Pressure Reducing Valve Station - Honeywell AUTOMATIC CONTROL Engineering Manual

The disc trap (Fig. 106) is a device with only one moving part.
As steam in the chamber above the disc cools and condenses, the
disc snaps open releasing condensate. These traps cycle
independent of condensate load. Systems using these traps usually
have a means of recovering heat from the condensate. They are
used primarily on high pressure steam systems.
CHAMBER
ANNULAR
PASSAGE
DISCHARGE
PORT
OUTLET
Reprinted by permission from the ASHRAE Handbook—
1996 Systems and Equipment
Fig. 106. Disc Trap.
The orifice trap (Fig. 107) has no moving parts. It relies on
only a little steam flowing through the orifice because of the
low density of steam compared to condensate. It must have
constant supply pressure, constant demand, and constant
ambient temperature to operate.
INLET
SCREEN
Reprinted by permission from the ASHRAE Handbook—
1996 Systems and Equipment
Fig. 107. Orifice Trap.
The piston trap (Fig. 108) relies on a pressure change caused
by steam or condensate entering the trap to open or close the
piston or port. A small amount of condensate bleeds through
an orifice in the control disc into the control chamber. When
steam or condensate that flashes to steam enters the chamber,
the port is closed until the pressure in the control chamber falls.
The port then opens allowing condensate to pass.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
INLET
ORIFACE
DISC
(VALVE)
INLET
M15073
ORIFICE
FLOW
M15074
371
VALVE
OUTLET
Reprinted by permission from the ASHRAE Handbook—
1996 Systems and Equipment
Fig. 108. Piston Trap.

PRESSURE REDUCING VALVE STATION

Pressure reducing valve (PRV) stations are used in steam
systems where high pressure steam, usually at 50 to 125 psig,
is distributed for low pressure requirements (Fig. 109).
HIGH
PRESSURE
MAIN
PRESSURE
REDUCING VALVE
MANUAL
VALVE
PILOT
VALVE
BALANCE LINE
Fig. 109. PRV Station.
If steam is supplied at less than 50 psig, a single stage PRV
(Fig. 109) is required. For 50 psig or higher main pressure, a
second stage is added, in part because critical pressure drop
(50 percent of the absolute inlet pressure) through a single stage
PRV will cause noise and wear problems.
Since steam at 125 psig has a temperature of more than 350F
and little heat energy is dissipated through a PRV, the 5 psi steam
leaving the PRV station will be superheated (temperature is more
than 227F) for some distance downstream from the station.
Selection of valve materials used to regulate the 5 psi steam
must take this superheat into account, particularly for a device
in close proximity to the PRV.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL
CHAMBER
CONTROL
DISC
PORT
INLET
M15075
BYPASS
SAFETY
RELIEF
VALVE
LOW
PRESSURE
MAIN
M15076