As plant personnel know, repairing compressed air leaks can be an expensive, labor intensive and never-ending process. This article discusses ways plant personnel can reduce and maintain their leak rate at a lower level without repairing leaks. It discusses how pressure/flow controllers, variable speed and variable displacement compressors, automation, and addressing critical plant pressures allow plant personnel to lower the header pressure, which eliminates artificial demand and controls the leak rate. More importantly, the article brings a new dimension to the idea of turning off the air to idle equipment by focusing plant personnel’s attention on the idle time within the cycle of operating equipment.
Reducing Your Leak Rate without Repairing Leaks
During our audits, we have found leak rates as high as 6400 scfm, which exceeded 50 percent of the total demand. Rather than being an exception, we find 50 percent leak rates common in some industries. In others, we have found that a more reasonable 25 percent leak rate can represent a demand as high as 4500 scfm. At an energy rate of \$0.05/kWh, a leak rate of 6400 scfm costs approximately \$568,000 or \$89/scfm annually. Taking into consideration other associated costs, such as cooling water, maintenance, etc., the annual cost approaches \$110/scfm.
At a cost of \$110/scfm, why aren’t plant personnel repairing air leaks? Some of the most common reasons given are:
- We didn’t know how much leaks cost.
- Management hasn’t made it an issue.
- We need a shutdown to find leaks.
- Unless a leak affects production, production won’t give us time to repair it.
- We don’t have the manpower.
- It costs more to repair the leak.
- We used to repair leaks, but we never saved any energy.
- New leaks appear as fast as we repair them.
While compressed air system auditors have been pointing out the high cost of compressed air and promoting compressed air system efficiency for more than 20 years, it wasn’t until the inception of the Compressed Air Challenge in 1998 that plant personnel could readily find training in compressed air systems. The Compressed Air Challenge offers its “Fundamental of Compressed Air Systems” and “Advanced Management of Compressed Air Systems” classes throughout the United States on a regular basis.
While conducting compressed air system audits across North America and Europe, we have found that most plant managers are interested in saving energy. However, the level of their interest in compressed air correlates directly with the cost of energy (\$0.02 to \$0.15/kWh). If the plant has a low energy rate, you may still be able to get management’s attention if there’s a large savings opportunity, or if retrofitting the compressor air system resolves a production problem.
We find that the programs most effective at reducing and maintaining a lower leak rate are the ones that provide employees with the equipment to find leaks during normal production hours and push the responsibility to manage and maintain the system down to lower level management and their employees. In these programs, upper management sets performance levels for compressed air usage and makes compressed air part of each manager’s performance review. In addition, each manager must report on his department’s air usage, as well as leaks found, tagged, and repaired on a weekly basis.
Often we find leaks on components of critical production equipment that are either expensive to repair or must be repaired during a scheduled production outage, which may only occur once a year. In these cases, it’s important that plant personnel not only repair the leak, but also identify and eliminate the root cause that created the leak. In addition, we must employ every method available to reduce the leak rate until plant personnel can repair the leak.
The opportunities to reduce the leak rate without repairing leaks include:
- Turn off the air to idle equipment
- Reducing plant pressure
- Regulating end uses below the header pressure
- Shutting off the air to a pneumatic circuit on production equipment during a portion of the production cycle
Reducing the plant pressure and regulating equipment pressure below the header pressure have the added advantage of reducing the air consumption of equipment, as well as reducing the leak rate.
Turn Off the Air to Idle Equipment
In most plants, when production equipment operators shut off their equipment, the compressed air doesn’t shut off. Consequently, the compressors keep delivering air to support the leaks, and the power company keeps collecting revenue — even though the equipment isn’t producing any parts. Plant personnel can eliminate this waste by installing electrically operated solenoid valves that shut off the air to the idle equipment when the operator turns the equipment off. In order to take full advantage of the opportunity, plant personnel should wire the automatic solenoid valve into the run circuit rather than the on/off circuit.
