8 replies to this topic

[MScudder]

Hello,

First, let me say I am new to the forum (and to the Chemical Engineering discipline) and I have been impressed with the knowledge that can be gleamed from this site. That being said, I am hoping to find some clarification to an issue I encountered.

I am working to purchase a portable (non DOT) 1250L nominal, 1430L total size vessel that will be used in various locations in my plant to blend chemicals (mostly flammable solvents) into and package from. During blending, the unit will operate at atmospheric conditions (it will be vented). During packaging, the unit will operate at roughly 12-13 psig (fluid will be transferred to smaller containers).

I would prefer that this vessel NOT be considered a 'pressure vessel' as I want to avoid the necessity of having an NB number, but I am a bit confused as to what defines a vessel as a 'pressure vessel'.

From direction internally, I had initially set my design pressure at 15psig and set my relief device (PRV) to relieve at 15psig as well with the goal of avoiding the 'pressure vessel' designation. When I did that though, my calculated relief valve size (fire case, two-phase flow, SuperChems modeling) was inflated to a point that it is not feasible.

Here are my questions:
- Does the 'design pressure' calculated MAWP, 'relief pressure', 'operating pressure', or some other variable (or combination of) determine if the vessel is considered a 'pressure vessel'?

- If I set my relief pressure to 15psig and use the MAWP (which is approx. 40psig higher than the design pressure) to determine the PRV size, will I run into an issue with having to categorize this vessel as a 'pressure vessel'.

Thanks in advance,
Mike

p.s. I am sure I left out some necessary details so, please, if there are any other details you need, I will do my best to supply.

[ARAZA]

Hi There,

The answer to your first question: If the design pressure of the vessel is more than 15 psig, than it has to be designed as per ASME section 8. div.1 period. No confusion. Design pressure is determined only by your maximum operating pressure. MAWP is calculated after you had designed your vessel, it is based on selection of the thickness of the material.

Question 2: You can set your relief pressure to 15 psig, no problem. But you CAN'T design your PRV on the basis of MAWP, if you do, you are violating the code API-520 / 521 which states that you have to calculate PRV orifice by set pressure. Set pressure = set pressure of the PRV + 14.7 + overpressure.

Sizing PRV higher than the set pressure leads to undersizing. In case of fire, vessel would not be relieved completely and there could be potential damage.

Hope this helps.

ARAZA

[fallah]

Question 2: You can set your relief pressure to 15 psig, no problem. But you CAN'T design your PRV on the basis of MAWP, if you do, you are violating the code API-520 / 521 which states that you have to calculate PRV orifice by set pressure. Set pressure = set pressure of the PRV + 14.7 + overpressure.

You can design your PSV on the basis of MAWP,if design pressure would be equal to MAWP.

Absolute relieving pressure (not set pressure) in psia=set pressure+14.7+overpressure (based on true MAWP)

[mishra.anand72@gmail.com]

Dear Cudder,

Hw r u??

In which country u r setting up ur unit?? What is the statutory guidelines in that country?? Hw do u move an exemption for not categorising ur vessels as pressure vessels?? Hw do u get CONSENT TO OPERATE for ur unit in that country??

Could u please think of using pneumatic operated pump to transfer fluid from vessel to containers. Is this something new?? Does it solve ur problem??

Could u please tell me more.

Bye

[Art Montemayor]

Mike:

You left out the size (capacity) of the vessel, the fluid involved, your calculations, the fill capacity, and the relief cases studied. ASME classified a pressure vessel as one that had a design pressure of 15 psig and 1 cu. ft. of capacity (I believe I remember this correctly) or more. This may have changed recently, but I think the 15 psig design limit is correct.

I request the size because I find it had to believe that you can’t find a PSV to fit on the vessel. You say that calculated relief valve size (fire case, two-phase flow, SuperChems modeling) was inflated to a point that it is not feasible”. I am not familiar with “SuperChems” and I don’t know how you set up your relief scenario, but you should be able to protect your vessel.

