This article establishes requirements for local pressure controllers in process plants. Main keywords for this article are Local Pressure Controllers Design Requirements in Process Industry. References Local Pressure Controllers. Definitions Local Pressure Controllers.
References Local Pressure Controllers
Reference is made in this article to the following documents.
American Petroleum Institute (API)
RP 551 Process Measurement Instrumentation
National Electrical Manufacturer’s Association (NEMA)
Standard for Electrical Equipment Enclosures
Definitions Local Pressure Controllers
For the purpose of understanding this standard, the following definitions apply.
Bellows.
A pressure sensing element generally of cylindrical shape whose walls contain deep convolutions, and for which the length changes when a pressure differential is applied.
Bourdon Tube.
A pressure sensing element consisting of a twisted or curved tube of noncircular cross section which tends to be straightened by the application of internal pressure.
Diaphragm.
A sensing element consisting of a thin, usually circular, plate which is deformed by pressure differential applied across the plate.
Explosion Proof Enclosure.
An enclosure that is capable of withstanding an explosion of a gas or vapor within it and of preventing the ignition of an explosive gas or vapor that may surround it and that operates at such an external
temperature that a surrounding explosive gas or vapor will not be ignited thereby.
Intrinsically Safe System.
An assembly of interconnected intrinsically safe apparatus, associated apparatus, other apparatus, and interconnecting cables in which those parts of the system which may be used in hazardous (classified)
locations are intrinsically safe circuits.
Pressure Element.
A device that converts a pressure value into motion or movement.
Suppressed Range / Suppressed Zero Range.
A range in which the zero value of the measured variable is below the lower range value.
Local Pressure Controllers Design Requirements in Process Industry
- Pressure controller sensing elements shall be Bourdon Tube, spiral, helix, bellows, or diaphragm type, depending upon the service and pressure.
- The material of the sensing element shall be hardened Type 316 stainless steel, unless process fluid requires the use of other material.
- Indicating pressure controllers located on Instrument Local Panels, or in the field shall have indicators for process pressure and set point.
- Self contained pressure controllers in utility services (for example: steam, air, water, or inert gas) for locally mounted (non-panel) installations, shall have an indicator for the measured variable mounted as close as possible to the controller.
- Suppressed zero range shall be used for pressure control of any process or system above 4.5 kg/cm. The span of the suppressed range shall not exceed 50% of the operating pressure.
- Balance-less and bump-less transfer switches shall be provided with electronic or pneumatic controller.
- Accuracy of process measurement shall be ± 0.5% of full span or better, unless otherwise specified.
- Electronic controllers shall meet the requirements of Hazardous Area Classification. If required they shall be provided with explosion proof enclosure or intrinsically safe system. Safety barriers may also be used.
- Controller shall be provided with NEMA 4X weather proof enclosure.
- The installation of pressure controllers shall be in accordance with API RP 551.