5 replies to this topic

[benoyjohn]

What is the design pressure for cryogenic propylene storage tanks. (-47 deg C)
What is the relief valve set pressure?
regards
Benoy

[Art Montemayor]

Benoy:

You ask for the design pressure of a propylene storage tank, but you don't state what temperature you want to maintain in the tank - presumedly, you would store the propylene as a saturated liquid (as opposed to a vapor or a subcooled liquid). The answer is set by the temperature you select. You cite -47 oC, but you fail to state that it represents your selected temperature; I assume that is what you mean, so the answer is the corresponding vapor pressure for pure propylene at -47 oC. You should be able to easily look the vapor pressure up, but if you are too lazy to do so, I'll furnish the answer as: approx. 14.5 psia (or atmospheric at sea level).

Most people I know store liquid propylene at ambient temperature in pressure vessels designed for approximately 200 psig or better. You have your choice - or your scope of work, which we don't know.

The corresponding pressure relief valve on the storage tank should normally coincide with the Maximum Allowable Working Pressure (MAWP) for the vessel - which we also don't know. The MAWP is not a design output, it is the result of vessel fabrication and is normally higher than the so-called "design" pressure. Although I can't furnish you specifics, I think I answered your questions based on the basic data you furnished.

Art Montemayor
Spring, TX

[benoyjohn]

Art:
Thank you. The storage temperature is -47 deg C. The operating temperature shall be close to atmospheric as you have mentioned. I presume such refrigerated tanks are designed per API 620 Appendix Q. The maximum design pressure specified for API 620 tank is 15 psig. I wanted to know what is the design pressure normally considered for propylene storage. Is it 15 psig or a lower value?

regards
Benoy

[Art Montemayor]

Benoy:

I'll repeat myself because I don't think you understood what I previously wrote: the "normal" (or maybe "most-favored") method of storing liquid Propylene is at ambient temperature - with a pressure vessel built to an approximate MAWP (Maximum Allowable Working Pressure) of 200 psig. These vessels are readily available and cheaper to build, operate, and maintain than the cryogenic /atmospheric tank you are describing. You fail to state your case and identify what your Scope of Work is, so I can't comment on what you can or should do about the type of tank or relief required. If it were my plant, I would use a standard, 200 psig "bullet" and store at ambient temperature. Cryogenic tanks are very expensive and a pain to operate and maintain. You employ this type of vessel when you have a process requirement or you have no alternative choice. You haven't stated what your position is or what your SOW calls for, so again I can't comment.

I hate to keep repeating myself, but either you give specific and total basic information or our comments are going to be bouncing all over the place with generalities or best guesses and assumptions. If you are going to use an API 620 vessel (or any vessel for that matter) you go by the manufacturer's MAWP and design to protect that design point with pressure relief. Note that I say MAWP - not design pressure. The design pressure is something totally different; it's not what is fabricated. The ultimate vessel product has a MAWP and that is what has to be known. Normally, the design pressure is below the MAWP and this is helpful in not having people that don't distinguish between both install the wrong set pressure on a tank. You can use the design pressure as the set pressure, but why do that and waste all the money and extra pressure that you have available in the MAWP? Perhaps we're not understanding each other. Let me know if I'm writing over your head and I'll slow down a bit.

[benoyjohn]

Art:
Thank you. I have understood your reply. You mean to say that it is more economical to store Propylene in spheres at ambient temperature. Is this statement true for all capacities? Will it make a difference if I have to store about 20,000 m3 of material?

regards
Benoy

[Art Montemayor]

Benoy:

No, I didn't mention a spherical storage tank. I mentioned a "bullet" which is a conventional, cylindrical, horizontal tank with ellipsoidal heads and normally used for LPG (Butane, Propane) service. Since I don't know the quantity of storage, I couldn't comment. That's what I meant by scarce basic data. For example, you mentioning 20,000 m3 of propylene means nothing if you don't reference the state and/or the materials density. I don't know if that figure refers to saturated Propylene at atmospheric pressure or at 170 psig; therefore, I'm unable to estimate the size of storage.

A sphere is normally justified only for rather large vessels - how large depends on how much you want to spend, your local fabrication and erection capabilities and a few things more. The basic reason ambient temperature storage is cheaper and more popular is that you don't have to pay for the cryogenic steels, the inherent insulation of such a large vessel, and the maintenance of such a cryogenic, insulated vessel. Also, don't forget that you may not want to "put all your eggs in one basket" - you may have need for storage flexibility and therefore may opt for having multiple, smaller tanks in order to minimize handling disruptions, shutdowns, and needs for inspections and maintenance. But we don't know your Scope of Work, so I can't tell if those are your needs.

The main point that I want to drive across is that ambient temperature storage relies on conventional, mild steel construction (516 gr70) and no insulation of special instruments and valves. Crygenic tanks are special and require special steels for the low temperature and special instrumentation. Therefore, it is easier to maintain an ambient temperature tank - especially if you are in a remote site with poor infrastructure or specialized technicians. Again, since I don't know where your site is, I can't comment - but I bring up the point just in case it's important.