NELES - THE VALUE OF SAFETY VALVES

Published: 25th September 2009 | Issue 6

Safety valves are used to protect processes, personnel and the environment against process disruption. Such valves are operated only in trip situations, but when they are required to operate, it is essential that they ensure that the valve is switched to the safety position. However, because these shutdown valves are rarely cycled, there is always concern over whether they will
operate when actually needed.

Introduction Of On-Line PST and It's Advantages

Partial stroke testing is technology whereby the emergency isolation valve is moved only slightly, so that it does not disturb the protected process, yet the movement is sufficient to detect the most critical problems with the emergency valve. When PST is carried out using an intelligent emergency valve controller, the test results can be analysed automatically after test implementation.

Partial stroke testing allows more frequent valve stroking without disturbing the process. In this way, the required SIL level for Safety Instrumented Functionality (SIF) can be maintained for longer periods. PST is designed to detect random hardware failures related to the final element. Detection is based typically on the change in valve dynamics, which can be seen when the latest PST results are compared to historical data. For example, Neles ValvGuard measures the breakaway pressure and load factor from PST and is one method of doing a historical comparison using an easily interpretable graph. Breakaway pressure indicates the pressure measurement level at which the valve starts to move during a valve test. Information can be used to analyse valve load changes.

Future Trends

One clear future trend seems to be the use of fieldbuses. FOUNDATION fieldbus and Profibus organisations are the most active in this area and are enlarging their specifications to cover Safety Instrumented Functionality. The reasoning is clear. Fieldbuses are used more often and have proved the technology.

New upstream processes are making more use of Fieldbus technology mainly because it reduces hardware, wiring and engineering costs and improves the asset management integration of final elements into the host system. Traditionally, on installation all control valves have fully supported Fieldbus communication technology, but emergency valve controllers are integrated into the asset management systems through HART communication, using separate communication hardware.

FOUNDATION-based emergency valve controllers would give much faster response times for the status information and easier integration of higher-tier diagnostic information into the host system, when compared to the parallel HART network. This would improve the cost efficiency of the project when additional communication hardware is not required. At the same time, operation of the device is much easier for the end-user.

Conclusions

The use of intelligent functions with safety valve controllers has created considerable interest in the recent past. Nowadays, automatic PST implementation is seen more often as a serious possibility, rather than as a possible source of spurious trips. Furthermore, the analysis results of PST implementation are being widely used as a part of the maintenance program. This trend raises the need for easier field-device integration up to control system level and expectations now lie in the wider use of fieldbus technology.

Metso Automation, Tel: 0870 606 1478

Reprinted with permission of Hydrocarbon Engineering,
December 2007.

Partial stroke testing is technology whereby the emergency isolation valve is moved only slightly, so that it does not disturb the protected process, yet the movement is sufficient to detect the most critical problems with the emergency valve. When PST is carried out using an intelligent emergency valve controller, the test results can be analysed automatically after test implementation.

Partial stroke testing allows more frequent valve stroking without disturbing the process. In this way, the required SIL level for Safety Instrumented Functionality (SIF) can be maintained for longer periods. PST is designed to detect random hardware failures related to the final element. Detection is based typically on the change in valve dynamics, which can be seen when the latest PST results are compared to historical data. For example, Neles ValvGuard measures the breakaway pressure and load factor from PST and is one method of doing a historical comparison using an easily interpretable graph. Breakaway pressure indicates the pressure measurement level at which the valve starts to move during a valve test. Information can be used to analyse valve load changes.

Valmet UK Ltd