Taking natural gas to market involves a highly integrated infrastructure of pipelines, processing stations, delivery/receipt points and transmission stations. Gas compressors, an important component in this network of transmission stations and storage facilities, are also integral to the operation of refineries and gas processing plants throughout the U.S. Maintaining these facilities is crucial to ensure a continual supply of gas to U.S. and foreign markets, and maintaining gas compressor foundations is a key component in operator maintenance programs.

Although compressors have been in use since the beginning of the 20th century, a large surge in the compressor and pipeline industry occurred in the late 1940s and 1950s. In the U.S., many compressor foundations built during that time are still in service today. These foundations, which support equipment vital to the gas processing industry, often are neglected well beyond their intended design life.

Many foundations were designed with the notion that the compressor itself would wear out prior to the foundation. Over the years, as the compressors were replaced, the foundations were only retrofitted for the new equipment and not upgraded. It is common for many of these equipment foundations to be in use for 10-to-20-years or more past their original intended design life. As a result, facilities on the Gulf Coast, the Rocky Mountains, and along the 300,000 miles of gas pipelines are subject to increased unplanned maintenance and potential foundation failures.

Not only are compressor foundations in service longer than originally anticipated, most were designed according to static conditions with minimal reinforcing steel and standard- strength anchor bolts. Nearly all of these foundations are showing signs of deterioration due to fatigue failures and most are in dire need of rebuilding.

Foundation and Compressor Types

Completed skid installation

Several types of foundations support industrial compressors. Concrete blocks on mat foundations and blocks on piers or tabletop foundations support many older or smaller machines. In contrast, most of the newer high-speed, high-horsepower compressors utilize a steel skid or a hybrid steel-skid-and-block on a mat. The foundation design is affected by many factors including the installation location (onshore or offshore), soil type, machine size, expected design lifespan, transportation constraints to the final location, or duration of service at a particular location.

Industrial compressors are classified by flow type, either intermittent or continuous. Intermittent flow compressors, also called positive displacement compressors, include reciprocating and rotary types, while continuous flow compressors include centrifugal and axial flow types. Reciprocating compressors typically are used where high compression ratios are required without high flow rates and the process fluid is relatively dry. Rotary types are specified primarily in compressed air applications, though other types of compressors are found in air service. Centrifuge compressors are used most often for wet gas compression, while axial flow compressors are utilized in high-flow, low-compression ratio applications.

Regardless of the type of foundation or compressor used to move gas in a process unit of a plant or a transmission pipeline station, the structural support is the lifeline of the system. These foundations are often weathered and aged from years of service, but only receive needed attention due to a major failure. Several conditions can cause failure including increased vibration levels due to lack of adequate support and dampening, as well as differential deflections that cause internal parts to wear out quicker than the expected lifespan. A foundation that does not support the equipment properly and cannot overcome the shaking forces will directly influence the cost of maintenance and the frequency of the mechanical repair cycle.

These failure events and related long-term costs can be readily mitigated using existing technologies. Strengthening or replacing older foundations reduces the maintenance cycle, as well as reducing planned and unplanned downtime, allowing facilities to realize real cost savings.

Rock drilling layout for additional reinforcing steel installation

Early Foundation Designs

During the early-to-mid-20^th-century, compressor foundations were designed with only static loads in mind, not accounting for related dynamic conditions or pulsations. The static designs featured minimal reinforcing steel around the perimeter of the foundation and at the connection between the block and the mat foundation. Inadequate by today's standards, these designs are susceptible to early fatigue and failure and often result in fatigue-related cracking.

Another characteristic of older compressor foundations is the anchor bolt design. Historically, standard anchor bolts, known as J-Bolts or L-Bolts, were constructed of standard strength steel rather than the higher strength materials used today. Even following proper design for embedment depth, the shape of the J- or L-Bolts can lead to failure by straightening and pulling out of the concrete, rather than the bolt itself or the concrete failing.

In addition to poor design conventions, many older foundations did not take into account the oil pans beneath the main frame of the compressor. The oil pans channel the oil away from the foundation preventing a possible spill during maintenance work. Many foundations, as part of the design, have an oil pan depressed into the foundation.

While the oil pans offer many maintenance advantages, they often are a weak point in the foundation strength. Oil-pan foundation design that does not take into account the foundation geometry often has heavy cracking, propagating at 45-degree angles, at the oil pan corners. Because these designs did not consider the concentrated forces in the foundation, they are inadequately reinforced in the direction perpendicular to the cracks.

Additional reinforcing steel and battered post-tension bolts

Grout design was also primitive in the early-to-mid-20^th-century and affects the performance of foundations for rotating equipment. A precision, non-shrink grout should be used on rotating equipment so that the void between the reinforced concrete foundation and the equipment-bearing surface is completely and permanently filled.  Concrete has a tendency to shrink as it cures and therefore will leave a small void or cause a misalignment of the equipment.

