Fortunately, compressor operations are constant and very predictable. Compressor applications and subsequent performance are based on specific physical conditions. Over time, the demand requirements can negatively affect a compressor’s performance efficiency. For example, deposits found in the air can lead to a loss of air pressure. Therefore, there is an increase in electricity needed to produce the required pressure for a system’s application.

Predictive maintenance programs, when utilized correctly, can alert the plant operator to these and other potential failures within the compressed air system. With the help of various types of control systems, companies can perform daily "observation maintenance" programs for compressed air systems and other rotating equipment. By reviewing daily readings of temperatures, pressures, and functions, plant operators can predict, not only where the failure could take place, but more importantly -- when it may happen. This advantage can prevent unscheduled repairs and costly downtime.

This article reviews the importance of establishing a predictive maintenance system and reviews strategies for selecting and managing an effective program.

How Predictive Maintenance Works

The first step in establishing a predictive maintenance program is to understand your goals. The overall goal of a predictive maintenance program is to maintain the compressed air system’s efficiency, which in turn increases reliability and extends equipment life. Predictive maintenance programs provide protection for each sub-system within the compressed air system, including air, cooling, oil, and driver (i.e., electrical motor, steam turbine, diesel engine, etc.). Here are some examples:

· For the air system, maintaining aerodynamic efficiency and the health of moving parts are primary concerns. Both can be compromised by poor air quality. If air contaminants come in contact with impellers, rotors or pistons, it can increase the wear rate of the moving elements and decrease the overall system efficiency.

· Air cooling systems can become contaminated simply due to the build up of deposits found in the water. Over time, this build up can cause a loss of heat transfer capability, resulting in an increase in air temperature.

· With oil cooling systems, if the lubricant is not kept clean and maintained at the correct temperature, metal to metal contact between rotating assemblies and bearings can occur. This can increase system vibration and required horsepower.

· With lubricated compressed air systems, improper changeout intervals for separators and/or oil coalescing elements can increase oil carryover throughout the compressed air system. This type of carryover can be detrimental to the compressed air system’s application.

· The driver system can have the same potential problems as the other systems. If the system becomes contaminated, its ability to work efficiently will be compromised.

· A predictive maintenance program will help prevent these potential faults in the system’s efficiency and help reduce both downtime and unscheduled repair costs. Although, in order to ensure the success of a predictive maintenance program, there needs to be an understanding of what variables can be monitored.

What To Watch For

A predictive maintenance program is monitoring complete system information and then using that information to schedule maintenance that can eliminate unexpected downtime and costly repairs.

When setting up a predictive maintenance program, the two most important variables to watch are temperature and pressure. In some cases, vibration is another variable that can be useful. These variables are very important in spotting breakdowns in the compressed air system’s efficiency. For example, a slowly increasing temperature can indicate a variety of maintenance requirements, including cooler core cleaning; overloading of system; and possible mechanical problems. A decrease in pressure may indicate reduced compressor performance or increased system leakage.

In addition to temperature and pressure, another important variable to monitor is differential pressure. Pressure drops are a result of air moving across a filter. Therefore, differential pressure exists wherever a filter element is found.

Filter elements, found in each of the compressed air system’s sub-systems, keep the sub-systems from becoming contaminated. Examples of filter elements include inlet air, oil, separator (rotary screw only), demister (centrifugal only), and oil coalescing , particulate, and activated charcoal.

As mentioned earlier, wherever a filter element is found; so is a drop in pressure. The one thing to keep in mind is, with time, these pressure differentials will increase. For this reason, it’s a good practice to replace the filter elements before the cost of increasing pressure drop exceeds the cost of a replacement element. Another critical concern is that these filter elements must be able to maintain the air quality required for the compressed air system’s application. Replacing the filter elements will help avoid unscheduled downtime and costly repairs.

These three factors, temperature, pressure and differential pressure, contain the data needed to compile a successful predictive maintenance program. The next consideration is to select the type of controls that would best suit your application. You can begin the decision process by reviewing the advantages and limitations of the different types of controls.

Selecting Predictive Maintenance Controls

With a predictive maintenance program, accuracy of the system’s maintenance predictions is the most important factor in deciding the type of control system to use. The risk of compressor damage and unscheduled downtime increases as measurement accuracy decreases. Currently, the control methodologies available provide a wide range of accuracy, allowing you to decide the most appropriate methodology for your application. It is also helpful to utilize your compressed air system manufacturer to help select the predictive maintenance system that best meets your needs.

The least expensive control method is through the use of gauges. This methodology requires an operator to manually record all gauge readings daily from the compressor. By reviewing the readings, the operator is then expected to interpret the information and make their own prediction regarding the compressed air system’s needs. Unfortunately, this control method requires a lot of responsibility from the operator maintaining the compressed air system’s efficiency. If the readings are not taken regularly and the data not reviewed looking for trends, there will be no predictive maintenance benefit. To add to the difficulty, gauges that monitor several functions are sometimes unlabeled and occasionally require operators to gather multiple readings to ascertain compressor functions. Therefore, this type of predictive maintenance program, while inexpensive, is the most inaccurate.

The next methodology in monitoring controls is the use of discrete inputs. This type of control utilizes switches that allow a binary decision -- yes or no -- to be made about the condition of the measured input. An alarm is then used to alert the operator when the actual value, whether it is a change in pressure or temperature, exceeds the set point.

