Rupture Disc to alleviate ignition???

[tbarkerjr]

I have a tank with deicing fluid and methane gas which collects at the top of the tank. I want to be able to alleviate any the pressure created from an incidental (static friction, etc.) ignition of the methane gas. Would a rupture disk be the right direction? Would you base the analysis around the tank MAWP or the pressure created from the ignition of the gas? Any help would be greatly appreciated.

 

[BigInch]

If it ignites, your a little late to be thinking about relieving the pressure, unless you're designing an internal combustion engine.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MortenA]

Is this for sewerage treatment? If theres no other gasses than methane then its acutually a bit difficult getting methane to burn (sounds vierd but its true). LOL/UEL is 5%/15% in air at atm. conditions and "room temperature" (CRC 67th ed). Unless you have the right mixture it wont ignite!

In denamrk we now make "city gas" by adding air to methane (in order to reduce caleorific value because all old city gas apperatus is design for a gas with a fairly low caleorofic value).

Best regards

Morten

 

[BigInch]

Morten, its a de-icing system.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[skearse]

I would agree with BigA, RD would only allevitae the pressure AFTER a problem. But, to answer your other question, you'd have to base it on the tanks MAWP and then size it based on the allowable overpressure (check API 520 and ASME Codes [if applicable]).

To help reduce the risk of an occurence in the first place, I would consider adding an inert atmosphere, such as nitrogen, in the tank vapor space.

 

[JimCasey]

There ARE "Blast Panels" available from the same manufacturers that make rupture discs, but as BigInch pointed out, you're better off not to have an ignition anyway.
DeIcing fluid is probably flammable (EG or Methanol both are) so if you get a detonation in the tank you are likely to spread burning liquids all over the place and be interviewed in an unfavorable light on the evening news.

Nitrogen or CO2 blanketing system in the vessel will prevent the atmosphere in the tank from ever being able to burn.

Consult with Groth, Protectoseal, or Varec about flame arrestors, condervation vents, and blanketing systems. Groth has a really good blanketing regulator as I remember.

 

[gerhardl]

This is at least a triple problem:

First: To prevent ignition. As adviced by several above this is here the best, and in practise only solution.

( Explosion panels are available to relieve ignition and following explosions, (pressure wave exceeding speed of sound) but only to be used where the explosion can safely be relieved to surroundings. Typical to be used where there can be an explosive mixture (air and gas or dust). Areas requiered tend to be very large for thin-walled vessels. - In my mind not to be recommendable solution here for first line protection.

Second: Relief of other 'normal' overpressure. Use normal control valves. Redundance solution if necessary.

Third: Prevention of explosion for relieved gas.

A fourth would be the building considerations in 'worst case': If there after all above actually should be a possible explosion caused by fire or unforeseen causes. For this case walls - placement - construction details lay-out, flame arrestors, emergency shut down valves, explosion panels, fast fire extinguishing systems etc. might be considererd.

Another specialist company for equipment here is Fike.

 

[JohnGP]

tbarkerjr,

I have seen rupture discs used successfully on the header for a catalyst bank - normally the air/metanol vapour mix was outside the explosive range, but when a plant upset caused the mix to be air rich it naturally exploded. The process was under vacuum, so I think the rupture discs were designed for about 10 or 15psig. But that wasn't for a tank.

Certainly the tank MAWP would need to be considered when sizing any venting arrangement, but I wouldn't be using rupture discs if this is a common occurrence.

Why not use flame arrestors? We used these to allow methanol storage tanks to breath without incident. Something like that shown in the link, combined with a pressure/vacuum relief valve -

http://www.varec-biogas.com/productSpecification.asp?pid=29

Regards,
John

Regards,
John

 

[ARenko]

I'm pretty late coming into this thread, but...

What you need is an explosion vent as others have mentioned. It should be sized according to NFPA 68. Of course the best option is to prevent the explosion in the first place - see NFPA 69.

I have a similar application - a tank with air and methane that will experience mixtures in the flammable range during the course of operation. While my preference is to use N2 instead of air in the process the industry won't accept the added cost and are willing to accept the risks (for now at least).

We intend to design our tank to withstand the deflagration (design pressure per calculation in NFPA 69) and use PSV or rupture disc for normal overpressure protection. The design pressure is based on the relief setting plus accumulation and a pressure ratio given by NFPA according to the situation.

 

[JohnGP]

Well, if we're going to reactivate the thread, I would like to add that the methanol tanks that I mentioned above have been in service now for just over 30 years, without incident, so the Varec flame arrestors and pressure/vacuum vents have seemingly done the job quite well for tanks breathing an air/methanol mix.

 

[rneill]

In the Oil and Gas Industry we routinely deal with the potential for a flammable mixture in the vapor space of storage tanks and there are two ways we deal with this. If it is permissible to vent these vapors to atmosphere then we install pressure vacuum vents in conjunction with emergency venting equipment in accordance with API 2000 "Venting Atmospheric and Low Pressure Storage Tanks". If unacceptable to release these vapors then we either install a vapor recovery system or take them to a flare system for disposal in which case we would install a gas blanketing system on the tank (using fuel gas) so on inbreathing the tank pulled in gas instead of air; this maintains the vapor space as too rich to ignite.

According to API 2000: "a flame arrestor is not considered necessary for use in conjunction with a PV valve venting to atmosphere because flame speeds are less than vapor velocities across the seats of PV valves (see API Publication 2210)."

I'm not sure how applicable this would be to methanol storage (e.g., flame speeds of methanol vapor) but regardless, the flame arrestor would provide for an extra layer of protection.

PV valves and similar venting equipment is supplied by Groth, Protectoseal, and Varec (and others), and I'm sure that any of these folks would be able to provide technical advise in this area.

http://www.protectoseal.com/vaporFlame/vfTankSafety.cfm

http://www.grothcorp.com/index.php

http://www.whessoevarec.com/