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Product category: Condition monitoring and vibration analysis
News Release from: AV Technology | Subject: CBM in petrochemicals
Edited by the Processingtalk Editorial Team on 24 April 2008

Condition Based Maintenance in
petrochemical plant

Condition Based Maintenance (CBM) is an important and integral function of petrochemical plant management and should be part of an all encompassing asset management function

CBM can be viewed as a system, the purpose of which is to control the condition of the monitored machinery within acceptable bounds by making maintenance interventions on the basis of measured condition A well implemented CBM system engenders a Measure-Assess-Improve regime within a plant, establishing the basis for improved equipment availability, enhanced machine reliability, reduction in downtime, improved productivity, relevant legislation compliance, and ultimately, cost reduction

In parallel improved equipment performance should bring important energy savings, increased personnel safety and improved protection for the environment.

According to leading industrial consultants AV Technology , out-sourcing condition monitoring offers significant benefits for petrochemical companies, giving them access to highly qualified maintenance practitioners with a wealth of experience.

Typically out-sourcing may be on a continuous basis with one or more engineers based permanently on site, or on a scheduled part time basis.

In both cases, the key to success is to build a strong partnership from the ground up, engendering trust with all parties concerned.

As relationships develop, transfer of knowledge through field based training increases the effectiveness of the overall CM process.

The Condition Monitoring (CM) techniques employed within a CBM system are well established and include: Lubrication Management, Vibration Analysis, Thermography, Remote Visual Inspection, Ultrasonic Leak Detection, Root Cause Analysis, Structural Monitoring and Acoustic Emission Monitoring.

However in isolation these techniques may have limited effectiveness and therefore a full plant-wide CM program should be adopted, utilising a combination of technologies as appropriate to provide comprehensive data from plant and machinery.

When conventional methods aren't suitable, good old fashioned innovation and ingenuity still have an important role to play.

CM lends itself to the trending of progressive failure modes that can be directly measured, such as changes in the dynamic forces between components, changes in wear and changes in performance.

In CM there are two thresholds; the level at which initial identification of a potential failure is made, and the limiting level before functional failure.

Exploiting the maximum potential of individual CM techniques is also an important factor.

For example, a lubrication management programme can provide a wealth of useful information and can cover aspects including lubrication selection, managing change-out periods and debris analysis, as well as lubricant procurement and recycling.

The key to implementing a successful CBM programme is not only applying the right CM technologies, but managing, interpreting and presenting the data in a clear and concise way so that management can make risk assessment based decisions on when maintenance should be carried out.

A pre-requisite for effective CBM further includes the integration of recommendations, following data analysis, with the existing maintenance function.

Simply producing reports and recommendations without such integration limits the effectiveness of the CBM process and may lead to doubts being raised regarding its true cost benefits.

Integration is achieved with a combination of the right technology, training and, most importantly, management reporting tools.

It is imperative to measure the effectiveness and efficiency of the integrated CBM programme against reliability and availability related KPIs.

There are a number of factors to be considered within the decision process, based on questions such as: When should maintenance be carried out? Can equipment be kept operational until the next scheduled shut down? What are the ramifications of such a decision? Although machinery identified as requiring attention may not fail before the next shutdown, additional and expensive damage may be caused by prolonging any maintenance.

It is important that any CBM programme should cover all areas of the plant and equipment, however small or large.

Gathering on site inspection data has traditionally been very laborious and time consuming, with engineers transferring information from trusty clipboards to spread sheets or log books.

However the data needs to be used to its maximum potential and made available to all parties concerned.

To achieve this there are a diverse range of powerful plant software tools that can fully integrate asset and maintenance management, condition based maintenance, equipment and basic care inspections and intelligent diagnostic agents, all using one system and one database.

Mobile data collection can be carried out using robust PDA data collectors that are fully customisable and extremely versatile, together with laser bar code scanners.

The CBM Process consists of six distinct phases:.

* Evaluation - in which potential applications are identified and screened for cost benefit and technical effectiveness.

* Set-up - in which the chosen techniques are implemented.

* Detection and Analysis - in which routine monitoring is performed, any faults identified and recommendations for remedial action made.

