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Cost control and efficiency improvement are key to successful plant operation—especially when it comes to volatile energy costs.
Most plant managers are keenly aware that upgrading from standard motors to NEMA Premium Efficient designs can save 4-6 % on their electric bills for appropriate applications. Yet, few of them realize that simple upgrades can improve pump efficiency 10-15% or even more. Often, these upgrades yield significant savings on pump maintenance and repair as well, which translates into less downtime and reduced environmental risk.
Available pump upgrades include custom coatings for internal and external parts, shaft and seal modifications, bearing isolators and bearing-housing breathers. Of these, custom surface coatings typically provide the fastest return on investment. When evaluating coating upgrades, however, it is critical to understand pump basics, as well as the pump’s operating environment and system requirements.
According to the U.S. Department of Energy (DOE), centrifugal pumps may be less than 50% efficient—but have the potential to improve by 20-30% through various upgrades and system changes. To create awareness of these facts, the agency is promoting “life-cycle costing” (LCC). As Fig. 1 shows, the acquisition price of a 30 hp pump that runs 6000 hrs/yr for eight years is a fraction of its life-cycle cost. Energy is by far the most significant cost, but maintenance and repairs total more than twice the purchase price.
Of course, some coating upgrades are worthwhile strictly from the standpoint of improved reliability and extended pump life. For most water and wastewater applications, a good enamel may be adequate, whereas specific epoxies may be required for potable water applications. Epoxies or other polymer coatings may also be necessary for harsher environments involving abrasives, acids, caustics, salt and petroleum by-products.
Choosing which pumps to upgrade
Not all pumps will benefit from reduced-friction or corrosion-resistant coatings. Appropriate upgrade decisions require both detailed knowledge of the application and a basic understanding of pumps and pump curves. Even experienced plant personnel may find the professional advice of the pump manufacturer or a qualified service center helpful when considering pump coating upgrades.
When evaluating pumps for coating upgrades, pay close attention to the pump curves of the units under consideration. Pump curves relate the head and flow that a centrifugal pump will produce to its efficiency and required input power. As pump head increases, flow decreases—until at maximum head (shutoff head) when flow is zero. If a pump is operating correctly, head and flow will intersect at some point along the curve. Efficiency dictates the shape and location of the curve, so any improvement in efficiency (including improvements as a result of reduced-friction coatings) will tip the curve up, effectively providing more flow for the same head (see Fig. 2). This will save energy in pumps that operate at full flow and cycle as needed.
Coatings that reduce friction do so by working on the flow-side of the head-flow relationship in centrifugal pumps. Thus, a high-flow, low-head pump would benefit greatly from a reduced-friction coating, whereas a pump with very high head and low flow would not.
Reduced-friction coatings can save energy in systems that regulate flow with variable-speed drives. In these cases, more efficient pumps can run slower and still produce the same head and flow, thereby saving energy.
On the other hand, reduced-friction coatings will provide no energy savings on those systems that use a modulating discharge valve to achieve a specific flow (e.g., heating and cooling loads). In such systems, the valve increases (or decreases) friction in the discharge line, which increases (or decreases) head to control flow. If pump efficiency were to improve (providing more flow), the discharge valve would close to compensate, recreating the friction loss that the coating eliminated. (Pumps in these systems, however, may still benefit from coating upgrades that extend pump life and improve reliability by resisting corrosion and erosion.)
Good candidates for reduced-friction coatings include pumps used in municipal water and wastewater industries, refineries and petrochemical plants and in HVAC circulating applications. Coatings are less beneficial—in terms of energy savings—for pumps that operate intermittently.
Industries with high abrasive-wear applications that would benefit from corrosion- or erosion-resistant coatings include: wastewater treatment and power generation that typically handle slurries; power generation that uses river or lake water for cooling; and mining operations that have numerous dewatering and production pumps that are subject to sand and gravel.
Depending upon the application and type of upgrade being considered, it is often best to first perform a cost-benefit analysis based on known energy costs, the projected efficiency gain, expected pump life and upgrade costs.
Unfortunately, first-cost or purchase price still seems to be a major factor in pump selection—and often rules out any “extras” that could extend pump life or improve its efficiency or reliability. When facing the reality of high operating costs for energy and repairs caused by erosion and corrosion, though, it is easy to see the value of properly designed and applied surface coating upgrades. Coatings that reduce friction and, thus, improve efficiency can be justified by reduced energy usage when operating conditions meet the previously discussed criteria.
The costs of maintenance and repairs—often, significant costs, that is—represent additional opportunities for savings. Pump upgrades that improve reliability provide benefits in reduced downtime and reduced environmental risk. The critical factor in evaluating most upgrade options is to understand the operating environment and the system requirements. When a pump upgrade is a good fit, the investment is easy to justify and will continue to pay dividends for the long run. MT
Most hard-finish coatings for pump impellers and volutes are paste-grade products that are spread or troweled on thickly enough to cover surface irregularities and resist abrasion. Many of them are available in thinner grades that can be brushed on. A few spray-on coatings, most of which are epoxy paints, are also available.
Polymer coatings are two-part systems that require accurate and thorough mixing. Most of these will cure at room temperature, although moderate heat can accelerate the process. Some polymer coatings contain catalysts that cure at lower temperatures for use on site in cold weather; others are specially designed for application on wet surfaces. Always follow the manufacturer’s recommendations or have pump coatings applied by a qualified service center.
Eugene Vogel is a pump and vibration specialist at the Electrical Apparatus Service Association (EASA), in St. Louis, MO. Tele: (314) 993-2220; Fax: (314) 993-1269. (Editor’s Note: EASA is an international trade association of more than 2100 firms in 58 countries that sell and service electrical, electronic and mechanical apparatus.)
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