through the system. If the normal system pump does not create sufficient volume on its own to generate turbulent flow, additional pumps may be required on a temporary basis to increase the flow velocity. Alternatively, if standby pumps are incorporated in the system, these could be used to augment the normal system pump flow. Similarly with filters, additional large-capacity flushing filters can be temporarily incorporated in the return line of the system if the normal operational filters are not suitable.
Fluid samples should be taken at regular intervals during flushing, and the flushing process continued until the target cleanliness level has been achieved. Wherever possible, the fluid used to flush the system should be the same fluid that will be used when the machine is put into operation. In this way, at the same time as the system is being cleaned by the flushing process the fluid is being cleaned as well.
Whether or not this is a valid approach to maintenance really depends on the consequences of any breakdown of the machine or device. It may be a perfectly acceptable approach for a TV remote control but totally inappropriate in other situations. It is unlikely that many people remove the batteries from their TV remote control once a week to check their voltage level and replace them when they have dropped to a predetermined level, as the consequences of the remote control not working are relatively minor. And perhaps more damage is likely to occur by constantly removing the battery cover or putting the batteries back in the wrong way round. In such a situation, therefore, breakdown maintenance (only replacing the batteries when they no longer work) would be an acceptable philosophy. Adopting the same practice for a nuclear reactor in a power station, however, would obviously be wholly unacceptable.
So, is breakdown maintenance an acceptable practice for hydraulic systems? The answer to that question is almost certainly no. At best, unexpected breakdowns will mean a loss of machine production and quite possibly a high cost of rectification.
Depending on the situation, unexpected breakdowns may also be potentially dangerous to personnel. Even something as simple as a burst flexible hose is likely to spray hot hydraulic fluid (which may be flammable) over the surrounding area and potentially cause contact injuries due to unrestrained whiplash movements.
So, almost without exception, it is possible to state that breakdown maintenance is not applicable to hydraulic systems. That is not the same, however, as saying that such maintenance does not happen in practice!
Preventive maintenance
Preventive maintenance (Fig. 8.10) involves predicting when components of a machine are likely to wear out or become unserviceable, and scheduling their replacement before this occurs.
For example, it may be possible to determine, through experience, testing or calculation, that the seals on a cylinder piston have a life of 8 months in a particular
PLANT MAINTENANCE
Fig. 8.9 Breakdown maintenance (Image courtesy of Eaton Corp.)
application. A maintenance schedule may then be drawn up to strip the cylinder down and replace the seals every 6 months. A ‘safety margin’ of some degree needs to be built in, as no lifetime predictions are likely to be 100% accurate.
This technique is often used on machinery that has a more or less predictable pattern of usage, such as passenger cars. In the main, passenger cars are used on roads (as opposed to off-road), in winter and summer, and at speeds nominally within national speed limits. Therefore, the interval between necessary oil changes or cam- belt replacements can be predicted reasonably accurately, and recommendations can be made in the service manual to carry out these maintenance activities after a set period of time or mileage. The same may not be the case, however, in relation to race or rally cars, the usage of which is likely to be much more severe and unpredictable.
Preventive maintenance can therefore work well when component lifetime expectancies are well understood and the machine operation is reasonably predictable. A further everyday example is the batteries in a camera. From experience we may know that after a period of use the batteries will have lost a certain amount of charge. So, before taking the camera to an important event, such as a wedding, we would change the batteries for new ones, or give them a precautionary top-up if they are rechargeable.
The disadvantages of preventive maintenance is that it is not always possible to predict component lifetimes or machine usage patterns accurately. If the component life is overestimated, then costly, unexpected breakdowns may still occur before the scheduled replacement time. If the component life is underestimated, perfectly serviceable components, with many hours, days or months of useful life left in them, will be replaced needlessly, thus increasing operational costs.
Preventive maintenance is a technique that is used regularly in relation to hydraulic systems. The most common example is probably the replacement of flexible hoses, where it is recommended that all hoses older than a stated age (typically 5–10 years)
PLANT MAINTENANCE
Fig. 8.10 Preventive maintenance (Image courtesy of Eaton Corp.)
