A partial stroke test (PST) application with solenoid valve (SOV) monitoring offers high levels of safety to avoid spurious trips and still allow testing of the valve. Other benefits include:
* Reduced installation cost in PST monitoring design.
* Easy implementation with existing ESD valves closed by the SOV.
About Valve & Automation positioners
Valve & Automation provides total valve and control solutions to the chemical, refining, mining & minerals, pulp & paper, sugar, steel, power generation and other process industries throughout South Africa. V&A iCon offers a wide range of control valve positioners, from pneumatic and analogue devices to advanced smart positioners for any industry. These are certified to ATEX, FM, CSA, INMETRO, GOST or NEPSI and provide solutions for HART, Profibus and Foundation Fieldbus, all SIL3 certified for applications with safety valves.
One particular application is PST, where the ESD valve is typically moved 10-20% and returned to its original position over a short period of time. As the most common dangerous failure mode in a static ESD valve is ‘stuck’, on-line PST done by the smart positioner SRD991 (intrinsic safety application) and SRD960 (explosion proof application) is key for safety. Moreover, the PST positioner offers operators a tool to enable predictive maintenance on the valves via data history and friction analysis. The test can be executed via the FDT-DTM based configuration and diagnostic tools VALcare and Valve Monitor.
Application challenge
For its new gas treatment plant, RasGas wanted to the highest possible level of safety on its ESD valves. These are contolled by SOVs connected digitally output to the SIS, in the case of emergency shutdown the SOV cuts the air pressure to the actuator. The positioner is mounted before the SOV and it can modulate the supply pressure to achieve PST. The positioner is used only for the PST and the SOV is in charge of the shutdown. For this project, the plant owner wanted to have feedback not only for the valve status, via PST, but also for the solenoid valve. Can the solenoid valve still react or not? This question is critical because it can have a severe impact on the functionality of the safety system resulting in adverse conditions for operators, plant equipment and the environment. In addition, an important requirement was to avoid, a spurious trip from the positioner, even in case of damage in the electronic board. Based on these challenging customer requirements, the design engineers developed a state-of-the-art PST positioner.
V&A iCon positioners already offer the standard features for a PST application with the status available under the 4th HART cyclic variable. In addition, the positioner has been pneumatically modified to become fail open, which means that even in a case of electronic damage the valve will remain open. Regarding SOV monitoring, the pressure between the solenoid valve and the actuator is measured by the positioned, therefore the positioner is able to detect the perturbation of pressure due to a quick test of the SOV. Any small SOV switches that are higher than 200 ms detected by the positioner are enough to show that the solenoid valve is working but not enough to move the valve, so the process is not disturbed. Finally, in case of emergency shutdown, a full stroke can be detected and a timer has been developed to measure the time to close.
Results
On critical emergency shutdown valves controlled by an SOV, V&A iCon provides a solution to check not only the valve status and the friction of the final element, but also the SOV in charge of cutting air to the actuator in case of emergency shutdown.
Moreover, the positioner is pneumatically designed not to initiate a spurious trip of the final element. Premium diagnostics, the PST status and the SOV status are available through HART communication. Finally, the cost of implementation has been reduced since the solution only requires a limited number of I/O from the SIS.
For more information contact Fred Venter, Valve & Automation, +27 (0)11 397 2833, [email protected], www.valve.co.za
| Tel: | +27 11 548 9960 |
| Email: | [email protected] |
| www: | www.profiafrica.co.za |
| Articles: | More information and articles about PI SA (PROFIBUS & PROFINET SA) |
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version