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vanuta
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Queries on Centrifugal Pump

07/01/2014 10:11 AM

I have a Centrifugal pump that is run using a fixed speed AC motor. The starter for the motor is a star-delta. I have a tank A on top of the centrifugal pump and the pump is fed from this tank and discharges to another tank B situated at the same height as the Tank A. Whenever the pump is started ( loaded) with a high density fluid, the motor trips. Whenever a low density fluid is used during startup and subsequently a higher density fluid is pumped, the motor works fine. Is there a way to determine the maximum density that i should start the motor with, such that when the motor first starts, it doesnt trip or do you think if i had started with a less fluid in tank A, the chances of the pump triping is low. Any advice/suggestion is appreciated.
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PWSlack
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#1

Re: Queries on Centrifugal pump

07/01/2014 10:31 AM

The power used by any liquid mover is the pressure difference multiplied by the flowrate and divided by the efficiency. The density of the fluid determines the pressure difference and the flowrate, so it is possible that the pump is undersized for the denser liquid. Without seeing any numbers, try starting the pump against a partially-closed valve in the discharge line to reduce the flowrate through the piping system and see what happens.

A centrifugal pump is generally a low-inertia device and unless it is huge and there are power supply constraints, star-delta starting is somewhat unusual.

The pump performance curve would be a good starting point for further investgation.

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sohail0110
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#2

Re: Queries on Centrifugal pump

07/01/2014 11:04 AM

you can ease the load on pump by reducing the flow rate of liquid .

just you have to arrange of a shutter valve to manually control the discharge of

tank "A"

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MJCronin
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#3
In reply to #2

Re: Queries on Centrifugal pump

07/01/2014 11:31 AM

I agree with the comments of Slack and sohail, reduce the startup load on the pump by either:

I believe that your pump is going to "run-out" conditions on it's pump cure while, at the same time, handling a dense viscous liquid...

- Reducing the density of the liquid at startup

- Using a throttling valve on the pump discharge to reduce startup flow, then opening the valve a short time later when the motor is not under such extreme strain. Almost all pump manufacturers recommomend this type of startup

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Tornado
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#4

Re: Queries on Centrifugal pump

07/01/2014 11:44 AM

Good answers thus far. Another possibility is to trim the impeller to a slightly smaller diameter.

Your pump is operating under a low head, high flow condition, placing it to the right on the pump curve, where motor overloading can occur.

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MOBI
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#5

Re: Queries on Centrifugal Pump

07/02/2014 5:26 AM

Whenever the pump is started ( loaded) with a high density fluid...........

What do mean by loaded, I am guessing that you do not mean that you started the pump with the discharge valve shut. If my assumption is correct may I suggest that you start the pump with the discharge valve shut and once the pump settles under "dead head" conditions, slowly open the discharge valve............and as pointed out you may have to throttle the discharge so that the pump does not reach its "run out" condition.

I was taught many years ago (more than I care to remember) that there is a good reason to always start up centrifugal pumps, particularly larger pumps, with the discharge valves shut, unlike positive displacement pumps which, of course, should be started with the discharge valve wide open. Many of these positive displacement pumps have a relief valve on the pump side of the discharge valve that will relieve pressure back to the suction side of the pump just in case the discharge valve or another isolating valve in the system is shut.

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PWSlack
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#8
In reply to #5

Re: Queries on Centrifugal Pump

07/02/2014 10:58 AM

<...relief valve on the pump side of the discharge valve that will relieve pressure back to the suction side...>

The forum doesn't know this exists yet as the original poster hasn't stated it.

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Wal
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#6

Re: Queries on Centrifugal Pump

07/02/2014 7:41 AM

I'm guessing here, because the OP didn't say, that this tripping when starting with the denser fluid from the get go is happening at the moment the star delta starter is switching to delta.

The motor takes longer to get up to speed when starting with dense liquid and the delta transition occurs too soon.

Try lengthening the time setting on the delta change over timer.

My guess.

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vaughn schueler
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#7

Re: Queries on Centrifugal Pump

07/02/2014 9:38 AM

Try starting against a closed or partialy closed valve and then open after pump is up to speed. you may have to do some automating.. Good luck.
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Users who posted comments:

MJCronin (1); MOBI (1); PWSlack (2); sohail0110 (1); Tornado (1); vaughn schueler (1); Wal (1)

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