Design for Maintainability

Maintainability can be defined as the ease in time and resources of retaining equipment in or restoring it to a specified operational condition. It directly affects the bottom line because it impacts operations, downtime, maintenance costs, and safety.

Maintainability is an important aspect of any system lifecycle, but some machine design engineers may not give it the direct consideration it deserves. This is primarily the result of a short-term view of capital project costs that fails to consider lifecycle costs and downstream activities.

There is an old belief among maintenance personnel that, "The designers and engineers have it for a year, but maintenance has to live with it for 20 years." There is considerable truth to this, as many times the machine builder or system integrator and the customer's engineering and maintenance groups are driven by different metrics, leading to conflicting interests. The result is a rather expensive proposition for a company in the long term, but not for this current quarter; and that makes the bean counters happy.

This article takes a broad view. It discusses some of the qualitative aspects of maintainability of instrument systems, but is also applicable to many other systems.

Before starting a system design, keep two principles in mind: Minimize maintenance from the beginning, and get customer maintenance people involved whenever possible.

Reliability is a cornerstone to successful system operation and directly affects system maintainability. The more reliable a system is, the less maintenance it will require. Allocate costs carefully. For example, maintainability is best improved if the difficult-to-maintain parts of a system are made more reliable. Identifying reliable components, installation types and arrangements, and vendors is a key step to improving the reliability of instrument systems.

Adding fault tolerance can reduce downtime to zero and allow repair off line. More components, however, will raise the overall maintenance rate.

Getting maintenance involved early in a project can improve maintainability. This seems like a no-brainer, but your customer's company politics, local turf wars, personalities, and sometimes even arrogance interfere with what is obviously a good practice. Involvement also brings ownership and an improved relationship between maintenance and engineering. At the very least, maintenance won't be able to complain about the system if they were involved in the design.

The somewhat overworked acronym KISS (keep it simple, stupid) applies to maintainability. While we live in an increasingly complex and sophisticated world, we should strive to keep it simple wherever we can. You can design something that is really cool, sophisticated, and elegant, but if it's too hard to maintain, it will waste everyone's time and end up on the junk heap.

Modular design divides the system into physical and functional modules, which can be arranged to facilitate design and maintenance. Easily replaceable modules with logical organization reduce repair time, troubleshooting, training, and engineering. Interconnectivity and interoperability should also be considered.

Modular design of software can do a lot to improve its maintainability.

Some thought must be given to future modifications or expansions. Nothing is static in a factory; improvements are made, additions are built, and things are modified. The design group must consider what the future may hold and make reasonable accommodations. Failure to do so may create a system that is easily maintained today but difficult to work on tomorrow. That won't make for good customer relationship building.

Using recognized national, industry, and company (if your customer has them) standards and codes is good practice. It improves maintainability by reducing variations of design and installation for maintenance personnel.

Standardization of components reduces inventory and improves reliability and maintainability. The use of commercial off-the-shelf components also should be considered, though care should be taken because some commercial components may not meet the necessary industrial requirements.

While many instruments are unique and the technology can change rapidly, standardization of instrumentation can have the same benefits of reducing inventory and improving reliability. In addition, training and engineering costs can be reduced. Successful standardization requires that an OEM or system integrator's customer use proper reliability engineering techniques and keep and analyze maintenance records to determine what should be standardized on.

Many systems consist of a variety of the same or different manufacturer's instruments. In some cases, these instruments are arranged in proprietary configurations; in others, the systems are more open, leading to some ability for spare parts interoperability, which can improve maintainability.

Out in the plant, system components must be easily identifiable. Labeling must be consistent, standardized, clear, and accessible. While not a substitute for drawings, system identification that provides the capability of tracing wiring, power sources, and identification of components without drawings provides for more efficient and safe troubleshooting. Proper identification is also a safety consideration because it can help ensure that the proper things are worked on and that hazards are properly identified.