Purchasing new-style safety valves requires some knowledge of your old boiler.
Figure 1: The purpose of safety valves, in 1909 and today. 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.
All figures courtesy of Bruce E. Babcock
Figure 3: An example of an old-style safety valve.
Figure 2: A modern-style safety valve.
Figure 4: Examples of what not to do when installing a safety valve.
Figure 5: A possible safety valve arrangement; the new valve satisfies the code requirements while the old-style valve performs the function for which it was designed.
Figure 6: The results of the tests performed by the National Board on new safety valves of both setpoint and blowdown. A sample of just five valves shows that it seems reasonable to suspect that even greater variations may exist.