The safety valve is the final element in the safety loop. It operates in the most challenging environmental conditions and is constantly in contact with process media in the pipelines.
Therefore, regularly testing these valves is important not only to know their condition, but also to fulfill the requirements of relevant standards, like IEC 61511.
The first automatic emergency shutdown (ESD) valve testing devices were introduced about 15 years ago. Since then, process operators and maintenance engineers have been able to monitor the condition of ESD valves online. Partial stroke testing (PST) for valves and the smart devices that perform this testing have proved their importance and gained a favorable reputation.
The main concern with smart valve controllers that run the partial stroke testing is the increased probability of a spurious trip. Any unexpected ESD valve trip may lead to an unnecessary plant shutdown, which can cost even up to millions of dollars a day. That’s why although the reliability of ESD valves is important, overall process availability has become even more crucial for managers and operators.
An option like Neles® ValvGuard™, which serves as the sole controlling element for the safety valve, has been available for more than a decade. Still, the use of a separate solenoid valve to perform the safety action on the valve has been the preferred choice of many plants around the world. The risk of a spurious trip with integrated devices is one of the main reasons why many plant operators want ESD and PST systems to be separated. Even with sufficient diagnostics features to prevent spurious trips, operators and the entire industry itself do not fully trust solutions with integrated functionality, despite the fact that they, like Neles® ValvGuard™, have a proven track record.
Metso recently responded by launching its latest solution – a partial stroke test device – that basically eliminates any spurious trip risk. This new product is part of Metso’s way to make a difference for customers. The new PST device is based on the SIL 3 certified Neles® ValvGuard™ VG9000, Metso’s intelligent safety solenoid, but is designed only to perform the partial stroke test. The fail-safe action still needs to be performed by a separate solenoid valve.
This new device, called a spurious trip prevention PST device or VG9PST, is powered from the distributed control system (DCS) using an analog 4 mA signal with HART communication for diagnostics. This is key to the solution. As 4 mA is the normal state, no signal change can stroke the valve to create an unwanted trip. The valve can only be stroked by the partial stroke test. Even then, the stroke size is restricted to 50% to prevent the valve from fully closing or opening. To visually distinguish the new PST device from the standard Neles® ValvGuard™, this device has a green cover instead of a red one.
Metso introduced the PST device purely for partially stroking the valve manually when needed. This eliminates the risk for a spurious trip and any unwanted or costly plant shutdowns during the testing, which is the biggest fear for many plant operators.
As a result, operators can confidently perform all necessary testing and better understand the condition of the valve, and whether it still fulfills the relevant standards. VG9PST eliminates unexpected surprises in a plant’s operation, thanks to its timely information.
All standard ValvGuard™ diagnostics from the partial stroke test are available in this PST device too, including the load factor and breakaway pressure history trends for predictive diagnostics. Yet, there are no sophisticated features, like a safety view, automatic scheduled testing or internal pneumatics testing, which are found in the standard Neles® ValvGuard™ VG9000.
Now plant operators have a choice. They can choose to have a fully integrated device for the safety valve testing and emergency action all in one enclosure, like the intelligent safety solenoid Neles® ValvGuard™. Or, they can select a solution that simply performs the PST, but eliminates any risk of a spurious trip. This is the new, green VG9000.