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Getting the most from your machines and foundations
Aronui Tecnologias SL
Alphatec Engineering (Europe) Ltd
Process plants around the world depend on millions of rotating machines:
and so on through a long list. The maintenance department is generally pretty busy keeping these machines together; repairing them, and making sure they are available for production.
The diagnostics of machine problems have been steadily improving since the electronics revolution, and catastrophic failures are getting fewer every year. Yet there is still a gap between the sophisticated diagnosis of vibration and alignment issues, and the understanding of how best to solve intractable and recurrent problems.
It has been stated frequently that a large percentage of the installed base of rotating machinery is operating in a less-than-optimal alignment condition. This percentage could be as high as 70% of all operating machinery. Mis-alignment causes excessive vibration, increased power consumption, reduced output, and wear and tear on components. But the major cause of mis-alignment - grout failure - is very rarely diagnosed. During the last 35 years or so our group of companies has regrouted and repaired the foundations for thousands of rotating machines in all types of industrial plant. Invariably, alignment is a found to be miles out, and the grout severely degraded. Using the techniques developed and refined in various Alphatec operations around the world, we are able to restore a machine to a condition which is close to original, or even better in cases where "original" was actually sub-standard.
Our work is based on a series of premises:
- The concrete foundation is the main vibration absorbing element of the installation, provided that it is of sufficient mass; that it is one integral whole; and that it is properly connected to the machine.
- Sufficient mass can be calculated from the traditional rule-of-thumb (6 times the weight of the machinery in the case of reciprocating equipment, or 3 times for general rotating machinery), or using a power based criterion. Our records suggest that 100 kg of concrete per kW of power input (in the case of a driver) or output is normally adequate to damp the vibrations of a reciprocating machine, and that 30 kg/kW will do the job for most other machines.
- The primary cause of a foundation's inability to attenuate vibrations is the cold joint. These occur when the concrete is poured in several lifts, and cannot be avoided by chipping the surface, water-blasting, or any of the usually mandated construction techniques. Use of an epoxy bonding agent is the only way to obtain full adhesion between pours, and its use is rare. Most foundations therefore have this inherent problem.
- The best way to ensure that the foundation is correctly connected to the machine is to use epoxy grout, and good anchoring techniques.
- The anchor bolts are designed to hold the machine down, and the grout is designed to hold it up. The grout must therefore have sufficient resistance to the anchor bolt's fastening force, which is inevitably much greater than the deadweight load of the machine itself.
- The shaft alignment must be as close to "zero-zero" as possible when the machine is operating in its normal condition, not when it is stopped for repairs, adjustment, etc. The question of offsets, both thermal and mechanical, must therefore be addressed before pouring the grout.
- In order to ensure the best possible connection between machine and foundation, full-contact grouting is always advised. The base of the machine should be encapsulated in the grout, to ensure good contact on the underside, as well as a measure of lateral and longitudinal restraint. Many clients prefer soleplate grouting, which is a second-best approach, and stems mainly from the many small failures which are built into a majority of installations, and the necessity to make subsequent modifications and adjustments. If the job is professionally done, these adjustments become unnecessary.
Below specific case histories may help to illustrate the above points.