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pump taking high current

pump taking high current

pump taking high current

(OP)
hello

we have three similar vertical pump in parallel operation with full load current of 75amps. One pump is taking current more than full load current 75amps. It takes 76-80amps. Others pumps takes 65-69amps @same time for same parameter.generally two pumps always in operation.

I have checked suction/discharge pressure which are same for both pumps. Flow meter is common for all pumps so we cant take individually but i think is same as pressure are same.

There is all parameter (vibration,temperature etc.) are within limit.

can any one suggest ,what may be the probable causes of high amps.

reagrds
pradip

RE: pump taking high current

Yes,

Not all "identical" pumps are actually identical.

Your one pump drawing more power will be pumping more flow than either of the other two.

Unless each actual pump has been individually tested to create a unique pump curve you won't find put why. It also makes a big difference on the shape of the pump curve and where on the curve the duty point is.

Less than 10% from duty point is not unusual for parallel pumps.

Can you post the pump curves and mark or note the duty point?

Other issues can be that the inlet to the three pumps is not equal and hence one pump has a lower inlet head than the other. Can you sketch out the physical inlet (it there is one) and outlet headers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: pump taking high current

(OP)
thanks littleinch for ur reply...

As I said these pumps are identical and generating same head (same discharge pressure)with same suction pressure.They have common suction and discharge piing.
usually two pumps run parallel.

RE: pump taking high current

Yes, but one is pumping more fluid for the same differential head, therefore more power is required. A variance of 3 or 4% is "acceptable" as far as the vendor is concerned.

One could also be slightly less efficient as well.

If you have the pump curves then post them along with the inlet and outlet headers and then we can see if they really are "identical".

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: pump taking high current

Has this rogue pump always been overcurrent?
Has the running clearance been checked?
What pump type are they? -- vertical pump means nothing without configuration description.
Is o/current reading on all phases, have all electrics been confirmed as Ok?

quoting LittleInch "Remember - More details = better answers"

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 taking high current

This may be due in part to a pump curve which is more or less flat at around the operating point. Try throttling the discharge valve on the high amps pump a little - that should even out the amps consumption between the 3 pumps ?

RE: pump taking high current

(OP)

hello all
thanks for your interest and reply

I have seen the all pump curves supplied by the vendor and it is similar for all three pumps .I will provide to you pump curve. Here difference in ampere is by 8-10 amps in both pumps which delivering similar head in parallel operation. As I said the pump is delivering same head, it mean flow must be same if it is identical.

layout is like pumps suction from common header first is A then B and then C and in discharge its reverse .
C is taking high current.

RE: pump taking high current

Look forward to seeing them. Check to see if each pump was actually performance tested or whether you simply have a "typical" curve for each pump.

"similar" and "identical" are not the same thing.

also we don't have any process conditions for these units. If the head developed is small, the it won't take much to increase the head of one pump versus another.

Also check Artisis' post and confirm the motor is actually good.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: pump taking high current

Unless you have individual flow meters, you cannot assume that all pumps are producing the same flow just because they are operating at the same head conditions. You have still not provided the pump configurations. If they are vertical turbine pumps with semi-open impellers or lateral wear rings, the lift setting can dramatically change the performance. Some vertical turbine pumps have a horsepower curve that increases at low flow. As noted above, a phase imbalance in the electrical supply or drive motor could confuse the situation. Normally, the ammeter is only on one phase.

Are there minimum flow spill-back lines? Does each pump have an individual spill-back or is there one common spill-back for the group? Are there suction strainers or screens? Is this a service prone to fouling, lime formation, trash or solids?

Johnny Pellin

RE: pump taking high current

Another great post by JJPellin.

Metering in larger pumping stations, particularly at the highest voltage level tends to be more complex and comprehensive than in smaller pumping stations on lower voltage level. Service technicians may need additional instruments to measure and record, for example, circuit and insulation resistance

RE: pump taking high current

(OP)
Hello jjpellin /LITTLEINCH

Thanks for reply

This pump has closed impeller and we have already checked all 3 phase current and t is like 76 78 & 76 for one pump and 66 67 & 68 for another pump.

