This article is about Design Requirement of Pressure Instruments in Process Industry, Pressure Gauges Design Requirements, Pressure Switches Design Requirements, Accessories, Pressure Measurement – Low temperature and Cryogenic Service, Local Pressure Controllers Design Requirements.
Design Requirement of Pressure Instruments in Process Industry
Specifications for Pressure Instruments
- Direct connected instruments shall have spiral, bourdon, or helix diaphragm elements or bellows type elements depending upon the service requirements.
- Pressure elements shall, in general, be 316 SS except where the nature of the fluid is such that requires other material; the primary element shall be suitable for the service.
- Sockets shall be of the same material as the primary element and shall be welded to the element using the same filler material.
- In general, pressure instruments and gauges shall be furnished with full-scale standard pressure ranges having the lower limit of the range equal to zero gauge pressure. Suppressed ranges may be utilized where required for measurement sensitivity. For pressure gauges the range shall be selected so that the normal expected operating pressure shall be between 33 percent and 67 percent of full scale, and the maximum expected operating pressure shall be less than 100 percent of full scale. Emergency overpressure conditions shall not exceed the over-pressure rating of the gauge. Transmitters
shall have a span such that the normal operating pressure indicates near mid-scale. - Direct connected pressure gauges shall be provided with blowout backs or disks. Solid front gauges shall be used on all services.
- Pressure instruments exposed to possible vacuum shall be protected for full vacuum.
- Where diaphragm seals are used with pressure instruments, they shall be continuous duty type, shall be directly connected to the pressure element, and furnished as a complete assembly with the instrument mounted directly on the seal. The process connection on the seal shall normally by 3/4 inch NPT. 2 inch flanged connection may be used for special services (e.g., seawater).
- Pressure instruments shall generally have screwed connections. Pressure gauge stem connections shall generally be ½ inch FNPT. Connections in piping or equipment for pressure and differential pressure instruments.
- Pressure instruments on steam and other high temperature services shall be installed with gage siphons when the instrument is closed coupled and/or mounted above the pressure connection with no trap in the connecting piping.
- Liquid filled gauges shall be used for discharge of reciprocating pumps, suction and discharge of reciprocating compressors, and in corrosive atmospheres.
- Sprague (or equal) gauge savers shall be used on pressure instruments where the operating pressure of the service to which the instrument is connected can rise above the maximum working pressure of the instrument.
- Pressure instruments shall be located so that the head of liquid between the instrument and the point of measurement does not exceed the manufacturer’s limit of impressed static head.
- Pressure, including absolute pressure, transmitters shall be designed for an overrange pressure of the greater of 7.0 barg or 150 percent of the maximum sensor range without affecting the calibration of the instrument. Differential pressure transmitters shall be designed for differential pressure overranges, in either direction equal to the body pressure rating without affecting the calibration of the instrument.
- All pneumatic parts shall operate with 0.28 bar air supply. 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.
- 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.
- The following units of measurement shall be used:
a. Gauge Pressure: barg
b. Vacuum: torr
c. Absolute Pressure: bara - Gold plated diaphragm shall be used in process streams containing Hydrogen.
- Monel diaphragm shall be used for caustic and salt solutions.
- Two valve manifold assemblies (block and bleed) shall be installed between the process block valve and the pressure instruments.
- Installation
a. Pressure connection sizes for the instruments shall conform to the following requirements:
i. Pressure and Differential ½ inch FNPT PressureTransmitters and Controllers:
ii. Diaphragm Type Pressure and Differential 2 inch Flanged Pressure Transmitters:
iii. Pressure Gauge (Bourdon Type): ½ inch MNPT
iv. Pressure Gauge (Diaphragm Type): Mfg. Standard
v. Draft Gauge: ¼ inch FNPT
b. Instruments mounted above the take-off connection shall be protected from hot condensable vapors of temperature above 65 deg C by installation of a pigtail (siphon).
c. 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.
d. 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.
Pressure Gauges Design Requirements
- Pressure gauges shall conform to ASME B40.1.
- Pneumatic air receiver indicating gauges shall conform to ASME B40.1, Grade 2A.
- Nominal diameter of pressure gauges shall be 100 mm (4 inch) or 115 mm (4-½ inch), and bottom connection shall be ½ in. NPT male. However, different sizes of gauges may be
considered for special applications. - Vibration proof gauges shall be used if the surrounding environment is subject to vibration. Liquid filled gauges shall be used. Gauges on reciprocating pump discharge or other pulsating applications shall be liquid filled or shall be provided with pulsation dampners.
- The gauges shall pass Vibration Test (A3.10) and Fatigue Test (A3.13) in accordance with ASME B40.1, Appendix A.
