Page 87 - Improving Machinery Reliability

Vendor Selection und Bid Conditioning 59

Figure 2-3. Phase-displacement-type torque-monitoring system installed on large
turbomachinery train.

Torque Measurements Warn of Performance Deterioration. Performance
deterioration of steam and gas turbines is often caused by fouling deposits on blad-
ing or nozzles. In steam turbines, evidence of these deposits is frequently not discov-
ered until the steam flow increases to a point at which no additional steam can be
passed through the unit and the turbine is no longer able to carry an assigned load. In
gas turbine air compressors, fouling deposits will reduce the amount of air available
for efficient combustion, and excessive fuel consumption or reduced load-carrying
capacity may result. Similarly, process gas compressors are often subject to poly-
merization. The resulting flow impediment can seriously affect process operations
and mechanical performance.
Turbomachinery performance can be restored by judicious application of on-
stream cleaning methods." Abrasion cleaning and solvent cleaning (water washing)
have been identified as the two principal approaches. Details of suitable procedures
and their relative merits have been amply documented in the literature5v6 or can be
obtained from the major equipment manufacturers. The problem to date has been to
determine conveniently and accurately when to initiate the on-stream cleaning
process. It is well understood that in pure economic terms, on-stream cleaning
should commence when the cumulative cost of lost power due to fouling since the
last cleaning cycle equals the cost of the cleaning procedure. This is where the on-
stream torque-measuring system comes into play. Configured to sense torque, speed,
and power at the load coupling, the device can be tied in with computer means
accessing steam or fuel flow meters and heat-rate tables. For example, the average
turbine efficiency could be calculated in one-minute intervals from:

N,, X 860
= Aia,, x Do