The Steam Engine Safety Valve: New Valves Require Knowledge of Old Boilers

Enlarge Image Image Gallery The purpose of safety valves for hand-fired boilers have changed from 1909 to today. A safety valve from 1909 (left) was meant to give notice of the highest pressure permissible, according to the American Society of Mechancial Engineers, and to give alarm that more water or less fuel was needed. (See "The Purpose of a Safety Valve" at the end of this article to learn more. Figures courtesy of Bruce E. Babcock

At almost every show I attend, I ask a few engine owners and operators if they are satisfied with their new-style safety valves. It would be only a small exaggeration to say that I get just two responses. “I have been using a new-style valve for 15 years and I haven’t had any trouble with it” or “They are junk!” I have cleaned up the second response to spare the reader the unpleasant expletives.

As the result of these conversations, I have attempted to understand why there is such a discrepancy between the reactions to modern safety valves. It now appears that there are a few simple steps we can take when purchasing and installing these valves that might improve our satisfaction with the new-style valves, which are the only ones currently available.

The History of Safety Valves on Hand-fired Boilers

To understand the issues involved in the selection of a safety valve, it is necessary to review the history of safety valves used on hand-fired boilers. I am referring to hand-fired boilers rather than historical boilers because the issues are determined by how the boilers are fired, not how they are constructed or how old they are. The requirements for a modern welded boiler made to the American Society of Mechanical Engineers (ASME) code are the same as for a 100-year-old riveted boiler, if both boilers are hand-fired. The requirements for a safety valve for a boiler that is automatically fired are dramatically different.

With automatic firing, the safety valve’s function as defined by Anderson Greenwood Crosby, a manufacturer of modern safety valves, is to protect life and property if all other safety measures fail. A safety valve on a hand-fired boiler, as defined by ASME almost a hundred years ago, is to give notice of the highest pressure permissible and to give alarm that more water or less fuel is needed. (The evolution of the purpose of the safety valve is summarized in "The Purpose of a Safety Valve," at the end of this article.)

When hand-fired boilers, such as found on traction engines, steam cranes and locomotives disappeared, most of the boilers that remained were automatically fired. The safety valve manufacturers adapted their designs accordingly. The old-style valves with bottom guided, beveled seats were capable of withstanding vibration and operating near their setpoint, and were replaced by smaller top-guided valves with flat seats.

At the same time, steam system designs were adapted so there was no need to operate within 10 percent of the setpoint of the safety valve. Not all old-style valves had beveled seats, but the ones that didn’t were designed much differently from the modern flat-seated valves.

Evolution of the Safety Valve

When I speak of a modern-style safety valve, I am referring to a valve shown in the second diagram of this article's image gallery. An example of an old-style safety valve can also be seen in the image gallery. Changes in the design of safety valves had a dramatic effect on their capacity in pounds of steam per hour. If a boiler built in 1920 required a valve capable of releasing 1,000 pounds of steam per hour at a pressure of 100 pounds per square inch, it would have been equipped with a 2-inch safety valve. Today, 3/4-inch valves are available to release that much steam at that pressure.