Emerson Exchange 365

As part of a Basic Process Control System (BPCS), control valves are dynamic, moving to desired positions that control process parameters. Shutdown valves used in a Safety Instrumented System (SIS), on the other hand, are normally dormant, coming into action only when demand conditions arise. In either situation, these final control elements (FCEs) are critical parts of process safety control.

As process operators examine their FCE, a common question is whether safety certification is required for all valves.

Today at Emerson Exchange, Mahendra Shah of Wood Group Mustang (WGM) and Emerson’s Riyaz Ali presented their analysis of when Safety Integrity Level (SIL) suitability is required for valves.

“The tools and mechanisms in the standard enable users to establish when valves with safety integrity level suitability are needed,” said Ali. “IEC61511 provides definitions of control and safety systems clarifying the requirements.”

“With this standard in place,” Ali continued, “we must then determine if the end user intends to use control valves as the final element of a safety instrumented function.”

Three different scenarios were presented, in which control valves were used as an on/off final element, dual purpose control and safety, and dual purpose redundancy and on/off.

Ali and Shah then explored the practical application of IEC 61511, taking into account that the SIS valve remains static for long periods of time and only moves when predetermined conditions are exceeded. For this reason, SIS valves are prone to sticking. Without mechanical movement, unreliability inherently increases.

At WGM, Shah and his team employ a SIL assessment process. The steps include identifying the safety loop protecting a specific hazard, determining consequences and frequency of a hazardous event, and applying a risk graph, and a layer of protection analysis or other methods to determine the desired SIL based on risk. As an example, Shah applied this methodology to each of the three control valve usage scenarios.

“Because unreliability of the final control element generally contributes 50 percent or more to overall SIF loop reliability, it is one the most vital safety critical loop components,” he explained.

“The final control element needs to be evaluated in totality,” Shah continued. “If it’s a shutdown valve, the complete assembly with actuator, valve, connection between valve and actuator, and pneumatic/hydraulic circuit components (accessories) are evaluated.”

At WGM, partial/full stroke tests, test intervals, diagnostic information acquisition and interpretations also play a key role in their analysis. Based on the risk graph and/or level of protection analysis, a SIL is identified for the SIF loop. This leads to a determination of a test interval requirement for the final control element of the loop.

Correctly identifying SIL requirements provides a number of benefits for the client. It results in a safer system with lower failures, decreases the cost of engineering, reduces operation and maintenance requirements, allows proper testing interval allocation, lowers risk, facilitates design consistency across the plant with compliance to standards, reduces cost of ownership, and lowers the need for labor.

Ali concluded the presentation by returning to the question of whether safety certification was required for valves. “If a control valve is designated to carry out a safety function,” he said, “then it should meet the SIL of the SIF loop.”