Awareness of energy efficiency is one of the minimum job qualifications for reliability engineers. In the summer of 1994, Jack Lambley, then an intern at the Imperial Chemical Industries (ICI) Rocksavage site in the UK, was assigned the task of quantifying the effects on power consumption for misaligned process pumps. A surplus pump was overhauled, and new bearings were fitted. This pump was reinstalled, and water was recirculated in a suitably instrumented closed-loop arrangement. Prueftechnik GmbH loaned Lambley a modern laser-optic alignment instrument.
As an undergraduate student, Lambley had learned how misalignment affected bearing load, and how bearing load increases caused exponential decreases in bearing service life. Following instructions from his supervisor, Lambley reviewed the engineering sections of SKFs general catalog, which stated that a 25% increase in bearing load caused the rated bearing life to be halved.
Lambley investigated the alignment accuracy and the methods in use at that time. He discovered that straight-edge methods were inappropriate for refinery pumps. Rim-and-face alignment methods were judged difficult and unreliable. Properly executed, reverse-dial-indicator methods required consideration of the bracket sag, and they would require more time to apply than modern laser techniques.
From data available at the Rocksavage site, he calculated that the typical misalignment consisted of 0.02 in./0.5 mm vertical and horizontal offset and 0.002 in./in. vertical and horizontal angularity. In 1994, lasers were known to be inherently more accurate than the best competing techniques.
Lambley constructed several graphs and tabulations, as shown in Figs 14. The resulting recommendations were to align machinery to within 0.005 in./0.12 mm shaft offsets and to limit deviations in the hub gap to 0.0005 in./in. of hub diameter. Lambley further documented that adhering to these recommendations would reduce ICIs power consumption by about 1%. He confirmed that laser alignment was faster and superbly more accurate. Lambley determined that laser alignment technology was bottom-line more cost-effective; he deserves credit for establishing these facts instead of repeating the opinions of others.
Fig. 1. Effect of parallel offset on power
consumption of a pin coupling at 3,000 rpm.
Fig. 2. Effect of angular misalignment on
power consumption of a pin coupling
at 3,000 rpm.
Fig. 3. Effect of parallel offset on power
consumption of a toroidal (tire-type) coupling
at 3,000 rpm.
Fig. 4. Effect of angular misalignment on
power consumption of a toroidal (tire-type)
coupling at 3,000 rpm.
Using data from a mid-size refinery:
Average demand: 27 kW/pump 3 8,760 hr/yr 3 $0.1/kWh 3 1,000 pumps 3 0.01 = $236,520/yr. And, with 1,000 pumps operating at any given time, this location could annually save approximately $250,000 in avoided power consumption.
The total cost for laser alignment instruments includes equipment costs plus training costs. The benefit is 8 man-hours of time-saving credit per alignment job. For gathering more data, thermography and infrared monitoring techniques are possible options. These methods have been used to quantify significant temperature increases in a coupling located between misaligned pump and driver shafts. You could compare the energy wasted by the rising temperature of a coupling to the energy loss, as described by Lambley. Regardless of calculation method, laser alignment will result in surprisingly rapid payback. Remember: In all reliability improvement endeavors, never let somebodys opinion get in the way of sound science and facts.
If you are like the majority of hydrocarbon processing industry facilities in the industrialized world, your worker and technician resources are probably stretched to the limit. Understandably, you may be looking for ways to simplify some of your traditional work processes and procedures. You may have had an experience that reinforces the contention in which high-tech tools are not always the answer. And hold the view that the back-to-basics thinking has considerable merit. However, decades of well-documented observation attest to the fact that misalignment has been responsible for huge economic losses. The more misalignment of the rotating unit permitted, the greater the rate of wear, likelihood of premature failure, and loss of efficiency of the machine.
As an inquisitive Lambley proved, misaligned machines absorb more energy than they consume more power. So, its always advantageous to update ones knowledge of shaft alignment and alignment tolerances. Competent vendors will assist you in illuminating the roadway to becoming reliability-focused. And indications are that only the reliability-focused facilities will be around in the future. HP
|The author |
Heinz P. Bloch is Hydrocarbon Processings Reliability/Equipment Editor. A practicing consulting engineer with now 50 years of applicable experience, he advises process plants worldwide on failure analysis, reliability improvement and maintenance cost avoidance topics. He has authored or co-authored 18 textbooks on machinery reliability improvement and over 490 papers or articles dealing with related subjects. For more on alignment, refer to Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley & Sons, Hoboken, 2011, pp. 153162.