Most industrial centrifugal pumps carry a significant capital equipment value. Maintaining them to run close to their best efficiency point (BEP) saves in downtime, repair costs, and overall power consumption. Proactive maintenance extends the useful life of a pump, reduces breakdowns, increases longevity and reliability, and extends the mean time between failure.
When a pump operates outside its BEP, flow separation and turbulence occur. A common result of turbulence is cavitation, which can cause damage such as erosion and vibration. Corrosion and chemical attack also impact pump performance and longevity. All of these factors can cause a pump to fail prematurely and depreciate at an accelerated rate.
While damaged pump components can be replaced, the capital costs associated with replacement and spare parts inventory can be cost prohibitive. The most cost effective and realistic solution is protecting and/or renewing damaged pump surfaces.
Damage to pump surfaces is highly predictable, occurring most often on specific parts such as casings and impellers. To rebuild damaged equipment and provide ongoing protection, maintenance professionals are turning to polymer composite materials, a new category of aftermarket products that offer durability rivaling that of metal coatings.
Three main categories of polymer composite materials for equipment repair are 1) metal repair materials, 2) wear resistant coatings, and 3) chemically resistant coatings. Often used together to correct operating problems related to wear, these materials make repairing pumps significantly more cost effective than replacement.
Metal Repair Materials
Metal repair materials provide physical performance properties similar to or better than those of the base metal undergoing repair.
These polymer composites consist of an epoxy matrix combined with steel, aluminum, or ferro-silicon powder fillers that allow the material to mimic the physical properties of the metal including color, strength, rate of thermal expansion, and ability to be machined.
Metal repair materials are ideal for pump casings and impellers that experience tremendous wear from corrosion, erosion, cavitation, sliding abrasion, and mechanical damage. For example, during a recent scheduled pump teardown, maintenance personnel noticed voids in the pump's casing that ranged from 1/8-in pits to gouges from 1/2-in to 3-in deep. Wear to the pump's volute results in less efficient performance and shortened the pump's expected mean time between repairs (MTBR).
Ordering a replacement casing would cost approximately $11,000 and would require some period of downtime until the part arrived. Traditional hard face welding repair techniques involved a similar capital investment and required downtime of five weeks. To solve the problem quickly and cost effectively, the maintenance team chose to use polymer composite materials that would repair the damage on-site, in a fraction of the time and at a fraction of the cost of the other options.
The metal repair itself was quick and easy. Maintenance staff prepped the interior of the casing by sand blasting it to a near white metal finish. A high compressive strength, non-rusting metal repair material was trowelled on and left to cure for just 20 minutes.
Once the maintenance team applied the metal repair material, the pump was back to its original dimensions. The job required no specially trained applicators. (More details on this application example will come later.)
In general, metal repair materials have a putty-like consistency and are applied with a trowel. However, some flowable liquid formulations can be molded and poured. Cure times for these materials vary, ranging from a few minutes to 24 hours. Length of cure is extremely dependant upon ambient temperature. Typically, a pump treated with metal repair materials can return to service as soon as the material is fully cured.Using metal repair materials reduces time to complete the repair, minimizes downtime for the pump, and simplifies re-installation. Cost savings come from the inexpensive nature of these materials compared to the capital expenditures required for replacement parts.
Wear Resistant Coatings
Wear resistant coatings are polymer composite materials that act as sacrificial and renewable working surfaces and protect the structural integrity of pump components.
Highly filled with ceramic beads, powder, or fibers, these materials provide unparalleled resistance to sliding abrasion and cavitation that far exceeds the resistance of uncoated metals. One type of wear resistant coating provides a low friction finish that enables a pump to operate at its BEP while consuming less energy, thus lowering associated energy costs.
These coatings are often applied in several different colored layers to help monitor wear during preventative maintenance checks. As colors begin to show through the top layer of the protective coating, maintenance staff can determine the rate of wear and the necessary proactive maintenance steps.
Continuing the repair application example begun earlier, pits and gouges to the metal casing had been repaired in less than one hour using metal repair materials. The casing surface was now smooth and unpitted. In a proactive effort to combat future wear to the pump surfaces, the maintenance team went one step further. They applied two coats of a white brushable ceramic wear resistant coating to the inside of the casing, then two more coats of the same coating in gray. These smooth, low-friction coatings help resist future sliding abrasion and cavitation, and provide a visual wear indicator for upcoming scheduled maintenance.
The entire repair cost the plant just $500 and took a total of five hours. When the pump returned to service, its glass-smooth, low friction surface resulting from this repair reduced the internal casing friction so the pump could operate at its BEP and consume less energy.
Chemically Resistant Coatings
Chemically resistant coatings are composite reinforced epoxies formulated to resist some of the toughest chemicals in the industrial marketplace.
Certain chemicals can cause corrosion on either the interior or exterior of industrial equipment, resulting in aesthetic and performance problems. Exterior corrosion can also make it difficult to disassemble equipment for preventative maintenance checks.
While all epoxies offer some degree of chemical resistance, chemically resistant coatings protect in severe environments - such as applications where acids are the materials being pumped through the system. These products are especially useful for pumps in secondary containment areas where there is a risk of chemical spill.
Chemically resistant materials leave a smooth surface and are ideal for preventing corrosion on and around the footings of a pump or at its base, where compromised alignment can lead to potentially disastrous problems like vibration and bearing failure. On pumps used to transport caustic acids, coated interior components will prevent acids from eroding the pump from the inside out.
Chemically resistant coatings are typically brush or roller applied and have a much thinner consistency than wear resistant coatings. These polymer composites are basically thick, chemically resistant epoxy paints that do not use special fillers to achieve their performance properties. They can be used to coat virtually any device, including pumps, gearboxes, and motors, and will protect metal surfaces from moisture and chemical attack.
Chemically resistant coatings typically cure in 24 hours, depending upon ambient temperature. When applied to the outside of the pump, heat generated by the pump helps the coating to cure faster, allowing the pump to return to service very quickly. These coatings make routine maintenance easier and faster by eliminating corrosion, extending the useful life of the pump and increasing the time interval between repairs.
Repair or Replace?
Repair. Repair. Repair. By using all three categories of polymer composite materials during routine pump maintenance, facilities can greatly extend the operating life of their pumps and reduce capital and maintenance costs.
With all new equipment, maintenance teams should seriously consider applying wear resistant and chemically resistant polymer composite materials to appropriate interior and exterior pump surfaces. This simple proactive effort can minimize surface wear, premature failure, and rapid depreciation.
Pumps & Systems, April 2007