Using a storage tank to recover PSV discharge

[Ogasprocess]

Hi all,

I am writing this post as i would like to get feedback on the feasibility to use an old oil storage tank as PSV recovery drum.
The PSV are installed on a stabilized oil export pipeline which due to integrity issues has been derated during production years. The pipeline is receiving the production of various sites.
The current maximum allowable operating pressure is close to the normal operating pressure and there is a risk to open the PSV's.
The idea is to reuse this forment oil tank to get a good retention capacity allowing enough time for operator response to stop export from certain sites.

According to API 521, it is suggested to design the KO Drum at a minimum pressure of 3.5 barg which is far away from the design pressure of a storage tank. Morevoer, there is a risk of oxygen entrance in the tank as there are open vents for tank breathing purposes.
Has someone seen this type of design before or someone knows if there is any API or other standards allowing this type of use of a storage tank?
Thank you very much for your feedback

 

[LittleInch]

Sounds like you need to look at API 2000 to determine the acceptable instantaneous flowrate and vapour emission through the open vents.

You would need to have the inlet line below the Low liquid level to avoid foaming, but a lot will depend on the volume of the tank versus the inlet flow rate from the PSV.

If used for blocked in flow, then the inlet flowrate should not normally exceeed the max flow of the upstream pump(s)

A KO drum is normally only used for two or three phase fluids - you state "stabilized oil"??

API650 allows use of atmospheric tanks for liquid storage. That's what you're doing here, but the devil is in the detail.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EmmanuelTop]

API 521 does not prohibit you from utilizing another system as a PSV discharge recipient, but you need to prove the following:

- Tank has more than sufficient capacity (reaching maximum e.g. 80% working level inside the tank) for all identified relief scenarios, and for the maximum duration of relief rate;
- Tank venting devices have sufficient capacity to account for vapor displacement during maximum relief rate on a hot summer day, including variations of oil quality (possible high vapor pressure material?);
- Tank inlet piping arrangement is properly designed to avoid static electricity hazards (relief rates for certain scenarios can be very high - not sure which cases apply for your pipeline) and some other operational issues mentioned by LittleInch;
- There are proper design and administrative controls put in place to ensure that the tank (including venting devices) cannot get blocked during any of the relief scenarios;
- Back pressure created on the PSV will not adversely affect performance of the PSV, during all relief scenarios;
- All associated risks are assessed, addressed, and documented.

Depending on the subjects above (and some others that will come along), it may or may not be feasible to utilize the tank for relief service. I'm not aware of any legislation that would explicitly prohibit such configuration - just really be sure that you can always rely on the tank and the associated system to be safe and reliable, as much as you would take care that the flare system is reliable and safe in a gas plant.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Open vents on this storage tank can be easily blanked off, but a design pressure of 350kpag is a bare minimum for this service(700kpag preferred), provided the vessel is continously purged. The vessel inlet and exit nozzles should also have the structural strength to withstand the mechanical loads due to the largest flowing relief loads.