7 replies to this topic

[Art Montemayor]

First of all, your terminology is used wrong; a thief hatch is not a relief valve. A thief hatch is nothing more than a nozzle with a blind flange (a "hatch") located on top of a hydrocarbon liquid tank. It is a left-over term from the days of yesteryear when condensate storage tanks were the targets of thiefs who would syphon out the fuel for use on their tractors, cars, or other vehicles. The hatch was used to measure the contents of the tank. This flanged nozzle could also be used today for an emergency relief valve or vacuum valve location.

You need to be specific. Just exactly what relief (both pressure and vacuum) do you have on your tank? If it is not blanketed, then what is the normal vapor space pressure in it? What do you do to/with this tank? Is it a field operation that uses routine tanker truck collection once or twice a week? We need more information in order to give some comments that make sense and don't waste our time as well as yours.

[djack77494]

Renoyd,
You have not heeded Mr. Montemayor's plea to use correct terminology. As he stated, a thief hatch is not a relief device and does NOT have a set pressure (positive or negative). A conservation vent (a.k.a. a PVSV) is commonly used on API650 atmospheric storage tanks; some type of venting capacity is a requirement of the codes.

I am further confused by your refering to the tank as if it is existing, but then saying it "will be a newly added tank". Which is it - new or existing? Also, what do you mean by a mushroom vent? Is this an open vent pipe that ends with a mushroom shaped cover, perhaps used to keep rain and dust from falling into the tank? If so, that is the provision that the original tank designer made for the tank's venting needs. Reiterating what Art has said, please provide complete information for a complete answer.

[Art Montemayor]

Renoyd:

Doug is right. What I said was "This flanged nozzle could also be used today for an emergency relief valve or vacuum valve location." I never said the hatch could be used for emergency relief purposes. There are many companies that sell PVSV for installation on existing "thief hatches". This nomenclature is merely traditional and means nothing as far as thieves are concerned. What I'm trying to establish is that we are talking about the same thing. We waste a lot of effort in just doing that - and don't get to the problem.

What you seem to have is an atmospheric pressure 12F API condensate tank for field installation. How you have a PVSV installed on it at the same time as you have an open, screened vent to the atmosphere is beyond me. You either have an atmospheric-vented tank or you have a closed, PVSV-protected tank. You can't have both -- one defeats the other. This is exactly the kind of erroneous information that we try to eliminate in order to generate sensible, logical, safe, and correct responses. In trying to guess what it is that you need, what I can say is:

If you want a closed tank, seal off the screened, open vent.

Ensure that the proposed PSVS (which you erroneously continue to call a thief hatch) is of the correct capacity to take care of your safe venting needs to atmosphere in BOTH pressure and vacuum modes. Do not forget that you also need emergency venting in the event of a fire or a "blow-through" case. I doubt the PVSV will be big enough to cover you in all these cases. I hope this helps.