8 replies to this topic

[coal]

Dear all,

I need to calculate the pump discharge pressure based on the following information given:

Fluid density = 1004 kg/m3

Pump supply pressure = 0.29 bar
Pump supply elevation = pump elevation = 50 m
Pump discharge elevation = 125 m
Total dynamic head = 135 m

Pipe size = 2 inch
Flowrate = 15.4 m3/hr

Below are my calculation. Please tell me if I perform the calculation correctly or not. Thanks.

(Assuming friction loss = 0)

Velocity = 2.11 m/s

135 m = - 50 m + 125 m + 0.29E5/(1004*9.81) + Discharge Pressure + Velocity Head
57 m = Discharge Pressure + 2.11^2/(2*9.81)
Discharge Pressure = 56.8*9.81*1004/100000
Discharge Pressure = 5.60 bar

Your responses are much appreciated.

Thanks and regards.

[Technocrat]

Dear Coal,

Can you please explain us with a diagram?

How do you get pump elevation of 50m or it is NPSH available? Please tell us suction pipe size you used.

How did you get total dynamic head of 135m?

You cannot assume friction loss zero but you have to calculate it for the calculated pipe size. Discharge velocity of 2.11 m/s is OK.

Total dynamic head = available suction head (use negative value for a positive suction) + head loss in discharge piping + pump discharge pressure + pressure in the receiving vessel.

Hope it helps.

Regards.

Edited by Technocrat, 07 December 2009 - 10:57 PM.

[coal]

Dear Technocrat,

Please find attached a sketch to represent the problem.

The suction pipe size is also 2 inch.

I need to calculate the pump discharge pressure.

I had tried to look for the solution on the web but the more I look the more confused I am.

Appreciate your help. Thanks and regards.

Attached Files

•  feeder.pdf   86.94KB   107 downloads

[demank]

I had tried to look for the solution on the web but the more I look the more confused I am.

Appreciate your help. Thanks and regards.

Hallo,
To prevent from confusing, you should consistent with the definition what is discharge pressure? what is dynamic head? etc.

For example, discharge pressure = the pump pressure on the outlet of the pump before entering the pipe line, or the pressure arrive in the discharge tank?

[Technocrat]

Dear Coal,

After looking at the diagram I have few doubts:
Is this a theoretical problem from a book? If not please explain:
a. how and why the supply vessel and the pump are elevated at 50m? This head is of no use for the pump since the pump suction head is not 50m.
b. what is the liquid level above pump suction line?

The discharge elevation is 75m only and not 125m. You cannot add 75m and 50m because the pump is also elevated to 50m level.

If it is a practical problem please tell us the application. It is interesting to solve a practical problem than a theoretical one. After your input I will again look into the matter.

Regards.

[SSWBoy]

Dear Coal,

After looking at the diagram I have few doubts:
Is this a theoretical problem from a book? If not please explain:
a. how and why the supply vessel and the pump are elevated at 50m? This head is of no use for the pump since the pump suction head is not 50m.
b. what is the liquid level above pump suction line?

The discharge elevation is 75m only and not 125m. You cannot add 75m and 50m because the pump is also elevated to 50m level.

If it is a practical problem please tell us the application. It is interesting to solve a practical problem than a theoretical one. After your input I will again look into the matter.

Regards.

He has calculated the relative elevation difference as -50 + 125 = 75m so I see no problem with this.

As you point out it is a little odd having a pump elevated that high, purely for ease of maintenance, support etc. pumps are typically located at grade. Also if you have the suction vessel at the same elevation you are surely going to have NPSH problems unless there are various LL trips for stopping the pump when the drum level gets low. I'd expect to see the drum being elevated in the region of 3m+ above the pump centreline.

For reference I have seen suction vessels located at 30/40m elevation with the pump located at grade.

Back to the initial question, as a quick calc the discharge pressure will be 0.29 + 135/10 - 75/10 = 6.3bar (at your elevation of 125m) I believe you have got the sign wrong on your 0.29*100000/1004/9.81 it should be a negative.

[Padmakar Katre]

Dear,
Your sketch seems to be incomplete. Always keep in mind to have the ditto picture of the scheme along with details of line sizes (suction/discharge).
Suction conditions like,
1. Fluid properties like viscosity, specific gravity,Flow,Temperature
2. Source vessel operating pressure,Source elevation.
3. Source levels e.g. LLL (Normally I take into calculations of NPSH) and HLL for calculating Design Pressure.
4. Suction Line size along with the equivalent length. As well suction strainer with its type.
5. Pump centre line elevation (Suction Eye).
Discharge Side,
1.Destination Pressure
2.Discharge line size and its equivalent length.
3.Downstream equipmennts like Heat-Excahngers if any and the Delta P across them as well the control valves and its DP and flow elements and their DP.
4.Destination elevation (density corrected if temp is different from suction).
While caluclating the suction side calculation do forward calculations i.e. Source P + Source Elevation (converted to Pressure Units)-Suction losses = Suction Pressure whereas to calculate the discharge Pressure calculate backward i.e. from the desitination w.r.t. the assumption cited above for discharge side calculations.

P.S. - Why the velcity is so high @ suction (Suction/Discharge line size is same) Just check standard velcities in pump ciruits i.e. suction and discharge side.
I hope this helps a little.

[sheiko]

Don't forget viscosity, rugosity, length,..etc as pointed out by Padmakar

With your assuptions and data i get Pdis = 6,2 barG (please don't forget to make it clear which unit you use)

For example, with absolute viscosity = 1 cP, absolute rugosity = 0,15 mm and Lequivalent = 170 m (with 100 m straight line, from the souce upstream the pump to the destination downstream the pump): Pdis = 4 barg.

Edited by sheiko, 08 December 2009 - 03:29 PM.

[coal]

Dear all,

First of all thanks so much for your replies and enthusiasm to be of help.

But I am unable to give further input for your doubts because actually I am asking the question on behalf of a friend of mine. He was unable to give me input as well.

Therefore I apologize for the inconvenience.
And thanks again for your help.

Regards.