Sizing a centrifugal pump in an oil storage tank battery

[Editty]

I enjoyed soaking up some knowledge from the pump calculation thread. I have a question sizing a centrifugal pump in an oil storage tank battery. I have a 400 barrel tank, they say it is (low Pressure/Atmospheric) It has a thief hatch on top to allow inbreathing and out breathing for thermal expansion of the liquid. My question is, since the tank has ounces of pressure on the tank, and is tied into a flare line, when sizing the pump (NPSH) can I use my liquid head and add the atmospheric pressure to the calculation, or since it is under a small amount of pressure I cannot use the atmospheric pressure.

 

[JJPellin]

You can add the static pressure (in absolute units) of the gas in the vapor space of the tank if you subtract the vapor pressure (also in absolute units) of the liquid.

Johnny Pellin

 

[Editty]

It just does not make sense. The tank is 20' tall, and I want to pump the tank down to 4', but need 150 psi discharge. The pump curve shows I need 9' NPSH required. So if I get to add, static pressure, let's just say (2.31 x 16'= 36')to the 9' That would mean I should be able to pull a vacuum. Sorry, I might be way off the mark, but thanks for your response.

 

[JJPellin]

If that pump has an NPSHr of 9 feet, pumping a liquid with a low vapor pressure, it can pull a vacuum. Most centrifugal pumps can.

Johnny Pellin

 

[Editty]

OK, so even though the pump curve says I need 9' NPSHr I can still pull the liquid down to 4' because of the other atmospheric pressure on top of the liquid.

 

[Compositepro]

NPSHr is an absolute pressure not a gage pressure.

 

[Editty]

Thank you for the help

 

[JJPellin]

You have still said nothing about the liquid in the tank or the vapor pressure of that liquid. You need to do the calculation. If the vapor pressure is very low and the losses between the tank and the pump are small, you probably can. You did not give me enough information to answer the question that you asked. In addition to the points already made, I should add a couple more. We require a minimum margin of NPSHa above NSPHr. For water, we require a 5 foot margin. Depending on the piping configuration, submergence may be the limit before NPSHr. If the submergence of the tank nozzle is too small, you can get vortexing and draw air into your pump. So, can you draw your tank down to 4 feet? Given the information provided, I have no idea.

Johnny Pellin

 

[LittleInch]

Use head in absolute units for all pressures and losses.

Start with atmospheric pressure at your elevation, add static head difference between low level in the tank to centerline of your pump (positive if the liquid is higher than your pump inlet), then subtract frictional losses from tank to pump and subtract liquid vapour pressure. If this is less than your 9ft then you're in trouble

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

 

[Editty]

The vapor pressure of the crude oil is 11 and the SG is .82
There is 4-6 ounces of pressure in the vessel to keep the light ends from flashing to gas.

 

[georgeverghese]

You should quote the vapor pressure as true vapor pressure (TVP) in psi absolute at max tank operating temp, and not as Reid vapor pressure (RVP), so which one is it you've quoted ?

 

[LittleInch]

11psi is quite lively. Depending on where your pump is (elevation relative to liquid level) and how far away it is from the tank and pipe size etc, you could easily run into problems.

As JJpellin says, you need a margin over NPSH to avoid cavitation. 1-2 m is common.

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