What is TDH?

[Roach]

If I have a guage at the outlet of my pump on a closed H2O system, and it reads 42psi with the pump on, and when the pump is turned off it reads 28psi, does that mean that I am getting the GPM that is seen on the pump curve at 42 or 42-28=14psi ( 97 or 32 ft of head)? If it is 97, then can someone explain why my flow pressure is more than my "static" pressure. I thought that static would be more since some of the pressure energy is converted to kinetic through velocity. What exactly is my guage reading, static or "stagnation" when the pump is running?

 

[Roach]

On the return line (low pressure side) feeding into the cooling tower. The pump is on the cooling tower supply line.

 

[KLH]

You mention "cooling tower". But you said it's a closed system so it's some type of closed circuit cooler I assume.

The pressure at the expansion tank should not change, whether the pump is on or off.

If the expansion tank is located too far from the pump suction, you will experience negative pressures at the pump inlet.

In most cases, you want the expansion tank as close to the pump suction as you can. This will almost always result in positive suction pressures.

Bell and Gossett has design manuals dealing with expansion tank locations and its affect on system pressures.

 

[Roach]

KLH
Yes, it is a closed circuit cooler. The expansion tank is on the opposite side of the closed circuit cooler from the pump, but the rated pressure drop across the cooler is 3.8 psi, and there are 2 or 3 90º elbows with that. With the pump off, I read 28psi, which I assume to be the charged pressure of the expansion tank. How is it that I create such a large vaccuum without the line losses available, unless I have something restricting flow(closed valve, dirty filter, etc.)?

 

[KLH]

Please explain how it is pipe, starting at the pump discharge. It will help me understand the system.

For ex. Pump, 30'of 4-inch pipe, closed circuit cooler, 40' of 4-inch pipe, expansion tank, 10' of 4-inch pipe, pump. Include valves, strainers, etc and filters (you mentioned these on last post) if you have them.

Why is the head so high? I think I saw 124' on an earlier post.

 

[KLH]

Please explain how it is pipe, starting at the pump discharge. It will help me understand the system.

For ex. Pump, 30' of 4-inch pipe, closed circuit cooler, 40' of 4-inch pipe, expansion tank, 10' of 4-inch pipe, pump. Include valves, strainers, etc and filters (you mentioned these on last post) if you have them.

Why is the head so high? I think I saw 124' on an earlier post.

 

[Roach]

This is a very long system that I don't have time to specify, but in general, I go from the pump discharge up 30 feet out 300 ft and feed two compressors in parallel. The return line is back 300ft and down 20 ft to a closed circuit cooler. Most of this is 4 and 5" pipe, with various gauges, strainers, elbows, tees, ball valves,etc. It is not worth calculating head. I read 42psi at discharge of pump, and 24" Hg vaccuum at inlet. This gives me a total of 124ft of head. This gives me the GPM that I was interested in finding out. I do not meet the NPSHR, but I am very close, and I am not cavitating, so I am OK for now, but I am looking at all possible restrictions to find out why I have such a big drop in pressure.

 

[KLH]

Like you said before, take some pressure measurements.

I would start from the expansion tank and work my way to the pump suction.

Good luck!

 

[YSLiao]

Roach

for the problem of your water pumping system(42 psi & 28 psi case), i have an experience for the closed circuit cooling water system ( air cooler), our discharge pressure is 6 barG, static head: 28 Meters, Tdh is about 35meters(simply say, 3.5 bar), the suction pressure is 2.5 barg. an expansion vessel is just above the pump with 3 meters height which pressure set point is 2.5 barg, the total length of the pipe is about 90 meters(main pressurized pipe) , the minimum suction pressure for the system i ever seen was 1.2 barg cause the suction strainer was clogged over 60% of the area during the commissioning stage. because your system shall be positive suction head, except clogging in suction side very very seriously resut in inadequate flow for the pump, i don't think we have chance to see the vacuum suction pressure.