page_title: Safety valve
section_title: "Pop" valves
hierarchical_section_title: Safety valve / Development of the safety valve / "Pop" valves
caption_attribution_description: The safety valve of the steam locomotive 60163 Tornado
context_page_description: A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve, which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve. A leak tight, lower cost, single emergency use option would be a rupture disk.
  Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion unless carefully operated.
  Vacuum safety valves are used to prevent a tank from collapsing while it is being emptied, or when cold rinse water is used after hot CIP or SIP procedures.
  When sizing a vacuum safety valve, the calculation method is not defined in any norm, particularly in the hot CIP / cold water scenario, but some manufacturers have developed sizing simulations.
context_section_description: All of the preceding safety valve designs opened gradually and had a tendency to leak a "feather" of steam as they approached "blowing-off", even though this was below the pressure. When they opened they also did so partially at first and didn't vent steam quickly until the boiler was well over pressure.
  The quick-opening "pop" valve was a solution to this. Their construction was simple: the existing circular plug valve was changed to an inverted "top hat" shape, with an enlarged upper diameter. They fitted into a stepped seat of two matching diameters. When closed, the steam pressure acted only on the crown of the top hat, and was balanced by the spring force. Once the valve opened a little, steam could pass the lower seat and began to act on the larger brim.  This greater area overwhelmed the spring force and the valve flew completely open with a "pop". Escaping steam on this larger diameter also held the valve open until pressure had dropped below that at which it originally opened, providing hysteresis.
  These valves coincided with a change in firing behaviour. Rather than demonstrating their virility by always showing a feather at the valve, firemen now tried to avoid noisy blowing off, especially around stations or under the large roof of a major station. This was mostly at the behest of stationmasters, but firemen also realised that any blowing off through a pop valve wasted several pounds of boiler pressure; estimated at 20 psi lost and 16 lbs or more of shovelled coal.
  Pop valves derived from Adams's patent design of 1873, with an extended lip. R. L. Ross's valves were patented in 1902 and 1904. They were more popular in America at first, but widespread from the 1920s on.
  Although showy polished brass covers over safety valves had been a feature of steam locomotives since Stephenson's day, the only railway to maintain this tradition into the era of pop valves was the GWR, with their distinctive tapered brass safety valve bonnets and copper-capped chimneys.
caption: Ross pop valve, from Tornado
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