However, in doing so they must take into consideration safety and maintenance issues, equipment adjustment and setup requirements, and startup procedures. Often plant personnel can address these issues by installing time delays and a jogging button in the run circuit and a time delay in the stop circuit. When plant personnel find that they can’t completely shut off the air to the equipment, then we recommend splitting the pneumatic circuit into multiple circuits and shutting off as many circuits as possible. Installing automatic solenoids can be an expensive project, so we recommend installing them over an extended period in conjunction with normal maintenance schedules. When management personnel attempt to have the operator shut off the air manually, it creates a management issue and only works for a short time — if at all. Our recommendation is to “eliminate management issues by doing it automatically.”
Reducing Plant Pressure
A good “rule of thumb” to remember is that “for every psi increase or decrease in system pressure, demand changes by one percent times the percentage of unregulated demand.” For example, if the plant is 100 percent unregulated and demand is 20,000 cfm, then if we reduce the system pressure 10 psi, demand will decrease by 2000 cfm — without repairing any leaks.
∆cfm = (TD*((P1- P2)/100)*(URD/TD)
∆cfm = change in cfm
TD = total Demand in cfm
P1 = original operating pressure
P2 = proposed operating pressure
URD = unregulated demand in cfm
20,000 cfm * 110psi–100psi * 20,000 = 2,000 cfm
In an uncontrolled system, if you repair leaks or reduce demand, the pressure will increase, which will increase demand. Using the same example, if plant personnel repair 2000 scfm in leaks and the pressure increases 5 psi, then the demand will increase 900 cfm. The total demand will be 18,900 cfm instead of the expected 18,000 cfm.
18,000cfm – (18,000cfm * 110psi-115psi *18,000) = 18,900 cfm
This example is part of the reason why plant personnel often find leak repair unproductive. The other reason relates to the compressor controls. If the plant had several lubricated rotary screw compressors operating in modulation, repairing 2000 cfm in leaks may only reduce power by 1.5 percent instead of the expected 11.4 percent. On the other hand, if the plant has centrifugal compressors operating in blow off or bypass, they may not even save the 1.5 percent. Plant personnel must coordinate the compressor controls and control the system pressure to realize the appropriate energy savings from repairing leaks.In order to lower the system pressure, plant personnel must first identify and address the critical pressures that are driving up the system pressure. In most cases, plant personnel already know where these critical pressures are located, but they aren’t sure how to address them. To identify the real cause of each critical pressure, plant personnel must develop a pressure profile of the system and use high-speed data loggers to signature map each end use associated with a critical pressure. This information assures success and allows plant personnel to select the most cost effective way to address each critical pressure. Some of the methods used to address critical pressures include:
- Dedicated storage
- Dedicated storage with metered recovery
- Larger pipe or tubing
- Larger cylinders
- Dedicated compressors
In a comprehensive compressed air system audit, the auditor will include these solutions in your “roadmap to success.”
In many plants, plant personnel protect against production outages due to compressed air by operating the system at a higher pressure, which provides time to start another compressor before the pressure falls below the minimal acceptable pressure. In this case, installing additional storage will allow plant personnel to lower the system pressure.
Some other ways to maintain a lower system pressure include:
- Lowering the compressors’ set-points
- Automating the compressors by either networking them together or installing a master controller with interface panels
- Installing a variable speed compressor and automating the other compressors
- Installing variable displacement compressors and networking them together
- Installing a pressure/flow controller along with sufficient control storage
If plant personnel only lower the compressors’ set-points, there may still be a significant variation in system pressure, which presents an opportunity to further reduce the leak rate without repairing leaks. Automating the compressors minimizes pressure variations by controlling the pressure within a single dead band of 4 to 12 psi. In some cases, plant personnel can reduce the upper end of this band further by installing additional storage.
In addition, plant personnel must install the automation’s control pressure signal point downstream of the cleanup equipment in order to eliminate the pressure variation due to changes in flow across the cleanup equipment (dryers and filters). Except when the variable speed compressor is operating at a capacity below its minimum speed, integrating one with existing compressors reduces the system variation to about 1.5 psi, as long as the control pressure signal point is located downstream of the cleanup equipment. When the variable speed compressor operates at a capacity below its minimum speed, we typically see a system pressure variation of 7 psi, which increases with insufficient storage. Installing a pressure/flow controller with sufficient storage has none of these issues and controls the system pressure within 1.5 psi. We can maximize the energy savings in a system containing a pressure/flow controller by operating most of the compressors downstream of the pressure/flow controller. The pressure variation with a pressure/flow controller increases to 3 psi whenever the automation unloads a downstream compressor.
