How to Improve Pump Reliability
By Eugene Vogel, EASA
Most control and efficiency improvement are key to successful plant operations – especially when it comes to volatile energy costs. Most plant managers are keenly aware that upgrading from standard motors to premium-efficiency designs can...
November 1, 2011
By Eugene Vogel, EASA
Most control and efficiency improvement are key to successful plant operations – especially when it comes to volatile energy costs. Most plant managers are keenly aware that upgrading from standard motors to premium-efficiency designs can save 4% to 6 % on the electric bills for appropriate applications. Yet few realize that simple upgrades can improve pump efficiency 10% to 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.
Life cycle costing
Centrifugal pumps may be less than 50% efficient, but have the potential to improve by 20% to 30% through various upgrades and system changes, according to the US Department of Energy . To create awareness of these facts, the agency is promoting life cycle costing (LCC). As Figure 4 shows (see pg. 19), the acquisition price of a 22 kW (30 hp) pump, that runs 6,000 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.
Pump upgrades usually address maintenance and repair issues as well as efficiency, so they can extend pump life and reliability, prevent expensive product from going down the drain, and even help avoid spills that can trigger costly cleanups and fines from regulatory agencies such as Environment Canada. In many cases, the reduced downtime afforded by the upgrade outweighs all other benefits.
In lean times, it may be difficult to allocate scarce funding to upgrading rather than just repairing a pump. But, as the example illustrates, the payback can be very quick. The resultant savings will then free up funds that otherwise would go for wasted energy and more expensive repairs later. If a pump is already scheduled for service or requires emergency repair, the incremental cost of upgrades may be minimal.
Plant expansions and new installations are also good opportunities to specify pumps with upgraded efficiency and reliability features. Most pump manufacturers can accommodate such specifications, and qualified service centres can usually upgrade off-the-shelf pumps as required.
For most common pump applications, coating upgrades are among the best ways to improve performance and increase the mean time between failure (MTBF). Though they may look like paint, their purpose has little to do with appearance. Instead, they are designed to reduce friction (thus increasing efficiency) or to resist erosion and corrosion (thereby improving reliability and extending pump life). In some applications, they accomplish both objectives.
Typically, coating upgrades are applied to the rough surfaces of the impeller and the volute to improve efficiency while protecting these relatively soft brass or cast iron parts from erosion and corrosion.
Abrasive wear is easy to spot when a pump is disassembled for repair or overhaul, but is often regarded as ‘normal for the application’. In fact, some plant managers think short pump life is so ‘normal’ that they routinely replace pumps at regular intervals. Most of the time, however, erosion-resistant coatings for the wet end could extend pump life and therefore significantly reduce the operating costs. Often the coating can even be reapplied periodically for much less than it would cost to replace the pump.
Depending on the application, coating upgrades may also be beneficial inside bearing housings or even on external pump surfaces.
Bearing housings: Coating the inside of the pump bearing housing can prolong bearing life by eliminating a destructive contaminant – rust. Even moderate contamination, according to SKF, will shorten the life of oil-lubricated bearings by a factor of four. During the repair process, rust can form on bare cast iron and steel surfaces within a few hours after parts have been cleaned and then exposed to moisture in the atmosphere. Even after assembly, moisture in the air can oxidize inside surfaces. Applying the appropriate paint or industrial coating can prevent rust from forming and being washed into the bearing by lubricating oil.
External pump, base and related equipment: No one would coat the outside of a submersible sewage pump just so it will look good. But applying a good protective coating now may mean the difference between reasonable reconditioning costs and expensive replacement the next time the pump is pulled up for service.
Choosing the right coating: Any of the coatings designed for pump wet-end applications will provide a smoother surface that reduces friction, even if the primary objective is protection against wear and corrosion. But some, including a number of polymer coatings, are specially designed to cure to a very hard, durable and slick surface that further reduces friction losses. The result is a pump that operates more efficiently and resists corrosion and erosion. Where energy efficiency is important, careful selection of the coating material will pay dividends in reduced operating expense.
Even small efficiency gains can have a big impact on operating costs. For example, consider a cooling water pump that moves 172 cu m/hr at 69 m of head and is 70% efficient (the assumed energy cost is $0.10/kWh).
(kW=cu m/hr x m x s.g.)/(480 x Efficiency) = (172 cu m/hr x 69 m x 1)/(480 x 0.7) = 35.32 kW x 1.341=47.36 hp
Annual energy costs =35.32 kW x 5,000 hrs x $0.10=$17,660.
Coating the inside of the pump volute could increase the efficiency to 75% and reduce the energy costs to $16,483, saving $1,177 per year.
Of course, some coating upgrades are worthwhile strictly from the standpoint of improved reliability and extended pump life. For most water and waste-water 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 byproducts.
Choosing the right 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 centre helpful when considering pump coating upgrades.
Pump curves: 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 will tip the curve up, effectively providing more flow for the same head (see Figure 5, pg. 20). This will save energy on pumps that operate at full flow and cycle as needed.
Coatings that reduce friction work on the flow side of the head-flow re
lationship in centrifugal pumps. Therefore a high-flow, low-head pump would benefit significantly 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, thus saving energy.
However, reduced-friction coatings will provide no energy savings on 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. (Of course, pumps in these systems 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 waste-water industries, refineries and petrochemical plants, and HVAC circulating applications. The 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: waste-water 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.
Surface preparation: Whether the bond is structural (mechanical) or chemical (valence), the coating must adhere to the pump to be beneficial. That means the surface must be prepared properly (see Figures 1, 2 and 3).
A strong mechanical bond requires a microscopically rough finish, which usually is obtained by blasting with sand or a grit like Black Beauty that can achieve an 0.08 mm (3 mils) anchor profile. Chemical bonding depends on sharing of electrons at the molecular level, so the surface must be free of contamination and blasting residue. In either case, the coating should be applied as soon as possible after the surface has been prepared.
Products: Most hard-finish coatings for pump impellers and volutes are paste-grade products that are spread or trowelled 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 are also available, most of which are epoxy paints.
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 the coatings applied by a qualified service centre.
Price is always a significant factor in pump selection and usually 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, however, it is easy to see the value of properly designed and applied surface coating upgrades. Coatings that reduce friction and thereby improve efficiency can be justified by reduced energy usage when operating conditions meet the criteria discussed above.
The significant costs of maintenance and repairs 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.
Eugene Vogel is a pump and vibration specialist at the Electrical Apparatus Service Association Inc. (EASA), St. Louis, MO. EASA is an international trade association of more than 1,900 firms in 58 countries that sell and service electrical, electronic and mechanical apparatus. For more information, visit www.easa.com, where you can find a list of Canadian chapters and members.
To view the full layout of this article with images, as it originally appeared, see page 18 of the April 2012 issue. The digital edition of this issue can be found here: