The lowly drain deserves respect

By Alan L. Hitchcox, editor

Automatic drains usually are specified as an afterthought. But they can be the key to reliable and economical operation.  

When air compressors ingest ambient air, they also ingest any water vapor that the air contains. All this water vapor carries through the compression process and remains entrained in the compressed air.

Almost every compressed air system today includes some arrangement, device, or combination of devices to protect downstream components from water that condenses as the air cools. These can range from simple, passive sloping distribution lines leading to a drop leg, through separators, aftercoolers, receivers, and water-removing filters to sophisticated air dryers.

A common characteristic of these arrangements is that they remove condensate from the main air lines and deliver it to an area where it poses no threat. Furthermore, this accumulated condensate must be removed before it reaches a volume that interferes with operations.

Airline drains are the components that provide this function. Some older systems still use manual drain valves that present two problems: Someone has to open them, and a lot of compressed air is wasted when that happens. A broad variety of reliable automatic drains is available to eliminate both of these problems - often saving enough in compressed air cost to pay for them over time.

All drains need attention

A major source of contamination problems occurs when a drain does not function properly in the system. Users don't seem to mind investing tens of thousands of dollars to control or limit system contamination, but when it comes to drainage, little thought is given. Half the job is getting the contaminant into a physical state that can be removed. The other half of the job is removing it. This is the work performed by the drain.

Perhaps one-third of all users give no regard to this important issue. Another third buy higher quality or premium drains, but pay no attention to the installation and application. The remaining third buy premium drains and apply them properly - and then forget them.

Most drains used today are either solenoid-operated or motorized valve drains. When they are working, these inexpensive drains get rid of effluent at various locations in the system. However, they waste a tremendous amount of compressed air in the process by keeping compressors from unloading or shutting off. Drains also are a significant contributor to fluctuating system pressure. Adjusting these drains regularly overcomes most of these problems.

If the inlet temperature causes the operating temperature to rise, there may be more lubricant in hotter weather. The result is that the effluent of combined water and lubricant becomes much more viscous and gummy in the winter than in the summer. This issue must be considered when selecting and applying drains.

Sizes and types

Most drains range in size from 14- to 1-in. ID to handle flows from 100 to1,500 cfm. In many cases, a drain failing in the open position can result in low pressure in production because there is not enough compressor capacity to support production and the loss through the drain.

If solenoid- and timer-type drain valves are applied properly, and operating personnel are disciplined enough to frequently adjust the individual valves, everything works as it should. Unfortunately, drains generally are not considered important enough to warrant regular monitoring. Consequently, the result is system contamination, wasted energy, or both.

Many inverted-bucket drains are used for compressed air drainage because of their low cost. Inverted-bucket drains were developed primarily for steam usage, so they do not tolerate the dirt and viscous materials common to compressed air systems. They work for this type of service, but generally do not provide the long-term reliability of drains designed specifically for use with compressed air.

Bubble troubles

One of the more serious causes of drain failure is trapped air bubbles in the line. When a slug of liquid covers the trap connection port in the bottom of the sump, and there is not enough liquid in the trap to actuate it, an air or gas bubble becomes trapped in between. The vent line is supposed to allow the gas bubble to pass.

Almost all drains have a vent connection, but virtually none has vent lines installed. When the gas bubble is trapped, the liquid backs up in the sump, and carry-over occurs. Cracking the pipe connection to the drain relieves the bubble, and nothing appears to be wrong. The trap connection is reassembled until the next time a gas bubble becomes trapped.

None is immune

Even premium drains carry some inherent drawback. Because most drains don't accommodate dirt or rust very well, the size of the through-porting of the drain largely determines if the drain will become plugged. If plugging occurs, don't look for a replacement drain with larger porting. Instead, question why substantial amounts of dirt and rust are in the drain system. In a compressed air system with inherently non-lubricated compressors and ferrous-metal piping, expect lots of rust and pipe dirt to be present. Most cooler housings and separators also use ferrous metals. These can be coated, and alternative materials can be used, but there will not be enough lubricant to prevent rusting in pipes, separators, and heat exchanger housings.

All drains require inspection and regular maintenance. More sophisticated drains have failure lights or audible alarms in the event of malfunction. The best choice is to have spare drains on hand and installation arrangements that allow you to easily rotate traps for maintenance and cleaning.

Few drain manufacturers suggest this amount of detail for the installation of their product because the perception is that buyers want a set-it-and-forget-it solution. This, of course, is an unrealistic expectation by the buyer.

Automatic drain uses no power

Well, at least no electrical power. The Robo Drain, from Air System Products Inc., Lancaster, N. Y., actaully scavenges a small amount of compressed air to operate. Its low profile allows installing it in areas where the vessel to be drained is only inches from the ground, and it needs no vent line in most applications.

To prevent air loss, the drain valve features positive opening and closing through magnetic force.opens and closes from magnetic force. A support and positioning sytem prevents side loading that could otherwise cause premature leakage around the valve stem. And to ensure no-clog operation, condensate, rust, and scale exit through a full-ported 12-in. ball valve.

In operation, condensate accumulates in a reservoir, and a stainless-steel float rises as the liquid level increases. Once the condensate reaches a predetermiend level, a magnet snaps open and allows air to pass through a control valve. This air then actuates a pneumatic cylinder, which opens the ball (drain) valve.

Once approximately 24 oz of condensate flows from the ball valve, the float disengages the magnet, which closes the control valve. A spring then retracts the air cylinder (thus closing the drain valve), and the Robo drain is ready for its next cycle.

For more information on the Robo Drain, visit www.airsyspro.com.