Centrifugal Ballast Pump - Flow Vs Amps

[LNGC]

Could somebody please help me with the following query???
I am currently training onboard and LNG carrier. I have a query regarding one of the vessel’s sea water ballast pumps.
Brief details of the pump:
- Electrically driven centrifugal ballast pump
- Speed 1180 rpm
- NPSHr 4.3 mlc
- Rated Capacity 2800 m3/hr
- The pump is regulated by throttling the discharge valve.
When operating the pump I have observed the following:
1) Closing the discharge valve results in increased back pressure, reduced negative suction pressure, reduced flow, but, the Amps increase.
2) Opening the discharge valve results in reduced back pressure, increased negative suction pressure, increased flow, but, the Amps reduce.
At 360 Amps I have calculated (by measuring volume discharged) that the flow is 4100m3/hr (end of curve), and at 400 Amps (design amps) the flow is 3000m3/hr. These calculations/measurements were made under exactly the same conditions (tank levels, ships draught, water density etc.)
Why do the Amps reduce with more flow through the pump and increase with less flow through the pump? I thought it should be the other way around?

 

[waross]

It usually is the other way around. You may find an answer if you consult the pump curves. Or try multiplying flow times pressure at each operating point and comparing. These results will reflect the pump curve.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigInch]

But not LNG density.

That's a very significant change in flowrate that probably has a very significant change in pressures going along with it. I don't think you've mentioned the discharge pressures at each flowrate. They are ...?

What's the density of your LNG at higher pressures and lower flows versus the density of LNG at lower pressures and the higher flowrates. I suspect you may not have enough pipeline end pressure to hold your flowrates back and pressures high enough to where the density of the LNG is not affected.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MartinSr00]

I'm assuming the motor is constant speed.

What you're saying is that the horsepower curve is sloped down with increasing flow. It happens all the time. Increased flow would increase friction losses in the suction line causing the suction pressure to decrease.

As you go beyond the best efficiency point, the efficiency is dropping off. The horsepower is proportional to HQ/eta, where eta is the efficiency. If eta is dropping off more quickly that HQ, horsepower goes up. If not, it goes down.

 

[itsmoked]

BigInch; He is shifting "sea water".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BigInch]

OMG Thanks. I'll see the optician tomorrow.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Artisi]

Sounds like the ballast pump/s could be axial flow impellers, this would account for the increase of power as flow reduces by increasing the head.

 

[electricpete]

http://www.eng-tips.com/faqs.cfm?fid=1543

This type of flow curve (decreasing power demand with increasing flow) is typical for axial flow pump and also certain regions (to right of BEP) of mixed flow pump. Those are general statements - Ideally as waross said locate the points on the pump curve as first step to investigation.

=====================================
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[TenPenny]

I'm assuming they are axial flow pumps, some are designed to run in either direction, so they can be used to trim the boat.

What make/model/size are the pumps?