How to increase NPSH of a pump?

[Halflife9]

I want to increase the rates for a propane system that uses a bullet for storage at ambient temperature in equilibrium (the bullet pressure floats with ambient conditions). The increased rates will require more NPSHa than I have with the current system for a single pump according to the curves. Is there an inexpensive way to increase the NPSHa without making major piping or vessel modifications?

One idea was to put a fuel gas pad on the bullet to increase the vessel pressure for additional NPSHa. Would this work or is the system just going to find a new equilibrium at the higher vessel pressure?

I have heard of vessels put under a Nitrogen pad or other inert gas to gain NPSHa, but this depends on whether the system is compatible with the inert gas.

Is increasing the vessel pressure with a pad even an option? If so what type of pad could I use for a propane system?

Thanks

 

[ash9144]

You could use nitrogen or nat gas to pressurize the vessel which would increase your NPSHa. You need to check the vessel design, relief scenarios, vent requirements to determine where to limit your pressures.

 

[MortenA]

The pressuredrop from the bullet to the pump is dependant on the flowrate and piping. So increasing the pressure in the vessel will increase you NPSHa!

Your fuel gas would most vertainly "contaminate" you propane (unless of course its pure propane).

But the rate that this will happen in may not be high?

Then theres the N2 suggestion.While a little N2 will mix with the propane i dont think that will be much. Worth examening closer.

Another suggestion could be to increase the elevation of the vessel=increased hydrostatic pressure.

Best regards

Morten

 

[MJCronin]

half...

A tank pressure increase is a bad idea.......

Your propane tank (bullet)has very little margin for any increase in pressure. (Especially if the tank was made in the USA). Propane tanks are built for sale in a very competitive market. You are almost at 250 psig if the tank is out in the sun on a hot afternoon

Find the manufacturer's plate on the tank and see if the design pressure is 250 psig.

Consider a new pump designed for propane service.... Its not that expensive.

Where are you located..?

-MJC

 

[Halflife9]

I am located in Corpus Christi, TX and we do see pressures as high as 220 PSIG on a hot summer afternoon, but the vessel is good for as much as 275 PSIG.

A little contamination of methane / nitrogen would be acceptable at the pump suction assuming it doesn't impact the fluids vapor pressure to the point of cavitation.

I need another 5-7 feet of head / elevation to make this work - I will look into increasing the elevation of the bullets.

I was just wondering if anyone had experience with a fuel gas or nitrogen pad for a propane system to increase NPSHa. Looking for any problems that I should avoid with this option.

 

[rmw]

If you end up raising the bullet, then you will have to revise the piping to the pump. When you do, upsize the piping so as to minimize the flow losses between the bullet and the pump. One or two sizes should be sufficient depending on what you have now. That will gain you some of what you are looking for. Remember the rules for reducing back down to pump inlet size-flat side of the reducer up.

Few pump systems I have ever seen were designed to aid in reducing npsh.

rmw

 

[maddocks]

Other than raising the bullet, there's very little you can do. A gas pad may work but will contaminate your LPG with non-condensables. You could try several other quirky suggestions that have worked for me in the past. Slow down the pump specific speed - this will lower your NPSH required. Try a small suction booster pump. Last resort, look at purchasing a vertical can pump and lower it into a sump to increase NPSH. Note that small amounts of cavitation are acceptable in hydrocarbon service.

 

[BenThayer]

i would stay away from adding pad pressure to increase this. you are asking for problems with the PSV. many PSV's should not be operated beyond 90% of set.