Data Sheet
Bulletin 71.4:1808
5
As long as inlet pressure remains below setpoint, the
pilot spring keeps the pilot valve plug closed. Inlet
pressure passes through the upper port around the
upper portion of the valve plug then through the hollow
passage in that valve plug. Force from the main spring,
along with inlet pressure bleeding through the pilot,
provide downward loading pressure to keep the main
valve closed.
When inlet pressure rises above the set pressure,
pressure on the pilot diaphragm overcomes the control
spring to close the upper port and stroke the valve plug
to open the lower port. The pilot then exhausts loading
pressure from the top of the main valve diaphragm.
The pilot exhausts only while repositioning the main
valve. The inlet pressure unbalance overcomes the
spring force and opens the main valve.
As the inlet pressure drops, the pilot control spring
begins to close the pilot valve plug and the exhaust
slows. This causes the inlet pressure to build in the
main valve diaphragm casing, allowing the control spring
to close the main valve. Once the main valve is closed,
the pilot valve plug closes and the exhaust stops.
Types 1808 and 1808A relief valves or backpressure
regulators may be installed in any position as long as
the ow through the main valve corresponds with the
ow arrow on the main valve body (Type 1808) or runs
in through the bottom connection and out through the
side connection (Type 1808A).
Table 5. Capacities for the Type 1808 Using Optional External Control Line and Type 6358B Pilot with High-Gain Restriction
3 to 18
(0,21 to 1,2)
3
5
10
15
(0,21)
(0,35)
(0,69)
(1,0)
0.9
0.7
0.7
0.7
(0,06)
(0,05)
(0,05)
(0,05)
6.0
4.0
1.2
1.0
(0,41)
(0,28)
(0,08)
(0,07)
1.0 (0,07)
37 000
37 000
42 000
52 000
(992)
(992)
(1126)
(1394)
15 to 40
(1,0 to 2,8)
15
20
30
40
(1,0)
(1,4)
(2,1)
(2,8)
0.8 (0,05) 1.1 (0,07)
53 000
63 000
82 000
101 000
(1420)
(1688)
(2198)
(2707)
35 to 125
(2,4 to 8,6)
40
50
60
70
(2,8)
(3,5)
(4,1)
(4,8)
1.4 (0,09) 1.9 (0,13)
102 000
121 000
139 000
157 000
(2734)
(3243)
(3725)
(4208)
80
90
100
110
125
(5,5)
(6,2)
(6,9)
(7,6)
(8,6)
1.6 (0,11) 2.2 (0,15)
176 000
194 000
213 000
231 000
258 000
(4717)
(5199)
(5708)
(6191)
(6914)
1. Set pressure is dened as the pressure at which the pilot exhaust starts to bubble (discharge).
2. Crack pressure is the inlet pressure at which the main valve starts audible ow.
3. Inlet pressure buildup over the set pressure to achieve wide-open capacity.
4. Capacities with inlet piping equal to body size and without outlet piping.
An upstream control line is not required because of
the integral pilot supply tubing; however, for a more
accurate relief valve or backpressure regulator, this
tubing may be disconnected for upstream registration,
and the main valve diaphragm casing tapping plugged.
For liquid service, the pilot exhaust should be piped
to the downstream line or to a safe location. For gas
service, the pilot must be piped to a safe area because,
in enclosed conditions such as inside installations,
exhausting gas can accumulate causing a danger
of explosion. A vent line or stack must be located to
avoid venting gas near buildings, air intakes, or other
hazardous locations, and the line or stack opening must
be protected against anything that might clog it. The
thrust effect of a venting relief valve must be considered
when designing relief valve outlet piping and anchoring.
Tables 4 and 5 give relief capacities at selected
set pressures for the Type 1808 backpressure
regulator or relief valve. Flows are in thousands of
SCFH ( 6 0 ° F a n d 1 4 . 7 p s i a ) a n d t h o u s a n d s o f N m ³ / h
(0°C and 1,01325 bar) of 0.6 specic gravity natural
gas. To determine equivalent capacities for air,
propane, butane, or nitrogen, multiply the Table 4 or 5
capacity by the following appropriate conversion factor:
0.775 for air, 0.625 for propane, 0.548 for butane, or
0.789 for nitrogen. For gases of other specic gravities,
multiply the given capacity by 0.775, and divide by
the square root of the appropriate specic gravity. If
capacity is desired for the Type 1808A, multiply by 1.27.