1、Maintenance ofInstruments past chair of SP76, and technical advisor for IEC SC65D of the U.S. National Committee.Diana Churchill Bouchard, a Scientist at the Pulp and Paper Research Insti-tute of Canada who has lectured at McGill, Concordia University and Universit du Quebec Trois-Rivires, contribut
2、ed to Chapter 16, Software and Network Maintenance. Robert Hasselbaum, former Director, Services Marketing for In-vensys Process Systems, Foxboro, Mass., contributed to the same chapter.Mr. Goettsche has more than fifty years experience in instrumentation, mea-surement and automatic controls. Holder
3、 of a BS degree from New Hampshire College, he is owner of LG Process Automaton Services. During his career, he has been employed in the following capacities: Electrical and Control Designer for Kennecott Copper Environmental Engineering Projects Group, Electrical it also covers some of the fundamen
4、tal principles, vocabu-lary, symbolism, standards, and safety. It suggests the necessary basic knowledge required of I some formed an Instrument and Control (I the more complex instrumentation Figure 1-3. Typical Gas Fired Electrical Generating Plant Organization Chart.PskdjkjdidP MAINTENANCE MANAGE
5、R PLANT ENGINEER PRODUCTION MANAGER ENVIRONMENTALAND HEALTH (CHEMIST) WELDER ELECTRICIAN I therefore, all engineering disciplines get involved with the instrumentation and control system. Those who were fortunate to get involved in early instrumentation and control systems have become the I they are
6、 the “bosses.” If one doesnt please these bosses, one may not have a job for long. Treat all “customers” with the respect and courtesy they deserve.The function of maintenance is to keep equipment in a safe operating condition. Italso must ensure that all instrumentation works when it is required to
7、 work. Emer-gency shutdown systems should be checked regularly as they must work every time. Improper maintenance could cause harm to personnel and the environment.Safety standards seek to minimize the frequency and consequences of acci-dents related to toxic, flammable, or explosive chemicals in th
8、e U.S. processing industries. Procedures established for safety management are designed to protect Why Maintain?11employees of process plants by preventing or minimizing the consequences of chemical accidents that involve highly hazardous materials. Detailed written safety information must be compil
9、ed so that the employer and employees who op-erate the process can identify and understand the hazards associated with the pro-cess. This information must be communicated to all employees involved with the process. Safety information must include information on the hazards of the partic-ular materia
10、ls used in the process and detailed information on the process technol-ogies and equipment used in the process.Emergency shutdown systems must be understood, checked regularly, and work when required. The documentation must accurately represent the actual in-stallation. Maintenance personnel must un
11、derstand the control philosophy and must follow the specific maintenance procedures that are necessary to maintain equipment and system integrity.An investigation of the various types of maintenance will emphasize its importance.Corrective MaintenanceWhen a device, equipment, or a system fails to co
12、nform to the manufacturers specification and the operation fails to meet its intended function, maintenance is required. All items must be repaired or restored to a serviceable condition follow-ing a failure or malfunction.Corrective maintenance is an activity that is not normal in the operation of
13、equipment and is expected to be performed by qualified personnel who are aware of the hazards involved. Such activities typically include locating causes of faulty performance, replacement of defective components, adjustment of service con-trols, or the like.Maintenance personnel should have suffici
14、ent documentation to understand and repair the problem. After logical troubleshooting procedures have determined the faulty component, repair can be completed. Remember to check the operation and calibration after repair and document the repair activities and parts used.Another name for corrective m
15、aintenance is remedial maintenance, which is defined as the maintenance performed following equipment failure, as required, on an unscheduled basis. Nonscheduled, unscheduled, or emergency maintenance is an urgent need for repair or upkeep that was unpredicted or not previously planned and must be a
16、dded to or substituted for previously planned work main-tenance specifically intended to eliminate an existing fault.Corrective maintenance is generally considered nonscheduled, unscheduled, or emergency maintenance if it affects the productivity of the plant. Normally, an emergency maintenance work
17、 order (see Figure 2-1) is written to document the parts and labor hours used and to be added to the history file. Corrective mainte-nance can be scheduled maintenance when the device or system does not affect the productivity of the plant.Work-around should not be considered corrective maintenance,
