Safety Instrumented Function loop 'final element' problems and solutions

June 18, 2016

Riyaz Ali from Emerson Process Management describes the problems and possible solutions for testing the final control element of Safety Instrumented Function loops.

Accidents in process industries have created concern as to whether functional safety protects personnel, environment, equipment, property and a company’s reputation and image. It is therefore imperative to design a safe plant to protect these interests.

A safety instrumented system (SIS) can be one or more safety instrumented functions (SIF). A SIF is designed to prevent or mitigate a hazardous event by taking a process to a tolerable risk level. A SIF is composed of a combination of logic solver(s), sensor(s), and final element(s). Final elements of the SIF loop such as emergency shutdown and blow down isolation valves remain in one position and therefore need regular testing to ensure they will operate when called upon. SIS failures generally fall into two categories:

  • Nuisance or spurious trips, which can result in an unplanned process or plant shutdown. Although a spurious trip is safe for the plant and personnel, the financial impact due to lose production can be substantial.
  • Covert or hidden failures remain undetected and permit normal process operation. This failure type is dangerous and contributes to the probability of the system failing on demand. Testing the SIF loop is therefore crucial in uncovering covert failures.

Conventional testing methods
In the past plant turnarounds were scheduled every two to three years. Although extended turnarounds improve economic returns by increasing production, they also mean that safety system final control elements are tested less frequently. In an attempt to get around this problem, many companies have devised methods for testing the SIS valves online so they do not have to shut down the process. 

Various methods are available to partially stroke the safety shutdown valves. While there are risks of spurious shutdown trips, the mechanical limiting method is the most popular. A major drawback of this method is the unavailable safety shutdown function during the test period. 

Figure 1: Partial stroke test, signature graph. Graphs from Emerson.

Digital valve controller
The main considerations around online testing while the process is running are the availability of the safety function during safety shutdown valve testing and avoiding the mistake of leaving them in test mode. In addition, the test should be simple, flexible, and reliable with protection against nuisance trip.

Digital valve controller or 'smart' positioners solve these issues. A digital valve controller can diagnose itself, as well as the valve and actuator that it is mounted to. The digital valve controller enables the user to define a suitable online valve stroke value from 1% up to a maximum of 30% from its standby position to minimize the effect on the process (figure 1). The test parameters are saved in the microprocessor non-volatile memory, minimizing mistakes and enabling consistent test results.

Figure 2: Digital valve controller as a safety device

Digital valve controller integration
There are various ways to integrate the digital controller into the SIF loop. The digital valve controller can perform the safety function, or the solenoid valve can perform the safety function and the digital valve controller be used for the partial valve stroke test and diagnostics only.

Figure 2 shows a typical schematic of the digital valve controller in a safety loop. Here the digital valve controller is used to perform the safety function as well as the online partial valve stroke test.

A 4mA shutdown signal allows the digital valve controller to record a demand stroke trend and verify the safety shutdown valve stroking time (figure 3).

Figure 3: Demand stroke with stroking time captured by digital valve controller

A more common approach is depicted in figure 4, with the solenoid valve performing the safety function and the digital controller responsible for the partial stroke test the safety shutdown valve. Both solenoid operated valve (SOV) and digital valve controller are powered separately. An advantage of this SIF loop compared to the previous example is the redundant shutdown path provided by the digital valve controller.

Figure 4: Setup of a SOV and DVC power separately

The digital valve controller can also monitor online the health and integrity of the solenoid valve (figure 5). A short on-off pulse from the safety system is required to verify movement of the solenoid valve plunger. As the plunger moves (de-energized), it creates pressure drop in the actuator, which is detected and captured by the digital valve controller as shown in figure 6.

Figure 5: Configuration and setup for solenoid valve monitoring


To maintain SIF loop integrity, it is essential that the digital valve controller does not cause a spurious trip. To avoid this, the use of a reverse relay with the digital valve controller for SIF loops increases plant availability.

Figure 6: Solenoid valve pulsing results

Conclusion
Digital valve controllers can perform safety functions, capture demand stroke time and are a great tool for partial valve stroke tests. Using these devices allows partial valve stroke tests to be scheduled as frequently as required and hidden failures such sticky movement and friction/torque changes due to process build up are easy to identify. In addition, the ability to monitor solenoid valve health has greatly improved safety loop diagnostics and reliability. 

Riyaz Ali is director of instruments for the Fisher business unit of Emerson Process Management. His career in process control instrumentation spans almost 40 years and he is currently involved in implementing microprocessor-based technology for field devices for process control and safety applications. Ali has a B.E. (Hons) in Chemical Engineering with a focus on digital valve controllers for process industries. 



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