Making a choice between these options depends somewhat on whether it’s a new installation or a retrofit of an existing one, but more so on the system capacitance, the benefits of a more accurate header pressure, the amount of unregulated demand, compressor performance, and return on investment, capital costs, and other management priorities. The auditor must take all of these items into consideration when he or she prepares your “roadmap to success.”
Regulating End Uses Below the Header Pressure
Equipment manufacturers prefer that their equipment operates below the header pressure so that its components operate off a constant pressure set by their regulators. Operating the equipment below the header pressure also reduces air consumption by reducing the pressure upstream of orifice type devices and decreasing the compression ratio, which partly determines how much air a cylinder consumes.
Plant personnel often find regulators set at a higher pressure than necessary or wide open. The reasons operators most often give for adjusting regulators are “it won’t work,” “it moves too slowly,” or “we needed a higher pressure to increase cycle times.” Upon analysis, we find one of the following: a large air leak, insufficient storage downstream of the regulator, undersized tubing and/or control valve, or the control valve located too far from the article it’s controlling. Resolving these issues will allow plant personnel to reduce the pressure, which will reduce the leak rate without repairing the leaks. This is especially important with the rampant use of plastic tubing and pushloc fittings.
Some years ago, an independent study determined that thirty-seven percent of pushloc fittings leak upon startup, so in an effort to eliminate a root cause of leaks, we recommend that plant personnel replace the pushloc fittings with a double back ferrule design that accommodates the plastic tubing. When a cylinder moves as fast as 0.04 second, the regulator prevents access to upstream storage, which results in a large pressure drop in the tubing between the regulator and the control valve. This is why cylinder manufacturers sell empty cylinders to mount at the inlet of the control valve.
On the other hand, plant personnel can first try installing larger tubing between the regulator and the control valve and moving the control valve closer to the cylinder. These modifications allow operators to lower the pressure, which reduces the leak rate without repairing leaks. In addition, these modifications have the added advantage of reducing the air consumption of cylinders and other components. Finally, we often find the reduction in air consumption that occurs after retrofitting the pneumatic circuits on the plant’s production equipment and reducing their regulator settings exceeds the demand reduction achieved by lowering the system pressure.
Shutting Off the Air to a Pneumatic Circuit on Production Equipment
When the opportunity exists, we often find that shutting off the air to a pneumatic circuit on production equipment during a portion of the production cycle offers the largest opportunity to reduce the leak rate — without repairing leaks. For example, in one plant with a couple hundred of identical production machines, we measured the leak rate at 2750 scfm. Most of the leaks were due to scored cylinder rods and damaged rod packing. Plant personnel estimated the cost to repair the 900 cylinders at \$1,100,000, while it cost \$195,440 annually to support the leaks. On this basis, the simple payback was 5.63 years — if plant personnel could maintain a zero leak rate.
Based on a more realistic leak rate of 1000 scfm, the simple payback was 8.85 years. Neither of these simple payback rates was acceptable. Working with plant personnel, we found that the cylinders only operated for three minutes out of a thirteen-minute cycle, which meant they sat idle 76.9 percent of the time. We decided to divide the pneumatic circuit on each machine into two circuits, install an automatic solenoid valve, and program the PLC to shut the air off to the cylinders when it wasn’t required. This project cost \$109,000 to implement, reduced the leak rate by 2100 scfm, and saved \$149,200 annually. This project had an acceptable 8.8-month payback. In addition, plant personnel are in the process of eliminating the root cause of these leaks by modifying the cylinders to accept a different, more durable type of packing.
In another plant, we found a production machine with five positions of which only one position operated at any one time. In this case, we decided to install an automatic solenoid valve at each position, and program the PLC to shut the air off to the position when it wasn’t operating. This reduced the leak rate by approximately 1100 scfm from 1500 scfm to 400 scfm.
For more information, Contact Chris Beals, Air Systems Management, Inc.