These pumps have not any minimum flow lines individually.

is it good idea to check motor by solo run test,before jumping on the pump?

what may be the probable causes for unhealthy motor taking high amps ,I mean, what parameter I should check?

RE: pump taking high current

Interesting pump curve. I still think they only actually tested one pump. When you look at the curve it has the same test number and the same deviation from a straight line. In fact with this pump, the one taking more power might actually be seeing less flow, as the flow decreases there us a small hump as the efficiency drips off faster than the effect of flow.

If possible can you dead head each pump separately for a few seconds and see if you get the same pressure. That will start to tell you if there is actually something different with the basics of each pump.

I don't know enough about motors to know what the tests could be, but try the deadhead thing first. If they are all the same it might be flow related, but you're gradually knocking things off one by one

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: pump taking high current

Are you sure that the problem is the pump with high amps and not the pumps with low amps. I assume that the motor is rated for 500 HP. If so, I would expect all motors to be running close to full load. There could be a problem with the pumps running low amps.

Johnny Pellin

RE: pump taking high current

Do you have tap points to check pressure at the pump inlets and outlets?
A slight variation in plumbing could cause a difference in load.
I would not assume that the motors are identical either, unless you have motor test data.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: pump taking high current

Good to see the pump curve, at the moment the only useful data supplied so far, we now understand we are not dealing with a few small process pumps but reasonably high flow high energy units -- makes a big difference to the thinking of the guys in this forum.

We are still waiting on pump type / configuration

An installation drawing etc. would assist.

How about an answer to my earlier question - has this pump always run at higher current yes / no.

Is the o/current pump running over maximum allowed current?

It's looks like developing into one of those enquiries where we need to squeeze every bit of info. from the OP step by step.

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 taking high current

Pump curve not flat, so would agree that flow may be the same at the same differential head.
These amp reading indicates 3.3kV 3phase line voltage
Vertical turbine pump ? Long shaft ?

A check on design case power for this rated flow of 8925usgpm and h = 424ft at e = 0.81, assuming liquid density is 1000kg/m3 for water tells me design case shaft power is 1185HP and not 519HP ?? What are we pumping here - LPG or LNG?


RE: pump taking high current

georgeverghese: case in point ponder

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 taking high current

(OP)
hello Artisi
yes ,this is running with overcurrent 76-80 amps (More than full load current 75 amps ) always and sister pump with 65-69amps ,it not like spike.This is vertically suspended (V3) centrifugal pump. What do you mean by running clearance ?
phase current already checked and its nearly same for all phase.

RE: pump taking high current

(OP)
hello EdStainless

suction / discharge tapping on the same distance from the piping for all pumps and it is just before and after the pump.
Motor is identical in all pumps.

RE: pump taking high current

(OP)
hello JJPellin

pump is equipped with 435kw motor ,
pumps running with following data
for pump A Differential pressure is 117psi
for pump C Differential pressure is 116psi
Total flow for both pump is 13805BPH

RE: pump taking high current

Are the pumps fed from the same busbar? If voltage to one pump reduces slightly then the current to that pump will rise: over a small range of voltage variation a motor behaves as an almost constant power load. Are the feeder cable lengths similar?

RE: pump taking high current

Artisi: Am trying to make some sense of this pump's operating point...

For a pump with an operating point of 4831usgpm (13800bph for 2 pumps), the head curve indicates approx 550ft. If diff press at this point is 116psi, then liquid density must be 485kg/m3. And corresponding operating shaft power is 500HP (373kW), which indicates amps = 71 (approx) at a line voltage of 3300V.

This matches up with the amps on the sister pump at 69-70Amps, so this pump seems to be running as it should be.

Some light liquid being pumped here as suspected ( not disclosed).

