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Vacuum formation in a pessure vessel

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SM2K

Chemical
Sep 25, 2003
7
Hi all,

we have a vessel (Diesel salt drier) here which is rated for an internal pressure of 110 psig and an external pressure of 2.5 psia. I am sizing a relief valve for this vessel and there are concerns about the vacuum generation on the vessel by a centrifugal pump which takes suction from the top of the vessel. The logic says that the pump will cavitate and stop, but

The quesiton is -

Is there a way to calculate the max vacuum a centrifugal pump can pull before it cavitates and stops? The fluid is diesel with a vapor pressure of 0.001 psia @ 110 °F.

Please let me know if any references are available.

Thanks,

SM
 
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Add an absolute pressure transmitter to the control system and a digital output to the MCC. Trip the pump on low suction pressure.

NPSH
 
SM2K,

Given your vapor pressure, a pump with typical NPSHR (and capable of putting up more than 30ft of differential head) may well be able to pull more vacuum than your vessel can take, especially considering that there is essentually no flow in some scenarios (vessel feed stops and pump continues to run). I think there is no difference that the pump takes suction from the top of a liquid full vessel, and you can still calculate a NPSHA at your vacuum rating in the same manner as normal. In any event if you are counting on stalling your pump, you better give yourself some significant calculational fat above any vendor provided NPSHR. I would not feel good about counting soley on this- pumps have sucked in vessels plenty of times.

I think you must either instrument the problem per JLSeagull, add vacuum breaking (N2?) equipment, or add stiffening rings and new code calcs to get a full vacuum rating.

best wishes,
sshep
 
I wouldn't be so quick to dismiss sshep's 3rd suggestion either until the vessel's actual capability is checked. Just because it is "rated" for 2.5psia external pressure doesn't mean that was the limiting condition.

Depending on the present design, it may just require a new set of calcs, but at worst one or more stiffening rings as well.

Cheers,
John
 
I have seen similar to your case, the vessel has HC and in case of vacuum pulling much air would cause explosive mixture and explosion. Nitrogen with PC signal is added to break the vacuum and prevent air entering to the vessel by inbreathing valves. I think that you can provide Nitrogen with Pressure control that is to be activated in case of low level.

Hope this would help

Cheers
SmartEngineer

 
On a small scale I have done this with back pressure regulators in the purge gas lines. No controls, nothing to reset. You need the gauge also, so that you know what is happening, and to turn the pump off.
But as to your pump, don't rely on NPSH to limit suction. The NPSH is the minimum pressure that the pump is rated for at inlet, not the minimum possible. No one will rate pumps so that they fail, hence all pumps will pull below rating. In some cases a lot lower.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
 
Thank you guys for all the expert comments. I have decided to go ahead do some calculations to find the maximum external pressure it could withstand using ASME formulae.

Still one of my question remains unanswered, i.e. is there a way to calculate to calculate the maximum vacuum that can be pulled by a centrifugal pump?

Again thanks for the responses!

Regards,

SM2K
 
with regards to what vacuum can be reached, it is a function of the fluid you are pumping and the NPSH of the pump.

if your fluid has essentially no vapor pressure at this pressure, take a look at the NPSHr for the pump at the maximum (most likely the lowest flow point operated on the tank).

But you say you pump from the top but do not note if it is a pump in the tank or if it is self-priming or something above the tank. Assuming that the tank is not vented, that the pump is above the tank, then use a calculation method like the one in figure 4c at:


solve for NPSHa with the minimum pressure you can have in your tank (-2.5 psig converted to feet absolute) and see if the NPSHa exceeds your the NPSHr of the pump, it it does, you can possibly suck in your tank.

but i agree with others, given that your tank has a MAWP of 110 PSIG, i'd be real surprised if you cannot re-rate it unless it is very long. then you may need a stiffening ring.

good luck!
 
SM2K,
To answer your question directly, the answer is yes, there is a limit. The maximum amount of vacuum that your pump can "pull" is that at which it stops functioning. As long as the available NPSH exceeds the pump's required NPSH, it will keep pumping. It doesn't care about the vessel's vapor space pressure because it doesn't "see" the vapor space. If the vessel is sufficiently high, the pump could empty the vessel and much of the suction pipe before it chokes. By this reasoning, the guaranteed minimum pressure is the vapor pressure of the contents, which you state is about 0.001 psia. (Not very reassuring.) With a vacuum relief sized for the pump's outflow + thermal inbreathing your problems are over. With that low a vapor pressure, you should not have a problem with air sitting above your liquid.
Doug
 
Hi all,

Although I had done some scratch calculations myself, I asked our mechinical department to do it "officially" and they have come up with the same result that I did. The vessel can easily withstand full vacuum and will just have to be rerated for FV.

Thanks for the replies,

Respectfully,

SM2K
 
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