These examples show how overlooking simple issues can carry catastrophic consequences.
by Mark Jennings
September 28, 2016

Editor’s Note: This article concludes a series by the author covering topics related to predictive maintenance.

This article, which wraps up this series on predictive maintenance (Pumps & Systems, June 2016, read it here, July 2016, read it here, and August 2016, read it here), discusses fasteners that secure dynamic equipment to a foundation. There are a myriad of horror stories about this topic, but this article highlights three specific examples, complete with horsepower (hp).

60-hp Pumps

Part of the job scope in Taiwan (as discussed in “How Does Your Facility’s Maintenance Program Rank?” Pumps & Systems, June 2016, read it here) was to assist the end user in establishing a basic vibration monitoring program. The consultant guided the facility through the setup of a basic program similar to the one shared in the previous articles—all analog.

At one of the user’s facilities during the first round of data collection, the predictive maintenance team noticed that more than half of the facility’s small-frame pumps were running at a relatively high vibration level. At the scale’s lower end, they saw greater than 1 to 1.5 millimeters per second (mm/s) root mean squared (RMS) at all monitoring points. At the higher end, they saw 2.5 to 3.5 mm/s. There were even readings in excess of 4 mm/s.

The facility set its alarm value at 2.5 mm/s and its shutdown at 3 mm/s. After collecting and reviewing the data as part of the training, the consultant asked the question, “Was there any work recently completed on the pumps that had higher readings?” The answer was “yes.”

During a recent outage, a vendor was hired to check and correct alignment on a group of pumps. Initially, all of the pumps ran smoothly, but as time passed, they ran rougher and rougher. So the consultant then asked if facility personnel had checked alignment. The answer: “No, the vendor had realigned the pumps during the outage. The pumps are in alignment.”

After convincing the facility to check pump alignment, the predictive maintenance team started with a simple soft foot check—dial indication on the motor foot, loosen the mounting bolt and see if there is any lift. In this case, there was no motor foot lift; however, it was shocking that a 135-pound technician could loosen the motor mounting bolts with a 10-inch crescent wrench. When asked if the bolts had ever been tightened with a torque wrench, personnel returned a blank stare.

If you don’t tighten the bolts at or near their yield, they will loosen over time. And that is what happened. The bolts were not tight and the motors shifted over time, causing alignment issues and a corresponding higher vibration.

The moral of the story: Always torque your mounting fasteners or at lease use a cheater to tighten them as much as possible. A 10-inch crescent on a 5/8-inch bolt will not cut it.

mounting dynamic equipmentTable 1. Recommendations for mounting dynamic equipment (Graphics courtesy of the author)

300-hp Pump

At a hazardous material remediation site in the Pacific Northwest, a transfer system was installed to move liquid waste over several miles from one storage area to another. The project dollar value was in the millions, and completion was being held up by high vibration from a pair of 300-hp multistage horizontal booster pumps.

The high vibration was not caused by an alignment issue. It was a resonance issue and, of course, a variable frequency drive was involved.

However, when the consultant’s team double-checked the common causes of vibration—such as alignment, loose nuts and bolts, electrical, and structural—they found that the washers that were part of the motor mounting were not rated for graded fasteners. They installed SAE Grade 2 washers under SAE Grade 5 bolts. And as you torqued up the bolts, the washers would collapse under the bolt heads, extruding into the motor feet bolt holes.

They discovered the issue when checking bolt tightness and torqued the bolts to specific criteria (a torque value for that size and grade of fastener). As the washer pulled through the motor mounting holes, it would affect the alignment. Basically, they could “snug up” the bolts and maintain alignment, but when they went to torque the bolts after the final alignment adjustment, they would be outside of the alignment tolerance. The fix: hardened washers—ASTM F436.

Basic torquingTable 2. Basic torquing criteria

125-hp Belt-Driven Overhung Fan

At another hazardous material remediation site in the Pacific Northwest, a facility that was being decontaminated and torn down used the original exhaust ventilation system to prevent the release of hazardous material. The system included eight electric, 125-hp, belt-driven overhung fans and two backup steam-driven (about 75-hp) fans, which took suction on a common plenum and released exhaust through a common stack.