Analytical instrument maintenance and its implications

Analytica Chimica Acta, 238 (1990) 45-53

Elsevier Science Publishers B.V., Amsterdam

45

Analytical instrument maintenance and its implications

A.A. Davis

ICI Chemicals & Polymers Ltd., Merseyside Operations, Castner Kellner Sire, P. 0. Box 9, Runcorn, Cheshire WA7 4JE (Great Britain)

(Received 3rd April 1990)

Abstract

The various aspects of importance for the maintenance of analytical instruments for process control are discussed.

This discussion is based on practical experience in a large industrial chemical company. Emphasis is placed on the

organizational aspects and the continuous attention that is needed. Also, the significant role of audits is stressed.

Keyword: Process analysis; Instrument maintenance

The aim of this paper is to introduce the sub- ject of analytical instrument maintenance and reveal some of its implications. Much of the author’s experience has been gained in Castner Kellner Works, which is the largest within the Merseyside Operations Group of ICI Chemicals & Polymers.

The principle products are chlorine and its associated by-products, i.e., “caustic” (sodium hy- droxide) and hydrogen. Considerable care has to be taken with the production of such chemicals, hence the factory is extensively automated from an analytical instrumentation standpoint.

During the past 10 years, analytical instrumentation has grown in stature and is now recognized and accepted as a most important branch of industrial instrumentation. Progress in technology has brought about significant improvements in design, materials of construction and in the

hardware associated with sample systems. These areas of improvement, when combined, have con- tributed to greater reliability and integrity of analytical systems and as a consequence the role now played by the analyser is of greater significance and importance in industrial processes.

As the reliability has improved and confidence has been gained, more analysers have been intro- duced on-line to control difficult plant parame-

ters, thus contributing to improved standards of quality control, safety and more efficient use of resources. This is a significant change in emphasis as opposed to the past, when equipment of this nature was used merely to guide plant manage-

ment with product quality control.

FACTORY SCENE

Today’s labour costs are high and often resources are limited in both quantity and quality. To remain competitive, is it necessary to function with a lean yet efficient and well trained work-

force. Ways of overcoming skill shortages must be sought in order to alleviate the problems created

as technology advances and equipment becomes more complex and difficult to maintain.

It is common knowledge that many factories

have a number of analytical systems that have fallen into disrepute, much to the annoyance of process management. Reasons for failure can be usually classified into two groups: problems associated with hardware, such as poor analyser design, overcomplex systems, poor sampling tech- niques and incorrect or unsuitable materials of construction, and problems associated with peo-

46 A.A. DAVIS

ple, such as lack of training, lack of commitment and a shortage of qualified artificers and technicians.

In addition, when substantial capital invest-

ment programmes are taking place, priorities are generally channelled towards completion of the general instrumentation to the exclusion of the analysis systems. The author believes that both aspects of the work should proceed along parallel paths so that both the general and analytical systems are completed to meet plant requirements. In some instances analytical systems may form part of a control or shutdown system, in which case the complete loop should be commissioned at the same time.

Usually when commissioning a chemical plant, it is broken down into “key” sections, e.g., com- pression, liquification and drying. Each section will have its own instrumentation, both general

and analytical, which should be grouped together to form “systems’, each system being commissioned in accordance with an established plan.

It is well known that analytical instruments are difficult to maintain; a solution which went some way towards solving the maintenance problems associated with this particular type of technology was to create a “specialist group”, the remit of the group being to look after all aspects of maintenance and development work throughout the factory.

A section was established complete with En-

gineer and Foreman to carry out the day-to-day and routine maintenance work. It was considered necessary to choose a Foreman for his leadership and man-management skills rather than for his technical knowledge, and the Engineer leading the group would supply technical “back-up”.

With this type of work there are many routine operations which have to be carried out frequently (e.g., adjusting sample flow-rates, removing blockages). These essential but mundane tasks tend to be neglected if personnel remain unsupervised (people tend to do what they want to do in prefer- ence to doing what they are told). Experience has shown that it is of paramount importance that these simple tasks are dealt with and problems are rectified as soon as possible. If analytical systems

are left unattended for longer than 2-3 days,

considerable effort is required to bring the equipment back to a healthy state (this can often necessitate several weeks’ work). It is therefore

easily demonstrated that attention to detail is vital, especially when analysers are carrying out major control functions on large capital plants, the im- plication being that long shutdown periods/delays are unacceptable.

It is this attention to detail which divides the

thin line between success and failure, and having a Foreman following the correct guidelines is there-

fore a vital part of the organization.

