Installation; Capacity Information - Emerson Fisher 1808 Manual

Type 1808
Relief Valve
As long as the inlet pressure is below the set pressure, the
pilot control spring keeps the pilot valve plug closed. Inlet
pressure passes through the pilot restriction and registers
as loading pressure on top of the diaphragm. Force from the
main spring, in addition to inlet pressure bleeding through
the pilot restriction, provide downward loading pressure to
keep the main valve closed.
When the inlet pressure rises above the set pressure,
the pressure on the pilot diaphragm overcomes the pilot
control spring and opens the pilot valve plug. The pilot then
exhausts the loading pressure from the top of the main valve
diaphragm. The pilot continuously exhausts gas when the
inlet pressure is above the set pressure. The inlet pressure
unbalance overcomes the main 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.
Backpressure Regulator
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.

Installation

Types 1808 and 1808A relief valves or backpressure
regulators may be installed in any position as long as the
flow through the main valve corresponds with the flow arrow
on the main valve body (Type 1808) or runs in through the
bottom connection and out through the side connection
(Type 1808A).
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
4
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.

Capacity Information

Gases
Figure 3 and Table 4 give relief capacities at selected set
pressures for the Type 1808 backpressure regulator or
relief valve. Flows are in thousands of SCFH (60°F and
14.7 psia) and thousands of Nm³/h (0°C and 1.01325 bar)
of 0.6 specific gravity natural gas. To determine equivalent
capacities for air, propane, butane or nitrogen, multiply the
Figure 3 and Table 4 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
specific gravities, multiply the given capacity by 0.775 and
divide by the square root of the appropriate specific gravity.
1. To determine capacities at set pressures or buildups
not given in Table 4, use one of the following formulas
and convert according to the factors in the preceding
paragraph if necessary:
Q = (P + buildup)
1
2. For pressure drops lower than critical (absolute
outlet pressure greater than one-half of absolute inlet
pressure), use the following formula:
520
Q = C (P + buildup)
g 1 abs
GT
where,
Q = flow capacity in SCFH
G = specific gravity of gas
T = absolute temperature of gas at inlet in °Rankine
(°Rankine = °F + 460)
C = sizing coefficient from Table 3
g
(P + buildup) = absolute inlet pressure
1 abs
= P
= P
C = C /C (See Table 3)
g v
1
ΔP = pressure drop across the valve in psig
520
C
abs g
GT
ΔP
3417
SIN
C
(P + buildup)
1 1
+ P + buildup
1 atm
+ 14.7 + buildup
1
Deg.
abs