This article establishes design requirements for pressure instruments in process plants. Main keywords for this article are Design Requirements for Pressure Instruments. Material for Pressure Instruments. Installation of Pressure Instruments. Definitions Pressure Instruments.
Definitions Pressure Instruments
Absolute Pressure – Pressure measured on a scale referred to a perfect vacuum. Zero absolute represents the absence or total lack of pressure.
Blind Pressure Transmitter – A pressure transmitter not having an integral readout device.
Diaphragm Seal – A thin flexible sheet of material clamped between two body halves to form a physical barrier between the instrument and process fluid.
Differential Pressure – Pressure measurement based on the differential of two pressures.
Gauge Pressure – Pressure measured on a scale that is referenced to atmospheric pressure. Gauge pressure measurement represents the positive difference between measured pressure and existing atmospheric pressure.
Needle Valve – Metering valve used chiefly for precisely controlling flow.
Pigtail -A 270 ° or 360 ° loop in pipe or tubing to form a trap for vapor condensate. Used to prevent high temperature vapors from reaching the instrument. Used almost exclusively in static pressure measurement.
Pressure Element – A device that converts a pressure into motion or movement. Purging-Elimination of an undesirable gas or material from an enclosure by means of displacing the undesirable material with an acceptable gas or material.
Radio Frequency Interference – An electrical noise that affects electronic circuits adversely.
Suppression, Suppression Zero – A range in which the zero value of the measured variable is below the lower range value.
Design Requirements for Pressure Instruments in Process Industry
- The maximum operating pressure shall be 65 percent of the measuring range.
- All electronic parts of pressure instruments shall operate on 24 V dc power supply.
- All pneumatic parts shall operate with 1.4 kg/cm2 air supply.
- All pressure instruments shall be protected against Radio Frequency Interference (RFI). Instrument function shall not be affected by operation of Frequency Generating Units, for example, walkie-talkie sets or cellular phones.
- Instruments shall have over-range protection to the maximum exposed pressure. Instruments exposed to vacuum shall have under-range protection to full vacuum.
- Barometric pressure change compensation shall be provided for instruments measuring barometric pressures.
- Graduations on charts and scales shall be as follows:
a. 0-100 linear for charts.
b. Direct reading of pressure scales. - Undesirable process fluids shall not be piped to any instrument located in a central control room. Pressure signals shall be transmitted electrically to the DCS in control room.
- A suppressed-zero pressure transmitter shall be supplemented with a full-range process pressure gauge, even if the transmitter is equipped with an output indicator.
- An instrument shall be sealed, purged, or protected by a diaphragm seal when measuring viscous or corrosive fluids, or fluids with solids. Diaphragm seal unit shall have a flushing connection and wetted parts made of corrosion resistant materials.
Material for Pressure Instruments
1. All wetted and pressure-containing parts shall be made of grade 316 stainless steel unless process conditions require a different material.
2. Gold plated diaphragm shall be used in process streams containing Hydrogen.
3. Monel is recommended for caustic and salt solutions.
4. Pressure containing parts of low melting metals (for example: brass or aluminum) shall not be used in instruments used for flammable, or toxic gas service applications, or both.
Installation of Pressure Instruments
1. Pressure Instruments installation shall be in accordance with SES R01-C01 requirements.
2. Instruments mounted above the take-off connection shall be protected from hot condensable vapors of temperature above 65 °C by installation of a pigtail (siphon).
3. Instruments used for measuring pulsating pressures of reciprocating pumps and compressors shall be equipped with pulsation dampeners to prevent premature failure of the movements or the pressure elements. Needle valves, floating pins, or porous metal devices are often used for this purpose. Pressure indicating gauges with liquid filled casing shall be used for pulsating-service applications
4. Where local indication is desired and blind pressure transmitters, switches, and locally mounted pressure controls are used, these instruments shall be supplemented with directly connected process pressure gauges, or output indicators, or both.
5. Consideration shall be given to the local conditions (sandstorms, dust, marine atmosphere, large variation in temperature, and high humidity) in designing the instruments.
6. All instruments shall be tropicalized to conform to manufacturer’s requirements.