POINT OF INTEREST Car service manuals are a classic example of where preventive maintenance procedures are used in practice.
For example, they specify that an oil change or cam-belt replacement should be done after a certain number of miles or kilometres.
are replaced. In some applications, however, the recommended replacement age may be much less. Another example is the filter or air-breather elements typically used on mobile applications where condition indicators are not fitted. In such situations the recommended period between element changes will be stated in the service literature.
Predictive maintenance
Predictive maintenance (Fig. 8.11) is based on measuring the performance of key components in order to predict when they will approach the end of their useful life. This could involve something as simple as a blockage indicator on a filter, or something much more complex such as an on-line debris analyser.
PLANT MAINTENANCE
Fig. 8.11 Predictive maintenance (Image courtesy of Eaton Corp.) DEFINITION
Predictive maintenance is also known as condition-based maintenance (cbm).
A blockage indicator on a filter or breather senses the pressure drop across the filter element, and triggers a visual indicator or electrical switch when the pressure difference reaches a predetermined value. This pressure difference is somewhat less than the pressure difference required to open the bypass valve in order to protect the element from total collapse. The indicator therefore tells the maintenance people that an element change is required before the filter actually goes onto bypass and becomes inactive as a contamination-removal device.
This is obviously a more precise means of judging when to replace a filter element than simply replacing it after a recommended period of use, but additional cost is added to the system by including the sensor and indicator. In the past, the relatively high cost of incorporating sensors to detect not only filter-element condition but also properties such as fluid cleanliness and condition, component leakage rates and noise and vibration has probably been the main disadvantage of predictive maintenance. In recent years, however, the cost of such sensors and monitoring devices has dropped dramatically. In addition, monitoring devices do not have to be permanently mounted in the system. Provided easy access is built into a system for taking pressure or flow measurements, fluid samples, temperature measurements, etc., then monitoring can easily be carried out on a routine scheduled basis using portable monitoring equipment.
Where machine up-time is critical, the cost of online sensors can normally be justified quite easily. Such devices could include:
• pressure sensors for monitoring system pressures
• flow sensors for monitoring system flow rates or leakage flows (e.g. drain flows for pump-condition monitoring)
• temperature sensors for remote monitoring of the bulk fluid temperature or localised hotspots
• reservoir fluid-level sensors
• fluid-condition monitors
• vibration monitors
• hose-condition sensors for monitoring the condition of flexible hoses.
The advantage of basing maintenance decisions on actual measurements rather than experience or best practice are:
• advance warning of impending component failure can be obtained, and rectification work scheduled for convenient times
• replacing a component before complete failure occurs reduces the risk of further failures caused by contamination of the system, fluid spillages or the cost of re-flushing the system
• the cost of carrying out necessary maintenance work can be justified by hard data
• the expense of replacing serviceable components ‘just in case’ can be avoided.
The key aspect of predictive maintenance is being able to determine trends in the data collected (i.e. to see how a property of a component is changing over a period of time).
For example, a temperature reading of 52°C (125°F) on the case of a pump does not really mean much on its own. However, if the temperature was 46°C (115°F) last week and 43°C (109°F) the week before, it is clear that the pump is starting to generate more heat than normal and should be examined or replaced in the near future.
In the days when most machines were controlled by an operator (as opposed to a box of digital electronics), skilled operators could often tell when the performance of a machine started to deteriorate. When a drill or saw blade started to become blunt, for example, the level of force the operator had to exert might change or the noise made would alter slightly, which would tell an experienced operator that the drill or saw blade would soon need replacing. However, since machines have become automated and skilled operators replaced by electronic controls, this capability has largely disappeared. The object of predictive maintenance is to try to replicate the experience and skill of an operator by sensors and software that perform a similar task.
