Keeping Compressed Air Systems Draining By William J. Ulrich Adapted
from Plant Engineering magazine
One of the most common service problems with downstream compressed air equipment is damage caused by excessive liquid–compressed air professionals often call this “Death by Drowning.” A flood of liquid goes downstream, overwhelms filters and dryers, and ends up in the compressed air equipment or manufacturing process.
The amount of water vapor in a typical compressed air system that turns into liquid during an average day is often underestimated. For example, a 100-horsepower compressor rated at 475 scfm will produce 48 gallons of water in a day when inlet air is 70ºF and 70% relative humidity.
Most of the water vapor, 75% or more, will be condensed and dropped out ahead of the dryer. If the remaining liquid isn't taken out of the system, it has only one place to go, downstream, where it causes poor performance or failure of drying equipment or at least increased maintenance costs. In some cases liquid backs up into an inactive compressor, if there are cross connections and no check valves in the system. This can cause extensive compressor damage.
The key to preventing liquid-related damage is reliable draining at all points where condensed fluids accumulate.
The simplest and earliest type of condensate drain, which is still used today, is a partially opened or cracked valve. Instead of repeatedly manually draining a receiver tank or separator several times per day, the air system operator very slightly cracks open a ball or other type of valve, allowing condensate to continually drain. There are several significant drawbacks to this technique. The cracked valve can easily clog with pipe scale. When this happens, the collection point quickly floods and "Death by Drowning" may occur. Overtime, the probability of this happening is high. Also, if a well-intentioned maintenance person shuts the cracked valve, the system may flood.
In any event, valve cracking is very expensive, simply because of the cost of wasted compressed air. If a valve is opened the equivalent of 1/16", it discharges 6.5 scfm or 3,416,400 cubic feet of 100 psig air per year. This wastes $683 annually, based on a cost of $.20/1000 cubic feet of air. In too many plants, multiple valves are left cracked across the air system. The hissing that issues from these cracked valves is, almost literally, the sound of money being wasted.
Some direct-acting float drains came about as adaptations of steam traps. These generally use a float with a lever arm that lifts a plug out of a seat and releases liquid. They have the benefit of only operating when there is liquid present in the system, so compressed air is not wasted.
There can be a problem because design constraints limit the mechanical force available. The force is small unless the float is very large and on the end of a long lever arm. Such large floats and lever arm are often found in the natural gas industry but seldom are practical in plant compressed air systems. Float type drains found in plant air systems are typically small. For example, a commonly used separator and float drain combination with 1” NPT air side connections and a nominal 150 scfm capacity has a drain connection of ½”, but an actual drain orifice of 1/16” at the end of the lever arm. It does not take a lot of dirt or pipe scale to plug this orifice. Such a drain is a low cost solution, but the system operator must closely monitor its performance and regularly clean the drain orifice.
Another choice is a timed solenoid valve. These devices typically have a 7/16" orifice, which is far more tolerant of dirt and pipe scale than those of float type drains. Solenoid drain valves operate with much more force than a direct-acting float because of the solenoid's energy; however, the flow path through the valve is not a straight line. Because the flow is non-linear, a strainer should be used to keep large particles out of the valve assembly. The strainer and valve should be cleaned regularly.
An exception to using a strainer is when the fluid is from a refrigerated dryer downstream of a coalescing filter. In this case the liquid is very clean and there is generally little oil or particulate matter. Both float drains and solenoid type drains work well in these applications.
The most dependable and robust type of drain is an actuated ball valve. These valves have a straight-through flow design and are virtually impossible to plug.
A normal motor-actuated ball valve operates on a timed cycle to open every X minutes each Y hours, more or less duplicating what the operator would manually do normally during a day. This is simple, but if the timer not adjusted properly, air will be wasted or the system will not be drained enough. There is also a noise issue when air discharges from a drain pipe at line pressure if no condensate is present. In some plants, dirty compressed air is considered a contaminant that needs to be avoided anywhere people are working.
To eliminate both the noise hazard and the waste of compressed air, automatic ball valve drains can be wired to respond to independent controllers, operating only when needed. This gives the dependability of a full-flow design with zero air loss operation. These valves may be connected to any kind of mechanical or electronic level sensing device. In addition, they also have the capacity to indicate or alarm if they fail to open.
Another variation on the zero-loss ball valve uses a float to trigger a pneumatic signal. This in turn drives a cylinder connected to the ball valve. A major benefit is that no electricity is needed to operate. So it may be used on remote or portable equipment or in hazardous installations where explosion-proof electrical equipment is specified. In operation, the reservoir is never totally drained, so there is no compressed air loss.
Another plus of demand type valves is that they are more compatible with a typical oil/water separator. A gravity type separator does not do well with large instantaneous volumes of condensate mixed with escaping compressed air. Air rushing into a unit disrupts gravity type separation and encourages emulsion formation. This inhibits separation and shortens the life of the charcoal or final filter on the unit.
There are other variations and combinations of valves and actuation systems on the market today, but these are the most widely used.
Some basic selection criteria are based on air system capacity. If the system is small or an end-point application is being drained, the most common device is a solenoid valve with a simple interval timer. These are low-cost items and work well when there is a small amount of fluid. The amount of air loss from excessive draining is usually not enough to justify a demand type valve.
For a large plant air system, the best solution is a demand type or zero-loss drain that operates only when liquid is present. In this case the additional cost of a demand type valve versus a fixed cycle valve is easily recovered through the elimination of lost compressed air.
Be sure the valve is at the lowest point of the equipment being drained. This seems obvious but is sometimes overlooked.
If the condensate is dirty, install a strainer ahead of solenoid valves. It is always helpful to install a block and bypass for maintenance.
Avoid cross-connecting or manifolding multiple drain lines ahead of the drain valve. This is an understandable and often practiced effort to drain multiple liquid accumulation points with a single valve, often justified in terms of reducing initial costs. However, each liquid accumulation point is likely at a different air pressure due to line loss and pipe friction. Liquid migrates to the point of lowest pressure and will accumulate there. The irony is that when the valve is checked, liquid may not discharge because it is backed up somewhere else. Also, cross-connections sometimes bypass filters, dryers, or other treatment equipment.
There have been many attempts to use check-valves, inverted bucket traps, or other devices to isolate the lines from each other when using a single valve to drain multiple lines, but this effort to eliminate additional valves seldom works in practice.
The best solution is a separate drain valve for each liquid accumulation point. The outlet should then go to a common drain for final processing by an oil/water separator or other device to deal with contamination.
Solenoid valves should be used with coalescing filters, after-cooler separators, and refrigerated dryers. Ball valves (fixed or demand) should be used with receiver tanks, single tower deliquescent dryers, drop legs, and after-cooler separators.
Maintenance is most often done by manually actuating the valve on a daily PM schedule. Depending on the operation, this can be done on a “drive-by” basis, where maintenance personnel push the manual override button on the valve to make sure it operates. Maintenance personnel can make sure there is no water backed up in the system by pushing the button a couple of times.
A good insurance policy is to use a high level device in the separator, receiver tank, dryer, or filter to independently monitor any liquid. If the drain does not operate for any reason, a signal will alert the maintenance people before a flood occurs downstream.
If the drain is at the end of a long run of piping, make sure that there is not an air lock in the system. This is done by connecting a small tube or equalizer line back to where the drain originates.