Negative NPSH?!

[nucleareng78]

Talked to the system engineer of a plant and he informed me that there is actually a vacuum at the suction side of this centrifugal pump we are dealing with.

I can't seem to grasp my mind around how this can be possible. Since all pumps have a NPSHR, how they heck can you size a pump when they informed me there is a negative NPSH at the suction end?

Current pumps have a NPSHR of 10 feet, I'm assuming cavitation is not occurring since they have not been destroyed internally.

Help!

 

[nucleareng78]

Cooling water is from the sea. This condenser cleaning system has debris filters on either side of the condenser to collect the cleaning balls. Our pumps extract these pumps out of the filters below the condenser. The pressure indicators on the discharge side of the pump say they operate at 10 psig and have a max operating pressure of 30 psig.

After messing around with some calcs I determines the NPSHa, when the water is 70 deg F, is around 10.7 ft of head. This is something I was hoping for since the NPSHr of our new pumps is 10.1 ft.

The outlet does have a condenser air removal system as well, could that tell me more answers? Basically these pumps do not discharge water into the ocean, but work in-line with the circulating water system. Maybe I answered my own question about NPSHa, since I converted to absolute pressure from gauge. Can someone review my logic?

 

[rmw]

If your ball recovery pump is on the condenser outlet side, I doubt that the water temperature is 70F, probably more like 120 - 130F.

I assume from your fairly non-detailed post that the CW pumps pump from the sea up to the condenser and if so, then your ball pick up system is on the outfall side, and the pressure could well be negative. The closer to the condenser outlet that it is, the more negative that it will be. Depending on the height of your condenser above the sea level, the pressure at the condenser outlet has the highest negative pressure in the waterbox or right at the outlet nozzle and the negative pressure decreaes returning to atmospheric at sea level.

rmw

 

[gravitate]

If the NPSH is too low then this brings the liquid closer to its vapour state and causes cavitation in the pump right? Would a negative NPSH mean the water is getting pumped in an upward directio?

 

[micalbrch]

You would not even see water anymore when the NPSH is negative. That's why a negative NPSHA is a theoretical term only.

 

[BigInch]

A backward direction.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[nucleareng78]

The balls get pumped 10 feet straight up after the extract them from the filter