Refrigeration / Valves & Pumps

Ice Breaker: Relief Valves 101

Safety Device Helps Protect Equipment, Workers

November 4, 2013

A safety device used on many refrigeration systems is the relief valve. Relief valves are used to release abnormally high pressure inside a vessel before that

Joe Marchese
Joe Marchese

pressure causes the vessel to erupt. There are three general types of relief valves:

• One-Time Relief;

• Ruptured Disc; and

• Pressure Relief.

One-Time

A one-time relief valve, sometimes called a “fusible plug,” is normally constructed from a fitting with a drilled hole filled in with a low-temperature, solder. At a specific temperature the solder will soften and pressure within the vessel will cause the solder plug to “blow out.”

Ruptured Disc

The ruptured disc contains a thin disc of metal designed to rupture at a certain pressure. Neither the fusible plug nor the ruptured disc reseal after opening. Consequently, all the refrigerant is discharged from the storage vessel or protected portion of the system, should they open. These relief devices would then have to be replaced.

Pressure Relief

Pressure relief valves are spring-loaded valves normally encased in a brass body with a neoprene seal. They are designed to automatically reset once the pressure inside the vessel reaches a safe level. They are located on a section of the vessel where refrigerant vapor is located. This allows only the vapor to be released from the vessel rather than any liquid refrigerant. Some pressure relief valves will also have a thread connection on the top to allow piping to be attached to it so the released refrigerant can be vented out of the building or mechanical room where the vessel is located.

A popular type of pressure relief valve is the spring-loaded “pop” type. When pressure rises above the spring setting, refrigerant will initially begin to seep through. When enough flow develops, the piston will pop open, allowing full discharge. The pop-type relief valve has its advantages, including simple design, low initial cost, and high discharge capacity.

There are limits on the length of discharge pipe from a safety pressure relief valve. The limits are based on pipe size and relief valve discharge capacity in pounds of air per minute. Pressure relief valves are designed to re-close as the pressure is reduced. However, the valve may not completely reseal. Manufacturers generally recommend replacement after it has been opened. Failure to reseal tightly is generally due to an accumulation of dirt and foreign matter that attaches to the valve seat disc while the valve is discharging. For this reason, it is impossible to predict the reliability of the relief valve resealing after it has discharged in service.

Compliance

All relief valves must comply with the ASME code for unfired pressure vessels. Discharge rates are certified by the National Board of Boiler and Pressure Vessel Inspectors. A code symbol is stamped on relief valves indicating this certification. It includes the letters “UV” in a clover leaf design. The letters “NB” are stamped directly below this symbol. The pressure setting and capacity are also stamped on the valve. The exact number, location, and type of relief devices required are set forth in detail in the American Standard Safety Code for Mechanical Refrigeration. Local codes vary somewhat in this respect, and should also be considered in designing an installation.

Never eliminate or seal off a relief valve, as it serves a very important safety function in the systems we service, maintain, and install. Without these safety devices, serious harm could occur to those working around these systems. 

Publication date: 11/4/2013 

Want more HVAC industry news and information? Join The NEWS on Facebook, Twitter, and LinkedIn today!

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by Joe Marchese

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

2014 MCAA Annual Convention

Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

Podcasts

NEWSmakers: Julian Scadden

Training is an ongoing process. Julian will discuss how you can generate maximum return on time and energy invested training by following a three part process. Listen to this podcast to get expert tips on training, tracking and follow up. 

More Podcasts

THE MAGAZINE

ACHRNEWS

NEWS 04-14-14 cover

2014 April 14

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe

SERVICE CALLS POLL

Which statement on service calls best applies to your business?
View Results Poll Archive

HVACR INDUSTRY STORE

plumbing-hvac.gif
2014 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research

 

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

DON'T MISS A THING

Magazine image
 
Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

STAY CONNECTED

facebook icontwitter iconyoutube iconLinkedIn i con