Negative NPSH?!
Negative NPSH?!
(OP)
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!
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!





RE: Negative NPSH?!
RE: Negative NPSH?!
Is there flow when the pump is off? If so, the downstream system is siphoning fluid through the pump.
Or you could have the case where the inlet piping is draining backwards and comes to a state of "vacuum lock" due to having a downleg or something.
RE: Negative NPSH?!
This pump is extraction sponge cleaning balls from a debris filter at the exit of the condenser. It may be that once it goes through the condenser it becomes a vacuum? Also, if the centerline of the suction is below the outlet of where it sucking water, would that cause siphoning?
RE: Negative NPSH?!
Rework the NPSHA calculation in absolute units and then compare it to NPSHR. That should clarify the situation considerably.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Negative NPSH?!
RE: Negative NPSH?!
That will be the day.... (specifying any pressure in absolte - and never get confused about it)
Best regards
Morten
RE: Negative NPSH?!
If the water/condensate temperature is at 120 F , the saturation pressure is 1.69 psia ( -13 psig), and adding 10 ft wc to that makes it a grand total of 5.69 psia (-9.0 psig).
RE: Negative NPSH?!
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Negative NPSH?!
rmw
RE: Negative NPSH?!
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Negative NPSH?!
Here's what's confusing, if this screw centrifugal pump has a NPSHR and the NPSHa is a vacuum, wouldn't that cause all sorts of problems? This is what's tripping me up..
RE: Negative NPSH?!
RE: Negative NPSH?!
I would also suggest some reading on NPSHa/r and analysing pump suction conditions.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Negative NPSH?!
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Negative NPSH?!
http://www.weirpowerindustrial.com/images/hidrosta...
There are two pressure indicators in the fiberglass piping on either side of the pump but I don't think that will help me much, or would it?
I have a circulating water hydraulic gradient from back in the 80's for the system. What exactly am I looking for on there?
RE: Negative NPSH?!
That, when converted to absolute pressure, should be the same as the pressure reading at the pump's inlet, when that reading is also converted into absolute pressure.
To convert the value of the inlet pressure to absolute, first add to that value the local atmospheric pressure, 14.7 psi at 0 ft MSL, 60F. To get NPSHA, in terms of pressure, subtract the water's vapor pressure (appropriate for its flowing temperature). Multiply the resulting absolute pressure by the density of water (62.4 pcf, if the flowing temp was 60F) at the flowing temperature and divide by 144. That's NPSHA in "absolute" feet. It must be at least equal or greater than the pump's NPSHR.
If the inlet pressure gauge not at the same elevation of the pump centerline, you may need to correct for change of elevation head, roughly 0.43 psi/ft difference * the specific gravity of the water at flowing temperature.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Negative NPSH?!
There is no such thing as 'negative NPSH', however.
RE: Negative NPSH?!
For the condensate, that fluid certainly is at vacuum conditions, typically at 1.5-2 psia at the hotwell water surface. There could be a 8 ft water leg in the hotwell and condensate pump suction pipe, but piping losses typically consumes most of that gravity head, so there is normally very little margin in the provided NPSH to the pump suction. To minimize suction pipe losses, no tees or wyes to be in the suction pipe, and each pump should have its own nozzle on the condenser hotwell, possibly with an anti-vortex baffle. One of the main sources of air inleakage to condensate is thru the seals on the condensate pump, due to the vaccuum conditions.
For the case where the condenser tube side cooling water outlet nozzle is at a higher elevation than the final discharge at the cooling tower or basin, then a vacuum can also exist at that nozzle, unless a vacuum breaker or other device is provided- such a device will also imply more pumping head to be added to the circ water pump performance requirements.
RE: Negative NPSH?!
Most large turbine systems, and nuclear units certainly qualify as that use some form of a siphon system where the circ water pumps put the water up to the level of the inlet tubesheets and mother nature does the rest on the down leg of the siphon pulling the flow through the condenser and back to lake, river, or cooling tower basin level. I suspect that is what you have, and if so, the cleaning ball pickup screen is located on the downleg side in the negative pressure zone. Your SE's comment makes perfect sense to me. If your system is what I think it is, you also have an elaborate air removal system on the outlet side to remove the air that is liberated from the CW as it heats up going through the condenser so that it doesn't break the siphon.
Give us some info here.
rmw
RE: Negative NPSH?!
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?
RE: Negative NPSH?!
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
RE: Negative NPSH?!
RE: Negative NPSH?!
RE: Negative NPSH?!
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Negative NPSH?!