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Motor Overload Shutdown 3

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SAmecheng

SAmecheng

Mechanical
Jun 7, 2007
9
I need to upgrade a motor to an existing pump system. Here are some specs, where should I start? The system is pumping heavier crude than originally designed for.

For over a year now we have been continuously having problem on the Booster shutting down on motor current overload. This occurs when another pump is running and when pumping low gravity crude oil. To be able to use the Booster with this setup, we have to close the discharge valve almost 3/4.

With the discharge valve 3/4 close, the amp readings is at 146 A and opening the valve 1/2 the amp reading will be around 156 A which is above the full load amps of the motor of 148 A and will shutdown the booster.
 
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Right. Running the booster at 1/4 of the maximum flowrate is not what you really want to be doing. Defeats the purpose of having a booster.

Two things are happening,
1. With the new crude, you are getting a greatly increased resistance in the downstream pipeline (system curve) you are pumping into, from a denser fluid in the case of overcoming static head and also an increased head loss due to the considerably greater viscosity of the new crude.
2. More pump power is required for the denser crude, so you hit max at a much lower flowrate.

The combination of the two is causing the motor to overloads at minimal flowrates.

You will need to determine the new system curve of the pipeline running the heavier crude and calculate the power needed to pump the heavier crude over a range of flows from optimum to maximum production rate, if different from the optimum, then upgrade the electrical system accordingly.

18-0346198325T.gif



 
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BTW, it sounds like you will need way too much power at full flows for the existing system and might be forced to remove and build a completely new system, rather than trying to upgrade the existing works.

 
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The upgrade would involve replacing wires and conduits and probably the vacuum type motor starter? Or is there more?
 
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Motors are matched to the pump capacity.

Strange coming from an electrical engineer to mechanical but is the pump itself suitable to handle increased power, flow, viscosity?

Last thinng you want is to see a broken pump after upgrading the motor. If the pump is properly sized and evaluated, it must have the load-torque curve and HP rating. Just match that.


 
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rbulsara, I should have mentioned that! Great work and I'll give you a star for picking that up. At roughly 4 times power requirements, it is highly doubtful that the existing pumps will work.



 
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Biginch:

Thank you! Teamwork is what this forum is about.
 
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I expect that the motor upgrade would involve a marginal increase in HP instead of a multiple of the existing motor HP. Some comments/questions to consider.

What is the HP of the existing motor, and what is the max HP required by the pump curve given the original crude gravity?

What is the required HP with the heavier crude? Estimate by taking the max required HP from the pump curve, and multiplying by the ratio of specific gravities. (new SG/original SG)

Given the new motor size, are your existing cable and starter still OK?

New motor frame size may require baseplate modifications.

I would expect the pump to be able to handle a heavier crude with no problems because the pump model was probably generically designed with water in mind. (but you should still check)

A gate valve at 1/4 open will reduce your flow, but not linearly. I would expect that you are still around 1/2 to 3/4 of your full flow.

 
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As pointed out above you have a greater head loss etc thru the pipe line and fittings for the increased viscosity, but just as importantly remember that the pump will derate in terms of flow, head and efficiency due to the viscosity - this results in a further need to increase input power to deliver the flow and heads required.

The Hydraulic Institute has charts available which allows you to calculate the derating which results from increased viscosity and there are tables around that will give you the friction losses thru pipelines for viscous liquids.
 
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One would not typically expect pump power requirements to increase drastically, since the specific gravity of the heavy crude might not really be that much greater than that of the lighter crude, but it is possible if the heavy crude viscosity increases enough to have a significant impact on the system curve of the downstream piping. The system curve needs to be carefully evaluated for the effects of the new crude viscosity and its probably increased head loss in the piping. Not knowing the new specific gravity, but expecting that it is within the usual range of heavy crudes and estimating increased density power requirements to be limited to a maximum of a 20% power increase, the overload now occurring at apx. 1/4 of maximum flow could indicate that there is a significant impact in terms of increased head losses in the piping system. Note this is a bit of a guess, given the limited information so far.

 
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BigInch -
I have the same thoughts on this and as we lack a lot of info' it's a bit of a guess anyway, however just a few more thoughts for discussion:

- the increased power required to handle "öil" is a function of the derating of the pump performance which is normally based on pumping water and the increase in friction loss thru the pipework and fittings, whereas the SG of the "oil" is a minor consideration as it is always less than 1.

A couple of quick "for instance",

Water - 2000 USgpm - 10" sch 40 pipe = approx. 20ft Head loss per 1000ft.

Oil- 2000 USgpm @ 1000SSU same pipe = approx 55ft Head loss per 1000 ft.

Pump derating for oil at 1000SSU compared to water with the pump at the same speed etc,

Power will increase maybe 20%
Head will decrease by approx. 10%
Flow will decrease by approx 5-10%
Efficiency will decrease by approx. 35%

Therefore, besides having to increase power input to handle the pump derating the friction head could more than double the discharge head when compared to water and at a lesser increase as oil viscosity increases with an increase in speed to maintain the same flow.

So, without more detail it is difficult to make any definitive commendations.
 
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Oh yes I agree, but since you already mentioned that viscosity affects the pump, I didn't want to repeat your comment.

Ya, enough quessing for the moment. I'm sure "South American Engineer (?)" has got enough on his plate right now too.

 
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BigInch...

It's 'San Antonio' Engineer and thanks for adding to 'her' plate! Greatly appreciated.
 
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A completely different approach to your problem would be to look at the system curve. If you're dealing with a crude preheat train, and it sounds as if that may be the case, then this is a common "method of attack". If you're hydraulically limited, you could depass heat exchangers, introduce parallel paths, reduce control valve dP, increase line sizes, etc. Many of these avenues could be very expensive and/or unsuitable in your case, so you'd need an analysis of your system to evaluate this option. If you've switched to a heavier crude, however, your heat transfer has undoubtedly been affected. This might be a good time to see how to best approach the problem from a system point of view. You need to meet your hydraulic AND thermal constraints, and if your crude unit is like most, you're probably backed up against a wall with both.
 
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rzrbk:

What is the max HP required by the pump curve given the original crude gravity?

Is this @ BEP?


[gorgeous]
 
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Max hydraulic horsepower required is dH_ft * Q_ft3/s * density_lbs/ft3 /550

Density = Crude Specific Gravity * 62.4 lbs/ft3

Divide that by pump efficiency at your flowrate to get pump brake horsepower.

Divide brake HP by motor efficiency to get motor input power and change to amps, depending on what voltage and phase you have.

 
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SAmecheng,

I would use the HP at the end of the pump curve. This will ensure that the new motor will cover the entire pump curve.

Being a retrofit, there may be some room to back off of this if required HP becomes too large, and flow rate at the end of curve is not probable.
 
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