Why do you want to use the MAWP of 65 psig instead of the 15 psig? If your state is a “stamp state” (requires that the pressure vessel carry an ASME stamp) then, you must abide by the ASME rules. I believe NJ is a stamp state. If you design your vessel for 14.5 psig, I believe you can use it as a non-pressure vessel. You can size a relief device set at 14.5 psig and protect the vessel. You should check with your Insurance Underwriter to make sure that you are covered under these conditions. If you are designing for the Fire Case and the PSV is too large (which I find hard to believe – but then you haven’t given us all the basic data), then you can resort to a rupture disc or a rupture pin device to protect the tank.

If you set the relief pressure to 15 psig you have to determine the PRV size at that set pressure. If you set the relief pressure at 65 psig, you will have to categorize this vessel as a 'pressure vessel'. You cannot set the relief pressure at 15 psig and use 65 psig to determine the PSV sizing. That is not logical. You must design the PSV size for the fluid and vessel conditions prevalent at the time of relief.

[MScudder]

Art,

Thank you for the detailed reply. To answer your questions:

- Total capacity = 1430L, with a liquid charge of 1250L.
- The fluid chosen as a worst case was 100% n-heptane.
- As far as relief cases studied, failure of a Nitrogen pressure regulator (sudden pressurization of vessel with unregulated Nitrogen) and fire. The fire situation was determined to be worst case and what the PRV is being sized to.

I should explain my earlier statement regarding the PRV size not being feasible. The current design of the vessel has an 18" bolted manway cover on the vessel top. Fitted on the 18" manway cover are the following flanged connections: sight glass (3" flange), sparger (2.5" flange), vent (1.5" flange), chemical in (1.5" flange), and PRV (flange size TBD). With the current design, a flanged opening of larger than roughly 3" will require a redesign of the vessel, which I would like to avoid (budget reasons).

[MScudder]

This unit is being placed in service in the United States.

I am actually looking for some clarification to help determine IF I can categorize this as an atmospheric vessel and avoid the pressure vessel designation.

I do not know how to answer your third question, sorry.

This vessel is being used for extremely pure solvents (parts per billion, parts per trillion scale) and transfering via a pump would require a purchase of a Teflon unit, which is not in the budget for this project.

Thanks,
Mike

Dear Cudder,

Hw r u??

In which country u r setting up ur unit?? What is the statutory guidelines in that country?? Hw do u move an exemption for not categorising ur vessels as pressure vessels?? Hw do u get CONSENT TO OPERATE for ur unit in that country??

Could u please think of using pneumatic operated pump to transfer fluid from vessel to containers. Is this something new?? Does it solve ur problem??

Could u please tell me more.

Bye

[Art Montemayor]

Mike:

Thanks for the explanation. Now I believe I see the physical picture.

You plan to purchase what I estimate to be a horizontal, 39" ID x 60" cylindrical vessel with 2:1 elliptical heads. The tank is on wheels and will be towed where it is required. To transfer the n-heptane content, you plan to use nitrogen pressure at 13-14 psig and evacuate the n-heptane through a dip pipe. On top of this vessel you have an 18" diam. manway. (actually this is a misnomer, because a "normal" man can't safely fit in an 18" opening) The situation gets very crowded because the manway occupies approx. 50% of the diameter and 30% of the length. This leaves very little space to locate a PSV. You probably are purchasing this vessel as a used unit that has no calculations or National Board # (or ASME stamp).

Since the fluid is Heptane, I believe that the pool fire fire scenario is probably the worse case and should be the dominating one. I visualize the portable unit being enveloped in a pool fire that would quickly raise the internal heptane vapor pressure to the boiling point at the set PSV pressure. The result would be that even if the vessel were an ASME coded and registered pressure vessel, the applied PSV would release to the surrounding atmosphere and the discharged vapors would incinerate immediately, adding to the fire around it. This is not a pretty thing to imagine, but it credibly would be the factual happening. The area would have to be immediately evacuated and fighting this fire would be very, very hazardous. The best thing to do would probably be to allow it to burn itself out - through the discharged vapors emitting from the vessel. The point here that I make is that regardless of the vessel's PSV set point, the fire-fighting results would be the same. Therefore, I would opt for a non-pressurized vessel with a buckling pin PSV set at approximately 13 or 14 psig and installed on the cylindrical upper portion of the vessel - not on the manway. I realize that this is very close to the max. designation of pressure vessel (15 psig), but the accuracy of a buckling pin (as opposed to a rupture disc) allows me to do this. I also realize that this is making you decrease the max. working pressure of the nitrogen, but that is a trade-off you have to inherit due to the low, non-pressure rating of the vessel. It is either this way, or a pump - and I now know you want to avoid a pump and all its problems/costs.

