Prevent flashing on a separator outlet 8

[Hasab]

Hi,
Appreciate suggestions regarding the below case.

The liquid outlet from the separator is saturated, that means any reduction in pressure will result in a slight evaporation of the liquid and the formation of gas bubbles. In the attached scenario, the level control valve induces pressure reduction, leading to the creation of gas bubbles, a phenomenon known as "flashing," which can adversely affect the control valve, potentially causing erosion and damaging it. In such cases, how can we prevent flashing, or what are the typical ways to handle this situation?

 

[shvet]

Picture is incorrect. Bubbles evolute only at surface liquid levels because of liquid column weight and therefore does not affect a control valve.
This is speculation aimed at unexperienced customers to force those spending money to a non-existing problem.

 

[Hasab]

Thanks shvet (Petroleum) for your point, i was thinking similarly to be confirmed that if there gonna be a flashing on the LCV or the pressure of the liquid coming down from the separator is high enough.

 

[LittleInch]

If the DP across the control valve is more than the static head between liquid level and Control valve it will flash.

Your only other option is to add a pump to raise pressure but that is very difficult due to the same issue.

The control valve vendor should be aware of this and build their valve accordingly. Choke valves on oil and gas Wells handle two phase flow without big issues. You will get an issue if you use a control valve not designed for flashing liquid.

Your downstream system then needs to deal with a two phase fluid, either another separator or a degassing vessel which is essentially the same thing.

This is all very common. What exactly is your issue?

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

 

[Hasab]

Hi LittleInch, thanks for diverse suggestions.

Yes it is possible also to elevate the separator to increase the static head if confirmed the DP across CLV is much higher.

Yes flashing/cavitation is common issue, i think liquid column weight as shvet pointed will be enough to prevent any harmful flashing, i will go to check and confirm this sound assume.

 

[LittleInch]

Hasab,

I very much doubt it, but you haven't given any pressures in the tank or downstream of the separator on the liquid side or static head here.

So lets say your static head from liquid level to control valve is 5m (pretty big). Your SG is 0.8
This gives you 0.4 bar to drop across your control valve before the liquid flashes as it exits.
That's not a lot.
Note that the pressure in the tank is irrelevant here as the liquid vapour pressure is the same as the gas pressure in the separator. All you have is head difference to prevent flashing.

You need to look at the whole system here, not one isolated little bit.

And specify the fluid conditions to the valve vendor.

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

 

[Snickster]

Fisher has anti-cavitation control valves that may work in your situation.

https://www.emerson.com/en-us/catalog/fisher-cavitrol-iii

 

[hacksaw]

Your level control valve only drains liquid by the direction of the level controller and its settings, to a destimation of some pressure that you have not specified.

Your back pressure control valve maintains the separator pressure.

Carry out a material balance on your process with unspecified properties, then check the sizing of your valves.

 

[LittleInch]

Those valves are not anti cavitation, but are able to handle the effects of cavitation. Different thing.

The downstream pressure of the liquid line is what controls or impacts your flashing across the LCV.

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

 

[georgeverghese]

Typically, control valves in flashing-cavitation service ( where pressure in the throat area is much less than bubble point pressure, and operating P2 is greater than bubble point pressure) are specified to be anti cavitation trim, which I believe is stellite hardened trim. But if you have sand or some other erosive particles in the liquid, you may need even harder surface trim, perhaps tungsten carbide.
Adding a choke valve downstream of this control valve will work at high flows to prevent cavitation induced erosion, but this wont be any good at low flows.

 

[Snickster]

Correct - not anti-cavitation per se but only built to handle cavitation.

 

[LittleInch]

Typically they are essentially a whole series of mini orifices stacked together or a series of pressure drops one after the other.

But you need some really hard material to stop it disintegrating under the constant battering it gets in service.

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

 

[1503-44]

You do not give us operating pressure data and stream components.
THESE ARE IMPORTANT.

Without data, I cannot be sure, but I think you are trying to operate at too high of a pressure and you may need to reduce the operating pressure of this vessel (reduce the gas PCV backpressure setting) and try to get more off gas flow while reducing the liquids vapor pressure. Alternately, you might benefit from changing this setup into using two, both a high and low pressure seperators. That will reduce the pressure drop across the level control valve of each vessel. NEED PRESSURE INFO to understand if that is a reasonable thing to try.

From

https://www.e-education.psu.edu/png520/m20_p3.html

https://petrowiki.spe.org/PEH:Oil_and_Gas_Processing

https://en.m.wikipedia.org/wiki/Separator_(oil_production)

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Hasab]

Thanks all of you my dears, i have got helpful insight from you.

Currently have discussed with the valve vendor and he is going to tackle the situation。

@ 1503-44 (Petroleum),thanks for detailed answer, i will see changing the setup option. we also will see if the upstream inlet temp. can be reduced to increase the margin for flash to happened.

@ LittleInch, there is gas phase pressure maintained by the PC will add up to the static head.

@ georgeverghese, thanks i will take your point also.

Thanks all i will back if need more suggestions.

 

[LittleInch]

Hasab,

The gas pressure plus head adds to the pressure of the liquid, BUT, in terms of flashing issues it DOES NOT. This is because the Vapour pressure of the liquid is the same as that of the gas pressure. You need to understand this or you won't give the vendor the right information.

Liquid coming out of a separator, even water, is very volatile and will emit gas / flash gas very easily.

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

 

[Hasab]

Okay, i see now, my understanding was incorrect.

Thanks LittleInch lot for you valuable information.