Safety relief valves; every steam boiler has one. It’s that valve attached to the steam space of the boiler that most people assume will never be needed, as accidents don’t happen with their boiler. Well thankfully for laws and codes, a safety valve is a precision valve that is meant to be the last line of defense before the unthinkable happens in a boiler. In this post, we’ll cover the general workings of a safety valve to help someone better understand how one should operate.
Every safety valve has a data plate that is attached to the valve. The data plate is like the VIN on a car— if it’s missing, both the safety valve and car are just scrap metal. This data tag contains important information about the safety valve to ensure that it’s the right valve for the application. The two main pieces of information to highlight on the valve are the set pressure and the LBS/HR relieving capacity.
Set Pressure and LBS/HR
The set pressure of the safety valve is the pressure at which the valve will open. This pressure setting has to be the same as or less than the max allowable working pressure (MAWP) stamped on the pressure vessel. The second number is the relieving capacity in LBS/HR. When the safety valve opens, it has to discharge a certain amount of steam to keep the pressure vessel from rising in pressure even with the valve open. Small and large boilers alike can have safety valves with setpoints of 150 PSI, but the LBS/HR capacity of a small boiler will be less than that of a large boiler.
So How Does It Work?
A safety valve has a circular disk that is pushed against a seat by a spring that is carefully set at a specific pressure. The safety valve will open when the pressure below the disk is greater than the spring pressure pushing down on the disk. One the disk starts to lift, the steam is then exposed to the huddling chamber (a chamber around the disk that allows the steam to increase its pressure on the disk by pushing against more surface area). The valve will now open all the way, relieving the steam.
So, if a safety valve is set to open at 150 PSI and someone tests the valve by putting 150 PSI of pressure to the inlet of the valve, why doesn’t the valve immediately close when the pressure falls below 150 PSI? As mentioned earlier, when the disk is raised, the steam goes into the huddling chamber which exposes more of the disk area and increases the force on the disk. This increase allows the valve to be held open with less force and once the difference in force is reduced, the valve will seat. This is how the differential is created, and it’s why a valve is not constantly snapping open and closed when the pressure is at its setpoint.
So How Do I Make Sure It’s Working?
When you test a safety valve, you will want to make sure that the pressure inside the boiler is within 20% of the setpoint of the valve. If you lift the valve when the pressure is not close to setpoint, it puts stress on the spring that can result in incorrect spring pressure in the valve. After lifting the valve handle to test, make sure the valve seats tight and has no leaks. Leaks can cause corrosion buildup that will affect the performance of the valve.
It’s important to regularly test the safety valve(s) on a boiler, log when they are tested, and change them out per good practice. Some boiler inspectors like to see safety valves changed out every year—this will require having a spare valve onsite. The valve removed from the boiler can be sent out for repair and calibration and act as a spare safety valve until it’s time to replace the one on the boiler.
Maintaining safety valves should always be a part of regular maintenance for the boiler. Visually checking the valve(s) daily for leaks and/or anything not normal is a good place to start. Lifting the valve should be scheduled and recorded if your facility is required or chooses to do it. On an annual basis the valves should be inspected and changed out with a new one and the old one rebuilt and put on the shelf for the next year. Safety is important in every facility and a regular safety valve inspection program will ensure the boiler’s last line of defense for over-pressurization is correctly maintained.