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[sdarone]

My Dears collegues,
Your input and help will be highly appreciated on the following subject.
I'm process engineer with some experience, and work for an Engineering company.
In the project i'm working on our client requested for us to include in our design an oil sewer system, which will cover the fractionation section of an alkylation unit.
This oily sewer will be used mainly for equipments and instruments preparation for maintenance, basically to drain instruments (LG's), and the leftovers of Hydrocarbons in the distillation columns, pumps, pipes, etc.
Once drained the column will be subject to steam out procedures.

The oily sewer system is a closed drain system no air gaps between the equipments drain valves and the sewer system itself (sight glasses are provided for monitoring), besides the buried pipes the oily sewer has a partially buried double wall tank where the HC are collected and pumped out to an aboveground slop oil tank.

The project design standards we are using allows the venting of the oil sump to the atmosphere (safe location), however, the version of these standard is dated on 1999. (prior to BP texas city 2005 and OSHA warning about the use of Blowdown drums)

My questions are the following:
1- Shall I consider the tank (or vessel in matter of fact) a Blowdown drum? the use is maintenance related, no pressure relief valve discharge is received in it.

2- Are the restrictions and regulations of underground storage tanks applicable here? i guess so, since even when the vessel contenst are pumped out to an above ground slop oil tank, some hydrocarbon will remain in the vessel.
The required volume i'm getting for this vessel is about 300 usgallons, which is small.

3- Since the HC in the column prior to be drained will be at nearly atmospheric pressure, most of the light ends will be vented to the flare through the column PSV bypass valve. Do I need to route the underground tank vent to the flare as well?, that will be difficult to do, probably a kind of vaccum system will be required.

Your help and guidance will be really appreciated.

Thanks

Sergio