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Join Date: Aug 2015
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Pump Load

09/30/2015 9:30 AM

Dear Sir,

We have four chilled water circulation pumps connected to a header. Two will run one standby and other used when one special unit was in operation. For each pump discharge side we have a valve. When we open the valve more than some extent, there will be some load on pump impeller and trip of pump was taking place. The line size was 450NB. With your experience can any one please let me know the reason for this. In the attached image the valves are highlighted.

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#1

Re: Pump Load

09/30/2015 9:54 AM

Under start-up conditions, the pump is probably running quite far to the right on its curve; i.e., higher flow but less pressure than designed. You may need to open the valve(s) more slowly.

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#2

Re: Pump Load

09/30/2015 10:02 AM

With the valves fully open the load on the pump exceeds the capacity of said pump motor.

The more water the pump discharges, the higher the load/AMP draw is on the motor.

Adjust the valves until the motors no longer trip. The pumps won't mind.

If this is a start-up problem, see Tornado's #1.

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#3

Re: Pump Load

09/30/2015 10:12 AM

Are the pumps positive displacement type? Centrifugal? Or?

Is the pump tripping due to electrical overload? Or is it just stopping?

If it is tripping due to overload:

Have you measured the motor current to determine if it is within motor nameplate DATA specifications?

Have you measured the motor supply voltage to determine if it is within motor nameplate DATA specifications?

If the motor current is within nameplate limits then the motor overload parameters need to be adjusted to the correct value required to prevent inadvertent tripping.

If the motor current exceeds nameplate limits and the applied voltage is within acceptable nameplate limits then the cause of trip is mechanical.

The pump may require more horsepower than the motor is rated for and a call to the pump manufacturer will yield the answer.

If the pump is just stopping but not tripping:

It may be that there is a loose connection in the control circuit or an operations permissive that is causing the stop condition.

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#11
In reply to #3

Re: Pump Load

10/01/2015 7:33 AM

No Sir...... There is so such problem with the motor connection....Its all about the load on the impeller......we already changes the impeller once but the same problem exists

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#21
In reply to #11

Re: Pump Load

10/02/2015 4:18 PM

Any mechanical load that is applied to the pump impeller is directly transferred to the motor and can be calculated by recording the motor current then dividing by 746 to determine the required HP needed.

However; The easiest solution is to contact the pump manufacturer and ask what motor specifications are required to meet the pump performance curve as applied to the application.

The impeller is designed to fit the pump casing so that it will produce a given flow and pressure for a given pump speed.

Unless the impeller is contacting the housing due to improper fit and/or backpressure and thereby causing abnormal mechanical drag, then changing the impeller will not solve your electrical overload issue.

The horsepower, speed, and torque curve of the motor must be matched to the pump horsepower, speed and torque curve requirements.

Example: If the application requires 250HP at 3600RPM with 90% of maximum torque available from 0-500RPM the motor design must match those criteria.

You must identify the type of pump and all mechanical characteristics of the pump, then identify all dynamics and requirements of the process system that it is installed in.

If they do not match then the correct motor must be installed or a gearbox with the correct gear ratio installed between the motor and pump.

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#4

Re: Pump Load

09/30/2015 10:17 AM

These are adjustable control check valves use to avoid surges, since there are 2 pumps operating at the same time, you need to synchronized the 2 with a balance discharge pressure, otherwise one will surge up the other resulting to a motor tripping.

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#5

Re: Pump Load

09/30/2015 10:19 AM

Just checking to verify for more information.

Can we assume when the pumps are running

  • all pumps running are the same size?
  • Same Impeller size
  • Same RPM?
  • Same Horsepower?
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#12
In reply to #5

Re: Pump Load

10/01/2015 7:37 AM

yes.....there are all with the same capacity......but only two will run in operation..........Its all about the load on the impeller......we already changes the impeller once but the same problem exist........Is any thing like back pressure in header or discharge head is not maintained properly ?

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#15
In reply to #12

Re: Pump Load

10/01/2015 8:20 AM

Change the impeller?

to a: Bigger diameter?, Smaller Diameter?, without the pump curve we can't determined if your going in the right direction. The pump curve should have the horsepower curve requirements on it.

