Page 91 - Improving Machinery Reliability

Vendor Selection and Bid Conditioning 63

Figure 2-6. Hot alignment monitoring system. (Courfesy Prueftechnik A.G., 0-85730
Ismaning, Germany.)

How to Review Critically Important Electronic Systems. As machinery systems
become increasingly more complex and unforeseen downtime prohibitively expen-
sive, reliability reviews must employ new approaches, techniques, and resources.
Because defects in electronic governors, safety shutdown devices, and certain instru-
ments can have a devasting effect on equipment reliability and plant profitability, the
petrochemical industry will have to borrow its reliability review methods from the
aerospace industry. One such review method is called “sneak analysis.”
The Sneak Analysis Department of the Houston branch of the Boeing Aerospace
Company defines sneak conditions as latent paths, timing, indication, or labels in
electrical hardware or computer logic. The conditions generally exhibit unapparent
cause-effect relations, and may inhibit a desired operation or initiate an unintended
action. A sneak condition is not caused by component failure, but is a condition that
has been inadvertently designed into a system. Some sneak conditions are evidenced
as anomalies or spurious operative modes. Historically, sneak condlitions have
escaped rigid design screening efforts and caused commissioning delay or loss of
equipment availability during operation or test.
Boeing’s experience with sneak analysis in aerospace, military, commercial, and
nuclear systems has shown that sneak conditions have distinct characteristics in all
electrical and software systems. In each project (!) sneak conditions were found that
had been missed by standard system verification procedures. They were found
through application of a formalized, computer-aided, topological approach to
detailed system analysis. Further, the analysis provided procedure checks, drawing
and document error disclosures, and design concern conditions.