STAFFING REQUIREMENTS

Before creating a maintenance organization it is essential to have a purpose and a set of principles against which to work. For example, improve the quality of analyser systems; assess the present needs and future requirements; determine standards which are consistent across the site; estab- lish the correct maintenance level; set up record- ing/back-up systems; and raise the capability of personnel through experience and training.

In order to carry out the work, an organization was devised to meet the requirements of the above

principles (see figs. 1 and 2). A small number of technicians and artificers

were recruited from within the Instrument/

Electrical Department to form the nucleus of the specialist group, the group being supplemented by contractors employed from an outside company. Contractors are mainly employed to carry out construction work and are recruited in accordance with the needs of the business (numbers vary in accordance with work loading). The technicians and artificers concerned have received training to

ENGINEER

I..; Fig. 1. Structure manning requirements.

ANALYTICAL INSTRUMENT MAINTENANCE AND ITS IMPLICATIONS 47

Fig. 2. Organizational requirements.

“practitioner level” (average level of competence) to deal with the many diverse and difficult situa- tions which arise. “In-house” on-the-job training is carried out, supplemented by courses run by the company and instrument manufacturers. Within the group both the technicians and artificers are expected to carry out maintenance and development/commissioning work on a rota basis.

All analysis systems are maintained and performance is monitored for a period of 6 weeks prior to final acceptance and handover. If any faults due to either faulty design or manufacture appear during this period, then these problems are re- ferred back either to the design group responsible or to the manufacturer. A 6-week “running in” period is chosen because it is considered long enough to reveal component failures and to show any design weakness. In addition, the process personnel operating the plant will be able to assess the credibility of the instrument and whether the equipment is performing to the design specifica- tions.

A “bath-tub” curve can be used to display the mortality rate, life expectancy and wear-out phase of an analyser (Fig. 3). How far the “bath-tub” characteristic applies to process analysers may be a matter of debate and no doubt many maintenance engineers can readily quote instances where, for example, serious process malfunction or maloperation has caused a large increase in failure rate of some of the analytical systems, but the author believes that on a well run plant which has itself passed into the useful life period, the instrumentation will have a “useful life” phase.

WORK

Maintenance requirements should be based on an understanding of how critical the equipment is to the plant concerned. The systematic application of maintenance procedures is aimed at providing the correct balance between cost-effective reliability of equipment and maintaining confidence and credibility in the measuring/ alarm systems, particularly for those systems which require attention to ensure correct functioning, for those system which, if allowed to fall into disrepute, lose credibility, for those system where “fail to danger” (i.e., unrevealed) faults could prevent the trip/alarm systems functioning on demand, and to provide fault records which improve performance and hence enable more informed judge- ments to be made on frequency and scope of checks.

PROTECTIVE SYSTEM

All protective systems, as the term implies, are installed to perform a specific task. As a result there are some key factors which need to be ad- dressed: first, capability (to do the job): range; response time; accuracy of measurement; and set point selection; and second, reliability: confidence in measured value; quality of display, e.g., graphics, indicator/ recorder; fractional dead time; and demand rate on alarm system.

The justification for each installation/loop usually emanates from a hazard and operability study with each analytical system being designated either

"Year-out" phase

"Useful life" random failures .

I I ,

I

I I )

Time (years)

Fig. 3. “Bath-tub” characteristics.

48

hazard justified (minimize likelihood and consequences of hazard) or operability justified (improve plant performance).

Hazard-justified systems Such systems will be installed on plants where

the potential for serious danger exists, e.g., toxic gas leak or pollution of the environment.

Operability justified systems These are systems installed to assist process

operators in the operation of the plant. The major- ity of systems will fall into this category.

Special maintenance These systems depend on local management

requirements, e.g., mass balance and accounting.

Hazard summary Control measures and a hazard summary are

given in Tables 1 and 2.

MAINTENANCE

With process analytical systems breakdowns are inevitable; 85% of faults can be related to sample systems and associated hardware and the remain- ing 15% to the analyser itself.

It is essential that the sample systems and installations follow a check list (see Table 3). The

introduction of such a list is one way of drawing the designers’ attention to detail which should be

TABLE 1

Control measures

Preventive

Accurate control

Density meters

Katharometers

Accuracy/reliability

Confidence

Legibility of information

Interpretation of

readings

Corrective Emergency

Intervention Safety

Radioactive Gas alarms

alarms

pH alarms

Infrared

alarms

Reliability FDTs ’

Speed of detection

Speed of intervention

a FDT = Fractional dead times.

