Rated Pressure vs Actual Pressure

[aida2011]

Dear friends,

I have a PACO pump (Grundfos) rated at 230 m3/h and 40 m head (4.0 bar). I enclosed the pump specs and curve for you all to analyze.

In actual running condition, the pump can deliver 230 m3/h but it cannot deliver the required pressure. The pump can deliver up to 1.2 bar only, far from the rated 4.0 bar. The maximum load recommended by the manufacturer is 63 amp, but at 1.2 bar the load is already 57 amp. The plant requires at least 2 bar, so we have to run two pumps to obtain 2.1 bar. If we run both, we will not have a spare pump.

The pump is not running on VFD. We need to throttle the outlet valve a bit to prevent motor load increase that would trip the pump. The motor power is 45 kW. Please I need your expert inputs.

Miss Aida Hanani
Plant Engineer

Malaysia

 

[ione]

What's the actual impeller diameter of your pump? The data sheet report the performace curve of the pump for three different impeller diameter and none of the them is capable to give 230 m3/h and 40 m head. You seem to have some problems with performance curves reading.

 

[LittleInch]

Before you get into taking the pump apart to see whether the impellor is the right size or not damaged, please double check to make sure the pump is going round in the right direction. It is very easy to misconnect a three phase motor and the pump will pump, but just not as well as it should. There is usually an arrow stamped on the casing showing the correct rotation.

If it is going the right way then there is something seriously wrong with your pump.

Why have sent two pump curves?

Is the second pump identical? Does this also have the same issue?

Do you have anyone in your plant to talk to as you are posting a few questions all of a sudden?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ione]

My bad, I had not seen the second set of performance curves.

 

[aida2011]

Hi all

Thanks for your answers.

I am sorry to have sent two pump specs and confused you all. The relevant one is the second pump specs.

LittleInch, actually the main reason I posted so many questions all of a sudden is because I was recently promoted from a trainee to engineering executive, and my job scope covers steam boiler, chillers, compressors, and waste water treatment plant. I was asked to solve many problems. That's huge for a 25-yr old female junior engineer. Of course there are people in the plant that I can talk to, but I would avoid some male engineers here. I would pefer to turn to supplier, or maybe this forum, if I can get support from you all. OK back to business

Yes, we have done rotation check and free-run test. I have no clue, the electrical engineer also wondered why the amp becomes high despite motor not running at full cap.

Best regards

Aida

 

[Artisi]

If the pump/s are rated at 230m3h/ 40 there is no way to get 230 at 1.2 bar . where are you measuring the total head.if the pumps are operating at 1.2bar then flow rate would be way left on the curve and probably cavitating.
Think you need to sit down with the original design cals./ design concept and redo all the friction head total head cals. and compare these figures with the pump selection. Check all detail carefully, ignore what you have been told/ believe and start from scratch.
you info is very limited and the guys here are trying to make sense out of the limited data and crystal balls aren't much use in pump design / trouble shooting.
How are you measuring flow, head, is the instrumentation accurate, setup correctly etc.
Remember, pumps are stupid machines, they can't think and they are completely unaware of what you need them to do- they simply run at the motor speed, pump product against the head imposed on them by the system within their hydraulic capabilities - no more, no less.

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.)

 

[Artisi]

Correction, to second line - that should read "way right" on the curve ......

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.)

 

[HPost]

Are you sure of the your system restriction? It is that restriction that will convert kinetic energy to head. If the head is not as high as you expect, the flow will increase and the pump will be operating away from its best efficiency point.

Have you tried creating a restriction to push the pressure up?

If you have done this and the pump can still only generate 12m of head, there is a problem with the pump.

It is very easy to generate pressure...if you have the right flow in relation to the restriction. The pressure generated is function of the impeller peripheral diameter, the flow is a function of the blade depth.

It is also assumed that the pump is turning at 1475 RPM.

