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Foundation Repair

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Precision
Alignment
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"The Bitterness of Poor Quality is Long Remembered
after the Sweetness of Low Cost is Forgotten"


Precision Alignment

M&M provides the finest in optical alignment and owns three complete Brunson Model 545 precision sight levels and all their attachments.  The knowledgeable use of this instrument allows us to work in increments as small as one thousands of an inch(.0001).

In the absence of a crankshaft we offer a full wire alignment service that is second to none.

We are specialists in the recommendation and installation of anchor bolt systems which play an important part in maintaining good alignment.  These vary in all sizes and locations and can range from the special super, 4140 shot peened and frangelized load monitored bolts with torque nuts and self aligning spherical washers down to the older mild steel but still very functional and simple J Bolt.

To understand the importance of precision alignment and foundation repair one must understand the nature of the industry and what is involved.transom.jpg (5667 bytes)

A typical reciprocating gas engine compressor is a large integral gas engine compressor in the 2000hp to 10,000hp range. A right angle drive integral gas engine compressor develops power with the vertical power cylinders and turns that power into work by driving horizontal gas compressor cylinders on the same crankshaft. By their design, they impose large unbalanced forces on the concrete foundation. Another style is the separable (or endcoupled) compressor which consists of a driver which can be a gas engine, steam or electric motor and a compressor unit that is attached together by a coupling or gear box. This type also places high stress on the concrete foundation.

Reciprocating compressors are used in gas processing plants to extract liquids or to make plastics. Another major use is in the nations transcontinental gas pipelines, where compressing the gas is necessary to move it up the line.

Reciprocating Compressor foundations have an important role. Without a stable foundation critical alignment cannot be achieved or maintained. Crankshaft alignment is a key element for a long trouble free life of a reciprocating compressor. The crankshaft sits in bearing saddles that must be flat and level to each other within .001 inch to .002 inch. For reference, the average human hair is .003 inches thick.

The bearing saddles tie into the machine base usually at each anchor bolt. The machine is supported on Epoxy or steel Chocks, which allows the machines dynamic loads to be transmitted down through the chocks, the epoxy grout cap, and the concrete block into the mat and subsoil below. level.jpg (3855 bytes)

Reciprocating compressors normally operate in the 200-300 rpm range and create both vertical and horizontal dynamic loading due to their mechanical configuration. Improperly designed foundations or foundations where the machinery has out lived the design life of the foundation have caused many crankshaft failures and excessive wear part usage. These failures can cost hundreds of thousands of dollars in repair expense and downtime costs.

A brief history on how reciprocating compressor foundation design has evolved over time shows the challenges faced in supporting critical alignment equipment. The first reciprocating compressors were mounted full bed on sand and cement grouts. In the mid 50's, we saw the development of the first epoxy grouts.

Full bed grouting with epoxy grout was a great improvement but there was a problem with the heat transmitted into the foundation from the engine oil pan at the bottom of the engine. The edges of the foundation cooled while the middle stayed warm causing the foundation, machine frame and crankshaft to hump in the middle. This problem called "thermal humping" led to the development of steel rails with steel chocks on top. This provided cooling benefits by raising the machine off of the epoxy grout cap but created new cracking problems in the epoxy grout cap. This is caused by the difference in the coefficient of thermal expansion between epoxy and steel. During heating and cooling cycles the epoxy grout expands approximately three times more than the steel and cause cracks in the grout, which allowed the machinery to move on the steel chocks. This movement produces fretting at the machine base/steel chock interface and the steel chock/steel rail interface which allows for more movement. By cutting the long steel rails into segments, steel sole plates evolved. This reduced some of the cracking problems of the long steel rails. In the mid 60's the first adjustable system evolved. A 1" thick steel chock was replaced with two" thick steel chocks with a .030" shim pack sandwiched between the two" thick steel chocks. These were bolted together at each corner and used on top of a steel sole plate.mm52s.jpg (12308 bytes)

An offshoot of this split steel chock design was changing the materials of construction from steel to high-density composite laminate material. This product evolved in the mid 80's in order to dampen the machinery vibration and to insulate the foundation from the operating heat (150F). Adjustably proved to be essential for maintaining long term critical alignment. The next step was to simplify the adjustment process by using high-density composite laminate plates or a poured in place epoxy chock. This provided quick adjustability, and conformed to the machine base without field machining.

