Pressure Indicating Systems (updated 08/07/04)

This article replaces the article titled 'Pressure Testing of Equipment and Setting of Safety Valves' which appeared in Safety Lines No 53 (March 2002). It focuses on the use and requirements of pressure gauges and pressure transmitters, and is not intended to cover all aspects of safety valve setting or pressure testing (e.g. temperature limitations).

The following guidelines apply in relation to pressure equipment within the meaning of the PECPR Regulations. Whilst every care has been taken over advice given here, the user is fully responsible for ensuring that applicable standards are met.

It is acceptable to verify safety valve pressures in one of the following circumstances:

• For a pressure vessel (other than a boiler), on a test bed or in situ (clean service), provided that the gauge or pressure transmitter used has been calibrated by an accredited laboratory and has a current certificate of calibration. The pressure range(s) covered should be clear on any certificate of calibration for a pressure transmitter.
• During a boiler inspection under steam, against the boiler gauge, provided that the gauge has been calibrated by an accredited laboratory and has a current certificate of calibration.
• During a boiler inspection under steam, against a gauge, which has been calibrated by an accredited laboratory and has a current certificate of calibration, fitted to the test point situated at the boiler pressure gauge.

During accumulation testing to verify safety valve capacity, the pressure must not exceed 110% of the maximum allowable working pressure, unless permitted by an appropriate standard for a multiple valve installation. Valves are to be set to a pressure not exceeding 100% of maximum allowable working pressure, but an appropriate standard may permit setting at up to 105% of the maximum allowable working pressure for one or more valves in a multiple valve installation or for an additional pressure relief device.

Any gauge used for the pressure testing of equipment or setting of safety valves must have all of the following features:

• Be of the Bourdon tube type;
• Be large enough for the scale to be read, from the normal reading position, to the accuracy required to ensure that the valve settings are accurate. If permanently fitted to a boiler, the nominal size shall be not less than 150mm (100mm for low hazard boilers);
• Have a scale extending over an arc of approximately 270 degrees; and
• Have a scale selected such that the working pressure lies in its middle third. If permanently fitted to a boiler, it shall have the working pressure clearly marked by a red line.

Any pressure transmitter used for the pressure testing of equipment or setting of safety valves (other than on boilers) must have all the following features:

• Have an appropriate operating range for the application under consideration;
• Be calibrated as a system - i.e. transmitter and display as a connected unit; and
• Be capable of having its display checked independently of its transmitter.

A gauge or pressure transmitter used for pressure testing of equipment or for verifying safety valve pressures must be traceably calibrated to confirm that its error does not exceed 1% of maximum scale value (or maximum selected range value of pressure transmitter) for indications between 10% and 90% of maximum scale value (or maximum selected range value of pressure transmitter), and otherwise 1.5%. Calibration must be carried out by a laboratory accredited by IANZ to ISO/IEC 17025. The gauge or pressure transmitter is to be calibrated against a reference gauge, which in turn has been calibrated against a more accurate gauge, and so on back to the national standard. For traceability, all links back to the national standard must be fully documented.