But for a motor rated at 435kW, rated amps would be = 82amps, so 76-80amps = 92-98% of motor rating.

The vertical turbine pump running at 76-80amps - intermediate shaft bearing problems?.


RE: pump taking high current

(OP)
hello georgeverghese

Thanks for reply

Yes you are right ,The process fluid is ethane with sp. gravity 0.435. Motor voltage is 4000V.
It might be problem with intermediate shaft sleeve but I think it should reflect in vibration and vibration is normal.
also we can not open the pump without ensuring all other checks which has potential to creates this problem. I can say overhauling of the pump is the last option .


@ScottyUK: I don't know pump fed from same bus bar or not ,will check with electrical people. If not what we can check ? what will be effect if feeder cable lengths will not similar?

RE: pump taking high current

Pradip,

My summary so far is:

You might have one pump out of three which is operating at a higher flow for the same head and or with lower efficiency, but pumps may not have all been individually tested. If possible do a blocked in pressure test to see if all pumps are running on the same curve. Vendors can allow 3% or more from rated duty and still be "in spec".

Hydraulic power from the curve is remarkably flat, so as noted above maybe the other two pumps are the opposite end of the allowable variation.

Your over amp motor is being fed with less volts than the other two when running ( hence issue of the cable length and bus bars). Try and get an electrician to measure volts when operating at the motor terminals. I'm no sparky, but I've not come across 4000V as a standard voltage before.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: pump taking high current

Can you post an internal assembly dwg of the vertical turbine pump with some dimensions which shows the location of the intermediate shaft bearing also ?

Shaft seals working okay on all 3 pumps? Which API seal flush plan ? Anything different in terms of barrier fluid consumption / seal fluid level depletion ?

RE: pump taking high current

(OP)
Hello georgeverghese

will provide you the drawing .shaft seal is working fine and have plan 13 and 76.No barrier fluid.

RE: pump taking high current

A layout of the installation would be helpful.
I have NEVER seen an installation with parallel pumps where the piping was identical for each. There is always some variation.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: pump taking high current

If this pump has always had higher amps, it could have the wrong impeller pattern or impeller diameter. Block the pump in and measure dead-head pressure. If the impeller was left full size and not trimmed, this will show up as high dead-head pressure. Measure the flow and pressure with the two good pumps running. Assume that each pump is pumping half of the total flow and plot the operating point on the curve. Run one of the good pumps with the bad pump and repeat the exercise. Does the point for the second test plot above the curve? If so, there is a good chance that the bad pump has an oversized impeller or impellers.

I don’t recall seeing how many stages these pumps have. How many stages do these have?

Johnny Pellin

RE: pump taking high current

Took a shortcut yesterday to get the pumping power of 500HP by using the pump curve, not realising that the confirmed operating density of 485kg/m3 is at variance with the reference density of 438kg/m3. This reference density is the basis for the Q-h performance and power curve.

So, the corrected power consumed should be 500 * 485/438 = 553HP = 413kW
Corrected current then corresponds to 71 * 485/438 = 79Amps

So the high amps pump is performing in accordance with the pump curve.

RE: pump taking high current

pradip80,

Responding to your earlier question -

If one motor has a significantly longer feeder cable than the other then the motor with the long cable will receive a lower voltage at its terminals, resulting in higher current draw. Similarly a small difference in voltage at the busbars can result in differences in line current.

If the supply voltage is abnormally high then the motor current would also rise as the stator core reaches magnetic saturation. This is rare in my experience.

RE: pump taking high current

In general, the guys saying that the high current draw pump is pumping more flow will most likely be correct. Pressures won't show you differences for parallel circuit. It most likely is piping configuration because these larger flows can be affected by the streams running straight through whereas the stream trying to enter the manifold from a branch may have more trouble getting in. This won't be picked up easily by pressure differences. Pump impeller diameter differences will be other possibility. I would look at the piping and try to restrict a little the flow on the high current pump.

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