Draining a drain drum

[LondonJeff]

We are a remote location, a new gas field, and have a drain drum, collecting gas condensate with TEG. The drum is in an open pit. The designers arranged pipework up to grade with a connection for a vacuum truck. But I have a concern that if the condensate is near to equilibrium, use of a vacuum truck would just vaporize it. I wonder whether slightly pressurizing the drum may be a better way for removal (design P 3.6 Kg/cm2 g). Does anyone have any thoughts on this please?

 

[EmmanuelTop]

Pressurizing the drum is very risky option, the most dangerous thing being blowby to the vacuum truck that is essentially an atmospheric container. You do not want that, unless you have the facilities to control the pressure and shutdown the whole process if such event or anything similar happens. I have seen so far only one utility station where the design took care of these concerns, but the residual risk is still high.

How deep the drum is buried, and what components does it contain? If contents of the drain drum are in equilibrium at atmospheric pressure (I assume the actual drain drum pressure is equal or very near to atmospheric) and you have a 2-3 meters total height of liquid during vacuuming process, I don't see any real concern because that is how vacuum trucks operate in 99.9% cases - pulling liquid from underground vessels and pits. Some amount of vapor will be generated but unless you are handling a very toxic component, that should not be a concern.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Maybe okay to do this, provided you've got the following well stitched up :

A) Be sure to positively isolate this drum from the process plant and the vent system while running this sequence of operations.

B) Also you've got to know when the liquid level has dropped below the level of the draw nozzle, and avoid pressurisation gas blowby into the vac truck.

Of these 2, agree the second will probably require more work than the first. Maybe better to pull up into an intermediate elevated drum with a design pressure of 3.6kg/cm2, then flow down into the vac truck by gravity from this intermediate drum, if you believe the condensate vapor pressure is too high for the vac truck pump.

Whichever route you choose, routine as this operation may be, would suggest a desktop hazop review of the entire operation and then draw up physical mods and operating procedures to suit. Some 20years ago, I was once told by the Process Dept manager that plant drains systems have ruined the careers of process engineers than any other - that remark was lost on me at that time, but not forgotten.