The success of rotating equipment often can be traced directly to the surface and structure quality on which it is mounted. The useful life of seals and bearings in most process pumps is compromised when shafts are misaligned or excess vibration is present.
Some estimate that more than 80 percent of sealed centrifugal pumps are pulled from service for an issue with a mechanical seal or bearing. A solid foundation can increase the mean time between planned maintenance for pumps and most other types of rotating equipment.
While many features and options can be incorporated into a pump baseplate, experts will typically agree on the following minimum requirements:
- Mounting surfaces should support the entire pump and driver continually through the operating range of the equipment.
- Baseplates should be stiff enough to resist distortion and deflection and maintain alignment of the pump and motor when installed properly per design.
- Driver and pump mounting-pad surface flatness should be parallel within 0.002 inch per foot (0.15 millimeters [mm] per meter).
- Final alignment should be maintained by allowing up to a 0.125-inch shim pack (3 mm).
- Mounting hole requirements, including size and location, should never present a bolt-bound condition that inhibits final alignment procedures.
Typical equipment baseplates and foundations can easily become compromised for a number of reasons.
Incorrect or failed grouting systems common with a hollow metal or cast-iron base can quickly cause excess vibration frequencies that vary by rotational speed, temperature, and equipment forces and moments.
Less expensive, non-reinforced metal bases can also warp and bend during shipment, requiring additional labor to straighten, or they are often installed with such deviations.
Corrosion and other forms of environmental deterioration of the mounting base or underlying foundation can create an unstable and varying mounting surface that can result in excess vibrations and cause misalignment of installed equipment.
Baseplate failures are often difficult to identify in the field but can be correlated to reoccurring performance issues in the mounted equipment. Identifying and eliminating hidden mechanical defects in baseplate systems can save time and frustration in root-cause evaluations.
Historically, baseplates have been constructed of metal and either cast or fabricated to meet specific design requirements. Inexpensive, hollow, cast-iron designs along with channel and bent/formed steel options often twist or diaphragm during shipment and may require surface preparation or large amounts of grout to complete the installation. Rarely do these designs maintain a tight flatness tolerance. More complex metal-fabricated designs provide increased rigidity and surface flatness but may still require a significant amount of grout to complete a reliable installation.
A popular alternative gaining ground in the pump industry is the use of solid polymer concrete baseplates. They provide the benefits that reliability engineers desire at an economical installed price point.
In addition to a flat vibration damping surface, the corrosion-resistant solid material can easily incorporate drip pans, containment rims, higher-alloy threaded hole inserts and alignment devices.
Often referred to as a polymer, granite or mineral cast baseplate, this material is a special combination of well-defined grain size aggregates bound to a thermoset epoxy or vinyl ester resin. Manufactured in a controlled environment during a transfer molding process, these materials can be cast to precision tolerances and offer high mechanical properties suitable for most industrial applications.
Polymer Concrete Benefits
The polymeric matrix of the material provides damping ratios that are 20 to 30 times greater per identical geometry than steel or cast-iron materials (see Figure 1). Rotating equipment is often associated with stability under the influence of various disturbances, and polymer bases can effectively minimize vibrations without increasing the component's mass size.
Cast mounting surfaces are typically flat within 0.002 inches per foot, which reduces soft-foot and speeds alignment. Threaded insert holes are typically cast in place or located with a computer-numerical-controlled machine to ensure precise location and perpendicularity, which eliminates bolt-bound conditions often occurring with inexpensive metal bases. Difficult-to-machine features can easily be cast in place, and maximum design flexibility allows for retrofit to any existing concrete foundation.
Advanced resin systems along with inert natural aggregates provide excellent protection from many fluids that would otherwise require an expensive alloy or corrode standard cast iron or steel. Polymer concrete systems eliminate the need for additional protective coatings or periodic upkeep and maintenance activities.