13 replies to this topic

[syn23]

Hi,

I am working on sizing psv for fire cases. After calculating the wetted surface area and came up with a relief load based on Q=21000*F*A^0.82
The relief load calculated is for vapor relief. Do I need to calculate the relief load assuming it full liquid relief. Assuming the same mass flow rate as the vapor relief, due to high liquid density the liquid relief rate is significantly higher than vapor relief.
I read API 521 section 5.15.3.3, where it states


5.15.3.3 Liquid
The hydraulic-expansion equations given in 5.14.3 may be used to calculate the initial liquid relieving rate in a liquid-filled system when the liquid is still below its boiling point. However, this rate is valid for a very limited time, after which vapour generation becomes the determining contributor in the sizing of the pressure-relief device. There is an interim time period between the liquid-expansion and the boiling-vapour relief during which it is necessary to relieve the mixtures of both phases simultaneously, either as flashing, bubble, slug, froth or mist flow until sufficient vapour space is available inside the vessel for phase separation. With the exception of foamy fluids, reactive systems and narrow-flow passages (such as vessel jackets), this mixed-phase condition is usually neglected during sizing and selecting of the pressure-relief device. The aforementioned exceptions are discussed further in 5.15.3.4. Experience as well as recent work in this area [53], [54], [55], [56] has shown that the time required to heat a typical system from the relief-device set pressure to the relieving conditions allows for the relief of any two-phase flow prior to reaching the relieving conditions. As such, full disengagement of the vapour is realized at the relieving conditions and the assumption of vapour-only venting is appropriate for relief device sizing. Experience has shown there is minimal impact on the discharge system for the two-phase transition period. However, the user may consider the impact of transient two-phase flow on the design of the downstream systems.
If a pressure-relief device is located below the liquid level of a vessel exposed to fire conditions, the pressure-
relief device should be able to pass a volume of fluid equivalent to the volume of vapour generated by the fire.
Determination of the appropriate state of the fluid can be complicated. A typical conservative assumption is to use
bubble point liquid.


Is it safe to size the psv for the vapor relief and not oversizing the psv. Since there are multiple psv for fire case in the same fire zone assuming it for liquid relief is increasing the header size more than 3 times and also of the Sump drum.

Thank you.

[latexman]

With the exception of foamy fluids, reactive systems and narrow-flow passages (such as vessel jackets), this mixed-phase condition is usually neglected during sizing and selecting of the pressure-relief device.

Got any of those 3 applications?

[syn23]

With the exception of foamy fluids, reactive systems and narrow-flow passages (such as vessel jackets), this mixed-phase condition is usually neglected during sizing and selecting of the pressure-relief device.

Got any of those 3 applications?

No, it is for amine service on the LNG plant (acid gas removal unit).

[latexman]

Then, "this mixed-phase condition" can be "neglected during sizing and selecting of the pressure-relief device."

Why is this not clear?

[syn23]

Then, "this mixed-phase condition" can be "neglected during sizing and selecting of the pressure-relief device."

Why is this not clear?

Because the group responsible for sizing the PSV have provided the "Liquid" as the governing case in the datasheet. Hence when I size the outlet line and the header, it results in increasing the pipe size almost three times. I have a hard time convincing the client and the group to re think over their basis.
One more thing I would like to ask you. For fire case does the psv protecting the equipment needs to be elevated above the fire zone (25 feet).
Thank you for your response.

[latexman]

Is the PRD located below the liquid level, or is it the typical top head mounted PRD?

The PRD does not have to be elevated above the fire zone.

[syn23]

Is the PRD located below the liquid level, or is it the typical top head mounted PRD?

The PRD does not have to be elevated above the fire zone.

Is it mounted above the top head of the vessel.

Thank you.

[latexman]

A discussion with the Engineer that prepared the governing case data is in order. You and they may learn something. Be sure and point out what was discussed above.

[pravin164]

Syn23,

I would like to add something on this topic.

First of all it shold be clear that whether your releiving system is liquid filled or some vepor space is there.If it is liquid filled system then it may realease liquid, two phase and finally vapour also and As explained in API that in most of the cases the vapour release is thgovering case for PSv sizing.Thats why API neglects the initial liquid expansion and two phase relief.

But If you refer DIER's HEM method for establishing the two phase relief load for liquid filled vessel exposed to external fire. The two phase relief load results very large flow rate compared t0 other two flow regime relief load i.e. liquid pahse and vapour phase.However the orifice area calculation must be performed for all the three regime relief load calculated i.e. liquid, two pahse, vapour phase, any one of the later two may result a goveringing case.

Thanks & Regards
Pravin

[fallah]

If it is liquid filled system then it may realease liquid, two phase and finally vapour also and As explained in API that in most of the cases the vapour release is thgovering case for PSv sizing.Thats why API neglects the initial liquid expansion and two phase relief.

pravin164,

Right; but it should be noted that vapour release itself will be happened in two different situations in fire case:

- Vapourizing liquid relief
- Vapour expansion relief

that as per API 521, between them the contribution of the first situation compared with the second one is normally governing.

Fallah

[latexman]

In addition, in most cases this transient fluid behavior (liquid to two phase to vapor) occurs initially between when the PSV starts "simmering" and then opens at the set pressure until the vessel pressure develops up to the first occurance of the 21% overpressure Code limit in the relieving event. If and when the vessel ever gets to the 21% overpressure Code limit, it will probably be flowing almost all vapor. Then, after that, the PSV will somewhat slowly cycle open, closed, open, closed as the vessel is repeatedly depressurized to the reseating pressure.

Of course, if there are reasons two phase flow will exist throughout the relieving event, it has to be sized for.

[fallah]

Then, after that, the PSV will somewhat slowly cycle open, closed, open, closed as the vessel is repeatedly depressurized to the reseating pressure.

latexman,

You may mean: It might happen because the rate of heat to be absorbed from the pool fire is going to decrease, while the wetted area is lessened due to the progressing liquid vapourization. Am i right?

Fallah

[latexman]

Fallah,

Yes, that is going on too. I think I was mentally stuck on the fact that PSVs are usually oversized, and that also leads to the typical open, closed, open, closed behavior. The effect you mentioned and the PSV being oversized are additive in this respect. Thank you.

[chemsac2]

Amine service is one of the common foamy systems encountered in process plants. I don't think acid gas absorber amine column PSV would see 2-phase as foam may not reach PSV nozzle. However, KODs, flash vessel PSVs may see 2-phase relief.

Check if any hydrocarbon can be present in amine stream.

Regards,

Sachin