Setting of pressure and vacuum valves - API 650 tank

[garfio]

I am looking for some guidance for the following situation:

I have a new tank farm with small API 650 tanks for gasoline and ethanol (diameters ranging from 12 to 60ft). I am planning on using pressure/vacuum valves to reduce losses (project is not in the US). Maximum vapor pressure will be 9 psia Reid and max temp will be 100 F.

I would like to know:
- How to determine an adequate set pressure for the pressure/vacuum valves, and what would be typical pressures for this situation.

- How these set pressures are related to the capacities given by manufacturers for their valves, and to the design pressure of the tank (I've seen some confusing info related to using 1/2 of the set pressure to determine capacity of the valves)

I'll appreciate any input.

 

[djack77494]

You will calculate a required relieving capacity based on the relief scenarios you develop. The PVSV must have a set pressure and capacity such that it can relieve the amount you just calculated without the pressure in the tank exceeding its pressure holding capability.

For doing PSV calculations for pressure vessels, this procedure is clear and straight forward. When you are working with "atmospheric tanks", the situation (to me) is much less clear. You need a design pressure for the tank, but you frequently do not have one. If you are buying a new tank, insist on the vendor supplying you with the maximum allowable working pressure for the tank. If you are working with an existing tank, that number is often not available. The only advice I have received in the past is that you must have the MAWP. I guess you need to pay an outside expert to come in, take measurements, then calculate an MAWP if you don't have the in-house ability to do this.

Next you must consider the PVSV characteristics. Unlike "pop action" PSV's, conservation vents START to open at their set pressure but do not reach full relieving capacity until a somewhat higher pressure. You must know or estimate that somewhat higher pressure so you'll have the set point low enough that the full open position is reached before the tank goes. As you can see, the situation is much "messier" than with pressure vessels. When I've previously whined about this, some of the experts like Phil Leckner have jumped in and said, "Sorry, that's just the way it is." So, I'm resigned to this unhappy state of affairs, but hope for the day when the codes will make it mandatory to at least list MAWP on the tank.

Good luck with all this. You'll need it.
Doug

 

[4Pipes]

I'd look at the design pressure of the tanks first. It isn't very much for API 650. Without the code in front of me, my recollection is that in round numbers you are looking at the pressure equivalent of the weight of the tank roof give or take. i.e. a head of water about 8 times the roof thickness. You should be looking for pressure vacuum valves and API 2000. There won't be any pop action other than the roof coming off the tank. Anderson Greenwood website would be a good place to start.

 

[garfio]

Thanks for the input. Actually I have the API 650 and know the requirements for tank design based on the pressure. The tanks will be designed to work at the required pressure.

My question is more related to the pressure that I should use for the P/Vs. The maximum vapor pressure is 9 psia, so theoretically, any pressure just above atmospheric pressure will be enough for my purpose. Then, should I use 1 oz/in2, 5, 10 as the set pressure? For what I read above, once selected the set pressure of the P/V valves, I need to get the maximum pressure from the manufacturer (based in my relief capacity)which will be more than the set pressure, and design the tanks with an adequate safety margin.

Are there any recommendations for this pressure? I remember seen some info from one oil company, but don't have that information now.

 

[djack77494]

You need to find a practical setpoint. Typical conservation valves (that can protect from pressure or vacuum excursions) might have a setpoint of (say) +6 & -3 in W.C. While you could theoretically reduce the setpoints to very close to atmospheric, doing so might result in a very large valve to achieve the required capacity. It could also result in frequent valve openings and closings, which is not desirable. Vendors' literature should be able to help; e.g. Protectoseal, etc. In general, the larger the tank, the smaller the MAWP and the closer the valve setpoints will be to atmospheric.

 

[123bjorn]

Hello !
If the installation is outside US, you may also have to consider the European PED directiv and the ATEX and CE
regulations for the vessels and the PVSV.

From Europe