What You Need To Know About Your Safety Relief Valve

Safety Relief Valves. A boring topic to be sure, but it’s one piece of equipment in your facility that should not be installed and forgotten. Why? Well I’ve always liked the phrase, “A picture is worth a thousand words.”

Glycerin Tank Explosion due to over pressurization

Tank explodes while being filled by Technician

Relief valves, the last line of defense against accidents such as these, are an important part of your facilities safety equipment. There are a number of things you should do to keep the valve ready to do its job should it become necessary.

First,  have the valve tested on a periodic basis.

The National Board of  Boiler and Pressure Vessel Inspectors is one of the governing bodies for the testing and/or repair of Safety Relief Valves. Within their Code, they recommend a written procedure be created  for testing these type valves. You, as the owner of the valve, can test the valve, but it must be done so in accordance with the National Board Inspection Code and your state’s and/or local regulations. We recommend that the test be performed by an authorized testing facility. A list of these facilities can be found here. You can also find other companies that perform this service by typing “relief valve repair” into your favorite internet search engine.

So how often should you have your valve be tested? Based on the National Board Code, which bases their inspection intervals on what type of service the valve is used for, the following intervals are suggested:

• Steam Service- Annually
• Air and Clean Dry Gas Service- Every 3 years
• Propane or Refrigerant- Every 5 years
• All others- Per inspection history
• For valves installed for liquid service, our experience suggests that these should be check annually.

Second, the valve’s outlet piping should be sized so that it is at least as large in diameter as the valve’s outlet. If not, the piping could form back pressure on the valve’s outlet thereby causing the valve not to  open properly. Also keep in mind that this piping should be oriented so that no liquid relieved through this piping can flow back and rest on the valve’s outlet port. Doing so, could allow corrosion to occur within the valve’s internal components and prevent the valve from opening.

Third, your maximum operating pressure, within the tank the valve is connected to, should be at least 10% below the valve’s set pressure.  This will prevent the valve from ‘creeping’ open. Relief valves are set to fully open at its ‘set’ pressure, but will begin to partially, or creep, open before then. This is normally 10% below its set pressure. If  your valve is allowed to do this, trash and/or corrosion can set in over time which then could prevent the valve from either closing completely or from fully opening, either of which is not a flavorable solution.

Pressure relief valves are an often neglected and overlooked safety feature in your facility. Take some time to inspect them. You never know when they may called on to save your life.

Is your product’s media fouling up your gauges?

We’ve all been there. Your pressure gauge that was operating just fine yesterday is now stuck at 30 psi. Thunk Thunk Thunk! (that’s you tapping the gauge with a rather large wrench just to make sure a good rapping won’t bring it back…) This will be the 3rd time this year you’ve had to replace the foul thing! Now what?

Glad you asked. First you need to make sure the wetted parts, that is the parts of the gauge that will come into contact with media, are compatible. Many of the gauge manufacturers have online charts that list what materials their gauge is constructed of along with the medias they are compatible with.  An example of this would be Ashcroft’s corrosion guide.

Sometimes though just having compatible materials is not adequate. Your only option then is to isolate the gauge from the media. There are many ways to accomplish this. The most simple way is to use a coil siphon tube.

Coiled Siphon Tube

This tube is installed between the gauge and the piping the media is located within. This type of isolation device is commonly used to isolate a pressure gauge from direct steam. The loop in the pipe is filled with water thereby isolating the gauge from direct contact with steam.

Another common option is the use of a diaphragm seal. This type of seal includes a flexible type membrane as part of its design. The membrane allows your media’s pressure effects through the membrane but not the media itself.

Example of Diaphragm Seal

Again, you will need to choose a membrane material that will be compatible with the media you are using in your manufacturing process.

Other types of isolation devices are the use of isolation rings, gauge guards, or isolation spools.