Selection must be done with care and understanding of safety and reliability standards to avoid the risks associated with an operational failure of a critical plant system
By S.A. Nagy, PE When touring a process plant, invariably you witness solenoid valves at work. They can be found switching instrument air lines on and off and handling the directional control of air pressure used to energize or de-energize larger air-actuated process valves. When these instrument lines and process valves form part of a system that is critical to the plant’s operation, it’s essential for the solenoid valves to be reliable and safe or else the system, and possibly the operation of the entire plant, could be adversely affected. In order to avoid the risks associated with an operational failure of a critical plant system, the selection of solenoid valves must be done with great care. Prior to 1996, there were no generally accepted safety or reliability standards that could be applied to the application of solenoid valves in critical or safety-related process systems. However, there has been growing affirmation of standards such as Germany’s DIN 19250/19251 and international standard IEC 61508. Armed with these standards and additional system-related specifications such as IEC 61511, safety and instrument engineers can specify solenoid valves according to their suitability for a particular safety integrity level (SIL) in their safety instrumented systems. IEC 61508 is usually credited as introducing the SIL concept. In 1996, however, the Instrument Society of America enacted ANSI/ISA S84.01 in response to an increasing number of industrial accidents. ANSI/ISA S84.01 is fundamentally the same as IEC 61508. It provides the requirements for critical safety instrumented systems including emergency shutdown systems. An SIL is defined in terms of the failure rate of a function and can be regarded as a measure of the safety of a given process. It defines to what extent the process can be expected to perform safely and, in the event of a failure, addresses whether the failure occurs in a safe manner. The rate at which a solenoid valve fails to function may depend on many factors: the number of redundant measures included in its design, the effectiveness of those measures under average conditions, the vulnerability of those measures due to exposure from outside forces or influences. However, it is important to note that no individual products, including solenoid valves, carry SIL ratings. Individual pieces of equipment can only be defined in terms of their acceptability within SIL-classified environments. It also should be noted that simply using a solenoid valve that is rated for a particular SIL class is no guarantee. The system designer still has to analyze all equipment and its collective impact to the SIL. Before a solenoid valve can be certified suitable for a particular SIL area, it must prove that it will be available to perform its function when needed. Methods such as those found in DIN 19250/19251 are employed to determine, for example, mean time between failure and probability to fail on demand. Using this reliability data in combination with statistical measurements for the valve, the ability to fail in a safe manner is demonstrated, and the appropriate SIL environment is determined. Selecting solenoid valves certified for use within a designated SIL area offers benefits. Since reliability calculations have been completed, the system engineer can select equipment faster. When the equipment has been approved by a neutral third party, such as the testing and assessment services laboratory TUV Rheinland, there is an assurance of accuracy. Plus, a report done by a third-party agency provides important independent information and facts, varying from the restrictions of use to the reliability statistics for the product. Such a report should accompany the purchase of any solenoid valve that has been certified for functional safety. Usually, the worst operating conditions for solenoid valves involve open-air, continuously energized and low-cycling installations, which should be considered when testing for reliability. An appropriate scenario might include type-testing in accordance with DIN 3394-1, five years worth of operational experience testing and aging and icing tests. Type-testing in accordance with DIN 3394-1 is essentially the pre-requisite for classification in accordance with DIN 19251/19250. DIN 3394-1 defines the basic requirements for control valves in technical gas systems and provides test parameters and acceptance criteria relating to function, freedom from leaks and wear resistance. DIN 19251/19250 is concerned with fault risk and considers the major valve elements in this assessment. Particular attention is given to the possibility of spring fracture, residual solenoid magnetism, stem fracture and stiction associated with moving parts. Certainly, the most significant results for assessing valve safety come from five years worth of operational experience testing. As for aging and icing results, they can be measured with a 350-hour test, during which time the solenoid valve and coil are in a controlled temperature environment. Reliable function is determined in both energized and de-energized states. While it is certainly feasible to design a solenoid valve to comply with individual SIL environment requirements, more often than not it is far simpler to design one in accordance with the highest possible requirement class, i.e., SIL 4 according to IEC 61508 and SIL 3 for ANSI/ISA S84.01. Solenoid valve designs with absolutely the highest safety margin and risk assessment class are available in the form of redundant systems. Their demand and use in a SIL 3 or 4 environment is a function of the safety instrumented system being targeted and the need for additional redundancy. Although it remains the function of system and safety engineers to determine the SIL environment for their safety instrumented systems, the right solenoid valve can make it easier to select equipment. As the old saying goes, a chain is only as strong as its weakest link. Likewise, a critical plant system does its job effectively and reliably when its solenoid valve does its part. It’s wise to choose a “link” with recognizable safety integrity.
Les Gutzwiller is vice president of technical services and Thomas J. Kuli is chief engineer at Robinson Industries Inc., Box 100, Zelienople, PA 16063, a leading manufacturer of custom-engineered and custom-built industrial fans. With four U.S. locations, Robinson Industries works in a variety of industries including mining, pulp and paper, cement, chemical process, steel, power and pollution control. Parts of this article come from AMCA Standard 204, "Balance Quality and Vibration Levels for Fans." A copy of the standard can be purchased by contacting Air Movement and Control Association International Inc., 30 W. University Dr., Arlington Heights, IL 60004 or calling 847-394-0150. Additional information is available by visiting www.amca.org and www.robinsonfans.com or calling 724-452-6121.