NPSHA in produced oil and water tanks (pressure-vacuum vent valve and thief hatch)

[engr2GW]

Hello,
I have a produced water tank (from oil and gas well) as shown in the attached diagram. This is a tank built to API 12F, MAWP 16 ounces/sq inch.
LIQUID INLET; is the liquid dump from a low pressure vessel
LIQUID OUTLET: pump at the grade level to pump the water to a pipeline
GAS OUTLETS:
1. Vent to flare have a back pressure valve (A) to keep 5 ounces per sq inch (osi) and open to go to the flare above 5osi
2. Atmospheric vent (B) opens at 8osi to relief pressure and at 0.5osi to let air in for vacuum
3. Thief hatch (C) opens for pressure relief at 12osi and for vacuum to let air in below 0.5osi

All the pressure gauges we use are in gauge pressure, does this mean that a 8osi (0.5 psi) reading in the tank gauge means 14.7 + 0.5psi to give the absolute pressure psia?
I ask because if the 14.7 psia is not added, very few pump NSPSR will be met.

Because even actual pressure vessels without any opening to the atmosphere, they operating pressure is usually added to 14.7 before plugging into the equation to calculate NPSH, so I think it should be the same for this system as described?/
Thanks for your help.

As much as possible, do it right the first time...

 

[JJPellin]

Yes. You have to add in the atmospheric pressure so that all units are absolute pressure. You will need to subtract off the vapor pressure. If you know in advance that the vapor pressure is always equal to the static pressure (liquid at boiling point), then all you have left is elevation head.

Johnny Pellin

 

[engr2GW]

Thank you very much JJPellin,
the fact that all pressure have to be absolute makes it easier to understand and add the 14.7.
I use the height of liquid column above the pump center line as elevation head, and I look up vapor pressure head in tables @ T

As much as possible, do it right the first time...

 

[LittleInch]

Produced water often contains lots of extra elements which affects vapour pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

But usually safe to ignore them for NPSHa calculations, because it is doubtful that they reduce the partial pressure of the water vapor in the tank, but unsafe for NPSHa if they increase the vapor pressure of the water/crude liquid mixture going to the pump.

 

[LittleInch]

We have no idea though. I've seen "produced water" coming in to these sorts of things basically bubbling and gassing off.

That's my point - you need to know your fluid.

Also engr2gw - there was no attachment....

But yes, you add 14.7 psi to guage pressure to get to psia if you're AT SEA LEVEL. If you're up at 2000m ( or even 1000m) this is a different figure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[engr2GW]

My apologies, I forgot to insert. File is now attached

LittleInch: Yes, a little gas flash off the liquid when dumped into the tank, hence the valves in the diagram and corresponding set points.

Here is what I did:
1. ADD- Pressure head: if the operating pressure of the tank is 8 osi (0.5PSIG), I say 14.7 + 0.4 , ~15psia to get ~34 feet
2. ADD- Elevation head ~10 feet
3. MINUS- Friction losses
4. MINUS- Minor losses
5. MINUS- Vapor pressure head

As much as possible, do it right the first time...

 

[engr2GW]

Thank you Sixty: The reason I don't ignore them is because the operating pressure of the tank are usually very little (although good operating height of liquid), not taking advantage of the Patm reduces the chances of meeting or exceeding the NPSHR

As much as possible, do it right the first time...

 

[1503-44]

Yes, but it helps you to get a safe design criteria. Then you might have some room to work with if the problem refuses to go away during operations. For example, these things can be very dependent on higher temperatures and you have not mentioned that yet. And on altitude, which has already been mentioned above. NPSHa requirements coming from mfgr info is usually only valid for cool water and new equipment.. If you are on the edge now, a lot can go wrong later. Pushing NPSH is one way to suffer for the entire system lifetime.

 

[TenPenny]

glad to see LittleInch beat me to the comment about sea level; blindly using 14.7 psi can lead to some problems, you do have to account for elevation.

 

[georgeverghese]

In the real world, produced water can vary from zero fraction oil to maybe 10-20% of oil in water by weight. Stubborn emulsions in produced water are more the norm than the exception, so use vapor pressure of the crude fraction, which will the same as the tank vapor space pressure, if you want trouble free operation of this pump.