, pump 2 operated without problems, and pump 1 wwas restarted and it operated without problems.

Any coments are appreciated.

 

[CGreenwood]

Actually, the pump sits only 6.17' above water level. Normal pool elevation is 590.00. Just wanted to clarify. Everything else sounds good.

 

[bimr]

One would suspect that air is being trapped in the suction pipeline.

You don't mention the diameter of the fitting on the pump suction. The intake pipeline is 12-Inch in diameter. Is it possible that the intake pipeline necks down from the 12-Inch?

"The Suction pipe should never be smaller than the suction connection of the pump, and in most cases should be at least one size larger. Suction pipes should be as short and as straight as possible. Suction pipe velocities should be in the 5 to 8 feet per second range unless suction conditions are unusually good.

Higher velocities will increase the friction loss and can result in troublesome air or vapor separation. This is further complicated when elbows or tees are located adjacent to the pump suction nozzle, in that uneven flow patterns or vapor separation keeps the liquid from evenly filling the impeller. This upsets hydraulic balance leading to vibration, possible cavitation, and excessive shaft deflection. Especially on high and very high suction energy pumps. Shaft breakage or premature bearing failure may result."

http://www.gouldspumps.com/pag_0005.html

The piping should be installed so that it is sloping upwards towards the pipe and should not have pockets that will trap air.

The 12-Inch intake piping would normally be rated for a maximum flow of approximately 3,400 GPM (8 ft/sec).

One would expect that you have an pocket in the suction pipeline. As long as the air is trapped in the pipeline, the air pocket is effectively reducing the pipe size. Once the water velocity gets high enough, the air pocket is forced towards the pump and flushed out. Then the pump will be able to operate at the higher flows.

"On pump installations involving suction lift, air pockets in the suction line can be a source of trouble. The Suction pipe should be exactly horizontal, or with a uniform slope upward from the sump to the pump as shown in Fig. 1. There should be no high spots where air can collect and cause the pump to lose its prime. Eccentric rather than concentric reducers should always be used, on horizontal installations, with the flat side located on top."

http://www.gouldspumps.com/pag_0005.html

You may be able to relocate the connections for the priming system to eliminate the air pockets.

 

[bimr]

I did not initially see the elevation drawing on the bottom of the pdf. However, if you review the elevation drawing, your problem most likely is caused by the orientation of the 24-Inch reducer.

Where the 24-Inch reducer necks concentrically, you have a spot where air will be trapped. You would expect more of a problem with Pump No. 1 because this pump is after this point in the pipe.

I usually add the note "no pockets" on drawings.

You may be able to add another suction connection just before the reducer to eliminate the problem.

Good luck.

 

[Artisi]

What is the vacumm level on the inlet side when the flow is at the point of "grunt and groan" ie,1000gpm - it could well be at a level causing a momentary NPSHa / NPSHr problem.

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.)

 

[Munequip]

BIMR - Do you have a link to an example of the suction connection? Are you referring to a suciton diffuser?


Artisi - We don't have the suction pressure recorded at the 1000 to 1200 GPM, but we do know that it was 9.2 PSIa at a stable flow of ~ 3100 GPM, and it was 8.55 PSIa at a stable flow of ~ 4000 GPM.

From memory of the last pump test I witnessed, the suciton pressure continual fell from a high when the pump was off to a low at the ~ 4000 GPM flow rate. In terms of the pressure transmitter reading, eveything appeared normal.

The suction pressure for Pump 2 is slightly higher than Pump 1, but that would be expected.

 

[bimr]

I was suggesting that you can add an additional 2-Inch connection on top of the 24-Inch intake pipe which you can then connect to the vacuum priming system.

 

[Munequip]

Thank you for the clarification on the connection point, and all of your ideas and comments.

 

[77JQX]

You might also want to consider the inertia of the water in the suction pipe - there's a lot of weight in 700' of 24" pipe. With a flat HSC curve, small speed changes can result in large flow changes. It may be that you are passing through the 1,000 gpm range too fast to accelerate the water in the suction piping.

 

[bimr]

The poster states:

"At a subsequent test, Pump 1 would lose prime at ~ 1000 GPM to 1200 GPM while operating at ~ 40 hz."

Since the poster states the condition appears at a steady state and with a velocity of less than 1 ft/sec in the 24-Inch suction pipe, inertia of the water mass is probably not a consideration.

 

[786392]

Dear Muequip Hello/Good evening,

Sorry for late entry in discussion& Hopefully the problem may have already been resolved.


If not then,Just check and rule out the possibility of physical obstruction in suction header coming floating,although very very remote one!


Best Regards
Qalander(Chem)