Safety in the Plant
Efficient periodic testing of safety relief valves
Pressure vessel legislation requires that safety relief valves are subjected to periodic testing in order to verify their functionality. Roger Bours reports.
W
CALIBRATED ASSIST DEVICE
SPINDLE
ASSIST LOAD
hile regular testing of safety relief valves is necessary, the process industries aim to reduce the downtime of the process and costs combined with the highest possible safety levels. If
a plant shut down is planned the pressure relief valves can be dismantled and shipped to a maintenance shop for testing on a test bench and eventual be overhauled. The testing procedures are usually according to API 527, a
recognised method of testing safety valves for operational fitness and leakage rate. Spring operated safety relief valves require the use of a test bench; therefore the valves need to be removed from the process. This is expensive because of the necessary process shut down, but also because of the cost of dismounting, the use of cranes or lifting devices/access platforms, labour costs, transportation to the maintenance shop, etc.
Reducing costs
Not every tested safety relief valve needs repair or maintenance so plant operators are constantly looking for reducing the costs so that when a safety relief valves need testing the plant does not require going off line.
To achieve this it is possible to install similar relief valves on an upstream change-over valve. One valve is in operation while the second is in stand-by. The stand-by valve can be maintained without having to
shut down the process. This method provides a full redundancy of the safety relief valves at an important investment-level. Where the valves exhaust in a common header system an outlet changeover valve with a fail-safe interlocking is required. Changeover valves have a pressure drop typically between 15 and 20 per cent which needs to be taken into consideration when sizing and selecting the discharge system. Alternative API-compliant selector valves with a maximum
pressure drop of only 3 per cent are available. Such valves are more compact and do not require oversized valves with higher resulting maintenance costs.. Identifying a safety relief valve’s set pressure while the
FORCES AT SET PRESSURE: SPRING LOAD = INLET PRESSURE LOAD +AUXILLARY LOAD
protected pressure vessels remains in operation is known as on-line testing or in-situ pressure verification. In-situ testing has become especially popular on processes, which are difficult and/ or expensive to shut down such as offshore platforms, cryogenic applications (LNG trains), nuclear and fossil power stations, etc. In-situ testing of safety relief valves provides generally a good alternative to bench testing, it must be mentioned that it will not suit every application and will not eliminate the need to remove safety relief valves periodically for detailed analysis, maintenance and/or replacement of their internal components. While systems for performing in-situ
testing of spring operated safety relief valves might differ slightly, all are based on the same basic principle of calculations. For all of them it is required to know:
SPRING
SPRING LOAD
❒ Exact inlet operating pressure at exact time of valve lift. This must be recorded when the disc lifts from the nozzle.
DISC
❒ The effective (measured) seat or pressure area and the manufacturing tolerances (not always easy to obtain from manufacturers).
SEAT AREA NOZZLE
PRESSURE LOAD
❒ The exact auxiliary load applied to oppose the spring force at the time of effective lift of the valve.
In situ testing principle of spring operated valves.
38
www.engineerlive.com
Only when all above are exactly known at exact the same time a reliable test pressure can be calculated. Accuracy errors on each
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44