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Close-off rating of three-way valves: The maximum pressure
difference between either of the two inlet ports and the
outlet port for mixing valves, or the pressure difference
between the inlet port and either of the two outlet ports
for diverting valves.
Pressure drop: The difference in upstream and downstream
pressures of the fluid flowing through the valve.
P
ressure drop (critical): The flow of a gaseous controlled fluid
through the valve increases as the pressures drop
increases until reaching a critical point. This is the
critical pressure drop.
Any increase in pressure drop beyond the critical
pressure drop is dissipated as noise and cavitation
rather than increasing flow. The noise and cavitation
can destroy the valve and adjacent piping
components.
B
ody rating (nominal): The theoretical pressure rating,
expressed in psi, of the valve body exclusive of
packing, disc, etc. The nominal rating is often cast on
the valve body and provides a way to classify the valve
by pressure. A valve of specified body material and
nominal body rating often has characteristics such as
pressure-temperature ratings, wall thickness, and end
connections which are determined by a society such
as ANSI (American National Standards Institute).
Figure 2 shows ANSI pressure-temperature ratings for
valves. Note that the nominal body rating is not the
same as the actual body rating.
B
ody rating (actual): The correlation between safe, permissible
flowing fluid pressure and flowing fluid temperature of
the valve body (exclusive of the packing, disc, etc.).
The nominal valve body rating is the permissible
pressure at a specific temperature.
EXAMPLE:
From Figure 2, a valve with an ANSI rating
of 150 psi (ANSI Class 150) has an actual
rating of 225 psi at 250F.
Fig. 2. Sample ANSI Pressure-Temperature Ratings
for Valves.
Maximum pressure and temperature: The maximum
pressure and temperature limitations of fluid flow that
a valve can withstand. These ratings may be due to
valve packing, body, or disc material or actuator
limitations. The actual valve body ratings are
exclusively for the valve body and the maximum
pressure and temperature ratings are for the complete
valve (body and trim). Note that the maximum
pressure and temperature ratings may be less that the
actual valve body ratings.
EXAMPLE:
The body of a valve, exclusive of packing,
disc, etc., has a pressure and temperature
rating of 125 psi at 335F. If the valve
contains a composition disc that can
withstand a temperature of only 240F, then
the temperature limit of the disc becomes
the maximum temperature rating for the
valve.
337
o
F
ANSI CLASS150
(STEAM)
212
o
F
ANSI
CLASS 125
ANSI
CLASS 150
ANSI
CLASS 250
NOTES:
1. FOR HIGH FLUID TEMPERATURES, THE VALVE AND/OR
PIPING SHOULD BE INSULATED TO PREVENT AMBIENT
TEMPERATURES FROM EXCEEDING ACTURATOR RATINGS.
M12224
275
o
F
300
300 350
250
250
200
200
150
150
100
100
50
50
0
0
FLUID TEMPERATURE IN
o
F
LINE PRESSURE IN PSI
400
400
Appendix A: Valve Selection and Sizing