Diaphragm
Seal Isolators Fill Facts
For 1/2 Century,
BRANOM INSTRUMENT COMPANY has been providing industry with
diaphragm seals and isolators mounted on gauges, transmitters
and recorders, or connected with tubing for remote mounting
of these instruments.
Diaphragm Seals
protect instruments from plugging, extremes of temperatures,
corrosive media, solidification, heavy pulsation or pressure
fluctuations, and are manufactured in a variety of styles
and configurations for specific applications. The type of
media helps determine the materials of construction, the
elastomers used for sealing, and type of fill material.
Whereas a diaphragm seal invariably introduces additional
inaccuracy in a system, Branom neutralizes much of this
through proper filling and mounting techniques and with
the proper materials and fill for your application.
The following
"Fill Facts" are designed to assist you in choosing the
proper device and fill for your particular application.
As always, please do not hesitate to call us on our Toll-Free
number for a specific recommendation or for current pricing.
1-800-767-6051
VACUUM to 100
PSI Service
Vacuum, compound
and low pressure devices require a diaphragm seal with a
large volume of fill fluid to operate, as well as a responsive,
highly flexible transmissive capability. All fills are affected
by temperature expansion and contraction; this and the flexibility
factor cause more absolute errors here and hysteresis than
in higher ranges. For normal off-line service, we offer
the Bellofram Type 12000-20 with nitrile or Viton(R) diaphragm
(0.22 cu in displacement) or the B-8498 (0.75 cu in). The
standard Conoflow 100 A seal (.307 Cu in) with a Buna N
or Viton(R) Diaphragm usually is acceptable as well. For
particularly difficult applications, the Conoflow #430 Seal
(3.0 cu in 1) has enough fluid to drive any known instrument
and range. For in-line applications, the Onyx Isolator provides
exceptional service in any vacuum, compound or pressure
requirement.
An important
consideration is the vapor pressure point of the fill for
a vacuum application. If the combination of pressure and
temperature reach the vapor pressure point, accuracy will
be unpredictable. Graphs are available for the various fill
fluids depicting this relationship.
Caution:
Never use teflon tape if the fill fluid is silicon: the
silicon will be sucked by the teflon. Use "Rector Seal"
instead.
1. 100 PSI+ Threaded off-line Diaphragm Seals
A. Non-Metallic
Nonmetallic Seals
& those with non-metallic lower housings (wetted Parts)
typically have a limit of 200 (Conoflow) or 500 PSI (Bellofram).
Temperature limitation is 125 deg F for PVC (Bellofram)
and 140 deg for Kynar, Polypropylene, PVC, and Teflon (Carbon
or Glass Filled). Typical is 1/4" Gage Port, 1/4" or 1/2"
Process Port. Careful thought should be given to the housing
of both the upper and lower housings; corrosive laden air
would suggest exotic plastics or metals and bolting for
both process and gage sides.
B. Metallic
Metals have a
standard pressure limit of 2000 (Conoflow) or 2500 PSI (Bellofram);
Conoflow makes 5000 PSI and even 10,000 and 20,000 PSI versions
(no flushing ports available, and special 316 55 bolts necessary).
The standard 2000/2500t seals are available as miniatures
(Conoflow 140K and Bellofram 12000 Series) or standard size
seals. The standard size seals have desirable options available
such as Bleed Screw ( almost necessary for a good fill),
Clean Out Ring, Flushing Connections, 0 Ring varieties etc.
Typical is 1/4" Gage Port, 1/2 or 1" Process Port. Teflon
0-Rings cannot be used above 2000 PSI.
2. In-Line
Diaphragm Seals
This type of
seal is recommended for applications that require a continuous
flow of process across the diaphragm to insure that pressure
sensing is not inhibited by solids buildup. The Conoflow
1OON series features an Easy Cleanout design, permitting
removal of the lower housing for inspection or cleaning
without loss of fill fluid, and for the use of metallic
as well as elastomeric diaphragms. The 100N is for all 1
to 3" pipe schedules and flange ratings. The Onyx Isolator
is of a clamp type design to mate with 150 ANSI or 300 ANSI
companion flanges (not supplied) in sizes from 1" to 20",
Carbon Steel, Carp-20, or 316SS housings. The sensing element
can be Buna N or Viton. All have a maximum pressure rating
of 30" Hg vacuum to 1000 PSI. Pressure is limited,
however, to the pressure rating of the flange (150 # = 350
PSI, 300 # = 750 PSI @ 125 deg. F). Note: ONYX is
the ONLY recommended brand; other types do not have the
modular seal and can lose their fill with the slightest
jarring during installation or operation. Any loosening
means catastrophic failure, except with ONYX.
3. Fill Fluids
& Service Temperature Limits
Fill fluids should
be selected by carefully considering their physical properties
at the extremes of ambient and process temperature to which
they will be subjected. Fill fluids must be chemically compatible
with the process fluid, to avoid potentially hazardous reactions
in the event of diaphragm rupture. Typically, hydrocarbon
based liquids must not be used with strong oxidizers such
as oxygen, chlorine, hydrogen peroxide, or nitric acid.
For these special applications, an inert liquid such as
Fluorolube is necessary.