An example from everyday life of predictive maintenance that uses a sensor is the battery alarm in a domestic smoke detector. Obviously the battery condition in such devices is more important than that in a TV remote control, for example. As a result, smoke detectors normally monitor the battery condition and flash a visual alarm or sound a ‘beeping’ noise when the battery needs to be replaced so that the smoke detector can be kept in an operational condition. Normally this happens around 3 o’clock in the morning!
Proactive maintenance
The final maintenance methodology considered here is generally known as proactive maintenance (Fig. 8.12), or sometimes design for six sigma (DFSS). The aim of DFSS is to reduce the possibility of errors or defects in a process or piece of equipment to a target level of 3.4 or less in every million opportunities (or 1 failure per 300,000 opportunities approximately). ‘Six sigma’ is a statistical term that defines this number.
PERFORMANCE LEVEL
TIME IN SERVICE
A B
Sudden catastrophic
failure
New installation
Fig. 8.13 Machine degradation
PLANT MAINTENANCE
Fig. 8.12 Proactive maintenance (Image courtesy of Eaton Corp.)
Mechanical machines tend to follow a predictable degradation curve (Fig. 8.13). The aim of preventive and predictive maintenance is to carry out repairs or replacements during the period between points A and B on the curve in Fig. 8.13, but as close to point B as possible (i.e. before rapid degradation starts to set in). This is especially true in hydraulic systems, as beyond point B wear often accelerates, creating a rapid increase in the number of contamination particles generated.
POINT OF INTEREST Taking steps to prevent a component or system failure from reoccurring is similar to applying proactive maintenance techniques.
There are times, however, when a machine or system will fail catastrophically with no prior warning (see Fig. 8.13). For example, this could happen as a result of a component manufacturing defect, operation of the system beyond its design capability, unexpected operational conditions (e.g. a lightning strike) or poor routine maintenance. Proactive maintenance aims to reduce unexpected breakdowns by analysing the design of a component, system or machine from the point of view
of what could go wrong with it, the likelihood of it going wrong and the resulting consequences. There are several well-established methods for doing this, and these are similar to the processes carried out for risk analysis.
Having identified all the possible modes of failure of a machine or component, the design is modified to ensure that those modes of failure cannot happen or their likelihood of happening is reduced to a minimum or acceptable level. If this is not possible, back-up devices should be designed into the machine to allow continued operation in the event of a failure of the primary device.
A relatively common example of this approach is to include a standby pump on a hydraulic system, which can be brought into operation in the event of an unexpected failure of the normal running pump. In this case, however, proactive maintenance analysis could also be carried out to reduce the likelihood of the first pump failing unexpectedly in the first place. Such analysis could consider, for example:
• monitoring any shut-off valves on the pump suction or drain lines to prevent the pump being started unless they are fully open
• preventing the pump from starting or shutting it down if the fluid level in the reservoir is below a predetermined minimum
• monitoring the temperature and cleanliness level of the fluid supplied to the pump and shutting the system down if these exceed acceptable levels
• using pre-set (rather than adjustable) components to avoid the possibility of incorrect settings, or using key-lockable adjustments on pump compensator or relief valves
• monitoring the pump inlet and outlet port pressures to ensure they remain within specification
• monitoring the pump drive speed to ensure it remains within specification, and taking appropriate action if not.
In practice, the cost of such sensing devices needs to be taken into account and balanced against the likely cost of an unexpected machine breakdown. The merits of the proactive approach will, therefore, differ from one situation to another. However, there are many examples of where modifications that cost very little can reduce the likelihood of human error or equipment failure, provided the modification is made at the design stage. For example, the addition of a dowel pin to the interface of a gasket-mounted valve costs very little, but prevents the valve from being mounted the wrong way round.
Continuing with the battery analogy for maintenance, remote-control keys for cars are often powered by a conventional silver oxide button-type battery. Such keys will obviously not lock or unlock the car when the battery power is exhausted. Using a rechargeable battery that is topped up whenever the key is placed in the ignition will reduce the likelihood of the key suddenly failing to operate.
