Looking after your AHU

Accurate data capture is essential as part of a predictive maintenance plan in modern manufacturing and selecting the right vibration sensors to carry out effective condition monitoring in specific applications is essential. 

Here, Chris Hansford, Managing Director of Hansford Sensors, looks at the management of air handling units and how modern vibration monitoring techniques can maximise their performance.

A brief history of vibration sensing

Vibration sensing has been employed for some time now; indeed we can be certain that it has been in use for almost two thousand years.  The oldest vibration sensor known to historians was a seismograph invented by Chinese astronomer Chang Heng in the second century AD.  This device responded to distant disturbances by depositing a bronze ball from the mouth of one of eight dragons placed at intervals around a large urn, thus indicating the direction of a distant earthquake and potentially saving lives by enabling the government to send assistance to the affected area.

Vibration sensing is therefore one of the oldest solutions in the engineering workplace, with a lineage that outstrips other technologies by centuries.  However, today’s vibration sensing systems are no longer sculpted in the forms of mythical beasts; the development of vibration sensing has been less concerned with visual impact and more concerned with enhancing performance. This is because the users of each application in which vibration sensors are used (and there are many) demand ever-greater productivity from each mechanical system.  There has therefore been a corresponding growth in the need for sophisticated vibration sensors that can maximise the performance of many engineering processes.  Today’s designers and engineers of vibration sensors have not only succeeded in providing devices of exceptional reliability but also in packaging that functionality in a variety of resilient enclosures to enable its use within a wide range of applications. 

Vibration sensing and Air Handling Units

Air control as a method of manufacturing medicine is probably older than even vibration sensing, though the drying of animal flesh and bone to produce powdered drugs no longer represents the height of pharmaceutical technology.  Today, successful and effective air handling in the pharmaceutical industry requires maintaining machinery that can stabilise air conditions within precise limits during the manufacture of supplies.  Consistent indoor air quality must be maintained, with no sudden changes to air flow rate, temperature and humidity that could potentially compromise production quality.  To ensure that these levels are carefully managed, machinery must be well specified, maintained and run, since a mechanical failure can make a degradation in air standards almost inevitable.  It is vital to ensure that a quality product is produced on any pharmaceutical production line, so there is a requirement to maintain air flow rate, temperature and humidity.  Costs generated by lost production can be especially high in the pharmaceutical industry, owing to such issues as penalties resulting from late orders, and customer dissatisfaction can eventually result in the greatest damage if valuable business is lost.  For these and many other reasons, the mechanical failure of air handling units must be avoided. 

Taking advantage of vibration sensing

Vibration sensing assists engineers in preventing air handling units from unnecessary failure but it needs to be carefully specified.  However, because vibration sensing has become established as a powerful predictive maintenance solution within modern engineering systems, expert suppliers can provide the right advice and specification for a growing number of applications.  In addition, training in the use of condition monitoring components such as vibration sensors is now provided by organisations such as BINDT (British Institute of Non-Destructive Testing).  This is important because, despite the fact that a vibration sensor offers high levels of performance and reliability, the component is only as good as its installer.  For example, when mounting a sensor there may be a choice between drilling, tapping or gluing but engineers need to understand and consider how these methods may affect the warranties on their equipment.  However, with the right advice and assistance, the rotating elements of air handling units can be cost-effectively monitored to enable vibration engineers to pick up early signs of any degradation in the equipment so that maintenance can be planned and carried out with minimal expense and disruption to service.

A closer look at vibration sensors

Accelerometers contain a piezoelectric crystal element, which is bonded to a mass.  When subjected to an accelerative force, the mass compresses the crystal, and this causes the crystal to produce an electrical signal that is proportional to the force applied.  This output is then amplified and conditioned by inbuilt electronics to produce a signal that can be used by higher level data acquisition or control systems either ‘online’ or ‘offline’.  An online system is one that measures and analyses the output from sensors that interface directly with a PLC.  An offline system is created by mounting sensors onto machinery and connecting them to a switch box; engineers can then use a hand-held data collector to collect readings.

There are two main categories of accelerometer: AC accelerometers, which are typically used with data collectors for monitoring the condition of higher value assets such as turbines, and 4-20mA accelerometers, which are commonly used with PLCs to measure lower value assets, such as motors, fans and pumps.  Both AC and 4-20mA accelerometers can identify misalignment, bearing condition and imbalance, while AC versions offer the additional capability to detect gear defects, belt problems, looseness and cavitation. 

