pump taking high current

[pkg80]

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

 

[pkg80]

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

 

[ScottyUK]

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?

 

[georgeverghese]

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?.

 

[pkg80]

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?

 

[LittleInch]

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.

 

[georgeverghese]

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 ?

 

[pkg80]

Hello georgeverghese

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

 

[EdStainless]

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

 

[pkg80]

i have here attached pump drawing.

 

[JJPellin]

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

 

[georgeverghese]

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.

 

[ScottyUK]

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.

 

[pumpingtips]

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.