Improvements to make your equipment better than new.
03/23/2018
As Pumps & Systems celebrates our 25th anniversary year, we continue our series of reprinted articles that remain relevant today. This article is an aftermarket story that appeared in our November 1993 issue.
Upgrade (up + grade, v.): to raise the grade of; to raise the quality of a manufactured product (Webster’s Third New International Dictionary)
A pump upgrade (also called a revamp or retrofit) involves changing mechanical or hydraulic design or materials to solve a problem or increase reliable run time. An upgrade is different than a repair; an upgrade improves the design beyond the original.
Rerates are a type of hydraulic upgrade, usually involving a change in pump head capacity. Repowering may involve repairs and/or upgrades. Philosophically, repowering is different from normal pump maintenance because the plant being repowered has decided to spend capital monies to extend the plant’s useful life.
Pump upgrade goals include:- decreasing plant operations and maintenance expenses
- increasing mean time between failures (MTBF)
- increasing pump and plant availability
- increasing pump efficiency
- complying with the latest legislative mandates (such as the Clean Air Act Amendments of 1990)
- minimizing the risk of fire or other safety hazards
- eliminating hazardous materials
The Upgrade Process
To identify upgrade candidates, pump users should review maintenance records to see which pumps were responsible for a disproportionate share of expenses or cause safety or reliability concerns. Once identified, work with the upgrade supplier to evaluate upgrades available for your particular pump. Provide the supplier with a maintenance history so problem areas can be addressed. The following are upgrade examples: Mechanical design upgrades- install a stiffer shaft/rotor to reduce vibration
- modify structural elements to remove natural frequencies from the range of pump forcing frequencies (rotational frequency, blade pass frequency, etc.)
- eliminate threads (a source of breakage) on pump shafts
- modify components to make assembly/disassembly easier
- convert mechanical seals to further restrict or eliminate leakage
- redesign first stage impellers to reduce cavitation damage
- redesign impellers to lower vibration for part load/peaking operation
- control “A” and “B” gaps to reduce pressure pulsations and vibration
- improve efficiency
- optimize blade number to reduce pressure pulsations and vibration
- increase pump head capacity to meet system requirements
- eliminate asbestos, an environmental hazard
- install impellers made of cavitation-resistant materials for longer life
- use hardened wear parts to increase MTBF
- eliminate leaded bronzes because of environmental problems with lead
- improve product-lubricated bearing materials
- change materials for seawater use
Boiler Feed Pumps
Boiler feed pumps are at the heart of most power plants, and economical plant operation depends on reliable pump operation. Many pumps from the utility building boom of the 1950s-1970s had larger capacities and more horsepower than previously supplied in order to meet increased plant size. As pump energy levels increased, so did the failure rate. Progress has been made through pump manufacturers’ efforts to address first stage impeller design, materials and rotor dynamics. As a result, good reliability of high-energy feed pumps is attainable. Some of the common upgrades for boiler feed pumps include:- The thrust collar nut can be modified to reduce shaft bending stresses and the possibility of shaft fatigue failure.
- The bearing housing can be modified to spring-load the thrust bearing.
- Several sealing options are available, depending on the application. Floating seal rings, serrated bushings, or mechanical seals are often recommended to minimize first cost while maximizing reliability and thermal efficiency.
- Non-asbestos spiral-wound gaskets replace original asbestos gaskets. Pumps with copper or iron gaskets can be modified to accept non-asbestos spiral wound gaskets.
- Improved first-stage impeller inlet designs expand the stable operating range to lower flow rates without cavitation damage, vibration, or pressure pulsations.
- Dry couplings (like the flexible disc or diaphragm-type) eliminate the need for periodic lubrication and the associated chance of failure.
- Instrumentation can be added to monitor and protect pumps.
- A shrouded diffusor eliminates breakage problems and allows improved alignment.
- Improved bearing designs are available, including “high stability” designs to eliminate half frequency (“oil whirl”) problems.
Circulating Water Pumps
Circulating water pump maintenance requirements vary greatly, depending on whether the pumps are used in freshwater or seawater. For most freshwater applications, typical problems requiring pump maintenance are excessive vibration and premature bearing wear. Circulating water pumps in seawater face additional problems due to corrosion. Material selection is critical, and the selection process must consider general corrosion as well as velocity effects, galvanic compatibility, and pitting resistance, plus manufacturability and cost. Upgrades for circulating water pumps include:- An inner column stop on pullout style pumps to hold down the pump element during starting, stopping, unit trips, and other transients.
- A flanged inner column with rabbet fits replaces screwed inner columns, resulting in better bearing alignment and easier disassembly.
- Bearing spiders provide stiffer bearing support.
- Shroud metallurgy upgraded in high-velocity areas eliminates erosion and corrosion damage.
- Inlet bell modifications lower the required submergence and reduce vortexing.
- Modified impellers optimize cooling water flow, increase plant output, or save pumping horsepower.
- Erosion in iron casing vanes can be repaired. Coatings can be applied to extend casing life.
- Keyed shaft coupling improves shaft alignment and eliminates problems associated with removing threaded couplings. Shafts can usually be re-machined and re-used.
- Rabbet-fit drive couplings replace body-bound bolt couplings and improve alignment repeatability.
Boiler Circulating Pumps
Boiler circulating pumps (BCPs) are in a particularly sever duty, handling 600 F water at over 3,000 psig. Many of the original pumps supplied in the 1950s and 1960s exhibited less-than-desirable life spans. In light of this, an upgrade program was developed that:- adds a graphite-impregnated bearing
- improves the primary sealing device
- incorporates other reliability “lesson learned”
API Process Pumps
Process pumps may handle hazardous materials, and as a result seal leakage is critical. Industry standards (API 610 7th Edition), as well as federal legislation like the Clean Air Act, address mechanical seal reliability and pump maintenance. There are upgrades available for process pumps to improve seal reliability and reduce emissions, including:- using a heavy-duty rotor (shaft and bearings)
- enlarging the stuffing box bore for better seal cooling
- using a heavy-duty cartridge seal (single, double, or tandem)