- Pressure gauge measuring element generally shall be C type Bourdon tubes. The minimum material of construction for elements (Bourdon tube, stem & movement), and sockets shall be stainless steel 316, unless otherwise specified.
- Differential Pressure gauges shall be of bellows type; dual bourdon tube type differential pressure gauges are not acceptable.
- Cases shall be solid front, weather proof, and furnished with a blowout disc. All cases shall be made of aluminium alloys, stainless steel or phenolic. Dial shall be white laminated phenolic with black graduation.
- Draft gauges should be the vertical, edgewise type. Their elements shall be made of phosphor bronze.
- Accuracy shall be:
Pressure Gauges ± 1.0 percent of Full Scale.
Draft Gauges ± 1.0 percent of Full Scale.
Diff. Pressure Gauges ± 2.0 percent of Full Scale.
Diaphragm Seal Type Gauges ± 2.0 percent of Full Scale. - Where seal elements are used, they shall be of the continuous duty type, shall be directly connected to the pressure element and furnished as an integral part.
- Standard gauge stem size shall be ½ inch NPT, except receiver gauges shall be ¼ inch NPT.
- The gauge window shall be double-strength shatter-resistance safety glass. The window shall have resilient gasket on the bezel side, held in place from the case side by threaded
retaining ring. - The pressure connection socket shall be provided with wrench flats.
- The pressure range of the element and the element material shall be stamped on the socket.
- The gauge pointer shall be adjustable without removing it from its shafts.
- A visible stop pin shall be used to restrict the upper limit of the pointer travel. The stop shall be located at the six (6) o’clock positions on the gauge front.
- Fill fluids used in liquid filled gauges shall be selected carefully, and accounted for both process and ambient temperature limits. Glycerin or Silicon fill fluids shall not be used in
applications involving strong oxidizing agents (for example: chlorine, nitric acid, or hydrogen peroxide) because of spontaneous chemical reaction, ignition or explosion. Fluorolube shall
be specified in these applications. Fill fluid shall reviewed and approved by the Owner. - The gauges shall be isolated from the following process applications by using Diaphragm Seals:
a. Slurries, viscous or corrosive services.
b. Flammable or toxic streams in high pulsating/vibrating service.
Pressure gauges shall not have suppressed ranges.
Pressure Switches Design Requirements
- Pressure switch measuring element shall be Bourdon tube, bellows, or diaphragm type, depending upon the service and pressure. The material for elements shall be 316 stainless steel, unless otherwise specified.
- External adjustment shall not be provided.
- Electrical contacts shall be the single-pole, double-throw type and hermetically sealed type, with appropriate rating.
- Local mounted pressure switches shall meet the relevant electrical area classification requirements.
- Pressure switches shall not be used on process service. Switch signals for process service shall be derived from transmitter analog (4-20 ma) signals.
- Connections for pressure switches shall be ½ inch MNPT.
- Electrical switch shall have a minimum contact rating of 115 volts AC/DC, 2 amperes. Any switch that initiates a device, for example an inductive trip coil of a motor starter, shall have inductance contact rating suitable for that service.
- Pressure switches shall have a NEMA 4X weather proof enclosure Pressure switches shall meet the requirements of NFPA 70. They shall be provided with
explosion proof enclosure. - Accuracy of pressure switch shall be at least ±0.5 percent of the set value unless otherwise indicated.
- Actuation value shall be adjustable externally.
Accessories
- ½ inch gauge siphons shall be provided for gauges on steam service, or fluids with temperatures above 200oC. Siphon shall be installed adjacent to pressure instruments when the instrument is close coupled and/or mounted above the tapping point.
- For pulsating service, oil filled gauges shall be preferred. Snubbers with restriction orifice or moving piston are acceptable.
- Diaphragm sealing units shall be used with gauges measuring pressure of highly corrosive fluids, or fluids that may harden at ambient temperatures,
Pressure Measurement – Low temperature and Cryogenic Service
- Special attention must be given to those instruments, which will be exposed to a considerably higher static pressure than normal during plant startup and shutdown periods. Suitable over range protection must be provided especially pressure gauges.
- Pressure gauges and direct connected pressure switches shall not be close coupled on low temperature and cryogenic service. This is to prevent exceeding the low temperature design limits of these items.
Local Pressure Controllers Design Requirements
- 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 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.
- Balance-less and bump-less transfer switches shall be provided with electronic or pneumatic controller.
- Accuracy of process measurement shall be ±0.5 percent of full span or better, unless otherwise as specified.
- Electronic controllers shall meet the requirements of Hazardous Area Classification. They shall be provided with explosion proof enclosure.
- Controller shall be provided with NEMA 4X weatherproof enclosure.
- The installation of pressure controllers shall be in accordance with API RP 551.