Pumps & Systems, December 2008
Most energy engineers realize the high cost of energy that extra horsepower air compressors waste while trying to maintain pre-set air pressures. They also realize the cost of leaks in their nitrogen, carbon dioxide and Freon systems.
This article focuses on how leaks can negatively impact the system as a whole, and how leaks can affect the environment and the well-being of the personnel who work around them.
Air Leaks: Negative Impacts on Process Control
The air/gas system is like a food chain; any one portion of the system that fails will impact the others. In biotech facilities, compressed air may control environments where humidity and temperature stability is a top priority. If the degree of humidity in a controlled bio-genetic research facility goes awry, it can ruin years of research. Loose connections or splits in the tubing can adversely impact proper control.
Since air operated heating and ventilating systems need to accurately control the heating and/or cooling process, facilities must perform compressed air/gas leak audits on a regular basis, or make arrangements with a competent and professional firm to have audits periodically performed.
In a compressed air and gas system, many critical components require validation of proper operational capabilities. Non-critical system components also need to be scanned and tested for leakage. They include, but are not limited to, relief valves, solenoid valves, flange gaskets, thread connections, filter/lubricator/regulators, welds, thread and quick connection devices. At any given time, they may be wasting energy and perhaps sacrificing proper process control.
Compressed air systems depend on supplying clean, dry air to equipment and components. Separators, receiver vessels, compressors and other components in a compressed air system depend on drain traps to automatically discharge the condensed water from the system. Regular ultrasonic tests must be performed on these drain traps to ensure they are not failing.
When a drain trap fails in the closed position, it causes condensate back-up. The air fed to the system will contain water that can be detrimental to the equipment. Rust, dirt and corrosion are additional consequences of not replacing failed drain traps in a timely fashion. Ignorance of plugged drain traps also adversely affects other portions of the system.
If a drain valve fails in the open position, large quantities of energy are wasted. Since most drain traps are piped into discharge manifolds and then to waste drains, it is not generally visually apparent that they might have failed in the open position.
A proper air leak audit should identify the components that are causing energy loss. Valves, solenoids and other sensitive equipment can plug or stick in an open position and eventually fail. Many times the gaskets between banks of solenoids begin to leak when water has not been drained from the compressed air system. Sometimes oil in compressed air systems can cause O-ring or gasket failures. If part of the system is outdoors and is subject to low temperatures, the air lines and the equipment to which it leads can freeze. Once frozen, the portions that freeze can crack and be permanently damaged.
Gas Leaks: Costly and Dangerous
Other gases are more expensive than compressed air. When analyzing a compressed air leak versus a nitrogen leak, remember that nitrogen is typically ten times more expensive than air. Even tiny nitrogen leaks are costly.
Moreover, if the leaking gas is volatile, such as natural gas, identifying and repairing the leak becomes an urgent priority. One plant had 22 natural gas leaks in one section of piping near the ceiling. The gas line was feeding an oven that had ignition points every 10-ft along the length of the equipment. A huge argon leak at one part manufacturing plant was along the feed line carrying gas to a welder. The potential safety hazard of these leaks far outweighs the actual cost. Should an explosion occur, aside from the physical harm it could cause workers in the immediate area, it would shut down the plant.
Inert gases such as argon, helium and nitrogen are non-toxic and do not burn or explode. However, they displace oxygen in the air and can cause injury or death at high concentrations. Should oxygen levels fall too low, individuals in the area or entering the area can lose consciousness or die from asphyxiation.
It is crucial that leaks be found and corrected before a small problem becomes a severe problem. The old truism reminds us, "Everything leaks, it is just a matter of when."
Performing a Leak Survey
The success of a leak survey requires three major elements: knowledge, planning and follow-through. Knowledge includes an understanding of the compressed air system, including all the subsystems and components. What are the sizes, types and ages of the compressors? Have they been properly maintained? What about traps and drains? Are pressure gauges working, and, if so, is there adequate pressure for the various areas of use? What are the assigned pressures for these areas? Can any compressed air applications be replaced by alternative, less energy intensive methods? For example, instead of using compressed air for cooling, drying or cleanup, try using low-pressure blowers or fans.
Knowledge can also include understanding the ultrasound instrument, including how it works and inspection techniques. If you are not familiar with the technology or how to use the instrument, there are training courses available that can help make you and other inspectors in the facility more competent and effective in inspections.
Planning includes using a map of the compressed gas system and its various components. If none exists, take digital photographs of each section with long range and close-up views and label them.