Emerson Daniel 762, User Manual

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Supply pressure should be set at, or close to, the relief pressure required. The valve is a
hydraulically balanced piston design which means that the forces applied across the piston
are proportional to the pressures. This is particularly important when setting the system
up: the gas pressure set point, minus the force exerted by the spring is the relief pressure
for the system.
The gas tank should be buried underground or insulated to keep the gas at a constant
temperature. Thermal expansion, caused by the increases in temperature of the gas will
change the relief set point.
5.8 Gas plenum tank installation and sizing
The gas plenum should be buried underground to keep the gas at a constant temperature.
Thermal expansion, caused by increases in temperature, will change the relief set point.
The effective volume of the gas plenum decreases as the valve opens by an amount equal
to the piston displacement. The size of the gas plenum determines the percentage that
the pressure relief set point will change as the valve opens. For example, a 12" valve piston
displacement is 554 cubic inches from a closed to an open position. Thus, when fully
opened, an equal amount of gas is contained in a volume that is decreased by 554 cubic
inches. From Boyles Law, we can see the proportional increase in pressure as: P
1
V
1
=P
2
V
2
.
For example, a 12" valve piston displacement is 554 cubic inches from a closed to an open
position.
Before the gas tank is sized, determine:
1.
Set point (PSI)
(1)
2. Valve size
3. Allowable over-pressure (PSI)
(1)
(typically 10%)
Gas plenum sizing formula Equation 5-1:
V =
PD
(
P
2
P
1
)
0.709
(
P
2
P
1
)
0.709
− 1
P
1
Valve required set pressure, valve closed
P
2
Maximum allowable overpressure, valve open
V1
Total gas volume (gas chamber and volume behind main valve piston)
PD
Volume displacement of main valve piston
Exponent
0.709 derived from K factor of Nitrogen (1.41)
No consideration is given to the volume of gas contained in tubing, fittings and the top
portion of the valve itself.
Installation prerequisites
User manual 69