Setting up a maintenance schedule
Preventive, predictive and proactive maintenance are not mutually exclusive techniques. That is, two or more of these techniques can be combined to avoid
POINT OF INTEREST Due to the consequences of failure, aircraft hydraulic systems often have at least two back-up systems capable of taking over in the event of a failure of the normal system.
unexpected breakdowns. If all these techniques fail, of course, then breakdown maintenance will still be required, but even so this can be pre-planned to a certain extent (e.g. by having available spare parts or components, prepared procedures and trained personnel).
As mentioned above, predictive maintenance has become increasingly popular in recent times, mainly due to the availability of low-cost sensors and digital communications. It is now possible to monitor machine performance in real time anywhere in the world. Commercial jet engines, for example, are monitored continuously by their manufacturer, no matter where the plane happens to be flying in the world. However, there will always be instances where examination by human beings is still worthwhile, and in such cases it is advisable to establish a maintenance schedule.
When setting up a maintenance schedule for a hydraulic system the first thing to decide is what properties need to be checked. The list could include:
• fluid level
• fluid temperature
• fluid cleanliness level
• fluid chemical analysis
• pump temperature
• pump noise
• pump flow rate
• pump case drain flow rate
• pump outlet pressure
• external leakage
• actuator performance (speed, force, leakage, temperature, etc.)
• flexible hose condition
• any abnormal symptoms.
Based on knowledge and experience, a timescale (daily, weekly, monthly, etc.) for checking each property can then be established. It will also be necessary to establish procedures for checking and recording each property to ensure that data is obtained and recorded consistently.
Consider, for example, the simple task of checking the fluid level inside a reservoir.
To check this manually involves reading the level of the fluid visible in the sight glass and recording it. If the level drops from one reading to another, this might indicate a leakage of fluid in an area that might not be readily accessible and therefore has gone unnoticed. However, it could also mean that the second reading was taken with all of the cylinders fully extended (and therefore containing the maximum amount of fluid) and perhaps with accumulators fully charged with fluid, whereas the first reading was taken with the cylinders retracted and the accumulators drained.
The correct procedure for checking the fluid level should therefore include the requirement that the machine state (cylinder positions, etc.) is the same each time a TOP TIP
Maintenance schedules need to define not only what needs to be measured or checked but also how it is to be measured.
reading is made. Similarly, for fluid or component temperature readings, the readings should only be taken after the machine has been operational for a certain period of time and preferably when performing the same operations.
Good maintenance practice
As stated several times previously, it is vital that the people involved in recording data, taking samples, using test equipment, etc., are properly trained in the jobs they have to perform and the equipment they have to use. No matter how comprehensive the maintenance procedures are, there will inevitably be occasions when maintenance personnel have to use their initiative, so it is important that they are properly equipped to do so.
Examples of good maintenance practice include the following:
• Pressure test points. Fit pressure test points into the system where pressure readings are likely to be required (such as when setting pressure-relief or pressure-reducing valves). Gauges permanently connected into the system may be subject to pressure peaks and ripple, and so tend to have a short life. They can be protected by ‘push to read’ or ‘twist to read’ valves, which normally isolate them from the system, or they can be removed completely and only connected when required via simple quick-release test points (see Fig. 9.3).
• Access points. Include access points for taking fluid samples from the system, normally from the pressure line or from the reservoir. Sampling from the
reservoir should ideally be via a pipe, taking fluid from the centre of the reservoir, away from where water or contamination is likely to settle.
• Flow meters. Flow meters are sometimes permanently connected into systems but more often need to be connected temporarily for test purposes. This process can be made simpler by including quick-release couplings and/or three-way valves to divert flow through the meter when required (Fig. 8.14).
Care should be taken, however, to ensure that the test equipment and
SHUT-OFF VALVE NORMALLY OPEN,
CLOSE TO MEASURE CASE
DRAIN FLOW
QUICK-RELEASE COUPLINGS
FLOW METER
Fig. 8.14 Monitoring pump drain line flow