Vibration Monitoring Can Boost The Efficiency Of Machinery Used In Cement Manufacturing By Reducing Downtime.
By Chris Hansford
The boom in infrastructure building worldwide means that cement production is on the rise. World cement volumes are expected to see a Compound Annual Growth Rate of 4.9 percent to 2017, according to CW Group in its latest survey of the industry. It predicts an increase in per capita cement consumption from 539 kg in 2012, to 645 kg in 2017, which would take global consumption beyond 4.5 billion tons.
This is a huge opportunity for cement producers. But, to capitalize on it, many will need to boost plant efficiency by maximizing the reliability of their production systems. Many cement plants still operate below their potential capacity, due in part to the time spent on machine maintenance. New technologies can improve machine reliability and plant productivity, but optimizing existing systems will have the largest effect on cutting maintenance time.
Cement manufacturing is one of the most aggressive production processes. Some of the chemicals used in portland and masonry cement production – particularly silicates, aluminates and aluminoferrites – generate very fine particles that cause havoc in rotating components like the motors, pumps and fans used in crushers, mills, precipitators, kilns and silos.
Despite protective covers, and sealing to high IP standards, it is impossible to prevent the ingress of fine particles, which can affect the efficiency of lubricants, leading to increased wear of bearings, shafts and seals, followed by the risk of premature failure.
Worn rotating parts lead to an increase in internal tolerances, causing mechanical imbalance, loose components and rubbing parts. This is seen in the form of vibration, which can be detected by sensors. Because these sensors can detect very low levels of vibration – and tiny changes in one vibration state to another – it is feasible to assess the rate of wear and take action before a problem develops.
Vibration sensors – otherwise known as accelerometers – contain a piezoelectric crystal bonded to a mass. When subjected to an accelerating force such as vibration, the mass compresses the crystal, causing it to produce an electrical signal that is proportional to the force. This is then amplified and conditioned to produce a measurable signal, which can be used by higher-level data acquisition or control systems.
Output data from accelerometers mounted in key locations can either be read periodically using handheld data collectors – for immediate analysis or subsequent downloading to a PC – or it can be routed via switch boxes to a centralized or higher level system for continuous monitoring.
In cement manufacturing, the key components to monitor – in kiln drives, crushers, screens, conveyor belts, raw mills, elevators, separators and blowers – are the motor and gearbox assemblies. In each case it is critical that the correct type of sensor be selected, for ease of installation and access, and for reliable operation and data collection.
When sensors are to be installed on a crusher, screen, conveyor pulley or drive, for example, a side-entry model may be preferred over a top-entry component, so that debris cannot damage the sensor. In high temperature areas, an appropriately protected sensor should be used.
To specify an accelerometer correctly, engineers must consider the vibration level and frequency range to be measured, environmental conditions (such as temperature) and whether corrosive chemicals are present. The best solution is to work closely with the sensor supplier, to ensure that the selected units perform correctly. Hansford Sensors has an extensive range of vibration sensors designed to withstand the harsh conditions of cement manufacturing, and offers multiple connection, output and environmental options.
Vibration sensors are usually easy to install and use – though an accelerometer is only as good as the engineer responsible for it. If poorly installed or maintained, it will not offer suitable precision or longevity. Condition monitoring depends on stability: a poorly mounted accelerometer may give readings that relate not only to a change in conditions but also to the instability of the sensor itself.
Modern accelerometers for vibration monitoring can operate over a wide temperature range, measuring both high and low frequencies with low hysteresis characteristics and high accuracy. They are robust and reliable, thanks to stainless steel housings that prevent ingress.
An accelerometer should be mounted directly onto the machine on a flat, smooth, unpainted surface that is larger than the base of the accelerometer. The installer must ensure that the surface is free of grease and oil, as close as possible to the source of vibration and perpendicular to the axis of rotation. This helps to ensure the most accurate possible measurements of vibration levels.
Correct mounting of the sensor is vital to ensure true readings. Where possible, mounting a sensor via a drilled and tapped hole directly to the machine housing will give the best results. If the housing is not flat, a spot facing installation kit allows creation of a flat surface. A good spot facing kit has the tools needed to mount a vibration sensor onto the rotating machine accurately – including a tapping drill, taps, tap wrench and a spot-facing tool. These kits are now available to allow for different mounting threads, including ¼, M6 and M8.
