6 replies to this topic

[Ashishtheone]

Pump Motor rating is 75 HP. As per pump curve @ head of 200 ft and 800 gpm the power consumption is 75 HP.

RV is to be sizing for the vessel at downstream of the pump with MAWP of 143 psig.

Normal suction pressure of pump is 5 psig.  

During a certain operation the suction pressure at pump would be 120 psig and the relieving pressure at the downstream vessel due to blocked outlet is 157 psig. 37 psi differential head corresponds to 90 ft of head.

And as per extrapolated pump curve the flow at 90 ft head is 3500 gpm.

 

This is a typical runout situation for pump. I need to determine the relieving liquid flowrate.

I feel 800 gpm should be the the relieving rate but I am not able to convince myself.

Any help is highly appreciated.. 

 

 

[fallah]

Ashishtheone,

 

A simple sketch could be very helpful...Anyway:

 

37 psi differential pressure isn't corresponding to differential head during relieving conditions. Then at first you should calculate pump's differential head during relieving conditions using pump curve and maximum suction pressure of 120 psig, let say, by considering liquid static head between pump discharge nozzle and PSV inlet nozzle and approximate friction loss inbetween if the length is considerable. Flowrate corresponding to this differential head on pump curve would be equal to relieving load due to blocked outlet.

[Ashishtheone]

Thanks Mr. Fallah,

Sorry I wont be able to upload a sketch. 

But problem can be summarized as follows

 

For the differential head when pump suction is 120 psig and discharge 157 psig the pump curve is not present and I will have to extrapolate from existing curve. 

And at his extrapolated point power required is too high compared to motor rating.

So how to I decide the relieving load?

Hope I am clear with the issue....

If still required I will try to get some sketches by tomorrow...

[fallah]

Ashishtheone,

 

As it is realized 157 psog is the pressure at the PSV inlet during relieving and to find the pump differential head for this situation you need to know the pressure at pump discharge nozzle which might be higher. Then please go in this way, it might the operating point fall on the present pump curve.

[Ashishtheone]

I agree with your point of view.... but lets just assume if I get the Isometrics and draw a system curve.

This system curve does not intersect with the pump curve.

 

In this case I assume system curve a horizontal line...

 

I am too much confused because I dont have isometric drawing with me   .

but I need to start with the assumption that there is no specific pressure drop in the piping from pump discharge to the vessel.

 

The only way to know the flow rate is via extrapolation of the pump curve.

unfortunately the point where the extrapolated pump curve intersects with the system curve requires too much power which is very high compared to the rated capacity of the motor of the pump.

what would you suggest in such a case?

should I size for 3500 gpm or 800 gpm.

Thanks for the reply though...

Edited by Ashishtheone, 11 May 2013 - 10:39 AM.

[Atttyub194]

Dear Ashishtheone

Good day

 

Before making any comment , I would suggest to please provide data sheet for the pump indicating name of pump manufacturer and a schematic

[fallah]

Ashishtheone,

 

I don't think you are allowed to consider a working point beyond the pump's end of curve even as corresponding point to the relieving conditions. Then try to shift that point such that falls on the present curve and needed no extrapolation on the pump curve by:

 

1) Accurate calculation of the pressure at the pump discharge nozzle. In this way, although you may not isometric dwgs, you can use elevation difference betweeen pump discharge nozzle and the PSV inlet to find the pressure just at the pump discharge. It likely is higher than 157 barg and will result in higher differential head.

 

2) Lowering the maximum suction pressure (120 psig)