10-7.1 Reliability and Maintainability

The value of any instrumentation system is largely determined by its use factor, i. e., the amount of operating time compared to the amount of downtime attributable to
instrument or component failure during normal operation of the system. The use factor is a function of both the reliability and maintainability of the instrumentation sys­tem. Reliability is concerned with the frequency of failure, whereas maintainability is concerned with the duration of failure (see Chap. 11).

Although in recent years gains have been made in component reliability, they have almost always been exceeded by increases in system complexity. With the certain evolution of MSI and LSI (medium-scale and large-scale integrated circuits), which • will increase com­plexity on a component level without reducing reliability, systems reliability in the areas of total plant control and monitoring will begin to improve. Presently, the day of totally reliable, maintenance-free systems appears to be still
in the future This condition has forced the electronics industry to develop the technology of instrument and system maintainability. Although instrument manufacturers have always considered easy maintenance as a desirable feature, the complexities of today’s systems have dictated renewed interest and analysis. Achieving the shortest possible problem analysis and repair time (maintainability) should be considered one of the most important goals in equipment design and installation.

10-7.2 Reliability

Reliability is primarily a function of instrument and system design However, two aspects of installation can affect reliability the quality of materials and of workman­ship (particularly in system and component interfacing and interconnection) and the operating environment

Problems in the quality of installation can be minimized by the employment of highly qualified, competent labor and the use of highly reliable installation hardware Using crimp-type terminals and connectors with the proper tooling greatly reduces termination problems Installation plans with step checkoff sheets or diagrams can improve installation efficiency and accuracy

Many reliability problems associated with installation are created by unsuitable environmental conditions. In determining how instruments, components, and systems are to be applied, the environmental parameters must be carefully considered during installation. The manufacturer’s recommendations concerning installation and operating conditions, such as temperature, humidity, cleanliness, electrostatic and magnetic fields, vibration, and nuclear radiation fields, must be given close attention. Where conditions exist that are contrary to those recommended, special packaging, cooling, etc, should be provided as required.

10-7.3 Installation for Maintainability

Assuming that all the equipment in the system is installed in accordance with the manufacturer’s recommen­dations, system performance now becomes a function of system maintainability.

(a) Unit Design and Equipment Spacing. Several fac­tors must be considered in the design of the system installation. The type and location of the plant determines the availability of maintenance personnel. Space and access considerations for the physical layout of the plant are influenced by the number and type of operating personnel on site during operation and also the type of test equipment required for maintenance Unit design is also affected by the general maintenance philosophy for each instrument or system, such as repair vs replacement, components vs modules, and on-line vs. off-line main­tenance. This, in turn, is governed to a large extent by the availability of replacement parts

(b) Cables and Connectors. When maintenance re­quires removal of the equipment from its operating configuration, the ease with which the removal is accom­plished is a prime factor in successful maintenance. All cables and connectors should be installed for easy removal without the use of special tools. It should always be possible to remove equipment without disturbing the operation of adjacent equipment Likewise, cables and connectors of the operating equipment should not interfere with the removal of the defective equipment Connectors should be indexed and coded so that incorrect interconnec­tion is virtually impossible.

(c) Displays for Maintenance. One of the quickest ways to determine maintenance requirements is through the use of a self-annunciating or display system. Where practi­cal, this type of failure alarm system should be used In instrumentation systems where self-checking is used, the installation of the display unit should blend as far as possible with the operational configuration of the system and should become conspicuous only when an alarm is actuated