Gene Vogel
EASA Pump & Vibration Specialist
A challenge to all service centers is finding ways to differentiate themselves from every other “fix-it” facility. To the extent that service centers can offer additional services to their customers beyond basic repair, they will be able to get their customer’s attention and stand out as a value-added provider. In the arena of pump repair, there are a number of modifications that can be implemented during overhaul that will improve pump performance and reliability. The icing on the cake is that providing these upgrades can represent a source of high-profit revenue. Following are several options to consider:
Surface coatings
Centrifugal pumps can benefit from advanced coating processes, both inside and outside. Specific areas include the inside of the pump volute, the impeller, the inside of the bearing housing (power frame), and the exterior of the pump. Each area is discussed below.
Pump Volute Coatings – Smooth coating the inside of the pump volute reduces a major source of friction loss and can improve efficiency by five to ten percent. See Figure 1. Centrifugal pumps have notoriously poor energy efficiency. Best Efficiency Points (BEP) are often below 80% and actual operating points average only a little above 50%. Translated into dollars, improving efficiency just five percentage points on a 25 hp centrifugal pump could save thousands of dollars annually. (Improving pump efficiency from 60% to 65% at $0.10/ kW-hr for 24-7 operation equals $2,095 in annual energy savings.) End users benefiting from these savings can justify additional cost of hi-tech, polymeric coatings applied during an overhaul at the service center.
Impeller Coatings – As with smooth coating the pump volute, efficiency can be gained by smooth coating the impeller. Care must be taken on low specific speed impellers (high head, low flow) that coating thickness is kept to a minimum and doesn’t restrict passageways. In addition to improved efficiency, an impeller which may be subjected to cavitation damage can be protected by hard coating the impeller. This will also increase mean time between failure (MTBF) and reduce operating costs.
Coating The Interior Of Bearing Housing – Cast iron housing will develop oxidation (rust) during storage. Moisture in the air is sufficient to cause noticeable oxidation deposits with only a few days of exposure. During operation, deposits are washed into the lubricating oil and make their way into the bearings, accelerating bearing wear. (Grease lubricated bearings are less susceptible to this type of contamination ingress.) Coatings should be impervious to detergents which may be present in organic and synthetic lubricating oils. This simple upgrade performed during overhaul will improve MTBF and reduce operating costs for applicable pumps.
Coating Exterior Of Pump – “Ordinary” enamel paints used by most service centers are adequate for pumps used in “ordinary” operating environments. When a pump is operating in adverse environments, specialized coatings will preserve the integrity of the pump frames, bolts and related hardware. Epoxy paints are a good option for many wet environments found in water and waste water applications. Specialized paints or coatings may be engineered for more aggressive chemical environments.
Shaft strength modifications
Internal pump hydraulic force causes shaft deflection, which reduces seal and bearing life. The radial load on the shaft changes with the flow-rate. Most centrifugal pumps are designed to minimize radial force when the pump is operated at BEP. But most pumps don’t operate at their BEP and shaft deflection is significant. Shaft material, dimensions and the presence of a sleeve at the seal location affect shaft stiffness and resulting deflection. See Figure 2.
Replacing the pump shaft with one constructed of a material with lower elasticity will reduce shaft deflection and improve seal and bearing life. When manufacturing a replacement shaft, increasing shaft diameters may also improve shaft stiffness. Of course, there are limited sections of the shaft which can be increased in diameter due to necessary assembly fits for the pump. However, increasing diameter on non-working areas of the shaft will improve overall stiffness.
If a pump shaft is fitted with a sleeve at the seal location, it is likely the pump was designed for a packing type seal where shaft wear at the packing is normal. If the pump design has been converted to mechanical seal, the shaft sleeve may still be present. For pumps with shaft sleeves and non-pusher type mechanical seals, the sleeve feature can normally be eliminated from a replacement shaft, adding significant stiffness to the shaft. A thinner sleeve, shrunk tightly to the shaft may also improve shaft stiffness somewhat. When shaft or sleeve deterioration is apparent, a change in material may also eliminate a recurrent failure mode and add value to the overhaul process.
Fitting bearing isolators
Most pumps use simple labyrinth slingers or lip seals to keep lubricant in the bearing housing and to keep contaminants and moisture out. Neither of these methods are effective in damp or dirty environments (exposure to weather constitutes a damp environment).
Bearing isolators are devices that form a seal where the shaft exits the bearing housing. Some can be provided with contacting seal seats and magnetic tensioning to form a moisture seal. Installation may require machining of the bearing housing and/or covers, and is best done during pump overhaul.
When external contamination has been found as a source of bearing failure, bearing isolators have proven to greatly extend MTBF and reduce operating costs.
Upgraded breathers
Any contamination and moisture that enters the bearing housing will reduce bearing life. As machines warm during operation and subsequently cool when not running, air in the bearing housing expands and contracts, pumping contamination and moisture into the housing. A breather designed with adequate filtering will eliminate ingress of contaminants and a desiccant will eliminate moisture. This is a simple upgrade that can have real value for your customer.
Seal housing, flush and quench modifications
The “science” of mechanical seals provides many options for performance upgrades. Basic improvements focus on seal material selection and seal flush and quench options which provide clean, cool liquid to the seal faces. Seal suppliers will work with end users or with service centers to help engineer a solution to the specific pump requirements. The technical expertise of service center personnel working with a seal provider is a most effective combination. In some cases, the options for seal modifications may be limited by dimensions of the seal housing and cover. An aggressive approach to upgrading the seal could include replacement of the seal housing with a custom designed and machined part.
When proposing upgrades to customers, it is important to explore operating conditions and possible liabilities that may be affected by changes to the pump. Material compatibility and hazardous environments ratings could be affected. Service centers who strive to work with their customers will find many opportunities for increased revenue by looking at options for pump upgrades. And compared to “commodity” repairs, these value-added services are not as price sensitive, which means better profit margins for the service center.
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