Keep the compressed air lines dry

Install and maintain condensate drains to keep water out of the distribution system.

By Chris E. Beals, Air System Management

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In brief:

  • Condensate shortens the life of compressed air-operated machinery and can contaminate product causing significant maintenance costs and loss of productivity.
  • The most effective way to eliminate the demand side open drainage is to install dryers and filters at the air compressors.
  • There are four different types of drains — the manual valve, the level operated mechanical trap or float drain, the electrically operated timed-solenoid drain, and the no-air-loss auto drain.

If allowed to flow out into the compressed air system, condensate shortens the life of compressed air-operated machinery and can contaminate product causing significant maintenance costs and loss of productivity.

Plant personnel do their best to protect air-operated equipment and product by filtering the air at the end use; however, this means they must drain and change filter elements in multitude of filters on a regular basis. To minimize this workload personnel will often crack open the petcock on the filter housings and drain valves on drip legs creating what the compressed air industry refers to as open drainage or intentional leaks. The compressed air industry uses these terms to describe leaks that can be reduced or eliminated by installing automatic drains at those locations.

The most effective way to eliminate the demand side open drainage is to install dryers and filters at the air compressors; however, one should only dry the air to the level required; therefore, some industrial sectors such as chemical plants, refineries, paper mills, and glass plants only dry a portion of their compressed air. Typically, there will be intentional leaks in the wet air headers of these plants that can be reduced or eliminated by installing automatic drains.

Intentional leaks are also found on the supply side of the system, where they are draining condensate from compressor intercooler and aftercooler moisture separators, filters, and refrigerated dryers. It’s critical to drain the condensate from these devices to prevent damage to compressors and dryers, particularly desiccant dryers, and to improve the quality of the air flowing out into the plant. In fact, if you don’t drain the condensate out of pre-filters on desiccant dryers or the separator in a refrigerated dryer, it doesn’t matter what type or size of dryer you install; you’ll have poor quality air.

There are four different types of drains — the manual valve, the level operated mechanical trap or float drain, the electrically operated timed-solenoid drain, and the no-air-loss auto drain. Each type of drain is offered in various styles.

Manual valve

While the most common manual drain valve is the ball valve, we have probably seen every type of manual valve used to drain condensate from the system. Theoretically, one may think the manual valve can be opened once a day or once an hour so there won’t be much waste, and, in some applications, such as an air receiver, this works. But from a practical standpoint there is too much risk of either damaging the compressed air equipment or ending up with poor quality air with this manual approach, so personnel typically leave them leaking a small amount of air all the time. Other than making sure the manual valve is open enough so it doesn’t plug up or open too much that it freezes up, the manually cracked open method is probably the most reliable drain and basically requires zero maintenance. This method wastes significant amounts of energy, and, even though it increases operating costs, it’s still commonly used.

Level operated mechanical trap

The two most popular level operated mechanical drains are the float drain and the inverted bucket trap. Neither of these styles of drains wastes air when operating properly, but they often require a great deal of maintenance and are prone to blockage from sediment in the condensate. As a result these style drains either fail open and waste compressed air or fail closed so personnel don’t realize there’s a problem until moisture shows up downstream. In addition, either style drain can waste as much or more air than a manual valve drain. While improvements have been made in float drains to make them more reliable, many of the same issues remain.

Electrically operated timed-solenoid drain

Years ago, personnel began migrating over to electrically operated timed-solenoid drains because they are typically more reliable than level-operated mechanical traps; however, they can, depending on the orifice size and settings, waste more air than a manual valve. In order to minimize the wasted air, they should be adjusted for only condensate drainage and minimal air drainage, but unfortunately the amount of condensate varies depending upon the compressed air system demand and the humidity. Constantly adjusting the drain times isn’t practical; therefore, the interval and duration of the electrically operated timed-solenoid drain is typically adjusted for worst-case conditions that occur during the most humid time of the year. The worst-case conditions may occur only for a month or two, so the electrically operated timed-solenoid drain wastes air for the other 10 months of the year.

Electrically operated timed-solenoid drains are the standard drain on most refrigerated dryers, and a moisture problem often is solved by adjusting the interval and duration of the electrically operated timed-solenoid drains on their dryers.

While one can purchase electrically operated timed-solenoid drains with smaller orifices, a ½-in. orifice is recommended for most applications to prevent the drain from plugging up. One air dryer manufacturer recommends setting the electrically operated timed-solenoid drain for an interval of 3 min and duration of 10 sec. Based on an estimated rate of flow of 270 scfm at 100-psi for a ½-in. orifice, the electrically operated timed-solenoid drain with these settings can waste as much as 15 scfm of average flow, which costs about $1,350 annually.

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