Are all relief valves the same

The All-Important Relief Valve

Every boiler needs a relief valve. Unfortunately, our industry discovered this fact of life the hard way. There was once a time in the early days of heating when boilers exploded like clockwork. The reason was simple: Those early boilers had no way of relieving excessive pressure. The introduction of the modern, spring-loaded relief valve changed all that and saved many lives.

A brief job description…

The relief valve’s job is to protect the boiler against the dangers of thermal expansion. Should the pressure rise to the boiler’s maximum working pressure, which is established by the manufacturer and tested and confirmed by A.S.M.E. (the American Society of Mechanical Engineers), the relief valve will open and release the excess water.

A modern relief valve uses a spring-loaded diaphragm to hold the valve closed. Manufacturers set the spring to push down against its side of the diaphragm with a certain predetermined pressure. On residential systems, this pressure is usually 30 psi, the maximum working pressure of most household boilers. Tall buildings generally need boilers that can operate at higher pressures. We set those relief valves to open at settings higher than 30 psi.

Think Safety!

Never use a relief valve that has a pressure-relief rating higher than the maximum working pressure of the boiler. And under no circumstances should you ever plug a relief valve to stop a leak, even if it’s only for a few minutes. Nor should you pipe the valve’s discharge to the outdoors. If you do, the relief valve might discharge, and the water that doesn’t drain from the line could freeze. A block of ice in the relief line can be as dangerous as a pipe plug.

Some contractors think that as long as they pitch the relief valve’s discharge line down, it will drain fully, but this isn’t what actually happens. Picture this: the valve pops open and the discharge line fills with water. Some of that water drains, but some also stays in the pipe where it can freeze. The water stays in the pipe because air can’t get into the top of the pipe (the relief valve end) to break the partial vacuum formed by the falling water. It’s the same principle that keeps liquid in a straw when you hold your finger over the end and lift it from the glass. This is why you should never discharge a relief valve to the outdoors.

Always connect the relief valve directly into the boiler, and never put it up on a nipple. You want that valve to be as close to the boiler as possible, so it can sense what’s going on and react as quickly as possible.

You should also keep the relief valve’s boiler tapping full size, and of course, make sure there are no valves between the boiler and the relief valve that could be accidentally closed. This seems like a silly point to have to bring up, but it has happened.

The proper setting and capacity…

In addition to the pressure-relief setting, manufacturers rate relief valves to release the full Btuh load of the boiler. This is very important because when there’s an emergency, the boiler’s “exit door” has to be large enough to safely relieve everything that wants to rush out. If all that energy can’t get out of the boiler in an orderly way, pressure will build inside the boiler and lead to a dangerous situation, even though the relief valve is wide open! Make sure that this all-important rating meets or exceeds the boiler’s D.O.E. Heating Capacity rating.

You should always treat relief valves as though they were as fragile as raw eggs. Never drop them on the floor or bang them with a tool. And never try to “recycle” an old relief valve by installing it on a new boiler. The risk you take is just now worth the few dollars you might save.

When we designed our B&G A.S.M.E. relief valve, we gave it a diaphragm that has about five times the area of those you’ll find in a “pop-type” relief valve. This larger diaphragm area gives you a greater operating force, and that helps to overcome the effect of fouling on the valve’s seat. B&G ASME –rated valves also feature a unique fail-safe disc that allows the valve to work even if the diaphragm should rupture. They’re an excellent choice if you’re looking for a valve that can be opened for periodic testing, and still last for many years to come.

Size, Capacity and Relief Settings For B&G A.S.M.E. Safety Relief Valves

Relief
Setting 

PSIG

Model Number

Capacity in BTU Per Hour

Iron
Body

Bronze
Body

30 3301-30

3,300,000

4100-30

4,100,000

790-30

790,000

1170-30

1,170,000

36 3301-36

3,800,000

4100-36

4,600,000

790-36

900,000

1170-36

1,330,000

45 3301-45

4,500,000

4100-45

5,515,000

790-45

1,065,000

1170-45

1,575,000

50 3301-50

4,900,000

4100-50

5,990,000

790-50

1,160,000

1170-50

1,710,000

75  


NOT

AVAILABLE

 

790-75

1,615,000

1170-75

2,385,000

100 790-100

2,075,000

1170,100

3,060,000

125 790-125

2,535,000

1170-125

3,735,000