Together these functions provide a comprehensive package of monitors that work together to provide extensive protection. Like the heart monitor, individual monitoring systems can be combined to provide broader protection. It’s usually not enough to monitor only one parameter because people, as well as machines, are susceptible to failure in many different ways. We know from experience that some condition indicators are capable of detecting impending problems earlier than others. With prior knowledge of how people and machines announce their maladies, we can better design effective monitoring systems.
You don’t have to fully understand how monitoring systems sense, process, display, and save information in order to utilize their outputs. As a decision maker, you only need to ask some basic questions:
•What is normal for this machine parameter? Historical data is invaluable when answering this question.
•What has changed? Each machine parameter provides a different insight into what is happening. A baseline vibration spectrum, like the one shown in Figure 2.5, is invaluable when determining what has changed inside a given machine.
Figure 2.5 Machine system and signal sources
•What is the information telling me about the machine? If vibration levels have increased on the drive-end of a newly installed motor, you might want to check alignment.
•How quickly is the change occurring? A step change in temperature might be signaling a loss of cooling, whereas a gradually increasing trend might be telling you a bearing is beginning to fail.
•How quickly do I need to react to this information? Is the measurement value at the alarm or danger level?
We hope that the rest of this book will help you answer these questions and make better decisions.
General Machinery Monitoring Guidance
If you are a newcomer to the field of machinery monitoring, you are probably overwhelmed by all the new concepts and terminology. To simplify matters, we will present a basic roadmap for machinery monitoring for centrifugal pumps, centrifugal compressors, steam and gas turbines, gear boxes, fans, reciprocating pumps, reciprocating compressors, and electric motors. (For equipment types that are not included in this section, contact the corresponding original equipment manufacturers for their recommended monitoring guidelines. Also, talk to other users of similar machine types employed in your industry about their monitoring experiences and best practices.)
Here are some machinery monitoring methods frequently used during field machine assessments:
•Vibration—This method uses dynamic data collected by measuring the motion of a vibrating surface. Analysis of this type of data requires complex signal processing and pattern recognition.
•Pressure—This approach can involve either static or dynamic data collected by inserting a pressure traducer into a fluid stream. This type of analysis also requires complex signal processing and pattern recognition.
•Temperature—This method is usually in the form of static data. Thermocouples or resistance temperature devices (RTDs) are often inserted either into a fluid stream or on or below metal surfaces to measure temperature. Portable infrared temperature guns can also be used to monitor machine surface temperatures. In some applications, thermography is employed to visualize temperature distributions across a machine in order to identify component issues such as failing bearings. Thermography has been successfully used to spot electrical problems in the field on motors and control panels.
•Oil analysis—Oil analysis requires that oil samples be sent off-site for lab testing. This step can lead to a delay in prompt information. Although most oil properties are usually determined by lab testing, some oil properties can be monitored in real time.
•Analysis of piping, duct work, and structural components—Equipment attached to the machine—such as piping, vents, duct work, and supporting structures—can be sources of information; they can path sources for vibrations, flow issues, and loadings.
•Electrical measurements—These measurements are usually in the form of dynamic data because electric power is sinusoidal. Voltage and current waveforms must be analyzed in real time so that they can be converted to real power, root mean square voltage, average current, power factor, etc.
•Performance analysis—The required data for this type of study are static in nature. They are usually taken with temporary or field mounted sensors.
Because there is such diversity in machine designs and applications, we will now provide more specific monitoring recommendations. Below is a listing of the most common process machinery along with recommendations as to what should be monitored and when. Keep in mind these are general recommendations; they should be compared with manufacturer’s guidelines and company best practices.
Monitoring Guidelines for Centrifugal Pumps
Figure 3.1 Typical Centrifugal Pump
Vibration Collection and Analysis: Most centrifugal pump vibration levels are monitored on a periodic basis using “walk around” data collection programs. Assessments are made by placing a vibration sensor, usually an accelerometer, with a magnetic base on the bearing housings. Overall vibration amplitudes are trended to see if any changes are occurring. If a significant change in overall amplitude is observed, a frequency analysis is performed in an attempt to identify the nature of the malady.
Inspection frequency: Monthly and quarterly inspection intervals are common.
Temperature Monitoring: Bearing temperatures can be taken at the same time vibration data is collected. Assessments may be made by using either absolute or relative criteria. For example, you may decide that if a bearing temperature exceeds 200°F (93.3°C), you will shut down (an example of absolute criterion). Or if you see a 20°F (6.7°C) increase in bearing temperature from one inspection to the next, you will investigate (an example of relative analysis criterion).
Inspection frequency: At the same time the vibration data is taken.
Oil Analysis: Oil analyses are typically reserved for critical machines with large oil reservoirs. Therefore, oil analyses are not usually warranted for most small spare pumps with small bearing sumps.
Inspection frequency: Monthly and quarterly inspection intervals are common.
Pressure Pulsation Analysis: This type of analysis is usually performed only when an internal pump issue is suspected or an unwanted pump/piping interaction is occurring. Clues as to when a pressure pulsation analysis is required are excessive piping vibration, broken bolts, high pulsations observed on a pressure gauge, high blade vane pass vibration in the vibration spectra, etc.