A.A. DAVIS

TABLE 2

Hazards summary

Preventive

measures

Corrective

measures

Emergency

measures

Means for

ensuring safe

operation under

normal operating

conditions,

i.e., prevents the

initiation of a

sequence of events

that could lead

to a hazardous

event

Means to avoid

an unprevented

disturbance

leading to loss

of containment

Examples: Examples:

Control-loop Alarm/operator

procedure action

Design detail Trip system

inspection Dump tank

analysis Relief system

Means adopted

for reducing

the scale of con-

sequences, i.e.,

reduce amount

discharged or

effects following

discharge

Examples:

Bunding (area

of containment)

Water curtains

Remote shutdown

valves

Restrictor plates

Gas alarms

Emergency

procedures

obvious but is often overlooked. Therefore, at the design stage of a critical loop, consideration should be given to duplicating and making independent

the sample point and sampling systems including the primary measuring devices provided that fi- nance permits. This is particularly important when installations are required to meet pollution and environmental legislation, in which event the systems must be of the highest integrity and reliabil-

ity. If duplicate systems are chosen, they must be

designed in such a way that the stand-by system can be switched into service without delay and the faulty equipment can be readily and easily re- moved for repair. When servicing duplicate sys-

tems, it has been found advantageous to carry a “serviced spare” which can be fitted into the loop at very short notice.

There are several important advantages to be gained from adopting the serviced spare philoso- phy. Better quality repairs can be effected in a workshop than in the field. This is especially so

with pH measurements. These systems are of high impedance and servicing equipment of this nature


TABLE 3

Sampling systems and hardware check list

Sample point

Wherever possible should be accessible.

Most suitable location on vessel or pipe should be chosen to

avoid fouling.

To be large enough bore to prevent blockages occurring, pre-

ferably not less than 1 in NB (nominal bore: internal pipe

diameter).

Access platforms should be provided where necessary.

Sample loop Most suitable material of construction for pipework valves and

fittings to suit particular application.

Loop should be kept as small as possible to avoid

distance/velocity lags.

Bypass “fast flow” sample loops should be considered.

Filtering should be considered if applicable.

Exit sample lines should be piped back into plant stream or

alternatively the effluent should be scrubbed before disposal

down any drains.

Where a nitrogen or air purge is used to flush sample lines,

permanent connections must never be made. It is vital that all

purge lines so used are disconnected after use.

Cabinets

Fire-retardant fibre-glass boxes with polypropylene hinges and

stainless-steel catches are to be recommended; alternative

hinges and catches should be chosen to suit the particular

environment for which they are intended. A stay or chain

should be fitted to any hinged doors to prevent them becoming

detached.

Care should be taken to select a box of adequate strength and

dimensions. It is preferably to use boxes which have rein-

forcement built into the fibre-glass (e.g., l&in thick plate

built into base of box for attachment and support).

Cabinets should. be mounted preferably on their own rigid

pedestal at a realistic height and in a location free from

vibration. Access should be left to rear of cabinet.

Electric or steam heating should be used if applicable to

prevent freezing, etc.

Purging cabinets with works air or nitrogen if applicable.

Layout of equipment inside cabinet

Equipment mounting boards should be easily removable and

accessible.

Material or mounting board should suit the particular environ-

ment.

All bolts and screws securing hardware to board should remain

captive.

All incoming and outgoing pipework should be fitted via

detachable bulk-head fittings through a manifold block bolted

to the side of the cabinet.

Correct materials or construction for all hardware, i.e., valves,

rotameters, pipes.

Drip trays and drains should be fitted to all wet analysers.

TABLE 3 (continued)

Services

Electrical:

All electrical equipment should be divorced from sample

equipment where possible.

All electrical connections should be glanded into cabinet

via sealed junction boxes to prevent ingress of weather.

All wiring must conform to particular factory safety re-

quirements; details of specification are available from the

Factory Electrical Engineer.

Plumbing:

All pipework and ancillary equipment supplies, etc., i.e.,

instruments’ air, water, nitrogen and plant samples must be

rigidly piped using the correct materials of construction

and all being properly clipped and bracketed to suit the

particular application. If in any doubt the particular plant

process manager should be consulted.

Safety:

It is most important that safety is always borne in mind.

The Factory Safety Officers should always be contacted if

ever in doubt.

in the field can only lead to disaster. The serviced spare can be installed without delay. The broken equipment can be repaired at leisure.

Planned maintenance is advocated as the best form of preventive maintenance. This maintenance is carried out on a routine basis from a series of schedules designed to meet the needs of the installation, the maintenance frequency being

decided by experience and recommendations made by hazard studies, etc. The maintenance takes the

form of weekly, monthly, 3-monthly and yearly checks. This work is preplanned by an instrument planner in conjunction with the Instrument Fore- man, the schedules and testing frequency being

decided by the Engineer in conjunction with the Plant Manager.

