14
MT 235 – EN ed.2015 (Rev.A)
4.0
ACCESSORIES
4.1 RELIEF VALVE
The relief valve is a safety device that discharges to atmosphere a certain amount of gas when the pressure at the control point
exceeds the set one due to short occurances, such as the closing of the on-off valves in a very short time and/or an overheating
of the gas with zero requested flow rate. The discharge of the gas to atmospehere can, for example, avoid the tripping of the
slam-shut device for transient causes not resulting from damages to the regulator. Obviously, the amount of gas released
depends on the extent of the overpressure with respect to the calibration pressure.
The different models of relief valves available are all based on the same operating principle, which is described below with
reference to the valve
VS/AM 65
(fig. 10).
It is based on the comparison between the thrusts applied on the membrane
24
resulting from the pressure of the gas to be
controlled and the thrust resulting from the calibration spring
20
. The weight of the moving equipment, the static thrusts and the
residual dynamic ones acting on the obturator
4
are also considered in this comparison.
When the thrust resulting from the gas pressure exceeds that of the calibration spring, the obturator
4
is lifted with the
subsequent discharge of a given quantity of gas. As soon as the pressure falls below the calibration value, the obturator returns
to the closed position.
The control and adjustment of the relief valve tripping can be done by following the procedures described here below.
Figure 10: VS/AM 65
4.1.1
DIRECT INSTALLATION IN THE LINE (fig. 11).
When the relief valve is mounted directly on the line, i.e. without the interposition of an on-off valve, proceed as follows:
1.
Make sure that the outlet on-off valve
V2
and the purge/vent valve
6
are closed;
2.
Connect to the valve
6
a controlled auxiliary pressure and stabilize it at the desired value for the opening of the relief
valve; open the valve
6
causing a consequent increase in the pressure in the outlet segment;
3.
Verify the opening of the relief valve
15
and, if necessary, adjust it by appropriately turning the internal adjustment
ring nut
14
(clockwise to increase the setting; counter-clockwise to decrease it).
4.1.2
INSTALLATION WITH ON-OFF VALVE (fig. 12)
1.
Close the on-off valve
16
;
2.
Connect to the connection point
17
a controlled auxiliary pressure and increase it slowly until reaching the expected
value for the opening of the relief valve;
3.
Verify the opening of the relief valve
15
and, if necessary, adjust it by appropriately turning the internal adjustment
ring nut
14
(clockwise to increase the setting; counter-clockwise to decrease it).
Figure 11: Direct connection to the line
Figure 12: Connection with on-off valve
