Maximizing compressor maintenance intervals and energy savings
For many years, traditional compressor maintenance was scheduled with nothing more sophisticated than a calendar. A pressure gauge measured the air pressure somewhere in the package and an hour-meter tracked the number of hours the motor ran.
A little more information was available from the compressor. The operating manual contained instructions such as check weekly, clean monthly, and replace annually. It may have also contained the phrase, “In dirty or dusty environments, maintenance intervals may be shorter.”
Today’s electronic compressor controls provide an abundance of information that allows the operator to maximize service intervals while minimizing energy use. Different installations have different site conditions, which dictate individual maintenance intervals.
The key to keeping overall operating costs to a minimum is to perform service only when required, without overextending the run-time on maintenance items.
Energy and maintenance can be as much as 85% of an industrial compressor’s life cycle costs. Other costs associated with delayed maintenance usually go unidentified. Electronic controls on compressors allow operators to monitor the condition of various maintenance items and replace them at an economical time.
The condition of inlet filters, separator elements, lubricant filters, and other operating conditions can be displayed on a compressor control panel (Fig. 1). Downstream filtration can be accurately monitored from compressor controls or from the filters themselves.
The following example, using a 200-hp rotary screw compressor, illustrates the hidden cost of untimely maintenance. A typical 200-hp screw compressor produces about 1000 cfm. The compressor rating, if rated to the CAGI/Pneurop test code, includes losses through a clean inlet filter. For the sake of this example, assume that the pressure on the inside of the filter is 14.5 psia. This figure is the pressure at which the 1000-cfm rating was determined.
As the filter gets dirty, it becomes harder for the air to enter the compressor. The pressure inside the filter decreases and so does compressor capacity. (Fig. 2). Another compressor may be required to compensate for lost capacity. Using a typical performance of 4 cfm/hp, the other compressor uses about 21 hp to make up for lost capacity at an inlet pressure of 13.3 psia.
The cost of this extra 21 hp is rarely taken into account. It is about equal to the average replacement cost of a 200-hp compressor inlet filter every week (Fig. 3).
Lower inlet pressures also result in higher compression ratios that generate greater temperatures inside the compressor. Higher temperatures lead to shorter lubricant life and additional expense.
Separator elements present a similar scenario. As the pressure differential across the element increases, horsepower required to overcome the differential and maintain the required discharge pressure also increases.
Many compressor manufacturers routinely use two-thirds of the available motor service factor. If the separator element is allowed to remain in the compressor until the differential pressure has increased 10 psi, the horsepower requirement increases 5% and a typical motor operates at or above its nameplate rating with the service factor included. The excess heat resulting from this overload shortens motor winding life and extreme heat melts the grease out of bearings.
|Fig. 1. Dirty inlet filters have an immediate effect on compressor output|
|Fig. 2. Changing compressor inlet filters too soon is uneconomical|
Check compressor specifications, settings, and current power consumption when operating with clean maintenance items. Check the drive motor nameplate for full-load running amperage and service factor. Multiply the full-load running amperage times the service factor to determine the maximum design load for the motor.
Average the ampere draw across all three legs of the drive motor wiring at full-load and compare the figure to the maximum design load. Use the rule-of-thumb that 2 psi equals 1% of full-load power to determine the maximum allowable pressure rise.
If the average across the three legs is 95% of the maximum design load, differential pressure across the separator element can rise 10 psi before the motor is overloaded. (Note: If oil carryover increases, change the separator element regardless of hours or pressure differential.)
Review the operator’s manual or check with the manufacturer to determine the pressure at which the control warns of high differential pressure. If that point is adjustable, change it to be within the motor limits. If it is not adjustable, monitor the differential pressure and note the pressure at which service must be done.
When the separator has to be changed, note the operating hours. Some compressor controls have adjustable alarms and prealarms for the separator element based on running hours and/or differential pressure. Adjust the alarms to reflect the operating conditions at the installation site.
If the compressor control does not have these features, install a pressure gauge to read the differential across the element and mark the gauge to indicate when the element must be changed.
Most electronic compressor controls also monitor the condition of the inlet filter with a vacuum switch. Check the factory setting of that switch. It is usually economical to change the inlet filter when the inlet pressure drops about 0.7 psi.
|Figure 3. Accurate pressure readings are necessary to determine when a filter is replaced.|
Monitor the operating hours and note them when the filter reaches the change point. Adjust alarms or install a vacuum gauge between the filter and the airend. Keep in mind that this interval may change seasonally, or even from one location within a plant to another. It is a function of ambient conditions at the compressor installation.
Downstream compressed air filters are available with accurate differential pressure displays. Using filters that read actual differential pressure, not just green and red zones, allows operators to calculate the most economical time to change these elements (Fig. 4). Use the same 2 psi equals 1% full load power rule-of-thumb to determine the optimum life of these elements. Energy consumption and the cost of maintenance can be significantly reduced by careful attention to maintenance intervals dictated by equipment monitoring. Timely maintenance improves compressed air quality, decreases downtime, and extends compressor life.
— Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, firstname.lastname@example.org
Energy and maintenance can amount to 85% of life cycle costs.
Dirty inlet filters are a main contributor to compressor inefficiency.
Monitoring and correcting all compressor functions significantly reduce energy and maintenance costs.
|The author is available to answer questions about monitoring compressors. He can be reached at 540-898-5500.|