Several years ago, while working in a refinery, I noticed a relief valve with its inlet block valve completely closed. I radioed an operator for assistance. Within minutes, two operators arrived and confirmed that the valve should not be closed. They opened the valve and thanked me for notifying them. In following up a few days later, the operators informed me that a Root Cause Analysis (RCA) revealed that their Management of Change (MOC) program and operating procedures had not been followed. In the event the relief device was needed to mitigate an over-pressure scenario, the integrity of the protected system would have been breached resulting in a loss of primary containment and potential environmental impact, ignition, explosion, fire, and injury to personnel. The injury could have been a single disabling injury or a fatality. The financial impact would have been significant.
Most accidents in the oil and gas industry are related to procedures that do not occur frequently but can have catastrophic consequences. Shut-downs, start-ups, and shift hand-over are all examples of times when incorrect procedures, a lack of concentration, or even slight mistakes can result in severe and disastrous consequences.
Valves play a significant role in safe operation of oil and gas facilities. The accidental closing of an important valve, as demonstrated in the scenario above, can lead to serious consequences. One potential solution to an accidental valve misalignment is employment keyed-valve interlocks. Keyed-valve interlocks prevent human errors and guide valve operators through a pre-defined valve operating sequence. Unique keys are used to allow only the correct valves to be opened or closed in the correct order. This assures the valve will be locked in one of two positions – open or closed – with one key trapped within the lock assembly and one key free.
To change the valve position (open-to-close or close-to-open), two keys need to be inserted into the lock assembly. The free key can only be released when the valve is in the open or closed position. Alternatively, a ‘single keyed interlock’ only allows removal of the locked open or closed key when the valve is in the correct locking position. By implementing valve interlocks, industries can create safer work environments and create cost savings as they avert product spills or losses due to incorrect switchovers.
Valve interlocks can be used in a variety of applications such as isolation for Pressure Relief Valves (PRVs), pig launching and receiving, decoking, inert gas systems, pump startup, flare lines, boiler blow-down, vessel filling, vessel isolation, and pump routing. Valve interlocks can be mounted without alteration to the corresponding valve; the characteristics of the valve remain unchanged. The valve interlock is installed as an integral part of the valve, with the original valve lever or hand wheel removed and replaced with a new sliding lever or hand wheel.
Typically, manufacturers of trapped key interlock devices also provide application guides showing common interlock problems and their recommended arrangements. Reliability and safety of the scheme critically depends on the possession of keys; therefore, duplicate keys must be controlled to prevent an unsafe operating sequence. For example, a lost key might be replaced only by a duplicate held off-site or might require ordering a replacement from the original manufacturer.
In addition to making manual valve operations safer in the field, trapped key interlocking can have an impact on your Process Hazard Analysis (PHA) and can provide additional value to your risk assessments. For instance, the intention of HAZOP/LOPA is to determine the level of risk inherent to the process; first by developing the potential consequence associated with an initiating event and then by determining the likelihood that the overall consequence occurs. The likelihood that a scenario occurs takes into account the probability of the initiating event occurring and the probability of failure of any safeguards or Independent Protection Layers (IPLs).
Similar to car-seals or chain-locks, keyed-valve interlocks can potentially reduce the likelihood of inadvertent valve operation. Unlike car-seals or chain-locks, keyed-interlock valves go one step farther and reduce the likelihood of operating valves in the wrong order. This produces the net effect of decreasing the initiating event likelihood and lowering the likelihood that the overall “bad day” consequence occurs.
No matter how foolproof you believe your operations are, human error will always pose a threat. Adding an additional layer of protection, such as keyed-valve interlocks, to your process is not only cost effective but can also save lives.
