pump pressure variables 6

[kahlilj]

Does anyone have ideas on how to reduce pump outlet pressure? We recently installed a pair of pumps (primary & backup up operation) which are delivering 80 psig pressure where we were expecting about 50-60 psig. I'm not sure why it's building that much pressure, but need to know ways/ideas on how to reduce it.

The centrifugal pumps are 20 hp electric motor driven, sized for 325 gpm at 150 TDH.

What do you need to know to give some reasonable suggestions?

 

[JJPellin]

I guess am not being specific enough with my terminology. I am a pump guy. A centrifugal pump will tend to perform to a curve of flow versus head. As the head is reduced, the flow will increase. As the head is increased, the flow will be reduced. In these statements, the head is the total developed head of the pump. This is determined by subtracting the suction pressure from the discharge pressure and converting from units of pressure to units of head.

You may be using the term head to describe the head loss in the system. That is based on the system curve. For a system curve, as the flow is increased, the head is increased and vice-versa. The operating point for a given pump in a given system is the intersection of the pump curve with the system curve. It is impossible for a typical centrifugal pump to have a curve such that 325 gpm at 150 feet and 400 gpm at 200 feet are both on the same curve.

It sounds like you decided to use a different curve based on the fact that you were getting more pressure at one point than you expected. But, if you can't measure the flow directly, you can't make that assumption. What is the suction pressure right at the suction of the pump? What is the discharge pressure right at the discharge of the pump? What flow rate would this correspond to on the original curve for the installed impeller and motor speed? What would cause you to believe that the flow rate is higher than this value?

I get the feeling that we have wondered off into the swamp on this one and we are getting bogged down. Something seems to be drastically wrong. Are you looking at 50 Hz curves but running with a 60 Hz motor? Are you looking at curves for a different size, speed or hydraulic design? I don’t see enough information anywhere in this chain to understand what is going on.

Johnny Pellin

 

[Pumpsonly]

Johnny

The OP specified a pump to produce 325 GPM with 150 diff. head and according to the pump curve should be fitted with a impeller of 6.75"

But the pump vendor supplied him a pump with a impeller 1 biginch over size at 7.75" and gave a flow of 400GPM at 400 ft diff head
on him system.

 

[kahlilj]

Johnny, I make no profession to be a "pump guy" & I respect your knowledge in the field. What I shared before was simply what I observed in the field and what I could interpolate from the suppliers pump curve based on field results. I do agree that something is not right. I have attached the pump curve and a table summarizing the field results. hopefully this will make results easier to follow for everyone including me!

 

[kahlilj]

Oops. Here is the other file summarizing the measured parameters. I guess I can only load one file per message?

Anyway we still have slightly more head than anticipated. Since we can't safely reduce the impeller size any less, we will install either an orifice plate in that 3" branch or open a bypass line put in between the 6" suction & discharge headers to reduce the overall line pressure.

 

[kahlilj]

ok. I'm not sure why neither file uploaded..?? trying to upload again...

 

[MechEngNCPE]

"Since it is a closed system, I'll go out on a limb and predict that the initial hydraulic analysis for pressure drop around the loop was conservative and now you're getting less of a pressure drop that you thought and that results in more suction pressure remaining than you thought you would have."

From a design engineer's standpoint, I'd have to say BigInch nailed it.

 

[MechEngNCPE]

Sorry, missed the more recent post, oversized impeller from vendor.

 

[BigInch]

I should know by now not to assume ANYTHING in these forums. The seem to be affected by Murphy's_Laws^2

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[kahlilj]

"I should know by now not to assume ANYTHING in these forums. The seem to be affected by Murphy's_Laws^2 " lol!

Or assume that you got incomplete info Sorry about that BigInch, but do appreciate your input as always - very insightful.

On a related note with regards to the pressure concerns, looks like we are good with the pressures in the 3" branch which leads to nozzles. There was an orifice further downstream in that branch which reduced the pressure to an acceptable level. We were not measuring at that location last week when we first started testing things out. Yesterday we did.

BTW, anyone have any idea why most previous posts did not include the files I uploaded? Is there a limit on file uploads to a thread?

 

[BigInch]

There was some "reworking" of the website going on over the weekend and quite a number of things went bad in the process. Probably had something to do with that.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[BigInch]

kahlilj, no problem. Sorry I might have mislead you, but I thought I was starting with the highest probability items first. As always when using probabilities, sometimes you win, sometimes you lose. From my perspective, when I get a question like that, its not often that I actually find something wrong with a pump. Often? No. In fact, I never have. It's always been a conservative pressure drop resulting in a pump selected with the wrong BEP for the job at hand, or visa versa. Faulty engineering, is more common than a faulty pump. That vendor is going to start making the good engineers look bad and the bad engineers look good. Not a lot of future in that.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[gr2vessels]

There is still a number of questions hanging above;- how did the wrong pump ended installed and started operating? It obviously was a long string of engineering failure. We haven't heard if the purchase specification was correct or not, how did the supplier end selecting/recommending a wrong pump, how did the wrong pump offer ending accepted (by whom?), how could a wrong pump be installed without proper engineering review and approval and then operated without proper commissioning/testing?
Just curious, I think we all can learn something of this.
Cheers,
gr2vessels