This article is about Instrumentation Accessories for Pressure Switches, Transmitter Installation For Pressure Measurement of Instrumentation and Control Systems as per International Codes and standards for Commercial Buildings, Plants and Refinery Projects.
Instrumentation Accessories for Pressure Switches, Transmitter Installation
Material Receiving General Checklist for Instrumentation & Control
Installation of Protective Accessories
Protective Accessories: The most obvious gauge accessory is a shutoff valve between it and the process, which allows blocking while removing or performing maintenance.
A second valve is often added for one of two reasons: draining of condensate in vapor service (such as steam), or, for higher accuracy applications, to allow calibration against an external pressure source.
Other accessories include pipe coils or siphons, which in steam service protect the gauge from temperature damage, and snubbers or pulsation dampeners, which can both absorb pressure shocks and average out pressure fluctuations.
If freeze protection is needed, the gauge should be heated by steam or electric tracing.
Chemical seals protect the gauge from plugging up in viscous or slurry service, and prevent corrosive, noxious or poisonous process materials from reaching the sensor. They also keep the process fluid from freezing or gelling in a dead-ended sensor cavity.
The seal protects the gauge by placing a diaphragm between the process and the gauge. The cavity between the gauge and the diaphragm is filled with a stable, low thermal expansion, low viscosity and non-corrosive fluid. For high temperature applications, a sodium-potassium eutectic often is used; at ambient temperatures, a mixture of glycerine and water; and at low temperatures, ethyl alcohol, toluene, or silicon oil.
The pressure gauge can be located for better operator visibility if the chemical seal is connected to the gauge by a capillary tube.
To maintain accuracy, capillary tubes should not be exposed to excessive temperatures and should not exceed 25 feet (7.5 m) in length.
The chemical seal itself can be of four designs: off line, “flow-through” type self-cleaning, extended seal elements, or wafer elements that fit between flanges.
The spring rate of the diaphragm in the chemical seal can cause measurement errors when detecting low pressures (under 50 psig, 350 kPa) and in vacuum service (because gas bubbles dissolved in the filling fluid might come out of solution).
For these reasons, pressure repeaters often are preferred to seals in such service. Pressure repeaters are available with 0.1% to 1% of span accuracy and with absolute pressure ranges from 0-5 mm Hg to 0-50 psia (0-0.7 to 0-350 kPa).
Presssure Switches
Pressure switches serve to energize or de-energize electrical circuits as a function of whether the process pressure is normal or abnormal. The electric contacts can be configured as single pole double throw (SPDT), in which case the switch is provided with one normally closed (NC) and one normally open (NO) contact.
Alternately, the switch can be configured as double pole double throw (DPDT), in which case two SPDT switches are furnished, each of which can operate a separate electric circuit.
The switch housings can meet any of the NEMA standards from Type 1 (general purpose) to Type 7 (explosion proof), or Type 12 (oil tight).
The Pressure transducer housing should be selected to meet both the electrical area classification and the corrosion requirements of the particular installation.
Corrosion protection must take into account both splashing of corrosive liquids or exposure to corrosive gases on the outside of the housing, as well as exposure of the sensing element to corrosive process materials.
The corrosion requirements of the installation are met by selecting corrosion-resistant materials, coatings, and by the use of chemical seals.
Installation Requirements
Branch connection up to and including the first block (root) valve shall meet the applicable Saudi Aramco piping codes. If more than one instrument are sharing a common root valve, means to isolate each instrument independently shall be provided.
Process connections for direct-mounted instruments shall be located where line vibration is negligible.
For low-pressure service, e.g., furnace draft, the instrument is to be mounted at the same elevation or above the pressure tap.
All pressure instrument installations shall be equipped with vent/drain valves.
For additional details of typical pressure instrument installations refer to the referenced Saudi Aramco Library Drawings and PIP PCIPR000, PCIDP000, PCIPR100, and PCIDP100.
Local recorder enclosures shall be NEMA Type 3 or IEC IP 44 as a minimum. Pressure and D/P switch enclosures for indoor use shall be NEMA Type 1 or IEC IP 22. For outdoor installation NEMA Type 4 or IEC IP 65 enclosures are required as a minimum.
International Codes and Standards for Pressure Switches, Transmitter Installation
- Flow Nozzle Material Selection Criteria Design & Flow Measurement
- Venturi Tube Material Selection Criteria Design & Flow Measurement
3. SAES-J-002 – Technically acceptable instruments, 23 June 2008
4. SAES-J-003 – Instrumentation Basic Design, 16 January 2008
5. SAES-J-200 – Pressure, 18 May 2008
6. PIP PCCPR001 – Pressure Measurement Criteria, April 2007
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