Dry and Wet Gas

Wednesday, 11 January 2011, DEGC project team conducts regularly meeting. When the issue of PWHT -Post Weld Heat Treatment- requirement for some pipe is raised up on that meeting, piping engineer ask me what the service for and whether the condition is wet or dry.

I said that the pipes are for fuel gas and dry. Actually, I forgot why that pipes are considered as dry gas. But I am sure, they must be in dry condition since the pipes leaving from glycol dehydration contactor.

The consideration of the gas condition –wet or dry- is based on the operating temperature corresponding to its dew point. Some reference state when operating temperature 10F higher than the dew point temperature, it is considered as dry gas. I think, we have an own adjustment with the margin or temperature difference.

Checking the pipe condition by HYSYS, the dew point is 30 F while the operating temperature is 117F, then obviously the pipes are dry service.

Dew point is a condition when the first liquid is being formed from the gas. Dew point temperature can be determined by input vapor fraction equal 1.0 on HYSYS simulator - imagine that the vapor fraction start reduce to 0.999999999, then input 1.0 -. at certain pressure, the dew point temperature will be automatically calculated.

Regarding the dew point temperature, the following service lines require dry condition. That is why, they shall be checked carefully.
1. Pipeline: the operating temperature shall high enough to prevent condensation due to pressure drop along the pipeline.
2. Fuel gas supply: generally, fuel gas supply requires temperature of 20F -30F higher than the dew point temperature.
3. Inlet gas to membrane: liquid to membrane cause membrane damage.
4. Etc. – please comment, if you have other opinion-

In some cases, a heater shall be installed to reach accepted operating temperature to avoid gas condensation.