If The Foundations Be Shaken

This article was originally published in the December, 2016 issue of COMPRESSORtech2. Subscribe for free to receive every issue in your mailbox or your inbox.

In the mid 1900s, reciprocating compressor foundations were expected to last 20 to 30 years. However, many of the machines installed in the 1960s or 70s are still running on their original foundations, which are in need of repair or replacement because of poor design, construction or maintenance [1].

Particular problems may signal that there is something wrong with the compressor foundation. These problems could be a broken anchor bolt, twisting movements of the machine frame or excessive vibrations.

Often, the machine has to be stopped in order to make these repairs — or else it might stop itself. In many cases, unplanned shutdown could be avoided by looking at maintenance records, the original construction and the as-built situation. There are signs to indicate the foundation is in bad condition. The many updated publications of API, ASTM, GMRC, ISO, DIN, CUR, EOTA, and other regulations and advice are helpful as well.

Moreover, the modern machinery’s performance (horsepower and flow capacity), accuracy of alignment, and environmental requirements with respect to sound and vibrations (due to stricter governmental regulations) have been increased radically over the past decades. This has made the need for increased attention to the foundation, anchorage and upkeep an important issue.

Without repeating all the facts and figures and describing all possible scenarios, this paper is intended to give some guidance on how to inspect a compressor foundation and what possibilities there are for repair. It will also cover some practical issues faced over the past 15 years.

Desk Research

To get a good understanding of the situation, first, desk research is required in order to gather all necessary information.

As-built construction

Especially when observing older configurations, an inspection of the foundation plan and the as-built construction drawings is required in order to check if the foundation and anchoring are designed and perform according to the relevant guidelines such as API recommended practices 686, GMRC Guidelines and PIP REIE 686. Also, check if there has been an update of the compressor or driver where the foundation has not been updated accordingly.

The As-Built Construction Should Be Reviewed On:

Concrete mat and block design

Information on the required dimensions and minimum weight of the concrete block can be found in the aforementioned guidelines. A rough check can be made first, and if it is found that guidelines were not followed, then a more detailed analysis may be required.

Rebar details

In general, there should be a minimum bar size of .75 in. (19.05 mm) (GMRC Guidelines for high-speed reciprocating compressor packages) [2]. There are also rules and practices that can be found in GMRC and API RP 686 to check rebar spacing and material. Again, a simple check could be enough to determine if further action is required and/or if the original design was made according to the applicable standards. Please note that other guidelines may provide different values and therefore one should determine which guideline is to be followed.

It is important to understand whether possible cracks could be related to insufficient rebar. For example, over fifteen years ago rebar spacing at the top of the foundation was advised as being approximately 12 in. (300 mm), while nowadays 6 in. (150 mm) is advised.

Concrete Properties

The concrete should have a minimum compressive strength of 4060 psi (28 MPa) (GMRC, API RP 686), although the design value in many cases does not reflect reality since concrete can be weakened by oil or just because the actual poured concrete was not at optimum quality. If the properties are in doubt, it is advised to have this checked in a laboratory.

Soil and pile setup

A foundation should never be installed without thorough analysis and characterization of the underlying soil. First of all, find out if a study has been done before designing the foundation. Also, look for any changes in the soil properties, for example, due to rising water levels.

For this, the advice of a geotechnical consultant is required in order to determine important elements like mass density, layer variation and stiffness of the soil. Based on this data and on the type and mass of the compressor system, a determination can be made as to whether pilings need to be used and the proper type of foundation for the job.

A tool that could be helpful in checking whether the design of a compressor foundation might be susceptible is the Compressor Foundation Analysis Tool (COFANTO). This program is available to R&D members of the European Forum for Reciprocating Compressors (EFRC).

Anchor type and dimensions 
It is of major importance to gather all information on the anchor bolts. Some installations incorporate outdated anchor bolt designs that can eventually lead to problems. At the very least, the following information is to be obtained on the anchorage:
  • Bolt preload and designed clamping force.
  • Bolt material, size, and type.
  • Designed stretch length/elongation.
  • Coating/surface treatment.
  • Pre-installed versus post-installed.
  • Pocket size and type information.
  • Edge distance of the anchor pockets.
  • Type of grout used for bonding.

Most standards and guidelines, such as the API RP 686, provide helpful guidance on the subjects mentioned above. The information gathered during research on the as-built construction must be combined with maintenance records and a field investigation to form a judgment on the foundation’s state.

Maintenance records

The compressor’s maintenance records are to be analyzed to see if there are any signs of:

  • Inability to hold alignment or anchor bolt tension
  • Loose or broken anchor bolts
  • Increasing vibration levels (above zone B/C of theEFRC guidelines) [3]
  • Crank case, crosshead, or support block damage and wear
  • Excessive bearing wear
  • Piping damage (excessive repairs needed for the attached piping)

These points could indicate foundation problems. All gathered information on trending vibration measurements, repair records, oil analyses and more will also be of great help in foundation troubleshooting.

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