Optimize compressed air reliability

Preventive maintenance, reduced operating pressure and proper size storage can maximize efficiency.

By Dean Smith and Alton Stokes, iZ Systems for the Compressed Air Challenge

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In brief:

  • Half of the compressed air produced in industrial systems is typically wasted through leakage, artificial demand and inappropriate use. An audit can identify this waste. Eliminating the waste can reduce the number of compressors required, freeing the existing unused compressors to provide backup.
  • When storage receiver capacity is too small, compressors will cycle too frequently, which shortens air end life. A small change in system demand will cause a rapid change in pressure, which will cause the compressors to overreact.
  • Properly sized system storage helps protect against the failure of the largest compressor, which means a backup compressor with adequate capacity must be available and capable of automatically starting, and sufficient storage capacity is needed to slow the rate of pressure loss while the spare compressor starts..
  • An automation system can manage compressors within a single control band by cycling only one compressor at a time. By monitoring the rate of pressure change, the automation system can respond to air demand changes without unnecessary cycling or compressors starts.

All rotating equipment will eventually fail, and the compressors that supply your compressed air systems are no different. Ignoring this fact may mean trouble for your plant production, or worse, cause a complete production outage. In a well-designed system automatic backup strategies to recover from this failure will be in place and, if implemented correctly, should cause minimal pressure fluctuations or compressed air system performance degradation.

Unfortunately, in today’s world of restricted capital availability, justifying backup equipment can be very difficult, especially in the case of facility support equipment. But without backup equipment, especially air compressors, the ability to maintain preventive maintenance schedules can be impacted, relegating maintenance of critical facilities equipment to off times like evenings or weekends, or not at all, so overall reliability is negatively impacted. Without adequate maintenance, the potential for a failure in the system increases exponentially.

Figure 1. Half of the compressed air that’s produced is wasted in the form of leaks, inappropriate uses, and artificial demand. (Source: Compressed Air Challenge)
Figure 1. Half of the compressed air that’s produced is wasted in the form of leaks, inappropriate uses and artificial demand. (Source: Compressed Air Challenge)

For firms specializing in industrial compressed air and vacuum audits, the most common marching orders from clients are to find energy savings and do something to increase systems reliability. Fortunately, these two basic needs work together in compressed air and vacuum systems. As identified by the Compressed Air Challenge, 50% of the compressed air produced in most industrial systems is typically wasted through leakage, artificial demand and inappropriate usage (Figure 1). This means, if sufficient waste can be found and eliminated from a system, one or more compressors can be turned off creating energy savings and providing a backup unit.

The systemic overview

The missing ingredient in virtually every compressed air system is sufficient storage receiver capacity, but how does this impact reliability? When storage values are too low, relative to compressor size, the compressors will cycle too frequently, which shortens air end life. Additionally, a relatively small change in system air demand will cause a rapid change in pressure, which will cause the compressors to overreact trying to maintain the system setpoint. To compensate for these fluctuations, the average system pressure level is often increased to ensure the pressure stays above the minimum acceptable level for production. This increase in pressure creates artificial air demand, one of the primary waste categories, where the higher pressure causes unregulated uses and leakage to consume more air than at lower pressure, but it also increases the work the compressors must perform to provide the required compressed air. Even if the compressors are rated for the actual operating pressure, every 2 psi increase in discharge pressure increases the work energy required by 1% (at a nominal 100 psig) increasing compressor component operating temperatures, decreasing the lifespan and increasing required maintenance costs. In worst case scenarios with minimum storage capacity, backup compressors will be left running unloaded, so they can quickly be available to maintain stable system pressure with the obvious negative impacts on operating and maintenance costs, and unit life.

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