The function of the liquid
fill fluid is to transmit the process pressure, which acts on
the diaphragm, from the diaphragm to the sensing element of the
Usually, this fluid is a light
instrument oil but other fluids can be used should oil tend to
contaminate the process in case of leakage.
The instrument/seal assembly
can be filled in either of two methods.
The instrument and seal can be
filled separately and then assembled. Back pressure, if
present, should be relieved by use of the fill/bleed screw
provided in the seal for this purpose.
The liquid filling operation
can be done by evacuating all the air from all the cavities in
the pressure instrument with the aid of a vacuum pump and then,
by the proper arrangement of pipes and valves, allowing the fill
fluid to replace the evacuated air in the cavities. The
cavity in the instrument flange of the diaphragm seal can be
easily filled through the instrument connection from an oil can
or other container prior to attaching it to the pressure
In the second method of
filling, the instrument and seal are assembled first, then the
entire internal cavities of the instrument and seal can be
evacuated and completely filled through the fill/bleed port by
the use of a fill adapter that is inserted into the port.
After filling, the bleed screw is inserted into the fill/bleed
port to seal it. If you use this method, it is strongly
recommended that an equal vacuum be pulled on the process side
of the diaphragm, especially on the Type T and Type V Zeals.
It is a fact that the smallest
amount of movement, required of the diaphragm, occurs when all
the air is removed from the unit.
It is also a fact that the
larger the amount of movement required to fully actuate a
sensing element of a pressure instrument, the greater the spring
rate of a diaphragm. The greater the spring rate - the
greater the inaccuracy of the unit. These facts are
inherent regardless of make or manufacturer of the diaphragm
The spring rate of the
diaphragm can be calibrated into the assembled unit by
recalibrating the unit after liquid filling. But, this is
possible only if the calibration range of the sensing element in
the pressure instrument is great enough to compensate for the
added spring rate of the diaphragm. In instruments of low
pressure range, the spring rate of the sensing element is too
small to allow for the compensation necessary for the high
spring rate of some diaphragms. In fact, the spring rate
of some diaphragms is so great that it is equal to and sometimes
even greater than the spring rate of the sensing element in low
The more precise and exacting
the process requirements, the more important it is to remove all
the air from a pressure instrument and to carefully recalibrate
the unit after liquid filling.
In the large Type T Zavoda
Diaphragm Seal (Zeal), the spring rate of the diaphragm is equal
to only two ounces of pressure per square inch, when all the air
is evacuated from the unit, and is so small that in most uses
this will not require the costly recalibration of the
- Depending on model used, Zeals can be welded,
bolted or threaded into the process system using any good
installation procedure. A hole is provided in the instrument
flange for the use of a pin spanner wrench when attaching a Zeal
onto a threaded process pipe.
MAINTENANCE - All Zeals in all series, except styles 10 and
11, are of the cleanout type and, by removing the flange bolts, can
be separated at the flanges without loss of the liquid fill fluid to
permit periodic cleaning of the diaphragm.
The cavity in the process
flange and the process side of the diaphragm must be kept clean
and free of any clogging matter, otherwise proper movement of
the diaphragm may be impaired.
The retention of the liquid
fill is accomplished by the diaphragm being firmly held by a
holding groove in the instrument flange.
CAUTION - At all times
extreme care must be taken not to nick or scratch the gasketing
surfaces or leakage may result.
DIAPHRAGM REPLACEMENT - When
removing a damaged diaphragm from the instrument flange, extreme
care must be taken not to dent, nick or otherwise damage the groove
are at the instrument flange gasketing surface face.
Install a new diaphragm by
inserting one point of the diaphragm into the holding groove
with the instrument flange gasket assembled in place on the
diaphragm and then press the diaphragm in with the fingers
pushing down on the thickened edge of the diaphragm and working
around the periphery until the opposite side has been reached.
Lightly oil all parts to aid
in installation, if necessary, but hammers or like objects
should not be used, as they may dent or nick the gasketing
surface and may result in leakage after assembling.
Zeals with the style numbers
10 and 11 do not have the holding groove in the instrument
flange, nor the self-holding type of diaphragm. Therefore,
to assemble a diaphragm into one of these models, simply
assemble an instrument flange gasket in place and insert the
diaphragm into the recess provided for it in the instrument
flange and bolt the process flange to the instrument flange.