Pipeline Terminal Facilities - PSVs with Liquids in Discharge 1

[cbnorthside]

We are designing a new Terminal Facility for a Client. The Client would like to send all thermal PSV discharges to a header back to the main heavy oil storage tank (bitumen from oil sands operations). In doing this, with a tank that is 17 meters tall, we will not have PSVs elevated such that they drain to the tanks. Therefore, we could see this bitumen back from the tank right up to PSV discharge nozzle, stagnant until PSV lifts to relieve thermal expansion overpressure. The terminal is a CSA facility, and the CSA Code is vague in details about what is and what is not permitted when it comes to liquids in PSV discharges. If it were an ASME designed facility, the wording is much more clear (an Appendix UG, Section UG-135 Installation Part (f) stating that we want to "...prevent liquid from lodging in the discharge side of the pressure relief device...".

Does anyone know if CSA Code have permitted the installation of thermal PSVs such that there will be stagnant heavy liquid in the PSV outlet tailpipes? Does anyone with Pipeline or Terminal experience know of this being done and accepted as a fairly common practice, because our Client tends to think this is permitted?

Thanks in advance.

 

[BigInch]

Clients tend to think pink pigs can fly, because no code says they can't. What's worse is that you appear to be prepared to do it, if you could somehow manage to stretch your imagination to believe that it isn't prohibited by CSA, even though you know it is strongly suggested that it is not a good practice by ASME standards.

I hate Windowz 8!!!!

 

[SNORGY]

What CSA Code, specifically, are you following?

 

[TD2K]

What type of flow rates are you expecting to see?

Normally I would not agree to liquids siting on the outlet of a PSV but thermal relief rates are usually quite low and, in my opinion, it's not an issue.

The bigger issue I can see is ensuring the lines are properly heat traced so that when you have to relieve, the outlet line isn't full of solid bitumen.

 

[SNORGY]

TD2K has it right.

This is one of those, "What else are you gonna do?" situations.

Especially if you are following CSA Codes (presumably Z662), which implies a Canadian installation which, in turn, implies that a Provincial Regulation may be in effect that discourages a CHD (particularly a sub-grade one).

A freeze-protected thermal PSV system is probably as good an idea as anything else.

 

[BigInch]

If limited ONLY to thermal expansion volumes, I didn't catch that before, then I might tend to agree. However "stagnant heavy liquid in the PSV outlet" is not a very good situation. Wouldn't it be better to drip it into a closed oily-water drain.

I hate Windowz 8!!!!

 

[SNORGY]

BigInch,

What you suggest is the best thing to do, if the client doesn't dig in and balk at the extra cost. To me, what it would come down to is site layout and piping elevations. Ideally, everything in the CHD header would slope to a common drain tank or tanks without pocketing. The elevations of the upstream reaches of the CHD header would be established on that basis. Certainly in recent years, buried tanks or knockouts are discouraged; however, at least in my Province (Alberta), there are local regulations that set out rules for how much "reservoir volume" you can have on a site without secondary containment. Our Alberta regulations, for example, would allow for a total volume for "containers" of 5.0 cubic meters (1321 USG) without secondary containment, with no single container having a volume more than 1.0 cubic meters (264 USG). These are exclusive of piping header volume, so a person could put in a healthily-sized header and one or several judiciously-positioned "containers" above grade to capture the thermal relief effluent and maintenance drains. I have indeed done this in a couple of midstream tank terminals, but invariably, the client and operators prefer that the thermals be routed back to the big tanks.

So, the above said, maybe other things come into play, like how "dead" the oil is and what the risk might be for a thermal PSV to stick open and give rise to flooding the containment.