Align pumps with laser accuracy

Misaligned pumps can affect energy efficiency.

By Heinz P. Bloch, P.E., Process Machinery Consulting

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In brief:
  • Intern approaches pump alignment with laser accuracy.
  • Tips to compare the energy wasted by a hot coupling to the energy loss.
  • Misalignment affects bearing load and excessive bearing load causes exponential decreases in bearing life.

In the summer of 1994, Jack Lambley, an intern at Imperial Chemical Industries’ (ICI) Rocksavage site in the United Kingdom, was quantifying the effect of misaligned process pumps on power consumption. He arranged to have a surplus pump overhauled and fitted with new bearings. He then had the pump installed in a suitably instrumented closed-loop arrangement operating on water. Prüftechnik loaned Lambley a laser-optic alignment instrument.

As an undergraduate student, Lambley had learned that misalignment affects bearing load and that excessive bearing load causes exponential decreases in bearing life. His supervisor, Steve Moore, had asked Lambley to read the engineering sections of SKF’s general catalog, which stated that a 25% increase in bearing load cut its rated life in half.

Lambley investigated alignment accuracy and methods the plant was using at the time. He found that straightedge methods were inappropriate for refinery pumps. Rim-and-face alignment methods were judged to be difficult and generally unreliable. Properly executed, reverse-dial-indicator methods required consideration of bracket sag, which would take more time. Still, from data available at Rocksavage, he calculated that typical misalignment was 0.02-in. vertical and horizontal offsets and 0.002 in./in. vertical and horizontal angularity.

In your reliability improvement endeavors, never let somebody’s opinion get in the way of sound science and facts.

In 1994, lasers were already known to be inherently more accurate than the best competing techniques. Lambley believed them to be 10 times more accurate.

The graphs and tabulations Lambley constructed are reproduced here, duly acknowledging ICI’s role. The recommendations coming out of the study suggested aligning machinery to within 0.005-in. shaft offset and limiting deviations in hub gap to 0.0005 in./in. of hub diameter. Lambley further documented that adhering to these recommendations could reduce ICI’s power consumption by about 1%. He confirmed that laser alignment was fast and accurate. He found that laser-alignment technology was bottom-line cost-effective. He deserves credit for establishing facts instead of repeating the opinions of others.

The following data is from a typical mid-sized refinery:

Average annual electrical demand for pump motors:

27 kW/pump x 8,760 hrs/yr x $0.10/kWh x 1,000 pumps = $23,652,000/yr

Figure 1. Effect of parallel offset on power consumption of a pin coupling at 3,000 rpm.
Figure 1. Effect of parallel offset on power consumption of a pin coupling at 3,000 rpm.

The ability to save 1% of this total represents a gross annual savings of $236,520. It ignores the cost of laser alignment instruments and appropriate training. Also, it’s reasonable to assume 7 labor-hours of time-saving credit per alignment job. Alignment work usually involves two craftspeople and a few hours of operators' and supervisor's time. Including the overhead contributions of operating and supervisory personnel, the total job typically consumes 15 man-hours with dial indicator methods and brackets, for which one needs to determine and take into account bracket sag. In contrast, it is reasonable to assume that laser-alignment by a two-person crew, including the inevitable participation of supervisory and operating personnel, takes perhaps eight man-hours. That would explain the seven-hour savings, which could also be expressed as a roughly 40% reduction in total man-hours for laser-alignment tasks.

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