wellhead choke valves 3

[hassann]

I have some confusion regarding the choke valves on oil and gas wellhead.
A. Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment for a fixed upstream pressure and temperature.
B. On the oil and gas wellhead and production facilities, decreasing the downstream of choke valve pressure (for example decreasing pressure on the production separator) can be caused the increasing the mass flow rate.
I think A and B are completely contradictory to each other.
Please share your ideas as possible.

 

[LittleInch]

Basically, if the mass flow increases when you drop the downstream separator pressure over a reasonable period of time ( minutes / hours) then it means the choke valve is not in a choked flow condition if nothing much changes upstream (Pressure / density etc).

This could be either the valve trim is more complex than a simple arrangement or the flow regime ("there is mostly liquid") is resulting in non choked flow.

Could be either IMO.

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

 

[TiCl4]

yes. there is mostely liquid.

I think the conversation is missing the forest for the trees. The majority (mass%, volume%?, which is it, Hassann?) is liquid, as stated by OP. Many equations that deal with flashing fluid choked flow assume a homogeneous liquid that is flashing, which can result in majority vapor flow even if inlet composition is mostly liquid. If the material stream here will not experience much flashing over that pressure drop due to liquid composition, then it makes sense that choked flow cannot realistically occur because the fluid dynamics are dominated by a liquid phase, which cannot become choked.

 

[hassann]

LittleInch (Petroleum)

All Thermo books says that under choked flow conditions, with decreasing of downstream pressure , the mass flow rate is constant if the velocity at the choke inlet is zero. and finally there is an important question: Is the velocity at the upstream of choke valve is zero in actual conditions?

Fundamentals of Engineering Thermodynamics 9th Edition, MICHAEL J. MORAN, HOWARD N. SHAPIRO, DAISIE D. BOETTNER page 357

.. When a nozzle is choked, the mass flow rate is the maximum possible for the given stagnation conditions.

 

[TiCl4]

Hassann,

No, all thermo books do NOT have that provision. Choked flow is defined as a Mach number of 1. That's it. Evaluations of critical pressure ratio for choked flow are typically taken with upstream stagnation conditions (see Perry's Chemical Engineering Handbook, Section 6-22 in the 7th Edition). Stagnation conditions are defined as the conditions if the flow were isentropically taken to zero velocity. So if a gas was a pressure P1 and velocity of V1, then P2 (stagnation pressure) would be:

P2 = P1 * (1+(k-1)/2 * Ma1^2) ^ (k/(k-1))

In reality, the gas has some level of velocity, and thus a starting Mach number (Ma1 above). So you can do your calculations to first determine stagnation pressure, P2 (Pnaught in most references), then determine your critical pressure by using the formula above, solving for P1, and using Ma = 1.

This mathematical evaluation doesn't mean the fluid flow is actually 0 velocity upstream of the choke point. It is just a means of actually evaluating the system.

Lastly, non of this particularly matters, because these formulae are for pure/ideal vapors. Typically, critical pressure ratios are in the 0.4 to 0.6 range. Your pressure ratio is ~0.16. In all vapor conditions, this would be choked flow. Therefore, you obviously have a very non-ideal (tons of liquid phase) system, and you need to stop trying to evaluate your system using single-phase compressible flow thinking.

 

[hassann]

TiCl4 (Chemical),

let us forget our expriment and suppose there is single gas phase.
Are you agree to my latest post?

under choked flow conditions, with decreasing of downstream pressure , the mass flow rate is constant if the velocity at the choke inlet is zero.

 

[Compositepro]

As I have already pointed out, it is not inlet velocity that is a factor, it is inlet gas density that has to be constant for mass flow rate to stay the same. Inlet gas density is proportional to inlet pressure.

 

[georgeverghese]

@hassan
Even if there is considerable kinetic energy in the inlet stream, say v=30m/sec for gas, perfect frictionless conversion of all this kinetic energy to pressure energy will only result in a miniscule pressure rise under isentropic conditions

P2/rho2 - P1/rho1 = (v1^2 - v2^2)/2.

Note that P is in N/m2 abs in SI units. You can approximate the pressure rise in this isentropic process by assuming there is no change in inlet density i.e isothermal conversion. At most, temp rise coincident with isentropic compression to P2 , with v2 = 0, may not be more than 5degK I suspect. Do the arithmetic and see for yourself.