To enable a schedule and testing system to

work effectively, it is of paramount importance that feedback is received from the artificers and

technicians carrying out the work and records are kept, drawing and maintenance instructions are kept up to date and paperwork is kept to a mini- mum.

1. DOES THE SCHEDULE WCUPIENTATION REQUIRE “OoIPIC*TION

(Y/N) IF YES, ACTIONS TAKEN OR CONT*CTS N*IIE AND EXTENSION:

3. FAULT HISTORY ENTERED INTO IRIS:

BY (SIGNATURE) DATE

Fig. 4. Computer list of job methods and schedule.


ALLOCATION OF WORK SCHEDULES

A computer-based maintenance information package developed within the C&P Division known as a Maintenance Management System is used to initiate the maintenance work. The system consists of the following: full plant inventory; call-up system for all routine work; task specification data library; daily and long-term planning facility; and on-line history.

The system makes extensive use of on-line transactions for all the above functions supported by batch programs to produce printed reports or carry out tasks that do not need an immediate response. All the Maintenance Management Sys- tem transactions are linked together to allow the user to move continuously through the string of transactions.

Having decided the maintenance schedule and frequency of checking for all the analysers on the plant, job methods are written, detailing the steps to execute the work. The above information is programmed into the computer, which then auto- matically prints out the job methods and schedule listing the analysers to be checked at a particular time (see Fig. 4). The foreman receives the information and hands it to the control technician or artificer who has been designated to carry out the work.

As the schedule will contain some critical loops and is most likely to contain an alarm/trip, authorization and approval from process must be sought to ensure plant is in safe condition before commencing work.

Should a major defect be apparent during the course of the work, this is attended to im- mediately. Faults of a minor nature are noted on a report sheet which is returned to the Foreman/ Planner when the schedule is completed. It is the Foreman’s responsibility to review outstanding faults and plan for completion as and when labour and materials are available in line with the plant’s requirements. Information is selected from every work order and passed to an on-line history record system which provides useful data for future analysis.

AUDITS/MONITORING OF STANDARDS

To apply frequent routine proof testing is expensive, which is often overlooked. Once a maintenance organization has been established, there should be rigorous monitoring of procedures to ensure that what has been intended has actually been carried out.

Audits can play a significant and prominent part in establishing whether procedures are in place and also highlight defects in equipment as site visits are made. The author considers that auditing should be carried out by a “team” and attaches considerable importance to the “team” make-up and the personnel involved. The team should consist of engineers and Foreman of mixed disciplines possessing the necessary skills and the experience to expose and probe in depth, thus revealing problems which may not be apparent to the owner of the section/plant being audited. It is to be recommended that one of the team members is a “visiting” engineer who is not normally associated or involved with the day-to-day running of the section or plant being audited.

Analytical system audits should be carried out annually to derive maximum benefit. If a greater time period is allowed to elapse then equipment could be falling into disrepute because the equipment is no longer useful, corrosion and the harsh environment have taken their toll and checks are too infrequent and or of an inadequate standard.

The benefits of an audit are twofold: it gives the opportunity to vet both the equipment and the paperwork systems to make certain they are up to date and adequate, and it provides the opportunity to review the usefulness and value of the equipment and to review the availability/ suitability of maintenance resources, both human and financial. Auditing shows up weakness and inefficiency, hence recommendations which arise as a result should be dealt with promptly.

If equipment is never tested, one day it will fail, possibly undetected, and the consequences of such malfunction could ultimately lead to a serious incident. Hence auditing should question spurious

52 A.A. DAVIS

alarms and trips in order to minimize the risk of neglecting genuine demands on the system.

DEVELOPMENT WORK

Development work is generated in two ways: it is either initiated within the works or takes the form of new capital projects. When the work is initiated within the works it is usually comes as a direct request from plant management or the result of a hazard study. Large captial projects are designed and handled by the Engineering Depart- ment.

In order to minimize design faults or to make changes to designs which experience proves necessary, close collaboration between site and the En- gineering Department is of prime importance. If possible, the analysis units should be assembled on the site for which they are intended. The benefits of this are twofold: personnel can follow the project through from the construction phase to commissioning and on through maintenance; and this gives the personnel a greater sense of achieve- ment as they will be maintaining equipment that they themselves have fabricated. Equipment built in this way is very much easier to commission and has fewer faults.

It is appreciated that owing to a shortage of manpower and skills it is not always possible to use the above strategy, in which case the sooner the site personnel are allowed to work alongside the contractor or site commissioning team, the more successful the work will be.

