Diagnostics are crucial
The key to achieving a higher SIL with less hardware is greater levels of diagnostics. A number of sensor manufacturers (e.g., ABB, Emerson and Yokogawa) have SIL-2-rated safety transmitters. These devices offer levels of diagnostics much higher than those in standard transmitters. The diagnostics can detect potentially dangerous failures that would prevent the sensor from operating on demand.
What sort of diagnostics are needed for valves in safety applications? Most safety-related valves are left in single positions for extended periods of time. The Pareto Principle (the 80/20 rule) and common experience indicate the most common failure mode of safety-related valves is “stuck.” You don’t need to completely close a valve to determine whether it’s stuck open; a partial stroke would suffice to check for the majority of failures. With such testing, a single valve can meet SIL-2 performance.
Let’s put this in a personal context: If you use your lawn mower every week in the summer to cut your grass, how confident are you that it would start each week? Now, if you leave it in the garage filled with gas over the entire winter without ever running, how confident are you that it would start in the spring?
At least nine companies offer packaged partial-stroking solutions — in alphabetical order: Asco, Drallim, Dynatorque, Emerson, ICS Triplex, Metso, Netherlocks, Safeplex, and Tyco.
Some are manual methods only and are often referred to as “jammers.” These need an operator to be physically present at the valve. Some manual methods require the insertion of a special key. Some end users have designed their own “home-grown” solutions.
Other methods are automated, so an operator needn’t be present at the valve. These solutions typically require the vendor’s specific hardware (valve, actuator or positioner) and software (used to record and analyze valve performance). Other automated solutions are controller retrofits that will work with just about any valve assembly.
At least one solution is incorporated into the SIS hardware itself and can work with most existing valves simply with the addition of limit switches or proximity devices. This means no additional controller hardware or software is required.
A safe way to save
The primary benefit of partial stroking of safety valves is saving money, either from reducing manual testing or eliminating the need for redundant hardware. In SIL-1 applications, manual test intervals can be stretched from the original one or two years to upwards of five years (naturally depending upon the failure rates and assumptions used in the modeling). SIL-2 and above applications historically required redundant valves. However, a single valve with partial stroking can offer the same performance as two standard valves. Considering not only the purchase price of the valve, but the total costs associated with installation, the financial benefits are considerable (e.g., tens or hundreds of thousands of dollars per assembly).
- “Functional safety: safety instrumented systems for the process industry sector,”ANSI/ISA-84.00.01-2004 Parts 1-3 (IEC 61511-1 Mod), ISA, Research Triangle Park, N.C. (2004).
- “Identification of emergency shutdown systems and control that are critical to maintaining safety in process industries,” ANSI/ISA S91.01, ISA, Research Triangle Park, N.C. (2001).
- “Functional safety of electrical/electronic/programmable electronic safety-related systems, Standard 61508, International Electrotechnical Commission, Geneva, Switz. (1999-2000).
- Fillion, E., “Digital approaches to safety instrumented systems provide faster ROI,” p. 55, Hydrocarbon Processing (Nov. 2006).
- Bingham, K., “Partial stroke testing of emergency shutdown valves,” PROCESSWest, p. 49 (Summer 2005).
- Lewis, C., “Coming of age: the economic case for large-scale use of wireless sensors is overwhelmingly favorable,” p. 51, InTech (July 2005).
- Gruhn, P., “Valve signatures and partial stroke testing,” p. 65, Hydrocarbon Processing (Jan. 2003).
- Gruhn, P., “Increase plant safety with online valve testing,” p. 39, InTech (Feb. 1998).
Paul Gruhn, PE, CFSE, is a safety product specialist at ICS Triplex, Houston, Texas. E-mail him at PaulG@ischou.com.