9
WORK TEMPERATURE
INCREASE TO THE SET
PRESSURE
UNTIL 100º C
FROM 101º C TO 250º C
FROM 251º C TO 500º C
MORE THAN 500º C
0 %
2 %
3 %
5 %
Correction factor shown above has to be equally
applied, even when the set pressure adjustment of that
valve was corrected to compensate the back pressure
effect in the release.
Therefore, the pressure to be used to test the valve in
the test bench, called “Cold Differential Test Pressure”,
is the inlet pressure which will open the valve in the
test bench at ambient temperature and atmospheric
pressure.
To perform an exact calculation of the cold differential
test pressure, the pressure which will pop the valve in
process, the increase for temperatures above 100 ºC,
and back pressure different to atmospheric have to be
taken in to account.
Examples of calculation of the cold set pressure
adjustment in back pressure and temperature
conditions:
A) DATA: Conventional valve (no bellows), set
pressure 20 barg, process temperature 260
ºC, and constant back pressure 1 barg, used
with vapours.
Adjusting the set pressure of the spring:
20 barg – 1 barg = 19 barg
From the temperature correction table we have to
apply a correction factor of 3% (as temperature is
between 251º and 500º C):
3 % of 19 barg = 0,57 barg (rounded 0,6 barg)
Cold set pressure adjustment for the valve to be
adjusted at the bench will be:
19 barg + 0,6 barg =
19,6 barg
B) DATA: Balanced valve (with bellows), set pressure
50 barg, process temperature 180 ºC, constant
back pressure 1,5 barg, used with vapours.
Adjusting the set pressure of the spring:
50 barg, as it is a balanced valve (with bellows), the
back pressure is not affected with the set pressure
adjustment, therefore back pressure is not deducted.
From the temperature correction table, a correction
factor of 2% as to be applied (as it’s between 101º and
250º C):
2% of 50 barg = 1 barg
Cold set pressure adjustment for the valve to be
adjusted at the bench will be:
50 barg + 1 barg =
51 barg
Is important to notice that in all cases (as it’s been
shown in examples A and B), temperature used for
calculations is inlet temperature in the valve, and that
the set pressure adjustment will be always multiplied
buy the temperature correction coefficient.
Cold set pressure adjustment = spring set pressure
adjustment x % corrector
The procedure is always the same and applicable to all
kind of valves.
6.4 POSITION OF THE ADJUSTING RING
To be able to reach the set pressure of the valve in
test benches when flow is small, the adjusting ring has
to be adjusted as it is shown below. Once the tests are
done, position the ring as shown in Table 1.
Disassemble the screw (14), turn the adjusting ring (7)
counter clockwise until it touches the disc holder’s bell
(8), then lower 1 or 2 grooves clockwise to avoid such
contact. This operation can be done through the outlet
flange of the valve, or by using a screwdriver through
the respective hole at the lock screw.
Once the ring is in position, assemble the screw
paying attention so the nut (32) gets placed between
the adjusting ring grooves. Once the screw is tightened
check that the ring is fixed so it can’t turn, but free
enough for it is self-alignment.
Never perform this operation with a valve that has
pressure, as an accidental popping may harm
anyone handling it.
6.5. SET PRESSURE REGULATION
If the valve opens itself at a lower or higher pressure
other than the set pressure, the compression of the
spring has to be adjusted, as shown below:
A) Disassemble the cap (3) or (50) if the valve
has a lever.
B) Loosening the adjusting screw net (13), the
adjusting screw will be released (11). Turning
it clockwise the set pressure will be increased
and turning it clockwise will decrease it.
C) Once the set pressure is adjusted, tighten the
nut and assemble the cap.
Never adjust the set pressure when the valve has
pressure, as the closing surfaces of the nozzle and
disc could damage if they are turned.