Correct installation is essential for reliable and consistent operation. A Vibration Monitoring System must handle issues including imbalance, misalignment, bad bearings, mechanical looseness, hydraulic forces (cavitation, resonance) and rubbing. To detect these faults, sensors should be located to ensure that horizontal, vertical and axial movement are measured effectively.
For horizontal measurement, vibration sensors should be mounted on the two motor bearings or pump bearings. This measures velocity in millimeters per second or mm/sec (Peak or RMS) to detect imbalance, and problems with structural rigidity or foundation.
For vertical measurement, sensors should be located on the motor or pump drive end bearings. This measures velocity in mm/sec (Peak or RMS) to detect looseness and problems with structural rigidity or foundation. For axial measurement, sensors are attached to motor or pump drive end bearings. This measures velocity in mm/sec (Peak or RMS) to detect misalignment between the motor and the fan.
In a gearbox, the sensor should be mounted radially on input and output shafts. This enables the condition of the bearings (g) and fan out of balance (velocity) to be monitored. Additionally, an optional axial accelerometer on the input shaft would give a good indication of the thrust on the shaft. For a typical crusher motor, accelerometers are mounted radially on the Drive End (DE) and Non-Drive End (NDE), in order to monitor the motor bearing condition (g).
Cement manufacturing applications typically use 4-20mA accelerometers connected to PLC systems. Each sensor is connected to a local junction box, then multicore (screened twisted pair) cable is connected to the PLC system for data trending and alarming.
With the sensors installed in this way – depending on accessibility – a local junction box can be installed close to the motor. Multicore cable is used to connect the junction box back to a main switch/connection enclosure.
In some applications the sensor cable is connected directly to the switch enclosure – a sealed industrial enclosure that has been designed to withstand harsh conditions. It is available in various forms such as mild steel, stainless steel or polycarbonate, depending on environmental conditions.
Increasingly, sensors are being hard-wired back to centralized control systems, with data monitored in real time. Although this is expensive, it is safer and more efficient than offline vibration monitoring, which nevertheless is still commonly used.
In offline vibration monitoring, maintenance engineers gather data manually using a single temporary sensor and handheld instrument, or a handheld monitoring device that connects to the outputs from permanently mounted sensors around the plant.
Regardless of how data is captured, it is critical to analyze different frequency spans, which are dictated by the fault frequencies of the fastest-turning component in the monitored machine.
Once the frequency span is known, the resolution needs to be set within the vibration software for spectrum analysis so that fault frequencies of rotating components are not mistaken for other – correct – machine frequencies.
Cement manufacturing is one of the toughest industrial environments – as well as being a tough, cost-conscious market. Vibration monitoring can help producers stay competitive, by ensuring that their machines remain in good condition with the minimum of maintenance.
Chris Hansford is managing director at Hansford Sensors Ltd, which specializes in the design and manufacture of accelerometers for monitoring vibration and temperature levels of industrial machinery, playing a key part in the role of predictive maintenance. For more information on the extensive range of Hansford Sensors products, visit www.hansfordsensors.com.
The cement industry is a punishing environment for machines, and for the sensors that monitor them. These pointers should help in selecting the most appropriate system.
- Identify the vibration level and frequency range to be measured. This is the first thing to establish before choosing a vibration monitoring system. Also, carefully consider issues such as temperature and humidity.
- Consider the working environment. The cement industry is a very demanding environment, so sensors must be sealed to a high level. High temperatures and corrosive chemicals are also common.
- Online or offline? An online system measures and analyses the output from sensors that interface directly with a PLC. In an offline system, engineers use a hand-held data collector to collect readings from machine-mounted sensors.
- Install close to the source of vibration. For best results, accelerometers should be as close as possible to the vibration source.
- Maximize stability. Accurate monitoring relies on stability. Sensor instability can be eliminated using spot mounting. There may be a choice between drilling, tapping or gluing, but consider how each method may affect warranties on equipment.
- Use the information that vibration monitoring provides. Warnings generated by vibration monitoring systems are often ignored in order to continue production. It’s better to perform simple maintenance straight away than risk more costly failure further down the line.