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!

 

[SNORGY]

Ask them (him) for clarification with respect to how they define "vacuum". I suspect what they mean is really "suction pressure below vapor pressure", which is not the same thing.

 

[tr1ntx]

Siphon effect?

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.

 

[nucleareng78]

I'll be sure to clarify what he defines a vacuum as.

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?

 

[BigInch]

There cannot be a negative NPSHA or a negative NPSHR as both should always be the equivalent heads of absolute pressures.

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

 

[micalbrch]

I'm sure the "vacuum" the system engineer is talking about is a negative gauge pressure. And a negative gauge pressure does not mean that NPSHA is negative, too. A negative NPSHA is only possible in theory.

 

[MortenA]

Biginch:

That will be the day.... (specifying any pressure in absolte - and never get confused about it)

Best regards

Morten

 

[davefitz]

just about every power plant has a condenser that operates at vacuum ( negative gauge pressure), and also has a condensate pump whose suction is also at negative gauge pressure, but postive 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).

 

[BigInch]

Ya. See how long that lasted.

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

 

[rmw]

Most large condenser cooling water outlet passes (waterbox) are at the top of a siphon leg in the return loop, so there is (or could be) a vacuum there depending on how high that point is above the bottom of the leg (surface of the water). I have seen condenser outlet passes where the vacuum pressure in the outlet waterbox was deeper than the vacuum in the condenser itself causing the condenser to leak out into the outlet water pass. Strange but true.

rmw

 

[Artisi]

Clarify before getting too excited.

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.)

 

[nucleareng78]

I've been trying to comprehends the circulations water hydraulic calculation document but still haven't found at what pressure the outlet of the tube side of the condenser operates at. Talking to other engineers we agreed the pump does operate in a slight vacuum from the outlet of the pump.

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..

 

[SNORGY]

Yes it would create problems. However I think they need to calculate the NPSHA again. Can they put a vacuum gauge on the pump suction somehow?

 

[Artisi]

Depends on what you call a vacuum, also - what is a "screw centrifugal pump"?
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.)

 

[BigInch]

Absolute(ly)!

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

 

[nucleareng78]

A screw centrifugal pump is primarily used in wastewater and slurries, but we are using it to transport condenser cleaning balls. See link below.

http://www.weirpowerindustrial.com/images/hidrostal_cutaway_850x817.jpg

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?

 

[BigInch]

Inlet head to the pump.
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

 

[TenPenny]

If we take 'vacuum' to mean 'pressure less than atmospheric pressure', then what's the big deal about vacuum at the pump suction?

There is no such thing as 'negative NPSH', however.

 

[davefitz]

sound like 2 different vacuum conditions at the condenser.

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.

 

[rmw]

Nuceng78, describe your secondary cooling system. Is it a cooling tower, a lake, a river, something else? Where are the pumps? at lake level, upstream of the condenser (where I would expect them to be), downstream - doubtful. Once the cooling water leaves the condenser, where does it go, to a lake, river, cooling tower? If you will describe the system, your hydraulic profile will be more appearant to us.

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