Standard Fills
for the Pacific Northwest:
Silicon
DC 200-10: -50 to +450 degrees F
Other fills available:
Silicon
DC 704: +30 to +520 degrees F (Bubble Free and Best Temperature
Coefficient of the Silicons)
Silicon
DC 710: -50 to +450 degrees F
Silicon
DC 550: -40 to +550 degrees F
Fluorolube
FS-5: -40 to +500 degrees F
(Recommended
for Chlorine & Oxygen Service but never to be
used in contact with aluminum or magnesium)
Halocarbon
Oil 11-14S: -30 to +450 degrees F
(Recommended
for Chlorine & Oxygen Service; but never to
be used in contact with aluminum or magnesium)
Food
Grade Propylene Glycol: -50 to +200 degrees F
Syltherm
800: -40 to +750 degrees F
Note: Glycerin
& Silicon are explosive in contact with chlorine. Flourolube
is explosive in contact with aluminum. Oxygen Service: Seals
must be oxygen cleaned and free from dirt/oil and the fill
must be halocarbon.
Other Fills
are also available; please call for a recommendation.
4. Accuracy
The accuracy
of a diaphragm seal and its instrument depends upon a variety
of factors:
- Basic Instrument
Accuracy Rating
- Temperature
All fills are
affected by temperature changes due to expansion and contraction
which, in a filled system, becomes a pressure increase
or decrease. The more volume of fill you have, and the
more area exposed to heat or cold extremes, the more you
will be affected by temperature changes. Therefore, you
want as little volume as possible to do the job; i.e.
small bore tubing and small fill chambers. Temperature
stability of the environment at the seal or along the
connecting line is very helpful to maintaining accuracy.
- Amount of
bubbles (entrained air) in the fluid
- Type of Fluid
(usually recommend Silicon DC 200-10 or the much more
expensive and viscous Silicon DC 704)
- Spring Constant
of sensing diaphragm (metal foils are less resilient than
elastomeric types)
- Pressure
Ranges: Vacuum and low pressure ranges have the greatest
potential for error. With higher pressures, errors may
not even be discernible.
- Filling Method:
A Vacuum Fill on both the Instrument and the diaphragm
seal is strongly recommended to eliminate errors due to
entrained air (bubbles collapse instead of transmit pressure)
inherent in simple filling.
- In differential
applications, the volume of the high and low legs must
be kept identical; temperature gradients must also be
weighted in the calibration phase, as well as attitude.
- A gauge keeper
is recommended with seals to lessen the chance of gauge
rotation and resultant loss of fill.
5. Remote
Mounted Instruments with capillary
Special Heavy
Wall, small internal diameter 1/8" 300 series SS tubing,
with or without additional protective 5/8" diameter SS spiral
armor, is used with appropriate compression fittings. Please
specify the exact capillary length and consult us on very
long runs (where perhaps a pressure transducer and digital
readout might be more cost effective, trouble-free and accurate).
It may be necessary to offset the measuring range to compensate
for any difference in height between the diaphragm seal
and the instrument at its final location (due to head pressure
of the fluid in the capillary).
RECOMMENDATIONS
Warning:
To avoid incorrect applications, all gauge components should
be selected on the basis of the fluid and operating conditions.
Improper use can result in damage to the pressure gauge,
break-downs and damage to equipment or personal injury.
For fluids not
listed in our manual (material life depends on fluid temperature
and concentration as well as other operating conditions)
please apply to the engineering department. The operating
pressure should not exceed the limit temperatures of the
used materials.
Operating
pressure: The maximum pressure should not exceed the
pressure rating of the diaphragm seals. For flanged diaphragm
seals the maximum pressure is the pressure rating of the
flanges. The maximum permissible pressure decreases as temperature
increases.
Temperature:
The operating temperature should be the calibration temperature
of the instrument and must be compatible with the used materials.
For operating temperature higher than 1500F the
use of a diaphragm seal with capillary is recommended.
Oxygen:
Oxygen diaphragm seals are manufactured and maintained completely
free of oils and grease. The fill fluid must be Fluorolube.
Maintenance:
If sediments are to be removed from diaphragm avoid the
use of tools that may damage the diaphragm and flush out
with a dilutent suitable for sediment nature.
Oxidizing
agents: Glycerin or silicone should not be used when
highly oxidizing agents are present, such as oxygen, chlorine,
nitric acid, hydrogen peroxide, due to danger of spontaneous
chemical reaction. In these cases the use of fluorates and
chlorates like Fluorolube are recommended.
GAUGE / DIAPHRAGM SEAL / VALVE ASSEMBLIES
- The measuring
instrument (gauge) should be directly connected to the
diaphragm seal whenever possible. Snubbers, valves and
other devices normally must be mounted on the process
side of the diaphragm seal, so as to not exceed the fill
fluid capacity of the diaphragm seal. The cubic inch displacement
of the seal must equal or exceed that of the devices mounted
to it. Annular Seals, such as the Onyx, are usually the
best choice for combination gauge, pressure switch and/or
transmitters on a tee, as they contain a large volume
of transmissive fluid.
- Mounting
a valve between the diaphragm seal and the gauge is a
poor location; if the diaphragm ruptures, it would be
necessary to shut off the process to repair or replace
the diaphragm. "Protecting" the gauge is not
as important as limiting the spill and being able to correct
the problem. The internal volume requirements of a shutoff
valve exceed the capabilities of most seals.
- Snubbers have
little or no positive value when mounted between the seal
and the instrument; often, the system will not work at
all, or a larger orifice must be drilled in the snubber
to make it work, which defeats its purpose. Instead, fill
the snubber with the gauge fill and mount it above the
ball valve. If it plugs, it can then be easily replaced
without shutting down the system.
Viton(R)
is a registered trademark of DuPont Dow Elastomers
