Operation
Why so many different diaphragm diameters?
PIC designs each seal to accommodate the largest diaphragm possible thereby affording maximum sensitivity.
Large diaphragms are more sensitive resulting in more accurate systems. They contribute less to the system spring rate and are less sensitive to ambient and process temperature changes. Larger diaphragms are capable of larger volumetric displacements. This allows their use of higher volumetric displacement instruments. This also allows them to accommodate the volume changes of the fill liquid in the instrument and long capillaries due to ambient temperature changes. Low ambient temperatures could cause enough contraction of fill to bottom out the diaphragm resulting in erroneous readings.
PIC offers an alternative to using "GAUGE" seals on transmitter applications and compromising the system sensitivity and accuracy.
A diaphragm seal is an isolation device designed to separate a pressure instrument from the process media while allowing the instrument to sense the process pressure. The diaphragm seal is attached to the instrument directly (i.e. close-coupled) or remotely (i.e. capillary line).
The classical diaphragm seal consists of a thin flexible membrane sandwiched between an upper housing and a lower housing. The upper housing is designed to attach to the pressure instrument and be filled solid (i.e. without air space) with a fluid which will act as a hydraulic medium. The lower housing is designed to adapt to the process connection and contain the process medium as it acts against the diaphragm.
The pressure is hydraulically transmitted to the sensing element of the pressure instrument. The sensing element moves as a result of the pressure, thereby accommodating some of the fill volume. The diaphragm is displaced an amount equal to the volume accommodated by the sensing element of the instrument. The higher the pressure, the more fluid accommodated by the instrument, resulting in more displacement by the diaphragm. So it is important that the diaphragm seal be capable of displacing at least as much fluid volume as the instrument requires for normal operation.
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Application
Diaphragm seals may be applied to most pressure instruments regardless of the sensor technology employed.
Some common sensor technologies include:
- bourdon tube
- bellows
- piezoelectric
- capacitance
- strain gauge
- force balance mechanism
Typical pressure instruments include:
- gauges
- transmitters
- switches
- controllers
- recorders
Many process applications necessitate the use of a diaphragm seal to protect the instrument. These may include:
- Corrosive and/or hot processes that would damage the instrument.
- Viscous processes that would clog the instrument and restrict or disable the sensor.
- Processes that might freeze or solidify in the instrument due to ambient temperature effects.
- Sanitary processes requiring easily cleaned process contact surfaces.
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Pressure and Temperature Limits
PIC offers a standard line of diaphragm seals that may operate at pressures from full vacuum to 10,000 PSI (690 bar) and temperatures between -100 to +570 degrees F (-73 to +299 degrees C). Maximum operating pressures are a matter of design and are dependent upon the temperature of operation. Increasing the temperature will lower the maximum operating pressure. All PIC diaphragm seals have maximum operating pressures base rated at 100 degrees F (38 degrees C). Materials of construction will also affect the working pressure and temperature. The maximum operating temperature is limited by the sealed system component with the lowest maximum temperature. The limiting component may be, but is not limited to, any one of the following: housings, diaphragm, gaskets, O-rings, or fill fluid.
Vacuum applications reduce the maximum application temperature of a fill fluid. Special fill preparation and procedures are required for high vacuum systems.
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Optional Materials of Construction
| UPPER HOUSINGS (Non-Wetted Material) |
| • 304 SS (304) |
• Carbon Steel (STL) |
| • 316 SS (316) |
• Monel 400 (MON) |
 |
|
| LOWER HOUSINGS (Wetted Material) |
| • 304 SS (304) |
• Inconel 600 (INC) |
| • 304L SS (304L) |
• Inconel 625 (I625) |
| • 316 SS (316) |
• Monel 400 (MON) |
| • 316Ti SS |
• Nickel 200 (NICK) |
| • Carbon Steel (STL) |
• Tantalum (TANT) |
| • Carpenter 20 (C20) |
• Titanium GR.4 (TIT) |
| • Hast-C22 (HAS2) |
• Teflon-25%GF (TFGF) (1500F max.) |
| • Hastelloy B-2 (HASB) |
• Zirconium 702 (ZIRC) |
| • Hastelloy C-276 (HASC) |
|
|
|
| DIAPHRAGMS (Wetted Material) |
| • 304L SS (304L) |
• Silver (SILV) |
| • 316L SS (316L) |
• Tantalum (TANT) |
| • 321 SS (321) |
• Teflon Coated Metal |
| • Carpenter 20 (C20) |
(for anti-stick purposes only) |
| • Gold (GOLD) |
• Titanium GR4 (TIT) |
| • Hast - C22 (HAS2) |
• Zirconium 702 (ZIRC) |
| • Hastelloy B-2 (HASB) |
• Buna N (NBR) (1250 PSI; 2500F) |
| • Hastelloy C-276 (HASC) |
• Kalrez 1050LF (KALR) |
| • Inconel 600 (INC) |
• Kel-F (KELF) (1250 PSI; 4000F) |
| • Monel 400 (MON) |
• Teflon (TFE) (1250 PSI; 3500F) |
| • Nickel 200 (NICK) |
• Viton A (VIT) (1250 PSI; 3500F) |
| • Nickel 201 (N201) |
|
|
|
| GASKETS |
| • Grafoil (GRAF) |
• Non-Asbestos (4401) |
| • PTFE (3510) (-400F to 2000F) |
• PTFE (K2) (-500F to 4500F) |
|
|
| O-RINGS |
| • Buna N (NBR) (-100F to 2500F |
• Kalrez (KALR) |
| • Teflon (TFE) (-400F to 3500F) |
• Viton A (VIT) (-100F to 3500F) |
|
|
Abbreviations are the material codes used for I.D. purposes on PIC labels.
|
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Features
PIC features the most complete line of diaphragm seals available. Our capacity to handle even the most custom requirements makes "PIC" The Wise Choice of diaphragm seal manufacturers.
The standard PIC line offers more than twenty types of process connections, four diaphragm styles including welding, and replaceable metal and non-metal diaphragms, as well as seven classes of diaphragms; because one size diaphragm does not fit every application.
All PIC diaphragm seals feature continuous sealing upper housings with a bleed/fill connection and cavity design to enhance vacuum filling. Removable and non-removable upper housings are available in replaceable diaphragm styles. Carbon steel parts are polyester powder coated for a superior finish and chemical resistance. All welded diaphragms are leak checked to 1.0 x 10-8 cc/sec to assure seals of the highest integrity.
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Services
In addition to a complete line of diaphragm seals, PIC manufactures welded capillary lines for remote mounting of diaphragm seals and low volume connectors for close-coupling. Our fill and calibration expertise is highly regarded as a result of our innovative fill procedures which will provide successful systems for your tough applications.
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