User Manual

8
of the gauge. If the friction error is excessive, the move-
ment should be replaced. One possible cause of exces-
sive friction is improper adjustment of the hairspring. The
hairspring torque,or tension, must be adequate without
being excessive. The hairspring should also be level, unwind
evenly (no turns rubbing) and it should never tangle.
NOTES:
1 For operation of test gauge external zero
r
eset, refer to page 17.
2 For test gauge calibration procedure,
refer to Figure 2 on page 18.
7.0 DIAPHRAGM SEALS
7
.1 General – A diaphragm seal (isolator) is a device which is
attached to the inlet connection of a pressure instrument to
isolate its measuring element from the process media. The
space between the diaphragm and the instrument’s pressure
sensing element is solidly filled with a suitable liquid.
Displacement of the liquid fill in the pressure element, through
movement of the diaphragm, transmits process pressure
changes directly to a gauge, switch or any other pressure
instrument. When diaphragm seals are used with pressure
gauges, an additional 0.5% tolerance must be added to the
gauge accuracy because of the diaphragm spring rate.
Used in a variety of process applications where corrosives,
slurries or viscous fluids may be encountered, the diaphragm
seal affords protection to the instrument where:
• The process fluid being measured would normally clog
the pressure element.
• Pressure element materials capable of withstanding corro-
sive effects of certain fluids are not available.
• The process fluid might freeze due to changes in ambient
temperature and damage the element.
7.2 Installation – Refer to bulletin OH-1 for information
regarding (a) seal configurations; (b) filling fluids; (c) temper-
ature range of filling fluids; (d) diaphragm material pressure
and temperature limits; (e) bottom housing material pressure
and temperature limits; (f) pressure rating of seal assembly;
(g) accuracy/temperature errors of seal assembly; (h)
diaphragm seal displacement. The volumetric displacement
of the diaphragm must at least equal the volu metric dis-
placement of the measuring element in the pressure instru-
ment to which the seal is to be attached.
It is imperative that the pressure instrument/diaphragm seal
assembly be properly filled prior to being placed in service.
Ashcroft diaphragm seal assemblies should only be filled by
a seal assembler certified by Ashcroft Inc. Refer to section
3.3 for a cautionary note about not applying torque on either
the instrument or seal relative to the other.
7.3 Operation – All Ashcroft
®
diaphragm seals, with the
exception of Type 310 mini-seals, are continuous duty.
Should the pressure instrument fail, or be removed acciden-
tally or deliberately, the diaphragm will seat against a match-
ing surface preventing damage to the diaphragm or leakage
of the process fluid.
7.4 Maintenance – Clamp type diaphragm seals – Types
100, 200 and 300 – allow for replacement of the diaphragm
or diaphragm capsule, if that ever becomes necessary. The
Type 200 top housing must also be replaced with the dia-
phragm. With all three types the clamping arrangement allows
field dis as sem bly to permit cleaning of the seal interior.
7.5 Failures – Diaphragm failures are generally caused by
either corrosion, high temperatures or fill leakage. Process
media build-up on the process side of the diaphragm can
also require seal cleaning or replacement. Consult Customer
Service, Stratford CT for advice on seal failures and/or
replacement.
WARNING: A
ll seal components should be selected
considering process and ambient operating conditions to
prevent misapplication. Improper application could result in
f
ailure, possible personal injury, property damage or death.
8.0 DAMPENING DEVICES
8.1 General – Some type of dampening device should be
used whenever the pressure gauge may be exposed to
repetitive pressure fluctuations that are fairly rapid, high in
magnitude and especially when transitory pressure spikes
exceeding the gauge range are present (as with starting and
stopping action of valves and pumps). A restricted orifice of
some kind is employed through which pressure fluctuations
must pass before they reach the Bourdon tube. The dampen-
er reduces the magnitude of the pressure pulse thus extend-
ing the life of the Bourdon tube and movement. This reduc-
tion of the pressure pulsation as “seen” by the pressure
gauge is generally evidenced by a reduction in the pointer
travel. If the orifice is very small the pointer may indicate the
average service pressure, with little or no indication of the
time varying component of the process pressure.
Commonly encountered media (e.g. – water and hydraulic
oil) often carry impurities which can plug the orifice over time
thus rendering the gauge inoperative until the dampener is
cleaned or replaced.
Highly viscous media and media that tend to periodically
harden (e.g., asphalt) require a diaphragm seal be fitted to
the gauge. The seal contains an internal orifice which damp-
ens the pressure fluctuation within the fill fluid.
8.2 Throttle Screws & Plugs – These accessories provide
dampening for the least cost. They have the advantage of fit-
ting completely within the gauge socket and come in three
types: (a) a screwed-in type which permits easy removal for
cleaning or replacement; (b) a pressed in, non-threaded
design and (c) a pressed in, threaded design which provides
a highly restrictive, helical flow path. Not all styles are avail-
able on all gauge types.
8.3 Ashcroft Pulsation Dampener – Type 1106 Ashcroft
pulsation dampener is a moving pin type in which the
restricted orifice is the clearance between the pin and any
one of five preselected hole diameters. Unlike a simple throt-
tle screw/plug, this device has a self-cleaning action in that
the pin moves up and down under the influence of pressure
fluc tuations.
8.4 Ashcroft Pressure Snubber – The heart of the Type
1112 pressure snubber is a thick porous metal filter disc. The
disc is available in four standard porosity grades.
8.5 Ashcroft Needle Valves – Type 7001 thru 7004 steel
needle valves provide varying degrees of dampening. These
devices, in the event of plugging, can easily be opened to
allow the pressure fluid to clear away the obstruction.
8.6 Chemiquip
®
Pressure Limiting Valves – Model PLV-
255, PLV-2550, PLV-5460, PLV-5500 and PLV-6430, avail-
able with and without built-in snubbers, automatically “shut
off” at adjustable preset values of pressure to protect the
gauge from damage to overpressure. They are especially
useful on hydraulic systems wherein hydraulic transients
(spikes) are common.