18、 but it can eliminate emergency maintenance requirements. A work-around is action required to complete the process run, even though all equipment is not working satisfacto-rily. Part of the process may have to be run on manual.A flow switch (a permissive) fails during a process run. Stopping the pro
19、cess when the failure occurs would cause product loss. In order to continue the run, the permissive input could be forced or the flow switch could be jumpered. When the run is complete, the problem must be corrected before another run is made.Fundamental Principles12Figure 2-1. Typical Maintenance W
20、ork Order.Why Maintain?13Establishing a comprehensive maintenance plan is a way of formalizing and codifying the accumulation of all of the procedures and methods that are critical to maintaining the production systems. It represents a way of assessing the accumu-lated maintenance activities and cap
21、turing the best solutions. Every maintenance organization has some procedures and methods that qualify as a best practice. A maintenance plan that encompasses all production equipment and systems across all company plants and factories not only addresses the preservation of current as-sets, but prov
22、ides a framework to organize best practice solutions and add others as they are recognized and validated.Lowering the cost of business is a primary focus in manufacturing. Optimal production processes, lean factory operations, curtailed capital expenditures, and preservation of assets are manifestat
23、ions of austere cost measures. Cost reductions have affected maintenance organizations with headcount reductions and scaled back training budgets. At the same time, maintenance staffs are under increasing pressure to keep equipment and automation systems up and running efficiently with a minimum amo
24、unt of unscheduled downtime. Maintenance resources in-clude people as well as specialized diagnostic equipment, software applications, communications, transportation, manuals, documentation, and access to supplier supported equipment.Corrective emergency maintenance could generate a scheduled mainte
25、nance activity for example, replacement of a whole unit or device to get the line or system working, followed by repair of the failed unit or device at a scheduled time. Care must be taken to ensure that the failed unit or device is either replaced or repaired for use at a later time. Some maintenan
26、ce shops do not have a good program to ensure that boards and equipment taken out of service are either re-paired and checked out immediately or discarded and replaced.Corrective maintenance includes logical troubleshooting, which is a search for the cause of a malfunction or erroneous behavior, in
27、order to remove the mal-function. Knowledge and experience are necessary to find the root problem. Qual-ified instrument technicians and instrument mechanics use logical troubleshooting concepts to ensure that the unit, device, or subassembly needs to be replaced and that the replacement corrects th
28、e problem.Board or equipment swapping until the unit responds correctly is not considered acceptable troubleshooting practice.Maintenance is not complete until the system has been repaired, reworked, or replaced and calibrated, tested, and, of course, logged into the equipment history files, as expl
29、ained by the following paragraphs.REPAIRThe word “repair” in a maintenance sense means to restore an item to service-able condition following a failure or malfunction.REWORKRestoring an item to a condition that exactly conforms to original design spec-ifications is called rework. This word is usuall
30、y applied to corrective action taken when an item has failed an inspection but requires a relatively simple operation, such as replacing a part or component, to enable the item to pass an identical inspection.REPLACEMENTWhen an item fails or malfunctions, a decision must be made to repair or to repl
31、ace it. If it is to be repaired, the proper maintenance work order and schedule must be submitted. If it is to be replaced, the proper material requisition must be Fundamental Principles14submitted, and the malfunctioning item must be discarded if it cannot be repaired. The component or spare part m
32、ay be replaced by a design equivalent replacement that is at least equivalent in performance to the item being replaced. It must also meet the requirements of form, function, and fit.CALIBRATIONTo ascertain outputs of a device that correspond to a series of values of the quantity that the device is
33、to measure, receive, or transmit is called calibration. The output must be adjusted to bring it to the desired value within a specified tol-erance, and the error must be ascertained by comparing the device output reading to a standard.TESTINGTesting consists of checking out a system to determine the