Inside the air handling unit

A typical air handling unit comprises a supply fan and an extractor fan, coils that circulate steam or hot water for heating and chilled water for cooling and air filters, encased within a large metal enclosure with removable panels.  The complete fan and motor assembly are contained within the enclosure, so engineers can easily gain access to carry out any necessary maintenance procedures by removing the panels.  The supply fan and extractor fan can either be direct-drive or non-direct drive.  Direct-drive fans may require a vibration sensor on both the drive-end and non-drive-end bearing, while non-direct drive fans may require the addition of two accelerometers on the journal bearings of the gear shaft.

A well-established, and cost-effective, technique for identifying imbalance and misalignment in air handling units employs 4-20mA sensors mounted on to the bearings and shafts, with the velocity levels being fed back to a PLC, allowing overall vibration trends to be monitored.  A dual output sensor can provide not only a 4-20mA output but also an AC output, allowing the engineer to take more-in-depth vibration analysis via a data collector.  In an air handling unit an M12 connector used in conjunction with separate M12 cable assemblies offers an effective option, as the M12 connector is of a smaller size than many alternatives, and the associated cable assemblies have a tight bend radius.  A further option is to use fixed AC sensors hard-wired to switch boxes outside the air handling unit, allowing data to be collected safely from the same positions on a regular basis.  The limitation of this option is that readings only apply to the moment in time that they were taken.  If there is limited access, for example, near the belt guards, it is worth considering that side entry sensors can be used, as well as especially compact small footprint sensors for small air handling units.

Specification and installation

When it comes to switch boxes, stainless steel units are often compulsory in pharmaceutical applications.  A well-established supplier will offer valuable advice on specification issues such as this, and offer important guidance on installation.  For example, accelerometers should be mounted directly onto the machine surface. This surface should be flat, smooth, unpainted, free from grease and oil, and larger than the base of the accelerometer itself.  The accelerometer should also be positioned as close as possible to the source of vibration.  If conditions (and product warranties) allow, the preferred mounting technique is to drill and tap a thread directly onto the machine so that an accelerometer with an integral ¼-28UNF, M6 or M8 mounting thread can be screwed into place.  This ‘drill and tap’ method can also be used to fix a mounting stud, to which an accelerometer can then be attached, and specialised installation kits are available for performing this task.  It is important to take care and use the right tools for the job, as tightening the sensor outside the appropriate torque levels can damage equipment or reduce its effectiveness.  For example, over tightening can damage the sensor by stripping the thread, while under tightening will lead to inaccurate readings due to poor contact with the vibrating surface.

If drilling and tapping is not an option, the next best thing is to attach mounting studs using adhesive.  The issue to be aware of here is that you may need to consider the temperature present within your application and choose an adhesive that is capable of coping with that temperature.  For applications in temperatures up to 100°C, a metal-filled epoxy adhesive would be the appropriate substance to use.

A good spot facing kit will give you all the necessary tools needed to accurately mount a vibration sensor onto the rotating machine, including a tapping drill, taps, tap wrench and a spot facing tool.  These kits are now available to allow for different mounting threads; ¼, M6 and M8.  Correct mounting of the sensor is vital to ensure true readings and, where possible, mounting a sensor via a drilled and tapped hole directly to the machine housing will give the best results.  However, if the housing is not flat, a spot facing installation kit allows creation of a flat surface.  

Having installed accelerometers as carefully as possible, ensure that the accelerometer cable is clamped to the body of the accelerometer itself with a cable tie.   This will not only prevent strain but also prevent the false readings that can be generated when loose cables make excessive movement.

If all of these specification and installation issues are carefully considered, not only will the air handling unit be able to maintain air control consistently to the benefit of the plant as a whole but also extend the operating life of equipment beyond recommended maintenance intervals, guarding against the expense generated when increases in vibration lead to machine failure, downtime and unwelcome reductions in production volume or quality. 

We have come a long way since Chang Heng’s ingenious seismograph and vibrations far smaller than earthquakes can now be detected instantly with accelerometers.  Chang applied an earthly, pragmatic approach to the challenges presented by earthquakes, contradicting the orthodox opinion that viewed such disturbances as representations of the gods’ displeasure.  While his elders awaited their fate, Chang preferred to take a more hands-on approach, and today’s engineers can follow in his footsteps by protecting their air handling units using vibration monitoring techniques, rather than crossing their fingers and hoping for the best.

Date: 31/05/2012