RUNNING COSTS OF ON-LINE ANALYSERS

By keeping a record of historical data associated with various types of analysers and systems, it has been possible to relate manning levels to the installed capital value of equipment. Using this technique, it is possible to estimate manning levels for carrying out both the maintenance and commissioning functions.

For example, if the total value of all work is $Z900000 and the estimated time to complete the programme is 2 years, given that the costs of maintenance/ installation/ commissioning rate/ tradesmen per year are S300000, then

900 000 manning requirement = 3ooOOo x 2

= 1.5 persons.

From past data and experience, maintenance costs can be obtained by taking 20% of the installed capital value of the equipment. Assuming a

TABLE 4

Schedule maintenance and calibration costs a

Item No. of Method of analysis Total Maintenance cost

installations hours per based on labour/ year materials at El8 h-’ (f)

1 Three dual systems PH 468 8424

2 One duplicate system Redox potential loo 1800 3 Three Radioactive density 18 324 4 One Thermal conductivity, katharometer 36 648 5 One Spectrophotometric 150 2700 6 One Turbidity 42 756 7 8 9

One multi-point Biamperometric chloralarm sensor 66 1188 One Conductivity (in-line system) 10 180 One pH, manual sampling 12 216

Maintenance costs: 16 236 Miscellaneous reagent costs: 2000

Total costs: 18 236

a The installed capital cost for the equipment indicated is ca. f100000.


f500000 total value over a 2-year period, the maintenance costs per year would be

&500000x20x1 100x2

= g50 000

An approximation of spares costs can be obtained by taking 20% of the installed capital value.

It must be appreciated that the cost of analyti-

cal equipment varies widely, usually in accordance with its complexity. The figures quoted above are by no means precise; they are intended only as a guide and a base upon which a maintenance organization can be founded.

An attempt has been made to list the installa-

tion and maintenance costs for a variety of on-line process analytical equipment which one could ex- pect to find at the sites of some of the larger chemical companies (Table 4). The costs are based on data acquired over a number of years and updated in the light of experience gained. The time spent on each of the systems is for an average level of maintenance, the standard such that it satisfies the criteria laid down for the hazard-

justified or operability-justified systems. Failure to maintain to the desired standard would be a seri-

ous threat to production or could result in a serious hazard or gas escape.

It is often said that the success of a system can

be judged by the number of complaints received. However, it is preferable to apply preventive maintenance rather than adopting a “leave it until it fails” attitude.

It must be appreciated that the cost of analyti-

cal equipment varies widely and that very few systems will be built using the same manufacturer’s equipment and components (most systems are tailor-made to suit individual conditions). Differ- ent site conditions could distort labour costs.

All the evidence to data points to the fact that

in most instances “on-line” wet chemical analysers

(those employing reagents to cause reactions, col- our changes, temperature changes, etc.) are in general (there are exceptions) unreliable, difficult and in some instances hazardous to maintain. In addition, other facilities such as the provision of a regular and reliable supply of reagents and calibration sample solutions are necessary; such a service in itself is often expensive and difficult to

provide. The author’s own experience is that if standard reagents can be purchased either from the manufacturer or a reliable alternative chemical

reagent supplier, this can be of considerable benefit. Using a works laboratory to provide a service often has its problems unless adequate warning can be given as to when the reagents are required.

Some wet chemical analysers require weekly replacement of reagents, others monthly, and this should be catered for when the preventive maintenance schedule for the analyser is first established.

It must be remembered reagents have a “shelf- life”, in which case if there is any doubt a reagent should be discarded and a freshly prepared solution obtained.

Wet chemical analysis equipment is usually complex and time consuming to maintain. The technicians who service this type of equipment should possess a high degree of skill and experi-

ence in addition to being dedicated to analytical work. Owing to skills shortage, in the future alternative ways of providing this more specialist type of maintenance will have to be sought. Instrument

manufacturers and specialist contracting organiza- tions may be called upon to supplement existing

work forces in order to provide cover on a 24-h basis.

Care should be taken in accurately specifying the scope of the work in order to prevent any misunderstanding occurring. Also, tight supervi- sion will be necessary to ensure that standards are of the highest order. In order to achieve this objective, “fixed-price” maintenance contracts will have to be sought and negotiated. Suitable contractors should be invited to tender in order that

contracts can be scrutinized to ensure technical

competence and scale of economy. In conclusion, experience has shown that the

systematic application of procedures by a special-

ist team is the best way of providing a reliable and effective maintenance service for analytical instruments.

The author thanks members of the Electrical/ Instrument Section and other members of the Castner Kellner Works staff for their kind assis- tance and support throughout this work.

Documents

Analytical instrument maintenance and its implications