3 replies to this topic

[aju_1807]

We have a tank where the PVRV is sized for teh PCV failure flow of blanketing line. The tank is having degassing boot in upstream. The feed from teh re-run vessel is transfered to tank through degassing boot. If the control valve located at the bottom of the re-run vessel goes wide open then the maximum liquid will pass to tank through teh control valve. Now, is it required to size PVRV for the maximum liquid inflow (due to control valve failure)? Actually, the control valve is huge and the failure of which results in very huge outbreathing flwo rate. Is it required to size RV based on this flow rate or it should be ideally handle by emergency hatch. We can go for self closing type emergency hatch which can avoid teh continuosu release of gases to atmosphere.

Please provide your feedback on this issue.

Regards,

Aj

[Techman]

Dear Aj,
It is my normal practice not to include the blanket valve failure in the sizing of tank PVRV. We ensure that the tank ERV is big enough to handle blanket valve failure.

PVRV is meant for normal venting. Blanket valve failure is not a normal event.

I have discussed this issue with PVRV suppliers, they recommend that blanket valve failure should be included in PVRV sizing. I personally disagree witht this. This may result in much bigger PVRV and under normal condition, the PVRV will then operate in flutter zone.

I don't recall any standard that says that blanket valve failure should be included in PVRV sizing.

[aju_1807]

Have you heard or ever used self closing type emergeny vent? we are proposing to use it to avoid the continuous release to atmosphere.

Any feedbak on this?


Regards,

AJ

[Art Montemayor]

AJ:

You can (& are free to) relieve a storage tank by either a PVSV (Pressure-Vacuum Safety Valve) or an EV (Emergency Valve). I've used a PVSV for emergency venting in some cases where the gases were noxious and the flows small when I needed sure, positive reseating of the device. However, like Techman, I normally would assign an emergency relief to an EV. The important thing is to ensure relief of the tank from a potential over-pressure.

EVs are normally supplied as "free-falling" (which I would never employ), chained, or hinged. An EV, in its simplest form is nothing more than a big, blind flange seated on a mated roof nozzle flange. The hinged variety, in my opinion, is the safest and surest EV and it is the only "self closing type" I have used (there may be others). However, I have had to rework or totally re-design some. For example, I've introduced Teflon sleeve bearings for the hinges and a special, tongue and groove design using a Teflon seat gasket. The trouble with EVs - like anything else installed on a storage tank roof - is that it is exposed to 100% of the weather and climate. steel gets rusty, paint peels off, grease dissolves away, maintenance is nil, etc., etc. Consequently, you are wise if you employ robust hinges with friction-free, maintenance-free features and a design that ensures the action will be guided and reseat safely after activation. Teflon fits these needs - at least for 5 to 7 years, depending on the climate.

If you want good engineering and reliability, look to use the more recognized brands in the Industry - names like ProtectOSeal, Groth, and others. I hope this experience helps out.