I would pass the above plan by your insurance carrier to make sure they agree with the design, procedure, and operating plan. The fact that you are carting around a very hazardous hydrocarbon (probably all indoors) already makes for a concern and fear of a surrounding fire. The relatively small size of the vessel is in favor of reducing the overall resulting hazard and fire. But these are issues that I'm sure you are very well aware of and are taking into consideration in the event of such a scenario.

I hope these comments and opinion help.

[Qalander (Chem)]

Mike:

Thanks for the explanation. Now I believe I see the physical picture.

You plan to purchase what I estimate to be a horizontal, 39" ID x 60" cylindrical vessel with 2:1 elliptical heads. The tank is on wheels and will be towed where it is required. To transfer the n-heptane content, you plan to use nitrogen pressure at 13-14 psig and evacuate the n-heptane through a dip pipe. On top of this vessel you have an 18" diam. manway. (actually this is a misnomer, because a "normal" man can't safely fit in an 18" opening) The situation gets very crowded because the manway occupies approx. 50% of the diameter and 30% of the length. This leaves very little space to locate a PSV. You probably are purchasing this vessel as a used unit that has no calculations or National Board # (or ASME stamp).

Since the fluid is Heptane, I believe that the pool fire fire scenario is probably the worse case and should be the dominating one. I visualize the portable unit being enveloped in a pool fire that would quickly raise the internal heptane vapor pressure to the boiling point at the set PSV pressure. The result would be that even if the vessel were an ASME coded and registered pressure vessel, the applied PSV would release to the surrounding atmosphere and the discharged vapors would incinerate immediately, adding to the fire around it. This is not a pretty thing to imagine, but it credibly would be the factual happening. The area would have to be immediately evacuated and fighting this fire would be very, very hazardous. The best thing to do would probably be to allow it to burn itself out - through the discharged vapors emitting from the vessel. The point here that I make is that regardless of the vessel's PSV set point, the fire-fighting results would be the same. Therefore, I would opt for a non-pressurized vessel with a buckling pin PSV set at approximately 13 or 14 psig and installed on the cylindrical upper portion of the vessel - not on the manway. I realize that this is very close to the max. designation of pressure vessel (15 psig), but the accuracy of a buckling pin (as opposed to a rupture disc) allows me to do this. I also realize that this is making you decrease the max. working pressure of the nitrogen, but that is a trade-off you have to inherit due to the low, non-pressure rating of the vessel. It is either this way, or a pump - and I now know you want to avoid a pump and all its problems/costs.

I would pass the above plan by your insurance carrier to make sure they agree with the design, procedure, and operating plan. The fact that you are carting around a very hazardous hydrocarbon (probably all indoors) already makes for a concern and fear of a surrounding fire. The relatively small size of the vessel is in favor of reducing the overall resulting hazard and fire. But these are issues that I'm sure you are very well aware of and are taking into consideration in the event of such a scenario.

I hope these comments and opinion help.

Dear MScudder Hello/Good Morning,

All above clearly and explicitly explained by Art. should be sufficient to help you making a correct decision.

One minor query I have.
what is the real reason/ interest about not calling this vessel a pressure vessel?

Once the Pool fire scenario gets built-up under any circumstance and

vessel is almost full i.e.more than 85% liquid fill and

simultaneously uncontrolled Nitrogen inflow pressurizes it(Be it a Worst case )

What strategy you should envisage to take care of these conditions.

Hope this proves helpful.