The Evolution of Valve Automation for Emergency Shutdowns

Valve failure. It’s an aspect of the fluid handling industry that no operator wants to experience, yet it happens. As the final control element of a safety-instrumented system, or SIS, emergency shutdown (ESD) valves are depended on when called upon. Yet they are often seen as the weakest link of an SIS, contributing to more than 50% of statistical failure data, and for an understandable reason. Remaining static or dormant for long periods of time, these valves slowly accumulate media buildup and corrosion, until the day they are suddenly required to operate. ESD valve failure can also be connected to systemic issues related to the valve automation package. Therefore, a consistent, quality valve monitoring process must be implemented into the SIS program to increase the reliability of the installed ESD valves. 

Though there are several different options from which an operator can choose, partial stroke testing (PST) has evolved to offer more safety and cost-saving benefits when combined with the latest technology available. Thanks to continual advancement in smart technology, digital position control transmitters have emerged as the most comprehensive PST solution for ESD applications, offering a broader diagnostic ability, an easier implementation and a more cost-effective maintenance program than could be achieved by traditional PST methods. The system’s automated diagnostics have the ability to eliminate the need for human dependency on critical areas of the plant, as well as the need for costly offline testing.

A review of partial stroke testing

Partial stroke testing allows processing plants to test the installed base of valves without having to close the valve and shut down the plant, as is the case with full stroke testing (FST). Since its inception, traditional PST methods have offered multiple advantages over other methods. For example, utilizing PST reduces the control element’s probability of failure significantly. Not only does it help to determine whether the safety function will operate on demand, it also exercises the isolation valve, decreasing the likelihood of valve sticking. From a financial perspective, PST is useful when there is a high-cost burden to close an ESD valve: it can extend the interval between full stroke tests it enables operators to plan inventory for maintenance turnarounds, and in some cases, it may reduce the need for redundant valve solutions. However, despite all of these benefits, the traditional PST method does have some drawbacks, including its potential for spurious trips. Also, it may not be an appropriate testing method for all final control elements because of the disturbance it may create.

Prior to the introduction of digital control transmitters, there were different PST techniques available, each offering a different set of these benefits and disadvantages. The most common still in use today are mechanical jammers and discrete valve controls (smart positioners).

The mechanical jammer is the simplest and least expensive option. It is a device that is fixed between the actuator and the valve or integrated into the actuator. When in test mode, a piece of the jammer locks the movement when the valve experiences 10-15% of travel. They are highly reliable because of their ability to resist vibration, but they are also the most manual option, requiring the device to be physically inserted into the valve assembly to prevent it from closing completely, subjecting the entire process to human error. Also, the safety function of the valve is unavailable during the test, posing a problem should an ESD occur during the process.

Smart positioners offer a more innovative technique that utilizes modern technology to automatically generate the PST function, either locally or remotely. They monitor valve movement proportionally, measuring the speed of its response and position. Additionally, smart positioners do have the ability to capture diagnostic data for use in maintenance, unlike mechanical jammers. However, smart positioners are the more expensive option. Also, smart positioners perform PST by bleeding air from the system via a pneumatic relay. By performing in this manner, the solenoid valve (SOV) remains untested by the positioner’s PST function. To test the solenoid valve, an additional test is required from the SIS system to “pulse” the signal to the SIS solenoid valve. With this method, there is no safeguard to prevent over-travel, which creates an increased chance for an unintended spurious trip.

Figure 1: Smart positioners have several inherent weaknesses when used for PST. All images courtesy of Westlock.

The latest technology

The digital position control transmitter represents an evolution in PST technology with an enhanced design based on SOV technology. It is a device that electronically measures a valve’s position and creates a feedback signal of its actual position, using analog and/or bus technology which is then used as an input separate from the SIS output, reporting a valve’s true position to the plant’s control system. 

Figure 2: Westlock’s Digital EPIC system.

Unlike mechanical jammers and smart positioners, a digital control transmitter can capture diagnostic information during FST, as well as during an ESD event without the need for limit switches, as it directly operates the SOV when executing the PST. Also, while expensive smart positioners may need a line-conditioner to communicate HART on the DO wires, which further increases the overall cost, digital control transmitters do not require this. Through the device, PST can be initiated in multiple ways: remotely via a maintenance PC with HART Protocol or discrete output; locally via dry contact push button or selector switch; or on a dedicated schedule. 

When monitoring the ESD event, digital control transmitters monitor the SIS signal to know when the ESD event occurred and document the event, providing 4 to 20mA feedback and HART so the user can validate the information received and be confident that the valve not only shifted, but went to the fully closed, tight shut-off position. 
 

Figure 3: Digital position transmitter set up.

In addition to gathering useful predictive data, digital control monitors have the added ability to mitigate spurious trips that were historically caused by traditional PST methods. A spurious trip is the accidental activation of the safety system due to process disturbance which could be caused if the PST moved the valve past the intended partial travel setting. Because PST is controlled by the digital control monitor, it utilizes a fast processor so that even on small, fast-moving valves, the position can be captured accurately and PST position will not be exceeded. The system measures overshoot in a baseline test and compensates by that amount in the maintenance PST. If supply pressure is out of range for the actuator, low supply pressure can prevent the actuator from being able to bring the valve back to the fully open position. The digital control monitor measures supply pressure and will not allow initiation of PST if out of range.

The science behind the data

The introduction of the smart technology present in digital position control transmitters allows operators to use the collected data for predictive diagnostics, rather than in hindsight. These transmitters focus on securing the data that provides the best insight on the probability of failure, for example break pressure to close or break pressure to partial stroke setpoint time. They then use that data to determine which automated/discrete valves are healthy and which are not during plant turn-around, before an emergency shutdown event occurs, saving the user from expensive and unnecessary service. In cases where an emergency shutdown event does occur, critical data is collected to analyze what happened and the historical graph is available for documentation and evaluation of valve performance. Additionally, critical data, such as break pressure and travel time, is analyzed and compared to previous baseline data to determine if automated valve system health is acceptable for reinstatement into service.

 
Figure 4: Monitoring the ESD event.

When providing diagnostics of online valve performance, some digital control transmitters have the ability to store up to seven full stroke tests in nonvolatile memory. Of the seven tests, five are for diagnostics, including baseline tests, dynamic baseline tests and maintenance tests which retain the four most recent valve travels. The remaining two tests are used to document history: the integrator test, which documents the function of the valve assembly as manufactured by the automation supplier or OEM, and the installer test, which documents the function of the valve assembly after being installed in the field by the contractor.

Conclusion

Partial stroke testing has come a long way from relying on mechanical jammers. Though the traditional methods still exist, the availability of smart technology has completely changed the way data can be gathered and used in the prevention of emergency shutdowns. The evolution of this technology has resulted in the reversal of how valve data is used, bringing advanced predictive diagnostics to the multitude of discrete automated valves in the plant. The data generated by digital control transmitters provides plant operators with valuable information that can help prevent unplanned shutdowns, organize maintenance resources for planned turnarounds and reduce inventory. Just as every other industry is and has been benefitting from the introduction of smart technology, so too is fluid handling with the introduction of digital position control transmitters.

Jason Moorehead serves as the Engineering Manager at Westlock Controls, bringing over 22 years of expertise in valve technology. Known for his passion for innovation and engineering excellence, he leads a dedicated team in designing advanced valve monitoring and control systems that set industry standards. Jason’s deep understanding of the field, combined with his leadership, drives continuous progress and innovation in valve control technology.