Cooling Tower Pump TDH

[Bill3752]

We are looking at replacing a cooling tower. To provide sufficient flow, we considering using pumps with TDH = 200 - 250 ft.hd. A lot of equipment in different operating units will be fed. However, some of the exchangers are design for 75 psig. Flow calculations show that at these exchangers the pressure will be < 75 psig due to friction losses, and to a lesser degree, elevation change.
Anyone familar with having a system where the pump TDH > design pressure of the exchangers?
Thanks, Bill

 

[Latexman]

I see the potential for a lot of open PSVs in your future Bill. What's the TDH on the old pumps?

Good luck,
Latexman

 

[BigInch]

Route the PSVs back to the cooling tower basin.

Independent events are seldomly independent.

 

[Latexman]

Bill,

Things are even worse than they appear on the surface of your question. A PSV in liquid service usually needs an inlet pressure < 0.7 x set pressure to reseat. So, if you get lucky with the elevation and frictional losses such that normally the PSV(s) see < set pressure, if any little transient pops the PSV(s) open, they will not reseat until the inlet pressure < 70% of the set pressure. I've seen where entire reactor lines have to have CW valves closed to get PSV(s) to reseat. The Plant Manager and Operations Director were non too happy about that!

Good luck,
Latexman

 

[rmw]

Are any of these PRV's on vessels with ASME or PED stamps? If so, you will have to increase the design pressure of those to a point where the shut off pressure of your pump doesn't exceed your design pressure - UNLESS - you can prove that there is no - absolutely no way to block the cooling water flow sending the pumps to shut off.

rmw

 

[BigInch]

Install an appropriately rated PSV on each vessel anyway.

Independent events are seldomly independent.

 

[Pumpsonly]

If I read you correctly, your concern is at some of HEs , the pressure of the CW is higher than the design pressure of the HE's. If that is the case, ins't installing a pressure control / regulating valve before those HE's will soft your problem.?

 

[Latexman]

If re-rating, modifying, or replacing the HEs doesn't work out, what about options that increase CW flow without increasing TDH. Larger headers. A second cooling tower at the opposite end of the headers. Breaking the system into two. There must be others.

Good luck,
Latexman

 

[MJCronin]

I am confused..... The TDH seems way high to me....

Most of the cooling tower water pumps I have seen run with a TDH in the range of 75 to 150 FT..... not 200 to 250 ft

Why does the original poster believe that the TDH of the pumps remains the same when they are re-purposed ?

The TDH required of centrifugal pumps is determined by the pressure drop throughout the system.

Enough pressure must be left over to push the water just to the surface of the distribution deck of the coolers.

Has some MBA placed these cooling towers atop a building somehow ?

We need pump curves and system layouts to make any sense of this