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A safety valve is a valve mechanism for the
automatic release of a gas from a boiler, pressure
vessel, or other system when the pressure or
temperature exceeds preset limits. It is part of a
bigger set named Pressure Safety Valves (PSV) or
Pressure Relief Valves (PRV). The other parts of the
set are named relief valves, safety relief valves,
pilot-operated safety relief valves, low pressure
safety valves, vacuum pressure safety valves.
Safety valves were first used on steam boilers
during the industrial revolution. Early boilers
without them were prone to accidental explosion when
the operator allowed the pressure to become too
high, either deliberately or through incompetence.
The earliest and simplest safety valve used a weight
to hold the pressure of the steam, (this design is
still commonly used on pressure cookers); however,
these were easily tampered with or accidentally
released. On the Stockton and Darlington Railway,
the safety valve tended to go off when the engine
hit a bump in the track. A valve less sensitive to
sudden accelerations used a spring to contain the
steam pressure, but these (based on Salter spring
balances) could still be screwed down to increase
the pressure beyond design limits. This dangerous
practice was sometimes used to marginally increase
performance of a steam engine. In 1856 John
Ramsbottom invented a tamper-proof spring safety
valve which became universal on railways.
Safety valves also evolved to protect equipment such
as pressure vessels (fired or not) and heat
exchangers. Safety valve term should be limited to
compressible fluid application (gas, vapor, steam).
The two general types of protection encountered in
industry are thermal protection and flow protection.
For liquid-packed vessels, thermal relief valves are
generally characterized by the relatively small size
of the safety valve necessary to provide protection
from excess pressure caused by thermal expansion. In
this case a small valve is adequate because most
liquids are nearly incompressible, and so a
relatively small amount of fluid discharged through
the relief valve will produce a substantial
reduction in pressure.
Flow protection is characterized by safety valves
that are considerably larger than those mounted in
thermal protection. They are generally sized for use
in situations where significant quantities of gas or
high volumes of liquid must be quickly discharged in
order to protect the integrity of the vessel or
pipeline.
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