 

[LittleInch]

Are you sure the 1.2 bar you mention is pump discharge and not downstream of your outlet valve which you are throttling? By sure I mean you personally have gone and looked at the pump during operation and can confirm where the pressure tapping is compared to the valve and not by looking at a drawing or relying on what an operator is telling you or what someone can see on their screen.

My suspicion is that what has happened is that the downstream system is not providing the resistance that the pump has been designed for and hence when it was commissioned the pump went to the far right of the curve and tripped. Therefore someone started to throttle the pump so it does the flow, but created a discharge pressure upstream the throttling valve.

If your pump is doing 230 and your pressure is only 1.2bar then there is something amiss with the design.

My calcs show your motor is providing 36kW of shaft power and if the pump isn't putting out 40m at 230 m3 then I don't know where this power is going.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JohnGP]

Hi Aida, to get much further you probably need to provide a sketch of the system setup, including showing where you are measuring flow and pressure. As Artisi points out, the pump operates at the point on its curve that corresponds to the head that it "sees" at its discharge (provided there are no issues with the pump itself). If you generate a system curve and plot on the pump's curve, that should confirm how much you need to throttle the flow to pull the pump back on its curve.

If you have a gauge on the pump, you can do a check on the shut-off head point on the curve by briefly closing a discharge valve, to confirm the pump can at least hit that point. Or looking at the curve, you wouldn't need to completely close the discharge to confirm it can attain 4.2 Barg at low flow.

Cheers,
John

 

[BigInch]

Just for the record, I'll go with LittleInch and my bet is that it's running backwards.

I hate Windowz 8!!!!

 

[BigInch]

Possible suction nearly blocked.

I hate Windowz 8!!!!

 

[DubMac]

Knowing Paco pumps fairly well, I would say check to make sure the impeller is not on backwards. It would not be the first or even the second time I've seen the "boys from Brookshire" do that.

Most likely though, Little Inch has probably got it right; check with your own eyes where the pressure measurements are taken from and read the gauge yourself.

 

[JohnGP]

Hey BigInch, not so sure about that. From the first post - "the pump can deliver 230 m3/h but it cannot deliver the required pressure". I don't think achieving that flow would be possible with either blocked inlet or running backwards. Think it has to do with measurement and interpretation of operating condition.

I once had a plant operator try to convince me that we had a problem with a scrubber pump because he thought the discharge pressure should be higher. He thought higher pressure meant higher flow in the circuit.

 

[aida2011]

Hi all

Thanks for your concern. PACO pump specialist will come and address the problem. Let see what he has to say.

Regards

Aida

 

[Artisi]

Please make sure we hear the result.

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.)

 

[LittleInch]

Aida,

Please take note of the advice above and before you bring in the vendor make absolutely sure yourself that the pump is only delivering 1.2 bar and that the pressure reading is correct and in the right place(i.e. upstream of any throttling valve).

If you can actually fit the impellor the wrong way round this is the same as rotating the wrong way.

We would like to get feedback on what happens next.

good luck

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

It never ceases to amaze me, after half a life time in the pump business when ever there is a problem it's always the fault of the pump. Well in 30+ years I can count on 1 hand the number of times it has been a pump problem - in all other cases it has been something else other than the pump. Wrong direction of rotation, excessive loses on the inlet side, completely wrong pump selected for the application, head calculation / estimation wrong, etc.

If the pump is running at correct speed - correct direction, NPSHa/r are compatible, head calculations are correct, inlet and outlet pipe work us free of obstructions then it should be operate as specified / required.

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.)

 

[DubMac]

Actually Little Inch, putting the impeller on backwards is not the same as wrong rotation. Think of the cutwater.

Also wrong rotation on many splitcases (Paco KPV included) can unscrew the seal sleeve nuts and have the impeller come loose if not properly pinned; impeller on backwards won't affect that if its spun in the proper direction.

 

[LittleInch]

Fair enough - the end effect is probably worse / going to decrease efficiency, but I agree with Artisi, I think there's something wrong with the system ,not the pump...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way