If The Foundations Be Shaken

Addressing Inspection And Repair Of Recip Foundations

Field Investigation

After all the data has been collected from the desk research, it is time to examine the compressor foundation on-site. First, the foundation and compressor are examined while running. More comprehensive investigations can be done while the machine is stopped; however, in general, this is only done when it has already been determined that there is something wrong with the foundation or anchorage.

Visual inspection (running compressor)

A lot of information can be gathered during a systematic visual inspection. Most common issues are quite easy to spot. One should look for the following issues:

Loose or broken foundation bolts

Loose or cracked foundation bolts are easy to spot. Very frequently, the compressor trips on a broken anchor bolt, and often, machine operators realize there is something wrong with the foundation if one of the anchor bolts fails. While this is indeed a strong indication, it is also possible that the cause of a bolt failure is to be found somewhere else. For example, sometimes it is easy to over-torque bolts with hydraulic tools.

Compressor looseness

Compressor looseness is easily spotted by air bubbles showing between frame-feet and grout in oil or water (Figure 1). These air bubbles indicate looseness, meaning that the anchoring system is not performing as it should. Oil also considerably reduces the required friction between the frame parts and the grout layer.

compressor bubbles
Figure 1. Bubbles indicate compressor looseness
Usage of shim plates

Excessive usage of shim plates often indicates that there is a need to “fill-up” space. This can be an indication that there is something ‘moving’ or a mistake has been made during the design phase. The maximum number and thickness of shim plates differ depending on the guideline, region, and the local engineering standards. Different guidelines agree on not using more than three shims in a pack, as more shims lead to spongy (soft) pads.

Unfortunately, the use of adjustable jacks, steel shim blocks or wedges is still frequently advised for alignment of pumps, skids, base frames or even compressors while they remain in place. These steel jacks make the equipment stand on “high heels,” forming a direct steel contact between machine frame and concrete foundation. Instead of having the necessary constant compression on the grout layer, the machinery stands on noncompressive steel blocks, which allows oil and water to penetrate between the steel and grout.

Even worse, neither the anchors nor the grout can function according to their design, as pre-tension is lost on the steel jacks and grout is merely an aesthetic cover.

Another problem with steel shim blocks in the grout is that they tend to corrode. This is a very common condition and causes serious problems such as cracked grout and a machine that is tilted out of alignment.

Edge lifting and delamination of the grout

Edge lifting is caused by the difference in the rate of thermal contraction between epoxy grout and concrete. Generally, the main reason for edge lifting is poor or inexperienced application and the usage of bad quality concrete.

Delamination often occurs between cementitious grout and concrete; mostly caused by the bad adhesive properties of the grout or due to poor preparation of the concrete block.

Edge lifting and delamination do not always form an immediate threat to the foundation of the machine since the part under the machine (if well grouted) is under constant pressure and therefore in better condition. However, oil and water can intrude into the foundation, causing more problems. It can also be a sign of poor application and therefore a reason to suspect more problems.

Cracks in grout layer 

Cracks in the grout layer may have different causes such as sharp corners, fast curing or thermal expansion. In many cases, these cracks do not form an immediate threat. However, it is important to look for their root cause and to seal it to prevent further damage.

Deterioration of the grout layer 

Penetration of (crude) oil and other fluids into cementitious grout will over time weaken its compressive strength and the adhesive capacity of cementitious grout in the anchor pockets [4]. This will continue up to a point where the grout will crumble between your fingers.

Oil penetrating along the anchors will eventually jeopardize the fixation of the machinery.

Cracks and cold joints in the concrete foundation 

Cracks in the concrete foundation can be due to various factors such as weather conditions, and thermal or aggregate expansion. They can be found at re-entrant angles such as the corners where the foundation of the crosshead support is connected to the main block, but also in the sump area, and, of course, cracks running from the anchors towards the outside of the foundation.

When observing cracks in the concrete foundation, one has to realize that vertical cracks are less “dramatic” than horizontal cracks. Horizontal cracks can cause alignment disorders while vertical cracks do not.

Figure 2
Figure 2. Foundation parts forming a monolithic structure.

The compressor and its foundation must form a tightly integrated structure. Vibration energy travels in the form of waves down and out through the foundation where the soil can absorb it. Breaks, cracks or separations in the integrated compressor/ foundation structure will prevent the vibrational waves from traveling downward [5].

Horizontal cracks create “separated” parts in the foundation and there- fore it will become unable to transfer vibrations into the soil. The monolithic structure (Figure 2) is disconnected.

The same problems can be caused by construction joints, also called “cold joints.” Because concrete does not bond very well to itself, separation can occur if different parts of the foundation are not poured continuously.

Concrete carbonation and spalling 

If carbon dioxide from the air reacts with calcium hydroxide in concrete, it forms calcium carbonate. This process is called carbonation. This is a slow and continuous process progressing from the outer surface inward and has two effects: it decreases mechanical strength of concrete and it decreases alkalinity, which is essential for corrosion prevention of the reinforcement steel.

Figure 3
Figure 3. Spalling concrete on a foundation.

Carbonation and rebar located too close to the concrete surface can cause rebar corrosion. The expansion of the iron oxides induces mechanical stress that can cause the formation of cracks, disrupt the concrete structure, or make outer parts of the foundation fall off (spalling, Figure 3) [6,7]. These phenomena are easy to recognize during visual inspection of the foundation.

Displacement of foundation

Instability of the soil can cause sink- ing or tilting of the foundation (Figure 4). Therefore, displacement of the foundation can indicate that the soil is not adequately supporting the foundation. This can be checked with tools such as a digital water level and tapeline, or more precisely by laser scanning. Trending measurements (measurements over time) can provide more suitable information.

Figure 4
Figure 4. Tilting foundation.
More comprehensive investigations (Running compressor)

The aforementioned visual inspection can be done relatively simply within a few hours. Depending on the information gathered from the desk research and the visual inspection, it may be wise to perform additional investigations.

Vibration measurements 

Vibration measurements and analysis of the machine system (which includes anchoring and foundation), will provide hard data to compare with the ISO 10816-8 classification table and the EFRC guidelines for vibrations in reciprocating compressor systems [8,9].

Additionally, an ODS (operating deflection shape) animation can offer a clear visualization of the movements of the complete installation. This can help you to recognize things as looseness or unwanted movements.