There are several different types of safety valves in use
but all are designed to open completely (POP) at a
specific pressure and to remain open until a specified
pressure drop (BLOWDOWN) has occurred. Safety
valves must close tightly. without chattering, and must
remain tightly closed after seating.
To understand the difference between boiler safety
valves and ordinary relief valves is important, The
amount ofpressure required to lift a reliefvalve increases
as the valve lifts, because the resistance of the spring
increases in proportion to the amount of compression.
When a relief valve is installed on a steam drum, it opens
slightly when the specified pressure was exceeded; a
small amount of steam is discharged; and then the valve
closes again. Thus a relief valve on a steam drum is
constantly opening and closing; this repeated action
pounds the seat and disk and causes early failure of the
valve. Safety valves are designed to open completely at a
specified pressure to overcome this difficulty.
Several different types of safety valves are used on
boilers; however, they all lift on the same general
principle. In each case, the initial lift of the valve disk, or
feather, is caused by static pressure of the steam acting
upon the disk. or feather. As soon as the valve begins to
open, however, a projecting lip, or ring, of the larger area
is exposed for the steam pressure to act upon. The
resulting increase in force overcomes the resistance of
the spring. and the valve pops; that is, it opens quickly
and completely. Because of the larger area now
presented, the valve reseats at a lower pressure than that
which caused it to lift originally.
Lifting levers are provided to lift the valve from its
seat (when boiler pressure is at least 75 percent of that at
which the valve is set to pop) to check the action and to
blow away any dirt from the seat. When the lifting lever
is used, raise the valve disk sufficiently to ensure that all
foreign matter is blown from around the seat to prevent
leakage after being closed.
The various types of safety valves differ chiefly as to
the method of applying compression to the spring, the
method of transmitting spring pressure to the feather, or
disk, the shape of the feather. or disk, and the method of
blowdown adjustment. Detailed information on the
operation and maintenance of safety valves can be found
in the instruction books furnished by the manufacturers
of this equipment.
STEAM INJECTOR FEED SYSTEM
The STEAM INJECTOR (fig. 1- 12) is a boiler FEED
PUMP that uses the velocity and condensation of a jet of
steam from the boiler to lift and force a jet of water into
Figure 1-12.A cross-sectional view of a steam injector.
the boiler. This injection of water is many times the
weight of the original jet of steam.
The injector is used to some extent in boiler plants as
an emergency or standby feed unit. It does not feed very
hot water. Under the best conditions, it can lift a stream of
water (that has a temperature of 120°F) about 14 feet.
The installation of an injector is not a difficult
operation because the unit is mounted on the side of the
boiler. The four connections (fig. 1-13) to the injector are
1. The discharge line to the boiler feedwater inlet
Figure 1-13.Injector piping.