The use of the correct spring is a matter of critical importance. Attempting
to employ a spring below its rated pressure range may result in a loss of lift
with resulting loss of capacity and a generally sluggish valve action. Using a
spring above or below its rated temperature range may cause a change in
spring characteristics and possibly failure of the spring itself. Before
changing the set pressure of any valve verify that the new pressure is
within the range of the spring and valve construction. When raising the set
pressure, check the applicable design code and the design pressure of the
protected equipment to ensure compliance with equipment design and
local requirements. Before reducing the set pressure, make certain that the
capacity at the reduced pressure is sufficient for the protection of your
equipment. Valves which are properly installed on steam, gas, or vapor
services will relieve with a pronounced pop when opening. After the excess
pressure has been relieved, the valve will generally reseat at approximately
5% to 7% below the set pressure. Actual blowdown will vary with each
valve and installation. There is generally no distinct pop with liquid service
relief valves as the opening will generally be proportional to the rise in
pressure. It is important for blowdown rings to be adjusted carefully and
accurately in accordance with the table values