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Set Point Testing
An investigation of safety relief valve set point testing techniquesBy E. Smith - BP Amoco Exploration, Sunbury, England, and J. McAleese - City University, London, England
IntroductionIn 1995/6 a BP group-wide work group met to consider various aspects of safety relief valve management and performance. One subject considered was the setting of these valves and, in particular, the use of on line set point verification (OLSPV).Traditional methods of testing such valves in BP Amoco group companies conform to the recognised industry standard API 576, and the usual procedure requires that all PSVs are removed from the plant periodically so that their condition can be evaluated in a workshop. Prior tore-installation valves are then "pop" tested on a test bench. On steam boilers the bench set pressure must also be proven in-situ by "floating" the valve. This method is both time consuming and costly. There are, however, methods of testing safety relief valves on line (with and without pressure), notably the Furmanite 'Trevitest" safety valve testing system and comparable in-situ test systems offered by e.g. Crosby and Consolidated. The benefits of these methods are lower costs and, where valves are not "spared", extended plant run times.Although these techniques are well established in the power generation industry on steam service confidence in process industries has been low and little controlled testing has been conducted. The Working Group proposed an investigation, carried out in collaboration with the vendors, to test the reliability of OLSPV and better define its limitations. This paper describes the resulting tests. At the same time the opportunity was taken to investigate the accuracy of the widely used bench testing technique.Most of the in-situ tests were performed using Furmanite's "Trevitest" system but a smaller number of tests (three valves on air and steam) were also carried out using Tyco Flow Control's Crosby system to provide confirmation of results and a comparison of the techniques.For the two participating vendors (Furmanite and Crosby) the apparatus, hardware and basic operating procedure were similar, differing mainly in the software and presentation of data. Other vendors may offer different in-situ test methods. (The inclusion of these two vendors in the programme does not imply approval and is not a comment on competing techniques which were not evaluated.)Note that, regardless of the method of set point verification employed, it is necessary to remove relief valves periodically in order to inspect and refurbish the valve internal parts and to inspect and maintain pipework. The necessity is greater on dirty, fouling or corrosive duties than on clean service.
Description of the on-line set point verification techniqueA hydraulic power pack supplies energy to a hydraulic ram located in the cross-head of a mechanical rig. The hydraulic ram displaces vertically upwards, overcoming the spring force and causing the valve to lift when the upward force exerted by the working fluid overcomes the downward force exerted by the spring. The force generated during this operation is measured using a strain gauged load cell and recorded on a portable computer where it can be converted into an equivalent pressure. If the test is undertaken with line pressure present, the calculated pressure is added to the line pressure present to give the as-found set pressure.Line pressure is measured by means of a calibrated pressure transducer connected to the portable computer. Valve lift is measured and recorded using a displacement transducer. The computation of this information, captured graphically on portable computer, leads to the determination of the set pressure, valve lift or displacement, spring adjustment rate, an indication of the effect of the nozzle and guide rings and the re-seat pressure.The accuracy with which the effective seat area of the valve is known has an effect on the accuracy of set pressure prediction. In this case the valve seat area was estimated based on average seat face width. A more accurate approach is to conduct a "fingerprint" test under known conditions which allow the effective seat area to be calculated and recorded for future use.
Test objective
The aim was to assess the effects of operating pressure, seat area, test fluid, and valve design on the accuracy of OLSPV and to compare actual lift pressure, bench set pressure and on-line set pressure.
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