This methodology loses some of its accuracy due to the fact that the set point is subjective. Normally, you look at the rate at which the measured input is increasing to determine how long you should wait before you change a filter element. But, because discrete input is binary, this rate of change cannot be determined. Therefore the operator must determine the set point based upon an estimated rate of change. This predetermined set point usually begins with a recommended value from your OEM. This value is then adjusted according to plant procedure and compressor application. For example, how long will it take an operator to change the filter elements once the alarm has occurred? An hour? A day? A week? Any of these times can be acceptable if it is factored into the set point.

Currently, the most accurate methodology for predictive maintenance is the use of analog inputs. Analog inputs are represented by directly measurable quantities such as temperature and pressure. To measure air pressure and temperature, this type of control system relies on electrical transducers and sensors. The use of this equipment is what makes the use of analog inputs the most precise methodology. It is also what makes the system more costly.

The analog input control uses a microprocessor panel to translate pertinent operating data so that it can be displayed in an easy to read format. Essentially, the microprocessor is providing a stand alone monitoring system. One of the advantages of this, is its ability to make “maintaining” accuracy easier. With time, pressure and temperature gauges need to be recalibrated to maintain accuracy. Microprocessor controls make this maintenance requirement less costly and time-consuming.

Putting It All Together

Since the development of microprocessors, the door has opened to allow plant operators easier methods to manage predictive maintenance programs.

First came the control system. The control system allows operators to connect the microprocessors of multiple compressors, dryers, and other accessories to a common controller. Many systems can accommodate up to eight air compressors to work as a system.

When compressor units experience shutdowns, it can be difficult to pinpoint the cause because several alarms may have been activated. Microprocessor control systems can monitor multiple alarms, and if shutdown occurs, recall the alarms to help identify the various problems.

Also, the microprocessor control system helps operators troubleshoot through its monitoring alarm system. Even if a warning alarm is activated when the compressor is unattended, and the system corrects itself, the alarm will remain in the microprocessor panel memory, with various operating parameters that were present at the moment of the alarm. This enables operators to reconstruct and evaluate the conditions when the warning alarm was activated.

Further, microprocessor controls can be added to older centrifugal, reciprocating and rotary units, streamlining data flow and eliminating complicated capacity control devices.

The next development, to help make managing compressed air systems easier, allowed plant operators the ability to monitor from a remote location.

This was made possible with the introduction of SCADA - Supervisory Control and Data Acquisition. One of the ways this technology is brought to life is through a communication box, which connects to the controller and allows various control and operating parameters to be passed along to a SCADA system or PC. Add these capabilities to a modem, and you now have the ability to operate from a remote location via telephone.

For example, an operator back at the office in North Carolina could call up and retrieve operating conditions of the compressors in Kentucky. Modem capabilities also allow a plant operator the ability to access vital operations of his facility through any PC that had access to the Internet.

This remote access capability allows compressor monitoring to take place in a centralized control room, integrating the compressors into the plant control systems. Two-way communication also provides the ability to retrieve maintenance observations and statistical data to develop operating trends that will enable plant operators to relegate "unscheduled" system outages as part of the routine maintenance program.

When trouble shooting occurs, this centralization allows maintenance staff to determine the problem in advance so they arrive prepared. This reduces the need for return visits saving time, effort and money.

Most recently, SCADA programs have begun offering automated paging services. With this ability, the SCADA program could essentially send a message to your beeper, alerting you to an alarm or warning condition on the compressor.

As technological advances continue, microprocessor control systems will move beyond basic regulation of compressor functions to offer operators an unbounded selection of integrated functions, that will not only extend the life of the compressors, but further increase efficiencies in overall plant operations. As for now, there are some definite trends emerging.

Looking Towards The Future

Most standard compressed air systems do not feature a direct preventive maintenance function. The most common protective functions will include inlet filter differential pressure gauges and alarm and trip controls on critical pressure and temperatures.

Similar to personal computers, initially people shunned away from microprocessor controls due to their high initial costs. But as sensor and controller prices fall, the initial cost of automatic predictive maintenance monitoring will decrease and more compressor owners will include the system on new orders. Most compressor manufacturers offer at least one design microprocessor system and have completely eliminated other older, less efficient control system units. In fact, some manufacturers offer micro processor control systems on compressor accessory units, such as air dryers, and conversion kits that can be retrofitted to older compressors. Such upgrades add significant benefits and maintenance reduction, especially on reciprocating compressors.

A growing trend among compressor manufacturers is to offer “time-based” predictive maintenance as a standard feature on compressed air systems. Instead of offering gauges, switches or transducers that track operation, the control system will simply keep track of the operation by using time for measuring temperature, pressure, and differential pressure. After a predetermined number of running hours, the controller will alert the operator that it is time to change the filter elements. Although time-based controls are more advanced than using gauges and switches, operators must still understand their limitations. Time-based controls are less expensive than analog inputs but they are still not as accurate. This is due to the operator’s responsibility in deciding the time operation for the filter.

Another growing trend within the compressed air industry is that compressor manufacturers will offer customers lease/rent options on compressed air equipment equipped with advanced predictive maintenance controls. Compressor manufacturers can then offer monitoring services to customers to help maintain predictive maintenance programs.

Regardless of what option a company chooses, it is important to incorporate a predictive maintenance program into any compressed air system operation. Predictive maintenance is essentially your operational safety net.

Reference :
Website http://air.irco.com

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