Corrosion-resistant polymer concrete technology provides better alternatives to conventional pump support systems in refining, petrochemical, and pulp and paper facilities.
Pump foundations and baseplates have always posed a challenge to plant maintenance personnel in refining and petrochemical facilities. Dealing with corrosion damage to metal baseplates and repairing corrosion damage to the concrete matrix of pump foundations - and preventing future damage from occurring - requires ongoing vigilance. Making repairs to conventional concrete pump foundations is labor-intensive and time-consuming, often causing unacceptable delays in returning equipment to service. Moreover, acid-proofing foundations for corrosive pumps under time-critical field conditions is difficult, and frequently prone to failure.
Conventional pump foundations typically consist of steel-reinforced concrete, to which a pump baseplate is anchor-bolted and grouted. The baseplate is drilled and tapped to accept the pump, motor and accessories. The mounting surfaces for the pump and motor are typically machined to a flatness of 3 mils/ft. to facilitate pump and motor alignment.
One of the drawbacks of metal baseplates is the possibility of warping the baseplate during installation onto the concrete foundation. If warping occurs, a condition known as "soft foot" results, which can be severe enough to require the baseplate to undergo costly field machining to return it to the required flatness.
For pumps that are used to circulate corrosive materials, incidental flange leaks and pump seal leaks expose not only the concrete foundation to harsh chemical attack, but also the baseplate. In time, this corrosion destabilizes the foundation and baseplate, which in turn causes increased vibration in the pump and motor supported by the foundation. Excessive vibration can cause the pump seals to leak even more, accelerating the corrosive attack and system destabilization ... and ultimately resulting in premature system failure.
Benefits of Polymer Concrete Material
A more satisfactory option for a pump foundation system takes advantage of the unique properties of polymer concrete. Certain relatively inert aggregates blended into thermosetting resins such as epoxy or vinyl ester, then properly cured, produce materials with high compressive, flexural, and tensile strengths with inherent resistance to a wide range of corrosive materials. The polymer concrete is prepared, poured into molds, and cured under controlled conditions to produce pump foundations with consistent properties. In order to minimize cost, the pump foundation is precast as a hollow box, typically with a 1.5-in. wall thickness, with a fill hole in the top wall to receive the grouting.
Because properly formulated polymer concrete exhibits very little shrinkage during the curing process, a pump foundation can be precast with metal mounting pads or threaded alloy inserts set at precise locations, thus allowing the pump, motor, and accessories to be mounted directly to the foundation. In this way, the precast box becomes not only the foundation for the pump, but also the baseplate fabricated as a single unit.
For pumps that handle corrosive materials, polymer concrete allows fabrication of a variety of foundation/baseplate combinations using minimal quantities of expensive chemical-resistant metal alloys, supported in and bonded to the polymer concrete. Up until now, conventional technology required baseplates for pumps in many corrosive services to be fabricated entirely from exotic alloys. This was very costly. Now, because of its unique properties, chemical-resistant alloy-enhanced polymer concrete can be used to replace most of the expensive alloys used for fabricating conventional baseplates, as well as for replacing the conventional acid-proofed concrete foundation in a single unit with corrosion-resistant material. Because polymer concrete is inherently rigid - and due to the hollow box design of foundation/baseplates - the potential for warping commonly associated with the installation of metal baseplates is virtually eliminated.
Speeding up the Foundation Repair Process
Precast polymer technology can be used to install a completely new foundation, or to repair a damaged existing foundation in a fraction of the time required for conventional repairs. Traditional foundation repair is a multi-step process wherein the existing pump, motor and baseplate are removed, the damaged concrete foundation is demolished, forms for the replacement foundation are installed, and repair material is poured into the forms. The repair material is allowed to cure, the forms removed, and the baseplate anchor-bolted to the foundation. Forms are then placed around the baseplate and grout is installed into the baseplate. After the grout has cured, these forms are removed.
Typically, one final step is then required: that of acid-proofing. It is during this last stage that conventional foundations in corrosive service often fail. It is a difficult challenge to seal the interface between the metal baseplate and the concrete foundation effectively. Differential movement plus equipment vibration often cause cracks to develop in the protective coating or acid brick mortar along the interface. Such a breach allows corrosive material to intrude behind the acid proofing and degrade the underlying concrete foundation, ultimately causing system failure.
