Valve monitoring system forms vital part of predictive maintenance solution

22 January 2013

A networked solution for digital valve monitoring and partial stroke testing on a large scale oilfield installation in the Middle East will feature a Rotork SVM (Smart Valve Monitoring) system.

The system has been designed to deliver predictive maintenance information from widely distributed areas for use by operators to help reduce shutdowns and improve overall plant efficiency.

This is made possible by the ability of the SVM to be integrated into an existing Ethernet and fibre optic infrastructure, enabling over 100 monitored valve actuators on wellheads at numerous locations to be networked over distances of 10 km or more.

The project sees SVM field control units attached to the ESD circuits for shut off valves, installed in sets of two and four on more than 50 wellheads. Analysis is performed on the SVM server computer in the centralised control room.

Communication between the wellhead sites and the computer is fully integrated within the operator’s existing network infrastructure. The SVM field units are incorporated into the Ethernet network that links other equipment and instrumentation at each wellhead site. A fibre optic link is then used to transmit the data to intermediate engineering and control stations at two manifold sites. From these sites, fibre optic links are again used to transmit data from all the wellheads to the centralised control room.

Partial stroke testing (PST) is a function used in a safety instrumented system (SIS) to enable operators to identify possible failure modes on a shutdown or emergency shutdown (ESD) valve without the need to completely close the valve which would disrupt the process. Partial stroke testing is an accepted hydrocarbon industry standard technique that is quantified in detail by regulatory bodies such as the IEC and ISA.

The partial valve stroke prevents unexpected failure on demand of the safety function and demonstrates that certain potential problems that would otherwise go undetected, such as spring fractures in the spring chamber of the pneumatic actuator, are not present. Consequently, the interval for testing for these otherwise undetected errors can be extended.

The SVM is said to incorporate several features that are not available from other systems, as well as providing detailed diagnostic data that allows the operator to plan for strategic preventative maintenance. The key to its reliable performance is its separation from the valve’s control system which enables operators to design the control system to suit the routine and safety requirements without having to compromise for the testing programme.

The SVM system is powered by the control signal to the actuator’s solenoid valve. The monitoring function is then provided by a pressure transmitter located between the solenoid valve and actuator, which records the instrument pressure changes while the valve is moving. Any change in the valve performance is detected and identified by a change in the pressure wave exiting the actuator. The self-contained design of the SVM enables it to be used with the most complex control mechanisms and makes it impossible for the SVM to prevent the valve from closing on demand.

Analysis of the 4-20mA signal from the pressure transmitter during the partial stroke test is performed using SVM software to confirm the correct functioning of the valve or identify a fault. To achieve this, the output pressure curve of the transmitter is compared with the pressure signature of the actuator, obtained during commissioning. Identified faults can include valve obstructed, damaged actuator cylinder, seized valve or failsafe spring failure, stem shear or disconnected valve, stiff valve, increased breakout torque, damaged valve seat, internal cylinder corrosion, exhaust restriction and sticking solenoid valve.

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