Relief Valve Sizing - Aqua Ammonia tank

[MJCronin]

To all,

My question deals with sizing of a relief valve for a 19% Aqua ammonia (also called ammonium hydroxide) tank.

The tank is ~55,000 gallons, 14 ft diameter and was purchased as an ASME Section VIII pressure vessel. MAWP is 30 psig. The tank is located in an area remote from any other plant components.

As I see it, the pressure relief valve for this storage device should be sized for 1) possible fire case - (API-520 methodology) or 2) maximum fill rate case -100 gpm.

My question is this:

If I size the PRV for the fire case, I would get a 6" x 8" valve with a large orifice ( this is due to the large tank area) or if I size the valve for the "fill rate" case, I get a 1"x 2" valve due to the low fill-up rate.....

Does this all sound reasonable ?

Am I missing any design considerations here ?

( see

http://www.larocheind.com/pdf/products/HorizTank.pdf

for a typical tank layout)

Help from anyone with experience is appreciated !!!

MJC

 

[CHD01]

I have not checked your calculations, but asssuming you are correct:

Yes, you can expect to get quite different relief valve sizing results for different over-pressure scenarios. You must select a relief valve that will be at least as large as the largest orifice area required. This would be the 6x8 inch valve. You could stop at this point and let the large valve also relieve the smaller over-pressure case. Or, if you feel the smaller relief case is pretty likely to occur and if your are concerned about the quantity of material that could be released through the large valve - then you could install a second smaller relief valve set a lower pressure than the large relief valve. Using two relief valves may also allow you to reduce the size of the largest valve since you are allowed to claim the capacity of BOTH relief valves. Be aware in doing something like this that you must satisfy code requirements for set pressure and pressure accumulation above set pressure.

Does this answer your question? The more you learn, the less you are certain of.

 

[TD2K]

MJCronin, you might want to look at an alternate technology to a relief valve for the fire case to avoid that large PSV. Rupture disk or better (IMO) is a rupture pin device would be something I'd consider.

Rupure disks are suspectible to fatigue failure if you operate them close to the set pressure, rupture pin (buckling pin is the correct name) does not have this limitation. Both of these will have a much larger capacity for a given size compared to a convetional relief valve.

 

[CHD01]

What TD2K says is correct. The negative is of course that rupture disks and rupture pins cannot reclose. For a 55M gal tank that could be a problem.

The more you learn, the less you are certain of.

 

[MJCronin]

To all,

Thank you all for your responses and comments....

The actual operating pressure of the tank is ~12 psig at a maximum of 100F. There is, therefore, reasonable margin between the actual maximum operating pressure and the MAWP of the vessel.

A rupture disc is a reasonable, inexpensive and prudent device to be installed in parallel with a smaller relief valve sized for the "fill" case. However, the "pipe away" from the disc to a safe location becomes an new design issue.....

We now believe that the "fire case" is not credible for this tank and the client will ensure this with a new location for an adjacent storage area. We simply will not require a large relief device for fire.

By the way, the PROTECTSEAL company (

http://www.protectoseal.com

)can supply a relief valve with the bucking pin technology

Thanks Again

MJC

 

[CHD01]

One comment regarding the rupture pin; this technology is at least 15 years old and it has still not replaced relief valves although vendors tout it as a marvelous new way of doing things. Why not? I'm still a little concerned about controlling and ensuring the right pin gets installed for the right relief device and the right vessel. I'm also still a little concerned about operations experimenting with the pin and artifically (or accidently) increasing the relieving pressure. So unless I have a need to operate VERY CLOSE to the MAWP I have chosen to use conventional relief valves. Also -if I use a rupture pin device, it would be with procedures modified to audit and control the installation, preventive maintenance and repair of these devices. Again, I think its a great addition, but like pilot operated valves it does not come without some trade-offs. The more you learn, the less you are certain of.