Weighted Relief Valve 5

[bob3420]

I have a question regarding the calculation of a weighted relief valve. I am protecting an atmospheric 2500 gal. storage tank, but we have done a pressure test on the tank at 1psig. We plan to use the tank with a nitrogen blanket with a pressure of ~5in WC. I plan to have a rupture disc rated for 1psig and also a safety relief valve. For the relief valve I plan to set it at 0.5 psig. I haven't been able to find a typical spring loaded relief valve at this low of a pressure so I assumed I would have to use a weighted relief valve. I've taken the cross-sectional area of my outlet pipe and multiplied it by the set pressure(0.5psig) and get a certain answer in pounds. Is this the weight of my relief valve, or do I have to somehow convert this pounds-force into pounds-mass? I would appreciate any help.

 

[TangoCleveland]

A couple thoughts: we use "lift disc" relief devices to protect vacuum piping and vessels from positive internal pressure. Something that I noticed, which surprised me, is that the force from 1 psig pressure will lift a 150 class flange, regardless of flange diameter. It is troublesome calculating flow through such a disc, though. I'd suggest looking through API 2000, and contacting a company such as Protectoseal. They make low pressure tank vents with certified flow capacities that should serve you well. Look at

http://www.protectoseal.com/.

Larry

 

[Ashereng]

I think I also once saw a weighted manway cover that can double as a pressure relief since it is weighted, and engineered.

Unfortunately, I don't have manufacturer's name.

 

[Montemayor]

bob3420:

What Tango is recommending is the appropriate and safe thing to do. I have used Protectoseal relief (both pressure and vacuum) on API 650, API 620, atmospheric water tanks and just about every type of low pressure (<1psig)tank. They are recognized experts at this and I would let them furnish the correct relief device(s).

What you must furnish is the correct (worse case scenario) capacity for the relief device. Do not relegate the vacuum scenario to a secondary, unimportant role. Many times vacuum is the cause of tank failure or rupture - and not pressure. Also, be sure to consider the scenario where your N₂ supply valve fails open. This might be the controlling pressure relief case.

Weighed manway covers are what are called "emergency relief" devices. ProtectoSeal also furnishes this type of relief.

 

[EGT01]

Bob3240,

Follow TangoCleveland's advice and contact a supplier of weight loaded relief valves...Protectoseal, Groth, Varec, Shand & Jurs to name a few.

In general, you specify the set pressure for a weight loaded valve the same as you would for a spring loaded valve. If you need 0.5 psig then it's the same no matter which type valve you select. The manufacturer will determine the appropriate loadings.

As added comment, you don't indicate to what code the tank is built or the tank's MAWP, but you need to be careful about setting your rupture disc at the maximum allowed. If the tank is an API 650 design, you are not supposed to exceed the tank MAWP during relief, in other words, 0% accumulation is allowed.

Rupture discs have a manufacturing tolerance so you need to discuss that with the supplier and you may want to consider specifying zero manufacturing tolerance, otherwise you may need to select a burst pressure that is less than tank MAWP to account for the manufacturing tolerance.

I can't say I'm familiar with using rupture discs on atmospheric tanks so I don't know what code allowances are for that type of device, but rupture discs certified for ASME pressure vessels allow a +/- 5% rupture tolerance outside the manufacturing tolerance. Again, you may need to make sure your rupture disc pressure is set low enough to account for that.

In the end, unless you really need a rupture disc, you probably would do better by using a device such as what Ashereng is suggesting.

 

[proinwv2]

All of the comments about using a commercial, rated valve are correct. Yes and don't neglect vacuum. If you want to know what vacuum collapse looks like, step on an empty coca cola can.

Further, a simple F/A calculation would only get you close to the pressure as you have to deal with the true effective sealed area, the "simmer" pressure, lift, reseat pressue and capacity at that lift just to mention a few.

This is not an area to tread in when the safety of your personnel and property is the issue.

Best


Paul Ostand

http://www.ostand.com

 

[JimCasey]

Unlike ASME safety-relief devices, a weight-loaded conservation vent passes rated flow at 50% overpressure. THe literature in the catalog will help you engineer the application. Example: it is unadvisable to expose the top of an atmospheric flat-roof vessel to alternating stresses so a prudent set pressure is less than necessary to overcome the weight of the top. Thickness of the top of the tank is the governing variable here.

Last time I compared the engineering sections in the catalogs, Groth had the best-presented information. But it has been years and Groth has since been acquired by CDC. Things may have changed.

CDC makes an appropriate low-pressure rupture disc. It uses a TFE membrane and a fabricated support girdle. The support girdle can be tuned to the exact relieving pressure, and when it buckles the membrane collapses onto a set of knife-blades.

 

[TangoCleveland]

JimCasey,
I'm curious about your statement that "a weight-loaded conservation vent passes rated flow at 50% overpressure." Can you provide some background or a reference for that statement? We are trying to determine available flow through weighted disks on a vacuum vessel if the vessel is exposed to positive pressure and are struggling with force balance due to the airflow out of the opening against the disk. Any comments would be appreciated.

Larry

 

[JimCasey]

Woops, I stand corrected. Rated flow is at 100% overpressure.

Here's a link to a sheet for a Groth weight loaded relief valve:

http://www.grothcorp.com/GPGpdfs/1260A.PDF

The lower table gives you the fudge factors for rated flow at other than 100% OP. example: at 10% OP the factor is 0.42.

Since tne drop through these valve is not critical and only marginally compressible, they don't have "pop" action like a coded safety-relief valve. They just start to lift at the set pressure and continue lifting as the pressure increases until they either match the necessary relieving capacity or they run into a mechanical stop.

When I was a rep I sold both Protectoseal and Groth at various times. The big 4 (Groth, Protectoseal, S&J, Varec) all work on the same principle and the flow vs overpressure would be equivalent.

If you really,really can't stand overpressure, there is an air-loaded valve. Normal installation would require a pressure switch on the vessel. When it gets to setpoint, it opens its contacts, interrupting current to a solenoid valve and venting the air-loading holding the diaphragm-actuated relief valve closed.

 

[TangoCleveland]

JimCasey,

Thanks loads! That's the kind of information I was looking for. I've only had experience with Protectoseal, and their documentation isn't quite as thorough as Groth's. NASA Glenn Research Center has several large vacuum research vessels that will benefit from this information.

Larry