6 replies to this topic

[ShaunHill]

I would like to look at vapour pressure of condensed liquids from natural gas (condensate).

I have this issue with specifying condensate pumps. If I have a natural gas separator operating at, say 300 psig and 40F the fluid in the separator has components ranging from methane (C1) to heavier hyrocarbons (in the C20 range). The liquid is mostly heptane (C7) and hexane (C6) but will also contain C1 to C5 (pentane) as well. A change in temperature or pressure will shift the equilibrium and cause components to move between the gas and liquid phase.

For the case where I want to determine the NPSHa for a condensate pump, I need to know the vapour pressure of my liquid.

What I have been doing is treating the operating pressure of the separator as the vapour pressure since I consider this to be at equilibrium. Then the NPSHa is only the liquid head (elevation of separator above the pump) minus pressure drop in the suction piping. I believe that this is conservative, since only a small fraction will vapourize when I drop the pressure or increase the temperature. The majority of the fluid will only start to vapourize at lower pressures.

Does this sound like the correct approach, or does someone have another method that may result in a less conservative design (but will still not cavitiate the pump)?

[gvdlans]

To me the approach described sounds like the correct approach. In my company we used this approach in similar cases (pumping of condensate from a vapor/water/condensate separator). It means that you have to carefully design the pump suction piping (large pipe size, limited length and bends etc.), and that you may need a type of pump that requires little NPSH. All of this is quite possible though.

[gvdlans]

I don't think so Ben! What you are saying may be true in case you would have let's say Diesel oil with a methane atmosphere above it, but this is a different case. Here we have a C1-C20 liquid that is in equilibrium with a C1-C20 vapor. ShaunHill already indicated himself that "A change in temperature or pressure will shift the equilibrium and cause components to move between the gas and liquid phase." So, in case pressure is reduced below the separator pressure, vapor will be formed. So, in case pressure inside the pump impeller is reduced below separator pressure, vapor bubbles will be formed inside the pump impeller. As you probably know, this is exactly what is called cavitation...

So, as I stated in my previous post, you have to carefully design the pump suction piping (large pipe size, limited length and bends etc.), and that you may need a type of pump that requires little NPSH. As well as provide sufficient elevation difference between the vessel low low liquid level and pump centerline.

[Art Montemayor]

Shawn,

I've done something very similar to your application many times. What you've received as comments and recommendations from the various respondents has merit and a lot of truth. Allow me to point out some features and experience in this application:

1) You don't need to know the vapor pressure of your liquid mixture. The vapor pressure, in the practical sense, is of nil importance in trying to figure the NPSHa of a saturated liquid. Note the term "saturated". The vapor pressure in the NPSHa equation cancels out with your system pressure and you are left, on a practical basis, with nothing more than the Hydrostatic head of liquid that exists over the centerline of your pump as the only positive measure of the NPSHa. To ensure that what I am saying is the practical truth, check with Goulds or other experienced pump manufacturers.

2) Doug is right and he is pointing directly to the above fact when he says "equilibrium". The system pressure is the vapor pressure.

3) gvdlans is also directing you in the right path. The type of pump is very important as well as the maximum static head that you can give the pump. For example, you could resort to a regenerative turbine pump (max. gpm of approx. 100) that inherently handles saturated liquids mixed with vapor due to the ability to expel the vapor and avoid the dreaded loss of prime (not "cavitation"! cavitation is the creation and subsequent implosion of created vapor bubbles).

My experience has shown me that although you are correct in taking precaution for the theoretical heat pickup (and potential vaporization) in the pump suction, the practical results (thank god) indicate that these can be overcome by today's efficient and well designed pumps. A healthy and generous hydrostatic head of liquid (3-5 ft) will keep most good pumps well primed and well fed. However, you should of course select an adequate model with a low NPSHr as well. A generous suction line size, a pump low specific speed, and minimum turns and pressure loss in suction piping are also pre-requisites that make for a good, dependable installation.

With the good engineering criteria that you reflect and the good advice you've already received, you should have no problem. I've sucessfully designed and pumped such troublesome liquids as liquefied CO2 (-20 oF) with a gear pump and only 1 foot of hydrostatic head - and no insulation on the suction line, day-after-day, for 365 days continuously. And no problem! I'm not advocating you go out and do this; I'm pointing to the fact that "God is good", in the practical sense, and that you can get by with it inspite of human failings. But you must furnish a hydrostatic head to begin with. The equilibrium (or saturated) pressure is of no help to the NPSHa.

I hope this experience helps.

Art Montemayor
Spring, TX

[ShaunHill]

Thank you to every one for the feedback.

I'm glad to have it confirmed that the conservative approach is the correct one in this case.

From the responses, I am concluding that the NPSHa is only the liquid head minus pressure losses in the suction piping.