Adjustability provides a tool that can be used to control reasonable elevation changes in the block, mat, and subsoil. The foundation block must be monolithic, properly reinforced and sized to provide the necessary mass in order to function properly by absorbing and transmitting the dynamic operating loads into the mat and subsoil below. Most foundations we see that need repair have very little rebar beyond a perimeter cage. This was a standard design close to 30 years. Currently ACI committee 351-2 a dynamic foundation design sub committee, is developing better foundation design guidelines, not only for right angle drive gas compressor, but also other types of dynamic machinery.

Most foundations we inspect are severely cracked horizontally and vertically. Sulfate contaminated engine oil has penetrated the cracks and attacked the cement paste in the concrete throughout the foundation. These are not the only problems that must be addressed, as we will list below.

When designing a repair for an existing foundation the following additional constraints must also be addressed:

Engineering vs. Economics
Allowable Equipment Downtime
Available budget
No current gas technical soil information
Physical barriers such as piping, bottles, equipment weight
Machine left in place during repair
What mechanical repair needs to be done concurrently

Most all foundations we see should be completely removed down to the mat and rebuilt. Economics does not allow this in most cases unless the concrete is completely deteriorated. Often, there is no updated subsoil study available at most sites. Additionally, physical barriers like piping, high-pressure bottles, and the equipment weight are also problems that must be worked around. The average reciprocating compressor weighs close to 500,000lbs and usually must be left in place during the repair process. Operating procedures and conditions must also be researched. If new cylinders have been installed changing the operating balance or the machine has been turbo charged and the horse power upgraded then a new or repaired foundation will not always solve the problem.

Over the last twenty-five years of repairing these foundations we have developed the following repair solution that provides for a long term repair yet meets the economic criteria demanded by our customers.

site.jpg (2625 bytes)The first step is to inspect the foundation, mat and building as well as review the maintenance records and as built drawings. This can provide a wealth of data, which can be correlated to explain cracking or help establish a demolition depth.

The deteriorated oil soaked concrete is removed with 30lb chipping guns with chisel points. Vertical post-tensioning rods are installed in cored or rock drilled holes to pull the lower block back into a monolithic state and to reduce any overturning moment. Horizontal and angled post-tensioning rods can also be added depending on cracking and foundation configuration. The old anchor bolts are upgraded to ASTM A193 high strength two piece bolts. This is done with a repair kit on shallow repairs and complete replacement on deeper repairs. When replacing anchor bolts completely on new foundations the bolts should go all the way into the mat or at least be staggered to reduce the chance of creating a stress plane at the anchor bolt termination point. The upper foundation is heavily reinforced with rebar and repaired with regular concrete if time permits or a modified concrete that unlike regular concrete will develop most of its strength and is ready for an epoxy grout in only 28 days. Regular concrete takes 21 to 28 days to fully cure to the point where epoxy grout will bond. If epoxy grout is placed on green concrete the heat generated by the epoxy grout during its cure will draw water from 2 to 3 inches below the surface causing a micro layer of water which prevents the epoxy grout from bonding to the concrete.

The new heavily reinforced upper block section is post tensioned to the lower block prior to pouring the epoxy grout cap. An adjustable support system is then grouted into the grout cap. Final machinery alignment is achieved prior to pouring the epoxy for the chocks. Once the epoxy has cured then the anchor bolts are torqued while the frame pull down is monitored with dial indicators. We normally see .001 to .003 pull down at bolt torquring due to seating of the machine. Excessive pull down should be checked to see if the alignment has changed. The anchor bolts should be re-torqued at 24 and 100 hrs and the pull down and alignment checked as well. We recommend a scheduled anchor bolt torque and alignment check every 6 months to 8 months of operation.

By implementing the repair plan as described above we have been able to take a 30yr old foundation, and upgrade it to the point where the machine can operate productively for another 30yrs.


M&M Precision Grouting P.O. Box 1873 - Sherman, Texas 75091
903.868.2711 - Fax  903.868.0189