When employing a precast polymer concrete foundation to repair a damaged existing foundation, the pump, motor and baseplate are removed, and the unsound concrete is chipped away with pneumatic chipping guns to expose the steel reinforcement in the foundation. The precast polymer concrete shell is then lowered over the existing prepared concrete foundation. The pump is bolted to the top of the precast foundation, and shims are placed under the bottom edges of the precast to ensure matching of the suction and discharge piping to the pump flanges.
If the unit turns out to be too high in elevation to match up the flanges, the unit can be trimmed in the field using a diamond blade and worm gear drive saw. The space between the inside wall of the precast foundation and the old concrete foundation is then filled with cementitious grout extended with pea gravel, using the precast grout fill hole. After the grout has cured (usually overnight), the grout fill hole is sealed with a precast polymer concrete plug with the same resin used to fabricate the precast foundation. The motor and accessories are bolted to the top of the precast unit, and the pump is returned to service. This type of repair can usually be completed in one or two days, depending on site conditions. By contrast, a conventional repair can take six days or more to complete.
Installing New Pump Foundations
In using precast polymer concrete technology to install a new foundation, a steel rebar cage is doweled into the concrete slab or footing - as would normally be done when forming and pouring a conventional concrete foundation. The precast polymer concrete foundation is then placed over the rebar cage, leveled and filled with standard concrete. This installation typically requires only a few hours to complete, and saves appreciable time in new plant construction.
Precast polymer concrete pump foundations are used mostly for standard ANSI pumps, since these pumps are commonly used in corrosive service. For this reason, standard-sized reusable molds are used to precast ANSI standard combination foundation/baseplate units. However, many end users are installing these units in non-corrosive services in new construction for enhanced constructability reasons alone.
Precast polymer concrete allows a further refinement of the technology in formulating removable, single-piece, motor-specific precast blocks, which are bolted onto the motor end of the precast units. These motor blocks have precisely positioned specially fabricated threaded inserts cast integrally into the polymer concrete body of the block to match the bolt patterns of standard NEMA frame motors. Changing to a different-sized motor for a given pump becomes a mere changeout of precast motor blocks.
The unique characteristics of polymer concrete allow greater versatility in designs. One example is the motor adjusting devices, favored by millwrights, that facilitate motor and pump alignment in the field. Polymer concrete also allows cast-in drip catch pans with integral drain fittings to be incorporated at the pump end of the precast unit, so that leaks can be collected and piped off to a collection basin or trench.
Vibration Signature Testing
Vibration signature testing is commonly used on rotating equipment to diagnose and predict pending failures with pump and motor operation. Readings are typically taken in three planes (horizontal, vertical and axial) and reported as velocity amplitudes in in./sec at various frequencies. It is generally accepted that the lower the velocity amplitudes on a vibration signature for a given pump system, the longer that equipment will operate before maintenance is required.
The vibration signatures observed on pumps mounted to precast polymer concrete foundations indicate that polymer concrete is a vibration dampener for rotating equipment. Velocity amplitudes in the range of 0.10 in./sec are considered desirable by rotating equipment engineers, but results as low as 0.005 in./sec have been reported on signatures from pump installations on precast polymer concrete foundations. (Pump and motor alignment must still be properly executed - and any pipe strain must be removed - or the signatures will not be acceptable in any case.)
The properties of polymer concrete also allow the fabrication of larger custom-made precast pump foundation/baseplate combination units for non-ANSI pumps. In fact, it is possible to fabricate units as large as 4 ft. wide and 14 ft. long, with stainless steel mounting pads for the pump, and the motor integrally cast into the top. The pads are first milled flat and coplanar, then drilled and tapped for the specific pump and motor.
Combining the disciplines of civil engineering and mechanical engineering has resulted in the practical application of polymer concrete technology to provide corrosion-resistant, precast polymer concrete pump foundation systems, which are more cost-efficient and quicker to install. Over time, these new designs may go a long way toward eclipsing conventional pump support systems in refining and petrochemical facilities.
Larry Welch is Industrial Products Manager for Structural Preservation Systems, Inc., in Houston, Texas. His responsibilities include the Polyshield® Foundation/ Baseplate System for industrial equipment. He can be reached at (800) 738-7659 or lwelch@structural.net.