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#17
In reply to #12

Re: Pump Load

10/01/2015 10:55 PM

If you changed the impeller and you get the same result it was not the impeller.

Its time to change the Engineer . ing approach on this. Did the system ever work? Ever?

Back pressure might arise from a partial or full blockage in the pipe works. Low pressure might be from a leak.

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#6

Re: Pump Load

09/30/2015 2:53 PM

Do you have check valves installed? If you don't then you may be back feeding one pump to the other?

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#7

Re: Pump Load

09/30/2015 4:40 PM

Just for general information:

Have you tried different valve opening rates?

What is your system operating pressure?

What is the style of discharge valve is installed? From the picture it appears it could be either ball or butterfly.

To repeat the above question: Do you have check valves on the pump discharge? It doesn't appear so from the picture.

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#8

Re: Pump Load

10/01/2015 12:54 AM

Dear Mr. yours bobby321

It appears to me that the delivery valve is leaking for that particular pump which trips. Is it for a particular pump-motor set tripping.? Further the picture does not show valve in Suction Side. Generally for pumps in parallel or stand-by operation, a valve will be provided in the suction side.

The tripping may be due to 1. Mechanical operation related issues. 2. Electrical related issues.

For mech. related tripping issues - if delivery valve is leaking, the pump will start against load - hence draw more current and over-load relay will trip. If over-load relay does not rip, the thermal relay will trip. The remedy is to have a good valve at the delivery which does not leak and a suction valve to be provided. Weak insulation of motor will be another possible reason

2. On Elec. related issues: (a.) Check for over-current relay and keepcorrect setting. (b.) Check for proper Thermal Relay. (c.) Check for voltage drop for the motor.

Pl. inform this forum - how it was solved.

DHAYANANDHAN.S

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#13
In reply to #8

Re: Pump Load

10/01/2015 8:14 AM

Sir......This was first observed on one pump....we check for the remaining three also same problem raised.....So one we changed the impeller for one pump still same issue.....For your information I am sending you the screen shot of the P&ID...Kindly go through it......Sir is there ant thing such that there is a back pressure in the pipe line ?

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#16
In reply to #13

Re: Pump Load

10/01/2015 9:18 AM

That picture is too small to be of any help!

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#9

Re: Pump Load

10/01/2015 1:29 AM

Pumps must be started with delivery valve fully closed and after staring, Valve is closed slowly, with an eye on motor current.

Valve can be opened upto a point where Motor amps limit is achieved and Valve must must be locked there and preferably a mechanical stopper is installed.

However, please see that Overload setting are correct as per Motor nameplate.

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#10
In reply to #9

Re: Pump Load

10/01/2015 3:30 AM

Step away from the keyboard! Far, far away!

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#14
In reply to #9

Re: Pump Load

10/01/2015 8:19 AM

Will try this.......Thanks

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#18
In reply to #14

Re: Pump Load

10/01/2015 10:56 PM

Awaiting more trouble shooting problems then!

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#19
In reply to #9

Re: Pump Load

10/01/2015 11:00 PM

So you close a fully closed valve slowly. Aha!

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#20

Re: Pump Load

10/02/2015 12:05 AM

yoursbobby321,

Pardon my bluntness.
This is an impossible situation.

1. You have no idea what the problem is, except that the pump motors are tripping something. In other words, you are not competent to even describe symptoms or ask questions.

2. Aside from a single photo, you can provide no useful information.

3. You changed impellers but see no difference and can't describe the results.

4. You seem to know nothing about pumps, process equipment and piping and instrument drawings, at all.

Abandon all hope of solving this problem yourself by consulting a FREE, anonymous forum and hire some competent help.

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Users who posted comments:

dhayanandhan (1); IdeaSmith (3); JAlberts (1); Legolaz (1); lyn (3); Original_Macgyver (2); phoenix911 (2); sgaharana (1); SHOCKHISCAN (2); Tornado (1); yoursbobby321 (4)

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