Calculation of Fire Case Relief Requirement for TEG Gas Contactor 1

[Wotcha]

I am attempting to size a relief valve for fire case vapour generation on a glycol contactor used for removing water from hydrocarbon gas in a compression train.

The principal problem lies with determination of the latent heat of the fluids in the base of the vessel - which are a mixture of water, TEG and hydrocarbon condensate.

The contactor operates at a pressure of approximately 40 bar.

 

[aminersson]

take a peek at API 2000..... but be aware that the geometry is predominantly assumed to be cylindrical/spherical look for the methodology behind this and not the examples presented and you'll be ok.

 

[EGT01]

Wotcha,

Not sure what your problem is but take a look at this thread to see if it helps any.

thread124-65447

 

[121202]

Wotcha,

In terms of the basic calculation methods, the problem your are considering is very adequately covered in API 520/521. However, I suspect that your question refers to general engineering practice rather than calculation methods?

The safety valve needs to be sized to accommodate the vaporization of the liquid contents of the vessel AND heating of the unwetted dry portion of the vessel. You need to separately determine the relief area required for vaporization and then add that to the area required for the unwetted dry portion of the vessel.

For vaporization of the contents I'd recommend that you assume that there is a 1" layer of condensate on top of the TEG. The condensate will vaporize long before the TEG\Water mixture reaches its boiling point. In fact I doubt if you'd get the TEG\Water mixture to boil at 40 bara before the vessel ruptured, so in practice it will contribute very little to the vaporization load. As far as the condensate composition is concerned I'd base it on the condensate collected in the scrubber directly upstream of the TEG contactor. This is reasonable, as condensate will have entered the TEG contactor due to carryover from the upstream scrubber.

The other thing that you must consider is the requirement for blowdown. Fire relief valves by themselves do not protect a vessel from rupturing. To stop a vessel rupturing you need to remove the heat input (i.e. by extinguishing the fire) and by reducing vessel stress levels (by reducing the internal pressure). You should therefore ensure adequate deluge coverage is provided and that a suitably sized blowdown system is also installed.

Hopefully this helps?

Rgds

 

[haripals]

We have installed Inergen gas system for control room and instrument equipment room in Offshore platorm. The control panel and actuators are non explosion proof, but exlosion proof required. Control panels are located in non-hazardous area.
Could anyone suggest how to resolve this.

Thanks

 

[MortenA]

Replace the equipment?

Inquire at the manuf. if there actually is a difference - or if the installed equipment can be Ex certified or you could challenge the need for Ex equipment since the control room is not in the Ex zone.

These are quite obvious statements so you most likely though of them yourself - but you should open this question in a new tread and not in some randomly selected tread dealing with RV sizing problems.

Best regards

Morten

 

[Wotcha]

Cheers for all the advice on my PSV problem - it all makes a lot of good sense. One further question though - 121202 - you mention that you doubt if the TEG/water mix will vapourise at 40 bara before the vessel fails. I've used HYSYS 3.1 to predict the bubble/dew points of the TEG/water mix and it calculated them to be 251 DegC and 296 DegC - which does not seem to be too high a temperature. What would the vessel failure mechanism be?

 

[zhangsteve]

Wotcha, the thing you have to be aware of is that TEG will decompose if temperuate is above 400 F (may be a little higher), and that is why the TEG regenerator reboiler has to be operated at a low pressure (typically 1 psig at TEG still condenser) so that the reboiler can be operated at 400 F.

As far as for a relief valve sizing, I am not sure how much HC condensate is in the liquid phase. Depending on how far away you design pressure (or MAWP) from operating pressure, vaporization of condensate alone may not make the relief valve open assuming condensate will vaproize first. There is a recommendation/guideline from Union Carbide (Dow Chemical right now) which recommends to use a empirical heat of vaporization to model the vapor generation due to TEG decomposition. I cannot rememeber what relieving fluid is based on, maybe CO2/H2O.

Hope this helps