The Weakest Link was of course a popular TV quiz show aired in many countries around the world, in which candidates were eliminated one-by-one. In the programme, contestants who provided incorrect answers were subject to nothing more than an inglorious “walk of shame” as they left the stage. In an industrial setting, however, getting things wrong can have far more serious consequences. Suppose for example that valves are incorrectly closed prior to performing maintenance work on equipment. Or imagine what might happen if valves were not operated in the correct sequence before opening a pig launcher…
Unfortunately, accidents like these do continue to happen, and in many cases operator failure can be identified as a major contributing factor. Indeed, it would seem that in the industrial arena humans are the weakest link when it comes to plant safety. This is why some end users are fitting interlocks to valves as a way of censuring operators follow prescribed sequences when operating valves.
If you are new to the topic, then Lester Millard (Managing Director at Alcatraz Interlocks) gives a very good introduction to the principles of valve interlocks in an article published on the Oil Review Middle East website. Here he indicates, for example, that: “mechanical key interlock systems are twin-keyed mechanical locking devices which operate on a 'key transfer' principle to control the sequence in which process equipment may be operated. This system is widely accepted as an effective safety management tool and is being adopted by many of the world’s oil, gas and chemicals majors.”
Examples of the application of valve interlocks are readily found on the Internet. The Smith Flow Control website for example gives compelling examples in tanker loading, ethylene furnaces, CO2 deluge systems, boiler bottom blowdown and pig trap interlocks as well use in safety relief systems. On the topic of safety relief systems they state: “An open path to relief must be maintained on Safety Relief Systems at all times during work exercises. A spare relief capacity, enables continuous production while maintenance procedures take place, and eliminates the need to isolate and shut down the whole process. Typically, twin or multiple safety relief valve systems are fitted with isolation block valves both upstream and downstream of each safety relief valve. Block valves, isolating the spare relief valve must be opened before the block valves are closed. Fitting a valve interlock to the upstream and downstream isolating valves on each safety relief valve (PSV), will ensure safety is maintained at all times.”
The importance of eliminating human error is further highlighted in an excellent article on the Netherlocks website. Quoting a joint work group of industry specialists, Netherlocks’ suggest that the root cause of many incidents often includes human failures in areas such as failing to complete or reverse isolations fully, failing to monitor the position of isolated valves, poor communication such as during shift handover and failure to check P&ID’s, such that valves are overlooked. They also provide some compelling case histories, showing how a partially blocked gate valve was not fully closed, leading to a significant release of a highly flammable liquid. In another instance a vent line between a heat exchanger and a reactor was overlooked, causing the release of hydrogen bromide gas. Such instances, they say, could have been prevented if a system of valve interlocks had been properly engineered and installed.
Do you have any experiences about valve safety or the use of interlocks? Then the author would be delighted to hear from you.