“Among our articles of lazy hardware, I recommend the faucet that stops dripping when no one is listening to it.”  — Marcel Duchamp

Piping systems need drain and vent valves. For start-up, shutdown, maintenance, and other reasons. A piping system without appropriate drain and vent valves won’t work properly and is an exposure hazard for maintenance personnel.

Still, no matter how necessary to the operation and maintenance of piping systems, drain and vent valves are a concern because of the potential for loss of containment. The consequences of loss of containment through a drain or vent valve can be as simple as the expense of wasted plant air or steam, or an inconvenience as released utilities that must be cleaned up before resulting in a slip, fall, and injury. Or they might be as severe as a toxic or flammable release that exposes personnel and the environment to serious injury or death.

Loss of Containment from Drain and Vent Valves

Concern about loss of containment through drain and vent valves has led to widespread adoption of policies to install plugs or caps on every drain and vent valve. While useful in limited circumstances, plugs and caps are not a panacea for loss of containment.

There are three primary causes for loss of containment through drain and vent valves. The first is when a drain or vent valve is opened while the piping system is out of service and then left open when the piping system is returned to service. The second cause is when a drain or vent valve is inadvertently opened while the piping system is in service. The third is a leak – the classic dripping faucet.

Here’s a brief discussion of these three causes of loss of containment through drain and vent valves and the usefulness of caps or plugs.

Inadvertently Left Open

Many maintenance activities require isolating a section of a piping system and then draining it before performing a line break. It is common to leave drains and vents open while performing maintenance on the piping system components. The error occurs when returning the piping system to service. Left open, the valve spews the contained fluid at full line pressure. Worse, because drain and vent valves often are manually operated, the spray makes getting to the valve to close it a challenge.

Some want to credit caps and plugs as a safeguard against this scenario. However, if someone forgot to close a particular valve, it’s unlikely that that unclosed valve would be capped or plugged. Caps and plugs offer no protection in this scenario.

Inadvertently Opened

There are also occasions when personnel inadvertently open the wrong drain or vent valve, exposing themselves to full line pressure. “Inadvertent” means that they intended to open a valve but opened the wrong one. In this scenario, the procedure would be to remove the plug or cap first, and then open the valve. The plug or cap would offer no protection against opening the wrong valve.

Fortunately, the response to opening a valve and discovering that it was unexpectedly under pressure is to slam the valve closed. The amount released is not likely to be much, and with the correct PPE and the practice of opening drain and vent valves slowly, there is not likely to be much exposure. This means training personnel to open all drain and vent valves as though they know they are under pressure, a universal precaution.

An exception is when a valve is inadvertently opened, but not by personnel. An example is when a heavy tool falls on a quarter-turn valve on a drain or vent line that points down. In the closed position, the handle lever sticks out to the side. When something heavy drops on that handle, it can be enough to open the valve. In this scenario, a cap or plug provides protection. Of as much use, however, to install a round handle on the valve, making it less vulnerable to a dropped tool or pipe spool.

Leaking Valves

Process drain and vent valves leak for the same reasons that a faucet drips: worn out gaskets, o-rings, or washers; deposits on internal parts; corrosion of the valve seat; or, most annoying, the valve is not fully closed. And like a dripping faucet, a leaking valve is often neglected until it can no longer be ignored. As countless plumbers and water utilities have pointed out, a slow water faucet leak of 10 drips per minute is almost a gallon per day, 350 gallons per year. A faster faucet leak at an annoying flow rate of one drop per second is almost 6 gallons per day, almost 2,100 gallons per year.

For a rarely used valve, it can be very tempting to “fix” the problem of a leak by installing a cap or plug. This avoids the need to take down the line to allow access to the leaking valve’s internals. There is a problem, however. A leaking valve will continue to leak into the space between the valve outlet and the cap or plug, eventually filling that space and bringing it to the same pressure as in the line. Then, when someone removes the cap or plug, there is going to be a spurt of the process fluid and the pressure may launch the cap or plug. Unfortunately, when all drain and vent valves are capped or plugged, there is no way for personnel to know which are in good condition and which are leaking and require planned maintenance. It also reinforces the need to train personnel to open all caps and plugs, like drain and vent valves, as though they know they are under pressure.


A specialized form of a vent valve is the bleed valve between two block valves in a double-block-and-bleed (DBB) configuration. It’s the two block valves that do the work of shutting off flow. In an emergency, these are the valves that must work, and they provide redundancy. The upstream valve is going to experience full line pressure and even the slightest leak in the upstream valve eventually is going to bring the pressure in the piping section between the two block valves to full line pressure. As the pressure in that section goes up, the second valve becomes more vulnerable to slight leaks. With time, the hazardous material in the line will leak past the second valve and accumulate downstream.

The bleed valve, when open, prevents the pressure in the section between the two block valves from building. So, even with a slight leak in the first valve, there is no pressure on the second valve to make it leak. The bleed valve is not there for the emergency, but for any lengthy period when the flow must remain shut off. This is why a DBB is required as part of the fuel supply in burner management systems, and why OSHA accepts DBB as a form of isolation in permit-required confined space entry (29 CFR 1910.146) but does not accept double blocks without the bleed.

When a DBB is fully automated, there is a habit of making the bleed valve a deenergize-to-open valve. After all, that’s what is expected to happen in an emergency. If the bleed valve fails open (a once-every-10-year event), however, the bleed valve will be venting at full line pressure. Therefore, it is important to consider the automation strategy for bleed valves when they are being installed. Are they for the emergency, or for something else?

Living with Drain and Vent Valves

Drain and vent valves are a necessary feature of piping systems, but they are also a source of loss of containment. When they are not needed, removing them also removes their associated hazards. When they are needed, treating them with the same care as other energy isolation devices helps to assure that equipment is fully restored before energizing it. Take the universal precaution of opening a drain, vent valve, cap, or plug as though you know it is pressurized with the process fluid.

As for caps and plugs, never lose sight of the fact that they are only helpful as safeguards under certain circumstances, and even then, they come with their own hazards.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.