PSV Sizing On N2 Receiver Vessel

[Pavan Kumar]

Hi All,

I have to size a PSV located on the N2 buffer tank. The N2 buffer tank receives N2 from Air Separation plant at 19 bar(g). The vessel operates at 19 bar(g) at 34 Deg C. The vessel is designed per PED 2014/68/EU with a Design pressure of 25 bar(g) at 34 Deg C. The maximum pressure that the vessel can see is 19 bar(g) and there is no applicable scenario. I want some guidance on this PSV should sized. External Fire case is not feasible as there is flammable/combustible in the vicinity. I have seen on some threads on this forum that such cases the PSV can be sized for External Fire Case(even if not feasible) to meet the code requirements. I am have worked with API 520/ 521 standards and can size a PSV for vapor filled vessels for external fire. I want to know the following:

1. How do I size PSV on a vapor/gas filled vessel when there is no credible/applicable scenario, but have to size and install a PSV oer Code requirements.
2. What particular section of PED should I refer to see if a PSV is mandatory on pressure vessels. ASME Sec VIII Div 1 mandates a PSV on pressure vessels.
3. What is the standard in European context that provides formulas for calculate the required relief rates like API 521.
4. Is ISO-4126 the standard that provides sizing formula for vapor sizing of PSVs in European context?.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1728505342/tips/Sketch_uw7gxs.pdf[/url]

Any guidance will be very helpful.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
Consider this resource (Leser) to support your work, Yes Iso 4126 is the reference.
Pierre

 

[shvet]

ASME Sec VIII Div 1 mandates a PSV on pressure vessels.

Incorrect. Such case is called "overpressure protection by system design", see ASME 13 para. 13.2 for instructions.
Why do you refer to the PSV codes? PED is the compliance code. What is the design code of vessel?

 

[pierreick]

Hi,
Probably good to read.
Pierre

 

[FMJalink]

Hi Pavan Kumar,
You are telling that the N2 buffer tank operates on 19 bar(g) @ 34 °C and that 19 bar(g) is also the maximum pressure the vessel can see. Usually one has a minimum and a maximum operational pressure for the N2 coming from an air separation plant.
Did you look at the maximum working (operational with one fault) pressure of the feeding system and the PSV (for full N2 capacity) setting of that system?
Success

 

[Pavan Kumar]

Hi shvet,

In correct. Such case is called "overpressure protection by system design", see ASME 13 para. 13.2 for instructions.
Why do you refer to the PSV codes? PED is the compliance code. What is the design code of vessel?

I agree but this requires a lot of documentation. I have had a chance to do this in the past. This is Greenfield project and I do not want to go this route. Can I use the Fire case scenario for vapor filled vessel to size the PSV to meet the code requirements as there is no other credible scenario?. The vessel is designed per EN 13445. This I guess is similar to ASME SEc VIII Div I?. And ISO-4126 is like API 520?. What document should I refer for equations for calculating required relief rate similar to the Required Relief Rate for Vapor Filled Vessel?.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi FMJalink ,

I checked the normal operating and max operating pressures are 18 and 19 barg respectively. The DP is 25 barg.

Pavan

 

[shvet]

Sure you can.
But I suppose that the margin between normal operating pressure and PSV set pressure is so high that N2 relief temp will be higher than metal creep temp. Therefore the firecase could be considered as a credible scenario but the overpressure scenario won't be able to be compensated by a relief device.
Quite common situation for "dry" vessels I should say.

The vessel is designed per EN 13445. This I guess is similar to ASME SEc VIII Div I?
Similar but not equal.

And ISO-4126 is like API 520?
Yes, but not equal. Note that some EU countries have propriate PV&PSD codes, e.g. see AD Merkblatt series. Note that API 521 is applicable in EU as ISO 23251.

Code compliance, aspecialy such complex and long-history as PV is, has no a simple answers. There are no shortcuts as every authority has own logic and hystory of the local industry regulation evolution. If you need some advice in PV&PSD codes applicability you obviously should not seek those at a "chem eng" forum as these issues are in mech engineers competence area.

Hope my idea is clear.

As a "shortcut" in such cases I sometimes practice installing a smallest PSV with no a credible scenario behind that. This can't be called "sound engineering".

Why are you avoiding filling some docs and drawings in? How many working hours would you save? 4? 16? IS it worth time spent on seeking for a shortcut?

 

[shvet]

Pavan, I aware you from oversimplifying the "dry" PVs.
PSV, let it be a common spring-loaded valve in your case, does not protect a PV from overpressure. This is a kind of a trap for the newcomers to the process engineering world.
PSVs (or PSDs as some forummembers insist on) protect PVs from a rupture caused by overstress, not overpressure. Overstress is caused by pressure and temperature combination and this point is critical for your case.

The weakest PV point max stress allowed and gaskets crit temp would be a most likely problem in your case governing a PSV design.

Note that this problem is valid for the "total liquid" PVs filled with liquids having low crit temp, e.g. light hydrocarbons, as those prone to become supercritical during uncontrolled heating like firecase.

 

[FMJalink]

Hi Pavan Kumar,
I assume there is a "no" missing in your original posting and you wanted to say ... no flammable/combustible in the vicinity. If there cannot be a fire, you do not have to design for it, unless requested to be included.

You appear to be in North America designing or making something for in the European area. Your role in the whole picture is not quite and therefore I will limit myself to general things.

The PED gives requirements to be complied with to allow pressure equipment to be put on the EC market and to be used. One of the things required is to put a PS and TS on it. That means the equipment/unit is allowed to be used up to that pressure and temperature combination. Exceeding that PS is where your safety valve has to safeguard your system against.

The other part is supply rate of Nitrogen, which determines capacity of the safety valve. That information will to come from the party involved with the air separation plant. That was why I asked what the full capacity safety valve setting is of the upstream system. If your PS is sufficiently above the maximally possible pressure, why put a safety valve on the pressure vessel?

It could be handy to downloaded the PED and read the applicable parts. It might help in the understanding the difference between PD and PS. It is not a large document.
Success

 

[FMJalink]

I have to correct myself on the statement: "If your PS is sufficiently above the maximally possible pressure, why put a safety valve on the pressure vessel?" That is not correct as the equipment/unit is allowed to be used up to PS.
There are two other documents to made or completed, that are the risk analysis ans the user instruction. They can contain specifics on the design parameters used.