* Correction - in which remedial action is carried out on identified defects in a planned manner.

* Technical review - in which the findings after corrective action are reviewed to verify (or otherwise) the diagnosis.

If the two do not match, the cause of the discrepancy is assessed and the monitoring methods or alarm levels adjusted to prevent recurrence.

* Long-term analysis - over time the cost-benefits of the programme should be evaluated to confirm that the expected benefits are being delivered.

CBM can also inform longer-term maintenance decisions, such as whether to replace equipment and what to include in shut-down work-scopes.

Recurring themes can be identified and priorities for Root Cause Analysis and improvement established.

Assessment of the effectiveness of CBM can lead to adjustments to the CBM programme as a whole.

VIBRATION ANALYSIS.

Vibration analysis is a crucial part of any CBM system and can be used to highlight a wide range of machinery and equipment.

The key to success is managing and interpreting the data from a very large number of measurement points.

The output from the data should allow clear decisions to be made regarding the maintenance requirements of individual pieces of machinery and equipment.

Hand-held data collectors are designed to automate this measuring and analysis process.

THERMOGRAPHY.

It is important that condition monitoring is carried out when plant and equipment is operational.

Thermography techniques provide a highly efficient method of identifying a wide range of heat related problems, especially where direct access may not be practical or safe.

This is particularly true in high temperature areas where equipment such as boilers, turbines and furnaces are operational.

Thermographic surveys play an important part in the Shell on-going condition monitoring programme at their Stanlow Refinery, and have proved to be an invaluable tool.

AVT have a long standing working partnership with Shell and in one example they conducted an in-depth survey on three furnaces at the refinery to confirm the condition of ductings, expansion joints and inspection doors, ahead of a planned two week shut-down.

Results from the analysis highlighted a number of areas where remedial action was needed, with failure prediction varying from highly imminent to 12 months.

Of particular concern was the heat pattern around a temporary box section welded on the side of one of the furnaces.

The preliminary recommendation was to replace the box section, but further inspection clearly showed that the refractory linings on the furnace wall were unstable.

This vital information allowed Shell to revise their safe system of work for entry and specify appropriate controls, including access through the roof of the furnace to reduce the risk associated with the unstable linings.

Once the remedial work had been carried out, further thermography images were taken and compared with the earlier results to ensure the maintenance work had been effective.

The results showed how the peak temperature of the box section had dropped from over 500C to below 170C.

REMOTE VISUAL INSPECTION.

Remote visual inspection is a highly versatile CM technique using remotely controlled video cameras, video probes or fibre-optic probes to obtain detailed visual information from inaccessible locations.

Recent developments in camera technology are allowing the technique to be used in a growing number of areas and in many cases, the inspection forms part of a wider investigative program which may involve debris retrieval, corrosion assessment or routine repairs.

Where applicable, video images can be used to support data from other techniques such as crack detection.

The multi-function capabilities of video inspection are highly effective is in the oil and gas industries.

Flare stacks are a common sight at oil refineries and perform a vital part of the overall process.

Carrying out effective inspections of the stacks and their support structure in order to assess life expectancy presents a number of challenging problems.

The flare support structure at the Shell Stanlow refinery supports four stacks, each 93 metres high.

On-site health and safety restrictions prohibit human access above a height of 27 metres and therefore the only practical way to carry out a thorough assessment of the condition of the structure is to use remote cameras.

To solve the problem, Shell called on the services of AVT who devised an ingenious method of fitting a camera to one of the flare stacks itself.

The stacks are made up of three sections and can be dismantled section by section and lowered to the ground via guides on the support structure.

AVT designed and built a special assembly so that a camera could be fitted to the top section of the flare stack.

The camera, which is fitted to a remotely operated boom assembly, is then lifted up with the top section.

With this section bolted in position, the boom can be moved so that the camera can be positioned inside the support structure.

The camera has extensive remote-operated pan and tilt, zoom and focus facilities, allowing the operator to focus in on key parts of the structure.

Remote inspection is also very effective for checking the condition of the flare heads and burners.

