Gas Hydrate Formation 1

[AreJay]

Afternoon all,

I am currently reviewing a gas compression package which has relief valves coming from the system to the flare. What has been asked is the potential for hydrate formation in stagnant lines.

When considering the layout of the pipes, there are some dead legs and some slanted horizontal pipework. The pipework is open to atmosphere and can have wind blowing into it. Would hydrate formation be a possibility if fluids were dropping out? I would assume that having self draining pipework I would be able to presume that if an accumulation were to form it would simply be a fluid accumulation rather than a potential hydrate i.e. the fluids would drop out but not instantly freeze?

If I knew the temperature profile through the pipework and the pressures along with composition would I be able to calculate the hydrate formation potential in stagnant pipes?

Thanks in advance

 

[zdas04]

First, hydrates do not form in liquid water. They form when there is water vapor present in conjunction with hydrocarbon gases. Second, they are a "high" pressure phenomena, at atmospheric pressure the hydrate temperature is really close to the freezing point of liquid water.

If you look at GPSA Field Data book figure 20-15 you'll see that the minimum hydration formation pressure at 32[°]F is 70 psia. For Methane it is 400 psia at the freezing point of water.

I've never heard of hydrate problems downstream of a PSV. Upstream is another issue altogether.

David

 

[ione]

The likelihood of having hydrate formation downstream a PRV is not that high, anyway if Joule-Thompson effect is capable to produce such a temperature that is lower than the fluid hydrate formation temperature, then this phenomenon could be an issue

 

[zdas04]

I've seen water ice downstream of a PSV due largely to the J-T effect, but never hydrates. If I ever see a flare header with 400 psia on it (the minimum pressure for hydrate formation in methane at 32F), I'll ask a lot more questions than hydrate mitigation.

David

 

[maddocks]

This is one of the reasons why compressor PSV's are located directly downstream of the discharge bottle but before the cooler. Typically the downstream JT temperature is still high enough to avoid hydrates. Hydrate formation is also a kinetic factor and won't happen instantly - at least, not enough to cause an operating issue. I don't think I'd be too concerned.

 

[dcasto]

go to here and download the hydrate program from the masters of hydrates

http://hydrates.mines.edu/CHR/Software.html

with it you can predict when you will hydrate off.

We may be mincing words, but at atmosheric pressure you can have hydrates of the other consitiuents in natural gas. ALL nat gas is not 100% methane.

I've collected the ice balls from leaking propane at a flange and taken that ice ball and packed it into a snowball and put a match to it, it sure makes the nubies at work nervous to see ice burning.

This propane mixture has a hydrate potential at 5 psia! (at -40f)

 

[zdas04]

That seems like information that would be really important to process design; much less so for field operations. At -40F everything that can freeze is already water ice. The field problem with hydrates is that they can cause ice-like substances well above the freezing point of water.

I worked on the periphery of a flow-assurance problem once where a well stream at 10,000 psig was in 6,000 ft of 40F water--hydrate point of the flow stream was 72F and the thermal entry length was under 200 ft, so the stream would be a 40F slushy a mile under the ocean surface. This was a serious problem. Worrying about whether hydrates will form at 25F doesn't seem serious since at that temperature the water vapor will condense and freeze as water ice which is a bigger problem than hydrates.

David

 

[Chatwin]

HI,

Hysys can be used to predict the Hydrate formation temperature of a particular gas composition / fluid composition presuming free water present.

To mitigate this, if it is a great concern, heat trace the stagnat lines to maintain a temperature abouve this point - put a safety margin in - say 10 degrees.

I hope this helps.

Regards

Alex Chatwin