Tank venting and thief hatch

[Shaheryar]

Looking for some comments/criticism:
I am working on sizing a vent for a large boiler feed water tank and am following API 2000.

The tank is normally vented so has vent on the top. The issue I am trying to resolve is; what is the sizing basis for the vent pipe i.e. to calculate the diameter?

My understanding was that I would assume a certain length of the vent pipe and select a diameter that does not result in a back pressure greater than the design pressure of the tank. A further refinement to this would be to limit the back pressure to 75% of the design pressure.

Based above logic the vent can be sized for normal flow. However, for fire case relief load, the vent size will be too large (>24"). In which case I could use a thief hatch, but have no idea how to size or select them. Does anyone know?

 

[Montemayor]

Shaheryar

What you are referring to as a “vent” is probably a conservation vent – and probably one that has dual pressure and vacuum relief. These type of conservation vents (CVs) are commonly supplied by such suppliers as ProtectoSeal and Groth – among others.

If all you are dealing with is ordinary boiler feed water, why are you proposing “a certain length of the vent pipe”? These CVs come in two main varieties: a local, open discharge and a “pipe-away” model. For common, pure feed water I can’t see a reason for a “pipe-away” model.

Based on engineering logic, the CV should never be sized on “normal” flow. The CV is there to protect the tank and it should be sized to relieve the worse, credible relief case. We can’t know your credible cases; you have to identify those. What is done by all experienced engineers to furnish relief for the fire case is to employ an emergency vent on top of the tank – this is usually nothing more than a large blind flange that is hinged or chained to allow relief but yet remain on the roof of the tank. The ProtectoSeal series No. 53300 is just such a device. I prefer the hinged variety.

You are probably a young engineer that has never been exposed to field jargon involving storage tanks, so I’ll explain what the term “thief hatch” means – at least in the “oil patch”, where it was first employed. This is usually an 8” or 10” nozzle on top of an API petroleum storage tank that used to be employed for measuring the height of the liquid inventory with a gauge rod. In the “old days”, it was common for a pure gas well condensate tank to be found isolated out in the oil patch and “thieves” would help themselves to the high octane condensate by use of this hatch and a long hose in order to steal the inventory. This nozzle was modified to have a locked cover or “hatch” – hence the name. Some lazy engineers still specify this nozzle by simply ordering what they believe is a “standard” API storage tank. There is no “standard” API tank, but some people believe this and order this nozzle because they don’t know any better about how to gauge a tank’s contents accurately. I doubt if you have a Thief Hatch, but if you do, it’s useless in a feedwater application. However, you might use it to install a good radar level detector.

If you know (& you should) your worse (highest) venting capacity, then ProtectoSeal or Groth can help you select the appropriate emergency vent size for your tank. They’ll even give you a sizing program, but why resort to sizing it yourself when they’ll do it over the phone and furnish you with a quote.

 

[djack77494]

I think an old fashioned "gooseneck" would suffice. Size the line so that the hydraulic losses from the tank's vapor space to the atmosphere are well under the design pressure of the tank. Of course, this is a very small value, but your line losses will be even lower. I see no reason for the additional cost, complexity, and pressure drop that a conservation valve will entail UNLESS it is imperative to keep atmospheric air from the tank contents. Venting volume would be the air volume displaced by the maximum inflow of liquid; incoming air to prevent vacuum conditions would be similarly calculated. In both cases, add normal thermal effects. (Tank hot after baking all day in the hot sun when along comes a sudden cool deluge, for example.)

If you are installing a manway on the roof, you could be ultra-conservative and have a blowoff cover (i.e. an emergency vent). If you cannot engulf the tank in a flammable substance, then I don't think you have a credible fire scenario.