Cementitious non-shrink precision grouts have been used for static and light dynamic loading, or for high-temperature exposure. Prior to 1955, cementitious grouts were the only choice for grouting rotating equipment. In 1955, epoxy grouts were developed for use in the petrochemical industry for applications such as reciprocating gas compressors and for rotating equipment in caustic areas. Epoxy grout has a much higher compressive strength compared to its cementitious counterpart and its real value comes from the resilience to lubricant oils and chemicals, and the ability to endure impact from vibrating equipment. 

Failure Modes of Foundations

Surface preparation with mechanical anchors

Grout - Typically, compressors run continuously until there is a need for repairs during an emergency or a planned outage. Because of continuous usage, components often must undergo maintenance and replacement every year. Many older machines have a full-bed grout that provides continuous support under the entire machine. Sometimes, as the machines undergo maintenance, they are removed from the foundation and then replaced after the machining work. 

In order to replace the machine on the same foundation, the grout is chipped off and the concrete surface exposed and prepared for regrouting. During the preparation of the new concrete, some good concrete is removed, thus decreasing the elevation of the concrete and increasing the thickness of the grout when the equipment is reinstalled. Over time, with each sequential epoxy grout replacement, the grout beds thicken.

While the compressive strength of epoxy grout is much higher than typical concrete, often by a scale of 3-to-4-times, the modulus of elasticity is also higher, causing the epoxy to creep (permanent deformation under loading and temperature) at a much faster rate than concrete. The differences of the movement between the two materials can increase wear on machinery parts, causing misalignment with the driver, among other problems. Once the epoxy grout is 12-inches thick, the lost concrete should be replaced and elevated so that the epoxy grout will be no thicker than 3-to-4-inches. This will ensure a firm footing for the compressor with as little material creep as possible.

Completed sole plate regrouting

Chlorides - As with any concrete foundation, compressor foundations deteriorate at an accelerated rate with the introduction of chlorides. During placement, fresh concrete can have chlorides cast into it from the water used for mixing the concrete or from the aggregates used to make the concrete. Chlorides can also become present in cured concrete through an external source such as ice melting from salted roadways or from sea mist.

The chlorides permeate through the concrete matrix and when kept in a moist, oxygen-rich atmosphere, oxidize the reinforcing steel. The oxidized rust can increase the volume of the steel by as much as 8-times, creating large tensile stresses inside the concrete.  These tensile stresses are manifested in concrete cracks, delaminations and spalls. Once the concrete cover over the reinforcing steel is lost, the deterioration rate increases due to the unprotected steel that is open to the atmosphere.

Voids and Cracks - The epoxy grout cap and the concrete are two separate materials, which must work together to support the equipment. The bond between the two materials is a critical part of the performance of the foundation. If the bond between the two materials is compromised and the grout is delaminated from the concrete, vibrations will not be dissipated by the foundation as efficiently and problems can worsen. In addition, the void can fill with oil or other contaminates, causing an increased rate in concrete deterioration and worsening the bond between layers.

Voids and cracks act as channels for chemicals to enter the foundation and attack the reinforcing steel causing rust and, ultimately, delaminations and spalls of the concrete.  Epoxy grout that is not bonded properly to the concrete can also fail due to edge lifting around the perimeter of the foundation. One method of ensuring a proper bond is to install mechanical anchors, also called wickets, around the perimeter of the foundation.  The wickets provide a way for the grout to interlock and increase the bond to the concrete substrate.

Compressor Parts - Integral parts of standard compressor design can also lead to failure in foundations. Sole plates and compressor frames with sharp edges on the underside and corners can create stress cracks in the epoxy grout. As the sole plate or compressor frame expands due to heat and/or vibration, strains on the grout and the sharp edges create concentrated stresses in localized areas that crack the epoxy grout caps. The common mitigation method is to radius the underside and corners to distribute the forces and reduce the stresses on the grout, or to create a small expansion joint around the perimeter of the sole plate to allow for small horizontal movements.

New Repair Materials and Concepts

Completed compressor mainframe regrouting with application of Superbolt's stud-bolt tensioners for securing frame

Concrete - Whether a new foundation or a repair, designers should specify the required concrete tensile strength to ensure the structure has sufficient strength to resist the dynamic forces. Standard concrete cannot receive epoxy grouts for approximately 28-days due to excess free water in the concrete. The foundation repair concrete should be a fast-setting repair mortar with the ability to receive an epoxy grout within 24-hours. These concretes are especially useful for foundations that will be repaired during a planned or unplanned shutdown, as they will greatly reduce the time necessary for the repairs.

Steel Fibers - Steel fibers, placed in the concrete at the time of mixing and cast into the concrete, help control plastic-shrinkage cracking and increase the tensile strength of the concrete. The increased tensile strength of the concrete allows the foundation to have more resistance to the cracking caused by the shaking forces of the compressor.