The use of a camera with a high quality zoom lens, mounted on a crane at a safe distance from the flame, allows a detailed inspection of the individual components to be carried out.

In addition, the high quality colour images make it possible for engineers to view the tip whilst changing flare operation settings to obtain detailed information on flare control.

DEBRIS RETRIEVAL.

An important associated part of remote inspection is debris retrieval.

'Debris' may include material left after construction or repair work or structural parts that have broken off.

It is not uncommon to find objects such as steel beams, tools, nuts and bolts.

In one refinery application, engineers knew that material had been left behind on the support structure but had no clear idea of what was there and its precise location.

Remote visual inspections identified where the debris was located, allowing an effective method of retrieval to be devised using electromagnets in conjunction with a camera suspended from a mobile crane.

COMPLEX CONDITION MONITORING.

Reciprocating compressors are some of the most critical and expensive pieces of machinery in refineries, petrochemical plants and gas transmission pipelines.

By definition they are subjected to extremely harsh high temperature working environments.

There are many factors that affect the lifecycle costs and they present a complex condition monitoring scenario in order to optimise maintenance.

Improved machine protection can be accomplished by monitoring critical temperatures, pressures, vibrations, and rod related parameters on a continuous basis.

Key parameters that give an overall health picture of the compressor are frame vibration, motor and crank-case vibration, rod-drop, load and run-out, crosshead vibration (knock), main bearing vibration and valve temperatures.

Monitoring trends in cylinder P-V traces provide additional important information.

According to industry studies, valve failures account for over 40% of the problems associated with reciprocating machinery.

In a Reciprocating Compressor, the valves are a pressure actuated "Poppet" variety and when a valve begins to fail, it usually begins to leak the process gas.

This causes the process gas to be re-compressed, further heating the gas.

This higher temperature process gas can be detected using a suitably mounted temperature transducer.

ACOUSTIC EMISSION.

This technology is well established in areas including crack detection in metals, blocked pipe and filter detection and high pressure leak detection.

More recently it is finding wider use in CM applications where, for instance, it is ideal for monitoring the behaviour of slow moving bearings and components such as valves in reciprocating machinery.

The technology can be used for both long term trend monitoring and short term evaluation.

The small AE sensors are effectively highly sensitive listening devices that detect high frequency stress waves propagated through a structure, providing a valuable noise 'signature' or pattern.

The sensors are typically either secured directly to the structure or mounted via external waveguides.

Monitoring and comparing trends in these noise patterns can give an invaluable insight to changes in wear behaviour, allowing appropriate remedial action to be taken.

Certified hazardous area AE systems further extend their capabilities.

LUBRICATION MANAGEMENT.

There is no doubt that a well implemented Lubrication Management Programme will improve equipment reliability in the same way that a healthy lifestyle keeps us fit.

Given the critical nature of lubricants and the frequency with which mechanical failure is related to the lubricating system, effective lubrication management is probably the most important single function within any CBM system, especially when combined with other effective maintenance practices such as alignment and filtration.

The correct choice of lubricant for a particular piece of equipment and application is the first and most fundamental part.

This has to take into account factors including materials and construction of moving parts, environmental operating conditions, including temperature, water resistance and cleaning routines, sources of contamination and legislative requirements.

Independent studies by numerous authorities over the years have concluded that contamination is the most prolific reason for failure of equipment.

The improvement in equipment reliability that will be obtained from a clean oil system is well documented, but unfortunately this key factor is often overlooked.

ROOT CAUSE ANALYSIS.

Understanding why components or equipment fail is vitally important and therefore root cause analysis is an important part of the condition monitoring process.

A prime example is highlighted by a recurring bearing failure in a large air compressor electric motor.

Preliminary investigations showed that the same bearing had been failing prematurely on a regular basis.

Rather than investigate the cause, maintenance engineers had routinely replaced the faulty bearings every two to three years.

Further root cause analysis investigation highlighted the fact that the failed bearing showed unexpected signs of overheating, consistent with the passage of electrical current.

It transpired that although the motor had originally been fitted with insulated bearings, at some stage a non-insulated one had been mistakenly substituted. Request a free brochure from AV Technology ...

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