This article is about Instrumentation technical engineering requirement for pressure measurement and design criteria in plant and petrochemical industry.
Table of Contents
1. Scope
This standard establishes requirements for material, design, procedure, codes and standards for
engineering and design of the pressure instrumentation.
2. Technical References
Reference is made in this standard to the following documents. The latest issues, amendments
and supplements to these documents shall apply unless otherwise indicated.
SABIC Engineering Standards (SES)
R01-E01 Field Instrumentation Design Criteria
R11-C01 General Instrument Installation Criteria
American Society of Mechanical Engineers (ASME)
B40.100 Pressure Gauges and Gauge Attachments
3. Definitions
Bellows: A pressure sensing element generally of cylindrical shape whose walls contain deep
convolutions and its length changes when a pressure 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 it.
Diaphragm Seal: A thin flexible sheet of metal clamped between two body halves to form a
physical barrier between the instrument and process fluid.
Overrange: Any excess value of the input signal above its upper range value or below its lower
range value.
Pressure Element: The part of a pressure instrument that displaces or temporarily deforms by
the pressure.
Pulsation Dampener: A device installed in a gas or liquid piping system to smooth out
fluctuations due to pulsating flow, pressure or both.
4. Pressure Instruments
4.1 General
4.1.1 For general design, guidelines and specification refer to SES-R01-E01.
4.1.2 Wetted parts shall be made of 316 stainless steel, as a minimum, unless process conditions
require different material.
4.1.3 Instruments used for measuring pulsating pressures of reciprocating pumps and
compressors shall be equipped with pulsation dampeners to prevent premature failure of the
pressure instruments.
4.1.4 Installation of pressure instruments shall conform to SES-R11-C01.
4.2 Pressure Gauges
4.2.1 Pressure gauges shall conform to ASME B40.100.
4.2.2 Pressure gauge minimum dial size shall be 100 mm. However, different dial sizes of gauges
can be considered for special applications, with SABIC approval.
4.2.3 The pressure connection socket shall be provided with wrench flats.
4.2.4 Pressure gauge measuring element shall be C type bourdon tube. However, other types of
elements, e.g. bellows, spiral or helical tube, or diaphragm, can be selected considering
range, accuracy and process fluid requirements. The use of tungsten inert gas (TIG) welded
tubes requires SABIC approval.
4.2.5 The pressure gauge measuring element and socket shall be hardened type 316 stainless
steel, as a minimum, unless process conditions require different material. The pressure
gauge scale shall be 1.3 times the maximum operating pressure.
4.2.6 Pressure gauge movements shall be stainless steel.
4.2.7 The pressure gauge case front shall be solid and furnished with a blow out back or disc. All
cases shall be made of aluminium alloys, stainless steel or phenolic. Other case material
shall be approved by SABIC.
4.2.8 The pressure gauge dial shall be white laminated phenolic with black graduation numerals
and pointer.
4.2.9 The pressure gauge window shall have safety glass with a resilient gasket on the bezel
side, held in place from the case side by a threaded retaining ring. Other material for
pressure gauge window shall be approved by SABIC.
4.2.10 Field pressure gauge shall have bottom pressure connection.
4.2.11 Panel mounted gauge shall have flush mounting case with rear pressure connection.
4.2.12 The pressure gauge pointer shall be adjustable without removing it from its shaft.
Movements shall be zero and span adjustable.
4.2.13 A visible stop pin shall be used to restrict the upper limit of the pointer travel.
4.2.14 Liquid filled pressure gauges shall be specified for reciprocating pump, compressors and for
any service where excessive vibration or pulsations are expected.
4.2.15 Liquid filled gauges shall have vapor space for thermal expansion of the liquid fill. Vapor
space shall be adequate to allow gauge to operate satisfactorily without leakage under the
full range of ambient and process conditions specified.
4.2.16 Fill fluids used in liquid filled gauges, diaphragm seals etc. 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 (e.g., Oxygen, Chlorine,
Nitric acid, or Hydrogen peroxide, etc.) because of the risk of spontaneous chemical
reaction, ignition or explosion. Fluorolube shall be specified in these applications. Fill fluid
shall be reviewed and approved by SABIC.
4.2.17 Standard range shall be selected such that the normal operating pressure falls between 30
percent and 70 percent of the scale range. The operating pressure shall not exceed 75
percent of the scale range. Failure to select gauge within these criteria may result in fatigue
failure of the bourdon or bellows.
4.2.18 Draft gauge should be vertical edgewise type. Their elements shall be made of phosphor
bronze.
4.2.19 The pressure gauges minimum accuracy class should be +/- 1 percent of span as per
ASME B40.100 Grade 1A.
4.2.20 Gauges supplied by vendors as part of other pneumatic devices such as current to
pneumatic converters, instrument air supply regulators, valve positioners, piston actuators,
and similar devices shall be vendor standard unless otherwise specified.
4.3 Pressure Switches
4.3.1 Pressure switches shall not be used without SABIC approval.
4.3.2 Pressure switch measuring element shall be bourdon tube, bellows, or diaphragm type,
depending upon the service and pressure. The material for elements shall be hardened type
316 stainless steel, as a minimum, unless process conditions require different material.
4.3.3 Electrical switch contacts shall be minimum single-pole double-throw (SPDT) type and
hermetically sealed. The contact rating shall be suitable for the specified service.
4.3.4 Pressure switch shall be selected to operate in a range that allows switch actuation to take
place above the midpoint of its range to provide better accuracy.
4.3.5 Pressure switch actuation, as the result of increasing or decreasing pressure, shall be
specified.
4.3.6 Pressure switch dead band shall be compatible with application. A large dead band may
prevent switch reset after trip has cleared.
4.3.7 Pressure switch accuracy shall be at least +/- 0.5 percent of the set value unless otherwise
indicated.
4.4 Differential Pressure Instruments
4.4.1 Specifications for the pressure instrumentation outlined under section 4.1, 4.2 and 4.3, shall
also apply for the differential pressure instruments.
4.4.2 Differential pressure gauge and switch shall be diaphragm or bellows type. Dual Bourdon
tube type differential pressure gauges are not acceptable.
5. Inspection and Testing
Instruments shall be tested and calibrated before shipment.