Any plant in the world, especially in chemical or petrochemical industries contains a rotary or rotating machine, rotary machines may be turbines, Compressors, Blowers, Fans, gearboxes, Pumps…etc. While the plant is running in normal condition, each equipment is a black box, we cannot see what is happing inside, what are the conditions of the internal parts like shaft, bearing, gears, seals …. etc. so, how can we monitor it?
To monitor or protect a machine black box we have to use machinery health monitoring system, which mainly based on vibration measurement, through the following lines, we will indicate some important principles and fundamental about machinery monitoring systems.
The first question,
what may happen in the rotary machine? or what are the expected problems.?
Rotary machine may be suffering from several critical problems, these problems will produce or generate vibration within danger levels, the expected problems may be:
• Angular or Parallel misalignment
• Bad or faulty bearing
And any problem from the mentioned above can effect on the machine performance and its life time.
What is the vibration?
Vibration means the state of an object moving repetitively back/ forward, right/ left or up/down and is generally expressed by Frequency, Displacement, Velocity, and Acceleration.
Vibration is very important phenomena, vibration can indicate the equipment condition, hence we can take suitable action to keep the equipment in good manner, through its life time.
In general vibration is very important phenomena, sometimes we generate vibration to measure physical quantities, there are many instruments work based on vibration such as:
Mass flow meters called Coriolis flow meter, the theory of this meter based on oscillation of tubes and measuring its frequency while it empty and then measure the frequency of oscillation while liquid pass through the tubes, the measured frequency can indicate the mass flow quantity, in this flow meters there is an excitation coil, that coil responsible for vibration(oscillation) generation.
level measurement via vibrating fork, and vortex flowmeters are another type of instruments that use vibration generation.
Vibration also can effect on the performance of industrial instruments even on the internal electronic integrated circuits. Each equipment shall work in specified vibration levels; any violation of the standard requirements will effect on the equipment performance.
Even musical instruments theory based on vibration, here we measured vibration to indicated the machine condition, then we can determine the required or proper action to keep the machine running normally, with maximum availability and reliability and to avoid machine breakdown.
As we mentioned before, Vibration could be expressed by frequency, displacement, velocity, or acceleration, so what is the difference between them.
Frequency: means the number of times that vibrating object generates a repetitive motion in 1 second. With measuring unit Hertz (HZ)
Displacement measurement is the distance or amplitude displaced from a resting position, and it is measured in mm or micrometer, non-contact proximity sensors (eddy current) are used to measure the displacement.
Velocity is the rate of change of displacement with respect to change in time. The SI unit for velocity is meters per second (m/s), Velocity vibration measurements are generally made using either swing coil velocity transducers or acceleration transducers with either an internal or external integration circuit.
Acceleration is the rate of change of velocity with respect to change in time. The SI unit for acceleration is meters per second2 (m/s2). Acceleration vibration measurements are generally made using accelerometers.
In other words, velocity is the integration of the acceleration, and displacement is the integration of velocity.
There are two types of monitoring systems
Permanent online vibration monitoring systems, these systems used permanent installed sensor may be contact or non-contact sensors to measure equipment’s vibration, and in general used with expensive, important machines like turbines, axial, centrifugal, or screw compressors, gear boxes, or expanders, also used in machines operate constantly and continuously, cannot be approached, deteriorates slowly, the most famous system irrespectively Bently Nevada, Emerson CSI, and Shinkawa.
These systems based on two types of sensors
• Contact sensors and used to measure the vibration in the form of acceleration or velocity, these types of sensor called casing vibration sensors, they measure the casing vibration.
• Non-contact transducers system (eddy current) which consists of proximity sensor, extension cable, and proximity driver.
• Each sensor is connected to a card in modular system like Bently Nevada 3500, Emerson CSI6500, the arrangement of the sensors inside the machine can help us to measure critical parameters like radial vibration, axial displacement, speed in Revolution per minute(RPM), and also can stop the machine in case of abnormal condition based on alarms and tripping levels according the machine and manufactures recommendations and standards.
• System like Bently Nevada can generate the rotative speed frequencies, these frequencies like 1X, 2X, 3X…. etc. can indicted the condition of the machine, like misalignment, unbalance, looseness, cavitation, resonance, or faulty bearing.
• We can also monitor the machine condition via the bearing temperature, temperature sensors like thermocouple or RTDs could be installed in journal or thrust bearing. also there are special cards or module specified for temperature measurement
Contact sensors like accelerometers or velocity meters it easy to be used, because we install it on the casing of the rotating machine, but non-contact sensors (eddy current) need accurate adjustment, like the gap between the shaft and proximity sensor, sensors orientation.
Using eddy current sensors, we can measure the following:
• Relative/radial shaft vibration
• Axial displacement (thrust)
• Shaft speed
Relative shaft vibration, we can measure the vibration in the form of displacement in mm or micrometer, it preferred to measure the vibration in two dimensions X and Y, oriented in 45 degrees from the center of the shaft, by measuring the X, and Y vibration we can indicate the orbit of centerline of the shaft, that could be used in vibration analysis.
Portable monitoring systems used with machinery that is minimal impact of breakdown, it easy to be measured, and deteriorates quickly like pumps.in this method you just need a portable device and suitable sensor can be accelerometer or velocity meter, fix it on the casing and measure the acceleration or the velocity.