Ice formation downstream LCV

[SZL]

We have drain boot connected to the gas headers upstream the dehydration plant. The liquid from the drain boot is sent to the Blow down system while the vapor is recycled back to the gas headers.
The pressure on the gas headers is around 400 psig and the temperature is between 100 to 120 F. The pressure of the blow down is 2 psig. The liquid level in the drain boot is controlled by level transmitter which sends signal to the level control valve to open or closed.
We are experiencing a problem that the ice is formed on the line downstream the control valve and the level increased at the same time while the control valve opens fully. However, after a while the level decreases and the ice melts and the level control valve start to control.
The valve is two phase globe valve, Is it really hudration???

 

[europipe]

http://en.wikipedia.org/wiki/Flash_evaporation

We always called it flashing.
You can reduce it bij slowing down the pressure fall after the valve, or use tracing.

 

[DRWeig]

Hi SZL,

Controls guy here, worked pipelines for a while. Might be flashing, might be hydrates. Need to do hydrate analysis and need to do P-T analysis.

In either event, I notice that you put this same post in the valve engineering forum. Please don't double-post, it causes lots of confusion. It's also against forum rules (click the link below my signature to read them). Suggest you red-flag this thread or the other one. OR, enter a post in one of the threads that directs the reader to the other one. Thanks!

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[SZL]

Thanks europipe and DRWeig, the valve is two phase valve not flash valve, is the two phase valve considered as flash?

i took a sample and i simulated the sample composition and it showed that the hydrate forms downstream LCV.

I think the heat tracing is short term solution but what about long term mitigation?

 

[Montemayor]

What is the liquid you are draining from the boot? Is it water – or does it have the properties similar to water? If the expansion of the liquid is causing a plug-up in the drain line, I doubt that it is a hydrocarbon. But I could be wrong. You don’t tell us the temperature downstream of the level blow down. Here, I am presuming you have rapid cooling due to expansion evaporation – as often happens with a liquid hydrocarbon. This is a normal refrigerating effect.

You also fail to tell us what you call or identify as ice being formed. Is the “ice” formed on the outside – or internally, in the drain line? I fail to see how you could know that ice is being formed inside the drain line. You certainly can’t see it being formed.

I suspect and speculate that what is happening is that you have liquid hydrocarbons (butanes, propanes, etc.) being expanded by the level control valves and these contain a level of water content. Upon expansion and subsequent cooling, the drained liquids form a hydrocarbon hydrate (a solid gel) that plugs the drain line. You are seeing ice formed on the exterior of the drain line. This is the solidification of the external humidity that condenses and solidifies on the external surface of the pipe. When the drain pipe warms up (and the external water ice melts), the internal hydrate gel dissolves and the drain line is freed up. If my guess is right, then the way to keep your drain line free is to inject a hydrate inhibitor into it – like methanol. This is what is conventionally done out in the field.

 

[SZL]

Thanks Montemayor for your reply,the liquid is hydrcarbon mostly ethane, propane and normal butane.

The ice forms on the outside of the pipe. We assume that the hydrate forms on the internal side of the pipe since the level of the boot increases and the level control valve opens fully indicating the flow is blocked.

I would like to know why suddenly the level decreases and the control valve control the level.

For the Blow down temperature, I will double check.

 

[SeanB]

Well this is just a theory...
But it sounds like as your valve opens you have approximately 400 psi delta P across the valve. You are getting a J-T effect and hydrate formation which is partially blocking the valve - which then causes the level to increase. Because the valve is partially blocked the delta P is now less - possibly reducing or eliminating the J-T effect. So now the hydrate begins to melt, allowing more fluid to pass through the valve, lowering the level.

So maybe you have to look at 2 stage control valve i.e. - two valves in series lowering the pressure in two stages.