Anchor Bolts - Anchor bolts for compressors are typically made of high strength steels such as B-7, all thread in the range of 105-120 tensile ksi. In earlier-designed compressor foundations, comparatively standard J-Bolts and L-Bolts were typically 36 tensile ksi. When designed properly, the increased strength of the bolts allows for greater clamping force of the machine to the foundation or a smaller bolt diameter. 

Research of compressor anchor-bolt design has shown that a termination point at the bottom section of the anchor bolt cast into the concrete will reduce the local tensile stresses in the concrete. An industry standard practice is to use a heavy hex nut, economical and readily available, as the termination point for an anchor bolt.

Spherical Washers - Spherical washers should be used whenever anchor bolt preload is critical, such as on vibrating equipment. Spherical washers allow the anchor bolt clamping forces to be transferred uniformly across the bearing surface, rather than concentrating at one side of the washer and nut if the anchor bolt is slightly misaligned. Spherical washers offer an economical method of ensuring the anchor bolt loads are transmitted to the foundation.

Epoxy Grouts - Epoxy grouts provide the most effective transfer of static and dynamic loads from heavy equipment to the foundation. Epoxy grouts are also impervious to the chemicals and lubricant oils typically used in rotating equipment. Because the linear thermal expansion coefficient of epoxy grouts is typically 2-to-4-times that of standard concrete and steel, appropriate material properties must be designed into the expansion joints.

Viscosity is another important property in choosing the appropriate epoxy grout, specifically viscosity during installation. A product that has desired properties during final cure, such as a high compressive strength and low creep, may not have a viscosity that will allow for straightforward installation. In this case, the epoxy will not make adequate contact with the entire bearing area of the equipment. 

Repaired reciprocating compressor foundation

Epoxy grouts range in compressive strengths from approximately 12,000 psi to 20,000 psi, much higher than precision cementitious grout used prior to the development of epoxy grout. Epoxy grouts, which offer a much greater dampening effect than concrete or cementitious grouts, also are an excellent choice for final grouting of rotating equipment.

Sealants - Oil resistant sealants, such as an RTV silicone sealant, are a cost effective solution for seal expansions joints, chock perimeters and the epoxy-chock interface with the compressor frame. RTV silicone will prevent oil and other contaminates from penetrating into the joints and interfaces of the foundation.

Post-Tensioning - Post-tensioning of a concrete block on a mat foundation adds considerable confining support to the foundation.  A reciprocating compressor on a block foundation oscillates as the machine operates and creates shaking forces and pulsations that must be resisted by the foundation. These shaking forces induce cycles of compression and tension as the compressor reciprocates. Cracks in the concrete form when the tensile force exceeds the tensile capacity of the concrete or during original construction due to plastic shrinkage cracking. The cracks will propagate as the tensile forces increase. As a crack opens, the reinforcing steel bridging the crack engages and the cracking can be limited. 

Properly placed post-tensioning will induce compression on the concrete block. The vibrating forces, i.e. tension, transferred to the block foundation must overcome the compression forces created by the post tensioning to engage the concrete in tension and create cracking. Post-tensioning a block foundation is a cost effective way of limiting the cracking and increasing the durability of the structure. The most common post- tensioning method is casting bolts or cables into the concrete. Post-tensioning also can be used to add confining force to the block foundation or to tie the block to the mat.

Foundation Repairs: Total or Partial?

Not all compressor foundations require total replacement. Often, foundations with minor deterioration may be repaired with high performance concretes, epoxy grouts and chocks. Deteriorated anchor bolts can be cored out and replaced with high-strength anchor bolts, and in some applications, it is possible to add post-tensioning systems to strengthen the foundation. With the proper team and sufficient planning, many partial repairs can be completed quickly.

Smaller compressor foundations that have heavy deterioration in the concrete and epoxy grout should be removed to the mat foundation and rebuilt rather than repaired. A total replacement allows many changes to be made including increasing the size of the anchor bolts or increasing the density of the reinforcing steel, changing the concrete material, adding post-tensioning systems, and installing epoxy chock systems in place of a full grout bed. For easy maintenance, replacement foundations should have an oil pan installed below the mainframe and adjustable or epoxy chocks used to support the machine. If the machine needs to be removed from the foundation for future maintenance, epoxy chocks can be easily replaced at minimal cost and time.

Foundation Repair Specialists

With any type of repair or upgrade to high-performance foundations, it is important to engage a contractor that understands concrete repair as well as has experience with repair systems and their application. Concrete specialists use repair methods that incorporate new technologies to strengthen and add serviceability to compressor foundations. Many of the products, materials and application methods used to upgrade these structures are unique to the industry and require the installation contractor to have extensive experience. Investing money in the foundation to restore its competence can decrease the maintenance frequency and long-term cost. A solid foundation allows a facility to plan for maintenance cycles because internal compressor parts will not prematurely wear out due to excessive vibrations or from misalignment problems.

For additional information, contact Structural Preservation Systems, Houston Operations, 1003 Clay Ct., Deer Park, Texas 77536. (281) 478-5300.