VAPOR RECOVERY UNIT (VRUs) PROCESS DESIGN BASIS

[Andrehzg]

Hi,

I´m working at the time in a project in wich we must consider an VRU to recover the HC vapor from the cold vents (blanketing gas) of the tanks.

I´ve been wondering about the process design basis to do this. I have several tanks in the area. And as the API 2000 recomends, we estimate the blanketing outbreathing gas to prevent oxigen to come into the tanks and safeguard the structural integrity of them.

Knowing this, the issue that we have is which should be the real process design basis of the VRU which recovers the HC vapors from these tanks. It is not real to assume that all the outbreathing flow will go simultaneosly to the VRU as the tanks have level control set in a normal operation level, so it is not real to assume either that this leve will fail in several tanks at the same time.

Resuming, my question then is which would be a conservative flow to be considered to design the VRU if we have several tanks connected to this systems, this tanks operates at a normal liquid level usually. I assume that there should be some prerrogatives we can assume not to over design this unit, as we do when we design a flare system.

Thank you in advance

 

[EmmanuelTop]

I believe you have to develop several cases in order to determine not only the maximum capacity but also the turndown capacity for the new VRU - otherwise you may end up in a situation where the new Unit will not be able to operate 90% of time, due to lack of feed.

I don't know what kind of tanks you have there nor which products are stored in, but here are a few thoughts anyway:

- Maximum flow would correspond to the worst "normal" operating scenario, not emergency conditions (unless you have strict environmental/legislation requirements which force you to recover 100% vapor in all cases). These worst conditions would correspond to the maximum ambient temperature and design inflow to the tank. In case of more tanks this would correspond to the maximum simultaneous inflow for all tanks which are designed to be in simultaneous service.

- Minimum flow would correspond to the lowest ambient temperature and the minimum product inflow, based on the upstream plant capacity. If it is not expected to operate at a throughput lower than design, the turndown capacity of VRU should be increased accordingly.

- Include some sort of recycle feature which enables you with sufficient degrees of freedom to operate the new VRU under conditions where the outbreathing flow is lower than the minimum flow. Or accept to vent the unprocessed gas.

Dejan IVANOVIC
Process Engineer, MSChE