Data Sheet

Bulletin 71.4:MR98
10
Typical relieving capacities in lbs per hour and kilogram
per hour of saturated steam are shown in Tables 9, 14
and 19.
Tables 7, 8, 12, 13, 17 and 18 give relieving capacities in
U.S. gallons per minute and in liters per minute of water.
Sizing
Determine Flow Rates for units used as differential relief
valves as follows:
Air and Steam Service
When the Capacity Information section do not cover
the actual service conditions, it will be necessary to
calculate the ow rate available from the relief valve.
Step 1.
From the Capacity Information section determine a
ow rate using the closest available data for setpoint
and build-up. With this information, calculate a ow
coefcient using the following:
Equation 1:
Q
C
g
=
P
1a
SIN
DEG
520
GT
) )
P
1a
∆P
3417
C
1
C
g
= Calculated ow coefcient
Q = Flow rate from tables (SCFH)
P
1a
= Inlet pressure from table (set pressure and
build-up) converted to absolute pressure
G = Specic gravity
T = Absolute temperature of gas at inlet,
°Rankine = °F + 460
P
1
= Absolute inlet pressure, psia = psig + 14.7
C
1
= See specications section
∆P = P
1
- P
2
, psid
Calculate the C
g
coefcient for the setting and build-up
nearest to the desired service conditions within the
selected spring range. Then use this C
g
to calculate
the approximate ow available for the actual setpoint
and build-up desired. Available C
g
will vary based
upon setpoint, differential pressure, build-up and
spring range.
Step 2.
Calculate the actual ow rate available using the
coefcient from Step 1 and the actual inlet pressure
(setpoint plus build-up) and actual outlet pressure
(if not atmospheric).
Equation 2:
Q
SCFH
=
C
g
P
1a
SIN
DEG
520
GT
) )
P
1a
∆P
3417
C
1
Liquid Sizing for Liquids Other than Water
Step 1.
Determine C
v
at build-up conditions of application.
C
v
=
Q
∆P
Q = Flow in GPM from capacity tables
∆P = Pressure drop in psi (setpoint and build-up)
C
v
= Valve sizing coefcient
Step 2.
To determine ow rate for liquids other than
water or ow rate for differential relief service:
Q = C
v
∆P/G
G = Specic gravity of uid
Maximum Allowable Pressure Drop for
Liquid Service
Pressure drops in excess of allowable will result in
choked ow and possible cavitation damage.
To determine maximum allowable pressure drop
for water:
∆P(allow) = K
m
(P
1
)
∆P = Valve differential - psi
K
m
= Valve recovery coefcient from table
P
1
= Valve inlet pressure psig
To determine maximum allowable pressure drop for
uids other than water, see Fisher
®
Sizing Program.
Installation
These valves may be installed in any position, as long
as ow will be in the same direction as that of the
arrow cast on the body. For dimensional information
see Figure 4.
Emerson Process Management Regulator Technologies,
Inc. (Emerson™), provides an instruction manual
with every valves shipped. Refer to this for complete
installation, operation and maintenance instructions.
Included is a complete listing of individual parts and
recommended spare parts.