So in real world applications, you could ignore the kinetic energy term in the feed gas

 

[TiCl4]

TiCl4 (Chemical),

let us forget our expriment and suppose there is single gas phase.
Are you agree to my latest post?

Quote:
under choked flow conditions, with decreasing of downstream pressure , the mass flow rate is constant if the velocity at the choke inlet is zero.

No. The correct statement would be "under choked flow conditions, with decreasing of downstream pressure , the mass flow rate is constant". It doesn't matter what the upstream conditions are. If you hit Ma=1, you have choked flow.

A simply way to prove this is the fact that flow can become choked simply by increasing pipe length of a flowing gas. At what point in the pipe would you say flow is zero speed?

The formulae use stagnation conditions rather than simple pressure because the velocity of the gas going into the obstruction matters. If you are going 0.99 Ma in a pipe, it will take only a very small pressure drop to induce choked flow. If you naively use measure pressure, it will seem that a pressure ratio of 0.99 induced choked flow. If you instead look at stagnation pressure, you will realize that you will be back in that 0.4-0.6ish range with only a very small pressure drop.

You seem to think the mathematical evaluation of a fluid's stagnation conditions means the fluid is actually stagnant. That could not be further from the truth.

It's clear you don't understand the fluid dynamics at work here. I suggest you pick up a good fluids book (I'm fond of Unit Operations of Chemical Engineering by McCabe, Smith, and Harriott) and learn there rather than trying to piece it together from internet comments.

 

[hassann]

TiCl4 (chemical)

Sorry for the delay in replying to you as I have been away from thermo books for a long time. In any case, I referred to McCabe's reference and, as you said, it was a good book. From Figure 6.1 McCabe version 5 (appendix) it can be seen that in all three figures the choke (nozzle) is directly connected to the reservoir, while in oil and gas well installations the distance between the reservoir and the choke valve (nozzle) is sometimes thousands feet. So in my opinion, process characteristics, such as pressure and density upstream of the choke, are not constant.(onlt for this case). also The figure I have drawn may help me to explain better. In any case, I am very grateful to you and others who took the time to share their points of view, and I apologize if the discussion was trivial and elemantary in your opinion.

 

[hassann]

georgeverghese (Chemical)

I would be very grateful to know your opinion about my above post. This post is slightly different from my previous comment about the zero entry speed of the nozzle.

 

[Latexman]

The correct statement would be "under choked flow conditions, with decreasing of downstream pressure , the mass flow rate is constant". It doesn't matter what the upstream conditions are. If you hit Ma=1, you have choked flow.

If I may. The "It doesn't matter what the upstream conditions are" is bothering me a little. If you have choked flow, and you raise the upstream pressure, density is increased and mass flow rate increases, all while velocity = Mach 1 = choked flow.

Good Luck,
Latexman

 

[hassann]

Compositepro (Chemical)

latexman (chemival)

Yes you are right. But the question is still raised whether in choke conditions (sonic flow) all of the fluid conditions such as pressure or density ... in the upstream always areconstant with the decrease in the downstream pressure? Or it may sometimes change depending on the location of the choke valve (nozzle) in the piping system? Please see my post of 26-Apr-24 at 1:25 PM.

 

[Latexman]

You have to characterize the entire system between reservoir and receiver (atmosphere). Use data of the reservoir and receiver, whether measured or assumed. Then calculate what happens. Maybe use Aspen+, HYSYS, Fluent, OpenFOAM, or others.

Good Luck,
Latexman

 

[georgeverghese]

Pls do not conflate the increase in pressure to reach stagnation conditions with the shut in tubing head pressure SITHP ( or closed in tubing head pressure CITHP) in this case of a production well choke valve. Stagnation pressure is the pressure reached by theoretical isentropic conversion of the kinetic energy component (only) of the feed stream. While SITHP is the pressure seen when dynamic frictional loss in the long vertical production becomes zero at no flow.

 

[TiCl4]

If I may. The "It doesn't matter what the upstream conditions are" is bothering me a little. If you have choked flow, and you raise the upstream pressure, density is increased and mass flow rate increases, all while velocity = Mach 1 = choked flow.

Good point. I should have put in the caveat that mass flow is independent of downstream conditions, not changes in upstream density. The wording about the upstream conditions not mattering were more in line with the fact that it doesn't matter if you are Ma = 0.01 or 0.99 upstream of the choke point - if you hit Ma = 1, flow is choked.