34、 attributes or per-formance characteristics. The operators should verify that the malfunction has been removed and that no additional malfunctions exist.RECORDINGThe corrective action, parts used, and hours spent in repair must be recorded on the maintenance work order (see Figure 2-1).INPUTThis inf
35、ormation must be input into the maintenance management system (MMS) files. The service life of the equipment can be determined by the history files of the equipment in the MMS.Service life is the length of time a mechanism or piece of equipment can be used before it becomes either unreliable or econ
36、omically impractical to maintain in good working order.Preventive MaintenancePreventive maintenance (PM) consists of scheduled inspection and upkeep that is specifically intended to prevent faults from occurring during subsequent operation. Inept PM, however, can also cause problems.Whenever any equ
37、ipment is touched, it is exposed to potential damage. It iseconomically unwise to replace components prematurely. A good preventive main-tenance program is the heart of effective maintenance, and inspection is the key to detecting the need for it.A PM program requires an initial investment of time,
38、parts, people, and money. Payoff comes months later. While there is little question that a good PM program will have a high return on investment, many people are reluctant to pay now if the return is not immediate, and some customers may perceive the PM ac-tivity only as, “That machine is broken aga
39、in.” PM supports a commitment of long-term life-cycle cost/total cost of ownership.Most preventive maintenance is scheduled; that is, maintenance is carried out in accordance with an established plan. Scheduled maintenance or preventive maintenance may be scheduled by hours, uses, sequences, or the
40、calendar. The use of performance intervals is itself a step toward basing PM on actual need instead of on a generality.Inspection is a key to detecting the need for PM. It should be nondestructive so that it will not harm the equipment. The two main elements of fixed-interval PM are procedure and di
41、scipline. Procedure means that the correct tasks are done, the right lubricants are applied, and consumables are replaced at the best interval. Discipline requires that all the tasks be planned and controlled so that everything is done when it should be done.Why Maintain?15Most organizations can est
42、ablish a PM program based on the fact that certain equipment or systems can be taken out of operation for the time required to give them a good checkup. Sometimes this is not practical. Seasonal equipment, such as air conditioners and heaters, requires special maintenance care at the end of each sea
43、son in order to clean and refurbish them when they are not being used so they will be ready for the next season. Failure patterns that have been developed show that equipment may fail within the first few weeks (infant mortality) and then will probably operate for quite some time before wearing out.
44、 Electronic components (once burned in) will last indefinitely until some outside force causes failure. A major cause of failures lies in the education and human reliability prob-lems of the operators and maintenance personnel. Failures are frequently caused by persons who push buttons and turn on e
45、quipment before they read instructions.PM records help determine the service life of equipment. Condition monitor-ing and prediction provide information on when problems are building to the fail-ure point. If the scheduled interval indicates that the interval should be decreased, or if a failure occ
46、urs before the scheduled maintenance or preventive maintenance is performed, the maintenance supervisors and maintenance engineers should in-vestigate. The investigation should determine a plan to ensure that the equipment will continue to operate until the equipment is replaced or retrofitted. Lead
47、 times of up to two years are frequently required to upgrade or retrofit a system. This in-formation should be passed on to management personnel. If a malfunction is found during PM, plant policy will determine if corrective action should be ac-complished as corrective maintenance or as a repair uti
48、lizing the Preventive Main-tenance Work Order (PMWO).Predictive MaintenancePredictive maintenance is a preventive type of maintenance program that an-ticipates failures that can be corrected before total failure. Predictive maintenance can also determine that a failure is about to occur.For example,
49、 vibration analysis determines the normal vibration that is ac-ceptable; when the vibration starts to exceed this point, maintenance can be sched-uled when the equipment is not being used in order to prevent a complete failure during operation. Noise, oil samples, additional heat, intermittent diagnostic er-rors, data highway retries, and unexplained glitches are other examples of warning signs for predictive maintenance. Most of the above can be measured, recorded, and annunciated.Unscheduled MaintenanceUnscheduled or nonscheduled maintenance must be done immediately when an emergen
