Cashco DA8 Differential - Manual (Page 3)

IOM-DA8/9 3
IV. PRINCIPLE OF OPERATION
1. Valve plug movements modulate the fl ow across
the valve seat to maintain the inlet pressure (P
1
)
equal to the sum of the differential pressure (∆P
DIFF
)
and the loading pressure (P
Load
). The inlet pres-
sure acting on the underside of the diaphragm
tends to lift the valve plug off the valve seat. The
loading pressure acting on the topside of the
diaphragm tends to close the valve plug against
the valve seat. The positive differential setting of
the range spring for the DA8 tends to close the
valve plug against the valve seat. The negative
differential setting of the range spring for the DA9
tends to lift the valve plug off the valve seat. The
inlet pressure tracks with changes to the loading
pressure. Each one psi increase or decrease in
the loading pressure produces a respective one
psi increase or decrease in the inlet pressure.
2. For a DA8/9 (single diaphragm) design, a com plete
diaphragm failure will cause the inlet process fl uid
to mix with the loading fl uid.
SECTION V
V. STARTUP
1. Start with the block valves closed.
2. Relax the range spring by turning the ad just ing
screw CCW for the DA8 (viewed from above)
and CW for the DA9; a minimum of three (3) full
rev o lu tions. This reduces the ∆P
DIFF
(differential)
pres sure set point.
3. If it is a “hot” piping system, and equipped with
a bypass valve, partially open the bypass valve
to pre heat the system piping and to allow slow
ex pan sion of the piping. Check for proper steam
trap operation if installed. Closely monitor inlet
(up stream) pressure via gauge to ensure not
over-pres sur iz ing. NOTE: If no bypass valve is
in stalled, extra caution should be used in starting
up a cold system; (i.e. do everything slowly).
4. Crack open the outlet (downstream) block
valve.
CAUTION
Do not walk away and leave a bypassed reg u la tor
unattended!
5. Slowly open the inlet (upstream) block valve to
about 25% open, observing the inlet (up stream)
pres sure gauge. Determine if the reg u la tor is
fl owing. If not, slowly rotate the DA8/9 regulator
ad just ing screw CCW (viewed from above) until
fl ow begins.
6. Continue to slowly open the inlet (upstream) block
valve until fully open.
7. Continue to slowly open the outlet (downstream)
block valve, especially when the downstream
piping system is not pressurized. If the inlet (up-
stream) pressure exceeds the desired pres sure,
close the inlet block valve and go to Step 2. Close
bypass valve approximately 25%, and re peat pro-
ce dure.
8. When fl ow is established steady enough that the
outlet (downstream) block valve is fully open, begin
to slowly close the bypass valve if installed.
9. Set the DA8 regulator set point (∆P
DIFF
) by turning
the ad just ing screw clockwise (CW) to increase
inlet pres sure or CCW to reduce inlet pressure.
The inlet (P
1
) pressure under these conditions
will approximate the desired dif fer en tial pressure
for the DA8. Turn the adjusting screw of the DA9
CCW to increase the negative differential setting
(i.e. CCW to change -10 psid to -20 psid) and
CW to decrease the setting. The negative differ-
ential setting is the difference between the load-
ing pressure (P
Load
) and the inlet (P
1
)pressure .
10. Pressurize the source of loading (P
Load
) pres sure
and allow to fi ll the spring chamber cavity. Slightly
open the bleeder valve (item 3) to vent any air as the
spring chamber is fi lling with a liquid loading fl uid.
11. Develop system fl ow and pressure and readjust
set point as required to obtain desired response.
Per for mance should be checked at minimum and
maximum fl ow levels.
12. Install closing cap where applicable. The standard
regulator relies on a double o-ring rotary seal (item
34) around the non-rising adjusting screw.
SECTION IV