Process creep is the term used to describe small changes within a process. Over time, however, these changes, while individually minor, can lead to significant changes from the original design. This can have a negative effect on your process equipment as it is now operating at a condition it was not designed to handle. Rotating equipment tends to be affected the most, but this can also affect other part of your process.
Consider, for instance, a process mixer. When these are installed in a process there is a given shaft diameter and length, impeller style and diameter, and rotating speed. At some point it may be decided to increase the speed of rotation or the impeller diameter. A few years later, these parameters are changed, again. And again. And again. Now a mixer drive is being asked to transmit torque into a process for which it was not designed. The power law tells us that speed increases power to the third order and impeller diameter to the fifth.
Filters within lubrication systems are commonly changed. Generally, micron (µm) size and beta (ß) ratio are the specific ratings considered. What started out as a 3µm ß≥1000 lube system has migrated into a 12µm ß≥100 filter. Instead of having a well protected turbine pump, particulate can now get into the bearings and cause them to wear prematurely. Fortunately, filter manufacturers have now started to publish ISO ratings, to prevent some of this, but it can still happen.
Instrumentation and process control can also be affected by process creep. Consider a flow control loop. If a differential pressure flow measurement is made to provide feedback to a control valve. Over time, the pressure and flow is adjusted. The instrument is now measuring below its turndown and the control valve is operating near the seat. This can wreak havoc on both the process and the control valve is continually adjusted. Of course, some of this can be tuned out, but the performance will not good (or reliable).
The best way to deal with process creep is to review process conditions and compare them with the original design. If a change is being considered, you will need to determine how these changes will impact your process. It is critical to understand how changes affect the equipment. In the case of the mixer little changes can have a big impact on power draw. Filter changes can lead to not having properly protected equipment. Instruments and control valves are no longer operating is a reliable range.
Beware of the Process Creep!
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