either a deteriorating internal or external component.
Figure 2.3 Trend Examples
4.Critical monitoring systems may also have built-in protection schemes, which can be readily programmed. These systems can provide either a remote or local alarm or alert whenever an undesirable condition has been detected.
5.Finally, for a monitoring system to be complete, assessment criteria are required to determine when the machine owner should be concerned or if action must be taken immediately. Without assessment criteria, there would be no need for monitoring systems because their outputs would be meaningless. If assessment criteria are set too low, then time and money are wasted. However, if assessment criteria are set too high, then human health, environment, and equipment are placed in jeopardy. One of the primary goals of this book is to provide proven and accepted assessment criteria to owners of process machinery.
Let’s put our newly acquired information about machinery monitoring knowledge to use by studying a hospital heart monitor (see Figure 2.4). Many of us have seen them in hospitals beeping and flashing incessantly. Their purpose is to continuously monitor key health parameters of patients in serious or critical conditions. If any of the key indicators are found to be outside normal values, an alarm will sound at a nurse’s station.
Figure 2.4 Typical heart monitor
Here, the patient is analogous to a machine; the heart monitoring system is analogous to a machine’s monitoring system, complete with remote alarms. Because this application is considered a critical monitoring one, i.e., a seriously ill human, continuous monitoring is employed with local and remote alarming capabilities.
Heart monitors typically include the following functions:
•Heart rate (dynamic data)—A heart monitor is a device that measures the heartbeat rate of a patient in real time. A pad with electrodes is placed on the patient’s chest. These electrodes must contact the skin directly in order to monitor the heart’s electrical voltages. Sensors detect a series of heart beats that are sent as raw signals to a signal processor, which processes the information and calculates the beat frequency. This heartbeat rate and digital waveform information are then sent to the monitor for display and storage.
•Blood pressure (dynamic data)—A blood pressure monitor must measure and report two key values that reflect blood pressure in a fractional form; one number on top and one on the bottom, e.g., 128/82. The number on top is called the systolic pressure, which is the pressure inside your blood vessels at the moment your heart beats. The number on the bottom is your diastolic pressure, which is the pressure in your blood vessels between heartbeats, when your heart is resting. The signal processor must be capable of detecting the maximum and minimum pressure in pressure waveform, storing the data, and then displaying on the monitor.
•Temperature (static data)—Temperature is probably the oldest indicator of health. By placing a thermometer, i.e., sensor, in the mouth or other location, we can assess the general health of the human body. It is common knowledge that 98.6°F (37°C) is an expected normal body temperature. Furthermore, any temperature over 100°F (37.8°C) is not expected and could be taken as an alarm, or alert, level. It is also useful to trend temperature to see when the body temperature began to rise. Gathering this information often requires some form of internal memory so that the data can be trended over time.
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
