Ashcroft DURALIFE 1009 Посібник з монтажу, експлуатації та технічного обслуговування - Сторінка 16
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7.0 DIAPHRAGM SEALS
7.1 General – A diaphragm seal (isolator) is a
device which is attached to the inlet connec-
tion 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
Displacement of the liquid fill in the pressure
element,
through
diaphragm, transmits process pressure
changes directly to a gauge, switch or any
other pressure instrument. When diaphragm
seals are used with pressure gauges, an addi-
tional 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 corrosive effects of certain
fluids are not available.
• The process fluid might freeze due to
changes in ambient temperature and dam-
age the element.
7.2 Installation – Refer to sales bulletin OH-1
for information regarding (a) seal configura-
tions; (b) filling fluids; (c) temperature range
of filling fluids; (d) diaphragm material pres-
sure and temperature limits; (e) bottom hous-
ing material pressure and temperature lim-
its; (f) pressure rating of seal assembly; (g)
accuracy/temperature errors of seal assembly;
(h) diaphragm seal displacement. The volu-
metric displacement of the diaphragm must
at least equal the volumetric displacement of
the measuring element in the pressure
instrument to which the seal is to be attached.
It is imperative that the pressure instru-
ment/diaphragm seal assembly be properly
a
suitable
liquid.
movement
of
the
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
seals, with the exception of Type 310 mini-
seals, are continuous duty. Should the pres-
sure instrument fail, or be removed acciden-
tally or deliberately, the diaphragm will seat
against a matching surface preventing dam-
age 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 diaphragm. With all three types the
clamping arrangement allows field disassem-
bly to permit cleaning of the seal interior.
7.5 Failures – Diaphragm failures are gener-
ally caused by either corrosion, high tempera-
tures 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: All seal components should be
selected considering process and ambient
operating conditions to prevent misapplica-
tion. Improper application could result in fail-
ure, 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 fluctu-
ations that are fairly rapid, high in magni-
tude 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 fluctua-
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diaphragm
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