Despite their importance in the economic well-being of concerned enterprises, machinery is often installed by the lowest bidder and using the cheapest available materials. A great deal of intellectual effort is expended on specifying the ideal machine for the job at hand, but very little on how it should be installed. As a result, a well-known standard has been introduced by the American Petroleum Institute with the API RP 686, essentially a distillation of the various standards used by large oil production, refining, and downstream companies like ExxonMobil, Shell, Texaco, and Chevron.
The API RP 686 has a whole chapter on concrete foundations for rotating machinery, comprised of 21 sections covering everything from general design parameters, geotechnical requirements, concrete mix design, drawing information, rebar, and fastenings.
A couple of important concepts are pointed out in this standard; foundation mass should be at least 3 times the machinery weight for normal common-or-garden equipment, and 5 to 10 times for reciprocating machinery. Based on our experience, we prefer to use a power-based formula, and suggest 30 kg of concrete per kW of power input for rotating machinery, and at least 100 kg per kW for reciprocating equipment. Other equipment not mentioned in this standard, such as turbo expanders, heat exchangers, and most of the piping racks and supports, may also benefit from proper use of an epoxy grouting.
However, concrete is subject to a wide range of problems during execution, and we have identified three basic concepts governing the repair of a foundation; cold joints, expansion cracks, and everyday concrete stuff – shrinkage cracks, minor stress cracks, and cracks produced by thermal expansion of the machine.
The GMRC (Gas Machinery Research Council) has also some quite interesting recommendation about anchor bolts length, diameter, tension, material, and termination, where they suggest to avoid using “J”, “L”, or square plate termination; we go further, though, and say that termination of anchor bolt into the foundation should be a clean fully threaded Grade 8.8 rod of the appropriate diameter.
In the end, every machine needs is a uniform foundation support and load transfer link, and this is provided by a grout, where it is important for the grout to be in good contact with both the underside of the equipment (or its mounting plate), and the concrete surface. This allows it to transfer static, dynamic, and thermal loads to the concrete over the widest possible area, thus protecting the concrete itself. It also allows for distribution of the machine's vibrations into the concrete, where the mass of the foundation, and the relatively loose connections between the molecules in the matrix, provide for a lot of energy loss in the process of transmitting the signal through the foundation. By the time it reaches the subsoil, there should be no energy left.
However, not all the grouts are the same. The evolution of materials for grouting has been under development in the last decades.
We have seen the incorporation of different additives to improve the properties of this in an attempt to achieve the necessary volume-change characteristics. One of the first methods was the Dry-pack (or damp-pack), a mixture which contains only enough water for compaction and hydration but not enough to permit settlement of the grout’s constituents.
Then, in the late 1950s we started to see the Epoxy Grout. These grouts are characterised by high strength and adhesion properties. They are also resistant to attack from many chemicals and are highly resistant to shock and vibratory loads.
Generally, the compressive strength of at least 8000 psi (55 MPa) is easily obtained for most epoxy grouts, but this number can reach 14500 psi (100 MPa) or higher, depending on the application.
Choice of grout material is very important to ensure complete integrity. Note that material cost differential of 10:1 (epoxy grout versus cement grout) has not changed. Nevertheless, Cement-based grouts performance is poor and a properly installed epoxy grout will function well for the remaining economic life of the machine, which can easily be 50 years or more.
Nowadays more and more companies, OEMs, and engineers understand the importance of using the right product with the right contractor for the job, such as did Fluxys in their LNG Gas Terminal in Belgium, where the last year we epoxy grout four BOG IHI Compressors with the quality that has characterised us.
If you would like to hear more on gas and LNG developments, be sure to register to attend the 2018 Gastech Exhibition and Conference and gain exclusive market insights by market experts.
Image courtesy of Alphatec
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