Pump Curves 2

[Homayun]

Dear Friends,

I am now really confused! THis sounds stupid, but..

When specifying a control valve that is in the discharge line of a pump, I use the pump curve to determine the pressure before the vlave (corrected for friction losses).

My confusion arises from the fact whether I should also take into account the suction pressure of the pump when determining the pressure before the control valve, or not!

In other words, Is the pressure before the valve, the pump suction pressure plus the pressure according to pump curve? Or not?

Can someone help?

Thanks

 

[katmar]

Yes, you need to take the suction pressure into account.

You should start at a point where you know the pressure - probably atmospheric pressure on the surface of the liquid in the supply tank, or the pressure in the vessel if it is pressurized. Then you add or subtract all the increases in pressure (eg static height or pumps) and losses (eg friction or static height) until you get to the valve.

If the level or pressure in the supply tank varies significantly look at both extremes.

In all problems - start with the knowns and work towards the unknown.

 

[NeedAHoliday]

Homayun,

The pump curves will show the total head a pump can generate with respect to the NPSHr, efficiency and power.

So saying take the generated head from the pump curve for your conditions, add the suction line pressure (at inlet flange to pump), and you have the discharge pressure aviailable for your pump.

You can then look at your outlet conditions and determine the losses up to the control valve. Subtract this from the discharge pressure (head) and you are left with the pressure before the valve.

For example if you suction pressure at the inlet flange ends up being 102.3kPaa and you have an available head of 100kPa from the pump, then the discharge pressure would be 202.3kPaa. If you had 20kPa worth of losses up to the valve then the pressure at the valve entrance would be 182.3kPaa (~81kPag)

Hope this short and quick answer is sufficient.

Cheers,

K.

 

[Homayun]

Thanks to both of you, Katmar and NeddAHoliday.

 

[sailoday28]

So saying take the generated head from the pump curve for your conditions, add the suction line pressure (at inlet flange to pump), and you have the discharge pressure aviailable for your pump.
NOT SO--Unless your definition of pressure is dynamic pressure, which would include velocity head.

The total dynamic head curve should be available,
This is the change in static head+kinetic energy head +pressure head across the pump from inlet to outle flange.

Adding the suction pressure/rho +V^2/2 +Z (elevation)at the pump inlet flange will yield the corresponding total discharge head. To get the static pressure at pump discharge flange subtract out the elevation head,and velocity head at the disch flange.
Regards

 

[NeedAHoliday]

sailoday28, your right I was talking of dynamic head not static, I did state total discharge pressure in my example, not static pressure, which is as per you response

total discharge head

.

After re-reading my post I can see where confusion could arise. I assumed Homayun would have sufficient knowledge to distinguish the context.