5 replies to this topic

[Epoche]

We have some PSVs that are being internally audited and I have a few questions about some items I found on their datasheets.

 

The vessels being protected by these PSVs are ASME stamped and need to follow ASME code. The only applicable relief scenario is a thermal case (hydraulic expansion of the fluid). As such, a small ASME stamped ¾” by 1” valve is installed at the top of the vessel.

 

On the datasheet, the relief scenario is stated to be thermal relief, but there is a section to state the allowable overpressure and it says 21%, which is the percent allowable overpressure for a fire scenario. On the datasheet, it appears the required capacity needed is around 4L/min of liquid. My confusion is coming from this being a thermal relief and the statement on the datasheet of 21% overpressure.

 

My questions:

1. For an ASME stamped valve being used for thermal relief, what is the allowable percent overpressure?
2. If this by chance was a non-code valve, I could then use 25%, right?

I appreciate any guidance that could be given.

[MTumack]

Epoche, 

 

I would suggest that the most critical case of "thermal relief" would be in the case of a fire, would it not? Its kind of like having a sore neck due to whiplash, or having a sore neck due to a car accident... one is a symptom and one is a cause. If it were not for the fire, how else would the fluid be super heated to a degree that causes an overpressure of the system?

 

I'd also likely be more worried about the potential generation of gases than the cubical expansion of the fluid, unless the fluid has a rather high boiling point.

[Art Montemayor]

There is a lot more confusion than what you state.

 

A thermal relief valve only relieves LIQUID.  It is not designed to relieve vapor or gas.  If you place the thermal relief device at the TOP of a tank, you must ensure that the tank will ALWAYS BE 100% LIQUID-FULL when it is loaded.  The thermal relief device is normally placed in a liquid flowing pipe for that reason.  This device is not designed to handle the Pool-Fire Case in a vessel because it will be subjected to increasing vapor generation within the vessel as the liquid level is being depleted.  This device only relieves unduly generated HYDRAULIC liquid pressure - usually from thermal increases external to the piping (or vessel).  These thermal increases are usually due to sun radiation or exposure to surrounding equipment - not to a pool-fire case.

[Epoche]

Epoche, 

 

I would suggest that the most critical case of "thermal relief" would be in the case of a fire, would it not?

 

That does make sense. Normally when I think of a fire case, I think of vapor generation. So, with my PSV in question, it is probable that a fire case was examined and it was determined that no vapor generation would occur and the only cause of concern would be hydraulic expansion. In this scenario, it does make sense that a 21% overpressure is being used.

 

For a bit more clarification, if the external source of heat was not a fire, but maybe something such as heat tracing, would this be a 10% or a 21% allowable overpressure scenario?

 

I believe I am about to answer my own question: I just re-read ASME Sec VIII, and it specifically states for the pressure not to rise above 21% of the MAWP when a pressure vessel “can be exposed to a fire or other unexpected sources of external heat”. And a hydraulic expansion scenario happens with an external source of heat; meaning 21% overpressure?

 

I appreciated your help, I believe I just needed a quick refresher on the definition of 21% overpressure.

 

 

There is a lot more confusion than what you state.

 

A thermal relief valve only relieves LIQUID.  It is not designed to relieve vapor or gas.  If you place the thermal relief device at the TOP of a tank, you must ensure that the tank will ALWAYS BE 100% LIQUID-FULL when it is loaded.  The thermal relief device is normally placed in a liquid flowing pipe for that reason.  This device is not designed to handle the Pool-Fire Case in a vessel because it will be subjected to increasing vapor generation within the vessel as the liquid level is being depleted.  This device only relieves unduly generated HYDRAULIC liquid pressure - usually from thermal increases external to the piping (or vessel).  These thermal increases are usually due to sun radiation or exposure to surrounding equipment - not to a pool-fire case.

 

As I mentioned above, I typically think of a fire causing some sort of vapor generation, so when I saw the 21% allowable overpressure, I instantly assumed fire and was wondering why this valve would be a thermal one. With the feedback I've received and the extra reading I did, I believe it was wrong for me to assume this was to involved fire, when it could possibly be any type of external heat (21% allowable overpressure doesn't necessarily = fire).

 

To comment on some of your concerns that you mentioned, the vessels with these PSVs will be 100% liquid filled. Because the actual source of external heat is not made known on these datasheets, I will look into what would happen if a fire were to happen and the time-frame needed for it to happen.

 

Thank you for your help!

 

[MTumack]

There is a lot more confusion than what you state.

 

A thermal relief valve only relieves LIQUID.  It is not designed to relieve vapor or gas.  If you place the thermal relief device at the TOP of a tank, you must ensure that the tank will ALWAYS BE 100% LIQUID-FULL when it is loaded.  The thermal relief device is normally placed in a liquid flowing pipe for that reason.  This device is not designed to handle the Pool-Fire Case in a vessel because it will be subjected to increasing vapor generation within the vessel as the liquid level is being depleted.  This device only relieves unduly generated HYDRAULIC liquid pressure - usually from thermal increases external to the piping (or vessel).  These thermal increases are usually due to sun radiation or exposure to surrounding equipment - not to a pool-fire case.

 

Ah,

 

I see where my confusion lies. All of my experience comes from Plants in Northern Alberta, all major equipment is provided with its own environmentally controlled building in order to avoid obvious environmental hazards, thus there is no potential for heat to radiate from the sun or other major equipment.

 

Obviously, this is not the case with every plant.

 

My apologies.

[fallah]

Epoche,

 

The ASME vessels you described are certainly equipped with heat tracing or being steam/hot oil jacketed. As ASME specified 21% over pressure is to be applied for any source of external heat regardless of it to be a pool fire or steam/hot oil jacket. Then for a liquid filled vessel if is subject to an external heat source a TSV of 3/4"*1", as specified in relevant data sheet is adequate and correct.