“Chains of habit are too light to be felt until they are too heavy to be broken.”  —Warren Buffett

Caps and plugs serve no process function.

It is a common practice to install a cap or plug on the end of a line that is open to the atmosphere, but the justification is for safety, not for process reasons. The process function is always served by the valve that is a little further upstream. “What if something gets past the valve when it shouldn’t?”

There are lots of reasons that material can get past a valve when it shouldn’t. A cap or plug protects against very few of them. They are not worthless, but they are not as worthwhile as many want to believe.

Reason for Material to Get Past a Valve When It Shouldn’t

There are three basic reasons for material to get past a valve when it shouldn’t: error, equipment failure, and struck-by hazards.

The most common reasons are error. Error includes inadvertently opening the wrong valve, inadvertently opening the correct valve when it is in an unsafe, pressurized condition, and inadvertently leaving a valve open while in a safe condition and subsequently introducing a hazard.

Less common reasons are valve failures. Equipment failure includes a valve seat failing while the valve is in the closed position. Rather than leak through the bonnet, which is more common, material leaks through the valve to the downstream piping. Another valve failure is the result of vibration, which gradually opens the valve.

The most dramatic reasons are struck-by hazards. This almost always involves a quarter-turn valve with a lever handle. An external event causes something to strike the handle, opening it.

No Credit for Caps and Plugs

Most lines that are open to atmosphere are vents or drains. They are equipped with vent valves or drain valves. When material is escaping through a vent valve or drain valve because of an error, no credit should be taken for a cap or plug.

Why not?

If the cap or plug was in place, it would protect against the open valve. However, if the error was opening the wrong valve, the operator would have removed the cap or plug before opening the valve, so the cap or plug provides no protection. If the error was opening the correct valve, but while it is still pressurized with an unsafe material, the operator would have removed the cap or plug before opening the valve, so the cap or plug provides no protection. Finally, if the error was forgetting to close the vent valve or drain valve before returning the line to service, it extremely unlikely that personnel would forget to close the valve but remember to replace the cap or plug.

So, while caps or plugs serve a purpose, it is not to protect against errors, and no credit should be taken in a PHA for caps and plugs as a safeguard in these circumstances.

The Down-side of Caps and Plugs

In the case of equipment failure leading to a leak to downstream piping, caps and plugs are a mixed blessing. On the one hand, they contain the material, preventing a steady release to the atmosphere. On the other hand, they contain the material, so that when the cap or plug is removed, there is a sudden release of the material in the downstream piping.

Fortunately, the piping downstream of a vent valve or drain valve is usually very short, so there is not much material to release. The release begs the question, though: was the material there because of a valve leak, or because at some point in the past the valve was opened then closed?

On the whole, though, the advantage of a cap or plug outweighs the disadvantage.

Why Use Caps and Plugs?

A few years ago, the Process Safety Beacon, a newsletter issued by the Center for Chemical Process Safety, published a piece about caps and plugs. It described an incident where a piece of debris fell from some temporary scaffolding, struck the handle of a quarter-turn valve, and caused a vent valve to open. The valve vented flammable material, which ignited, and the resulting fire killed one worker and injured two others.

In another incident, an operator was pulling a reel of steam hose into place when the hose caught on the handle of a drain valve. Hot caustic solution came pouring out of the drain. The operator was not wearing the appropriate PPE to approach the line and by the time he had suited up and closed the valve, several hundred pounds had escaped the drain.

The unambiguous benefit of caps and plugs is that they protect against random events like falling debris or hose entanglement. These are types of hazards that are not typically considered in a PHA, however. When they are, caps and plugs are mentioned, but the better suggestion is to replace lever handles with round handles, and if early in the design, arrange the piping to make the valve less vulnerable to struck-by hazards.

Using Caps and Plugs

Like many safeguards, caps and plugs are not universally beneficial. For many hazards, they provide no benefit at all. Under some circumstances, they might even be detrimental. However, they are of sufficient benefit that there should be a bias toward their installation.

When evaluating their effectiveness as safeguards during a PHA, consider the hazard. Does the cap or plug actually address the hazard? If the cause is an error, probably not. If the cause is a stuck-by hazard, probably so. If the cause is a struck-by hazard, however, there are solutions that avoid the hazard, and it is always better to avoid a hazard entirely than to rely on a safeguard to mitigate it.

Finally, treat all caps and plugs as though they are holding back a world of hurt, especially when removing them.