Is there a Relief Device Exemption for this Vessel? 2

[APIPapi]

Is there any code exemption to installing a relief device on a pressure vessel that is always open to atmosphere (no block valves in vent line)? We have a section VIII vessel, but it is treated as a flash vent tank that has a line routed to atmosphere. I don't see the need for a relief device because a standard atmospheric storage tank would work in it's place. I looked at ASME XIII and VIII and couldn't find anything applicable.

Thanks!

 

[georgeverghese]

Generally speaking, no need for RV in this case. Install an insect screen on vent exit. But you will have to calculate to show that backpressure induced at the vessel for the largest credible vent load does not exceed process design pressure.

 

[shvet]

PART 13 RULES FOR OVERPRESSURE PROTECTION BY SYSTEM DESIGN
13.2 PRESSURIZED EQUIPMENT FOR WHICH THE PRESSURE IS SELF-LIMITING
The decision to limit the pressure by system design is the responsibility of the user. The user shall request that the Manufacturer’s Data Report state that overpressure protection is provided by system design per 13.2. Pressurized equipment does not require a pressure relief device if the pressure is self‐limiting (e.g., the maximum discharge pressure of a pump or compressor), this pressure is less than or equal to the maximum allowable working pressure (MAWP) of the pressurized equipment at the coincident temperature, and the following conditions are met:
...(a) The user shall conduct a detailed analysis to identify and examine all potential overpressure scenarios. The “Causes of Overpressure” described in ANSI/API Standard 521 shall be considered. Other standards or recommended practices that are more appropriate to the specific application may also be considered. A multidisciplinary team experienced in methods such as hazards and operability analysis (HazOp); failure modes, effects, and criticality analysis (FMECA); “what‐if” analysis; or other equivalent methodology shall establish that there are no sources of pressure that can exceed the MAWP at the coincident temperature.
...(b) The results of the analysis shall be documented and signed by the individual in responsible charge of the management of the operation of the pressurized equipment. This documentation shall include the following, as a minimum:
......(1) detailed process flow diagrams (PFDs) and piping and instrument flow diagrams (P&IDs) showing all pertinent elements of the system associated with the pressurized equipment
......(2) a description of all operating and upset scenarios, including scenarios involving fire and those that result from operator error, equipment malfunctions, and instrumentation malfunctions
......(3) an analysis showing the maximum coincident pressure and temperature that can result from each of the scenarios listed in (2) do not exceed the MAWP at that temperature

 

[Latexman]

You don't need an exemption, the relief device IS the open vent line. Size and document each credible scenario showing the open vent line is adequate, and you've done your job.

Good Luck,
Latexman

 

[LittleInch]

In essence this is proving a negative and hence isn't covered by any code as such so the issue is simply to detail in a short technical note or report about why this tank does not fall into the requirements for a pressure relief to be installed.

Basically it looks to me that the max pressure it can see (just above atmospheric) is much lower than its MAWP and there are no circumstances where this could change (Fire, accidental valve closure, blockage, that sort of thing)

Then you can conclude that it doesn't need one.

Flash vent tanks can be something or nothing depending on what is being flashed and how much there is. Just define that and do a process calc or run to show what the max pressure is and you should be < MAWP.

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

 

[shvet]

@APIpapi
adding to said above
In other words - There is no a distinguished authority source that capable to take responsibility instead of a designer. A designer have to make a decision, take responsibility, document carefully, and obtain an approval from a final user. All that stuff called engineering )))

 

[ProSafPlant1]

Glycol dehydrator reboiler still columns with vents to atmosphere in the Alberta oil & gas industry used to be commonly designed without relief devices until a few fatal incidents due to freeze off in the 1980's. Now they all have relief devices.

 

[georgeverghese]

What is this " flash vent " stream ? Flammable or toxic or prone to plugging?

 

[MortenA]

@ProsafPlant1, i fail to see how a relief device would be more safe? Is it because the continuous flow of moist air would cause condensation in the vent that would freeze and eventually _block_ the pipe and since there is no flow in the PSV then this wont happen in the PSV tail pipe?

This would only have to be considered in harsh environments then. Although, if you do see a way that the vent could be blocked then this could lead to a requirement for a PSV. Now one other way could be a very short vent stack? Here the vented flow would not cool sufficiently to allow freezing?

--- Best regards, Morten Andersen

 

[ProSafPlant1]

georgeverghese, ideally the vent discharge is just water vapour, but typically can contain various components of the raw gas stream often both flammable and/or toxic, which is why they are now typically directed to containment / recovery.

MortenA, yes the continuous hot water vapour / steam flow was expected to keep the vent line free of ice, but condensate-soaked permeable insulation, build up of ice mounds to the vent line termination, and loss of heat tracing were contributing causes to vents plugging.

Some fixes were a glycol filled U-tube on the reboiler, or "thief hatch" type relief devices on either the reboiler or top of still column.

I've attached an old API P&ID which shows relief devices on the contactor and separator vessels but not on the reboiler / still column.

 

[georgeverghese]

@prosafplant, in the old days , glycol still vents went straight to atmospheric disposal. And the potential for blocked discharge on these still vents in modern installations where organics recovery schemes are implemented is the reason why PSVs' are installed nowadays.
The OP hasnt stated what this flash vent stream in his case is ??

 

[APIPapi]

It is steam from process venting off into a water “vessel.” It isn’t exactly a sparger, but the vented steam helps preheat the water in the atmospheric “vessel” somewhat before exiting to atmosphere.

 

[georgeverghese]

So what is the minimum or lower design temp of this water vessel ? Any chance this water may freeze due to controls fault or some Operations oversight ?

 

[MortenA]

@OP so, if you live in a very cold part of the world then this might be worth considering. I live in Denmark where frost is common but not very long periods of hard frost. We have _lots_ of atmospheric vessels with hot water/steam, one open vent and no PSV. On EU thats not against any code. But since you state that your vessel is a section VIII vessel. So couldn't you just de-rate it?

--- Best regards, Morten Andersen

 

[APIPapi]

It’s inside a heated building, and there is no U-1 or nameplate on the vessel, which makes it hard to size a PSV for it in the first place.