When a pump goes down, should you fix it, upgrade it, or replace it?
Key concepts: Pump maintenance is an opportunity to improve its performance. All economic factors must be weighed when repairing a pump.
Pump maintenance is an opportunity to improve its performance.
All economic factors must be weighed when repairing a pump.
Maintenance and obsolescence may dictate whether to repair or replace.
For those involved in the operation and maintenance of centrifugal and positive displacement pumps, there are numerous opportunities to improve performance. These improvements can be in the form of improved MTBR or MTBPM, lower energy consumption, increased parts life, and lower parts costs. While engineers like to dwell on the technical merits of what they do, performance improvement is always measured on the bottom line. The economics of what is done must be consistent with inhouse accounting practices.
When it comes time to repair a pump, routine procedure is to determine what parts to replace, write a requisition for them, wait for them to arrive from the manufacturer or distributor, install them, and return the pump to service. Part-for-part replacement may be the right thing to do, but also consider changing a pump subassembly, the entire pumping assembly, or the entire installed unit (Fig. 1).
Repair or replace
Each time work is done on a pump, there is an opportunity to improve its performance. Even though product life cycles are extremely long in pumps, manufacturers have made improvements in many pump designs to enhance efficiency, maintainability, and reliability—advantages that should be taken.
To make decisions properly, know what the economics are. Learn how internal performance is measured financially. Accounting personnel should understand maintenance and operation practices to better guide the decision- making process from an economics viewpoint. There are some typical factors that require both a technical and financial understanding before making a repair or replace decision.
There are several factors that should be evaluated.
Consider the pump's age, resource consumption (including motive power, flushing/cooling water, and waste treatment), routine maintenance time, repair time, parts costs, MTBR/MTBPM data, and past failure analyses. If the pump is old, is it now obsolete? With all the consolidations and acquisitions that have been happening in the pump industry, manufacturers are eliminating redundant product lines.
How much longer will the process or installation require this particular pump?
Energy. This category is the cost of energy, usually electricity. Depending on local utility rates, cost can vary between $250—$800/hp/yr ($0.04—0.12/kWh).
Shop costs. Beyond the hourly wages/benefits for maintenance personnel, consider the cost of supervision, heat and lights, and other overhead. Surprisingly, this factor can be between $150—$300/hr. If work is done by an outside vendor or service agency, use its rate plus internal handling and administrative costs.
Routine maintenance costs. Be sure to include hourly costs for normal adjustments, inspections, monitoring, etc.
Skill of maintenance personnel. Are the maintenance personnel really adept at putting pumps back together? For routine replacement of standard mechanical seals, only about 25% are done correctly. By contrast, 75% of four cartridge seal reinstallations are done correctly.
Inventory costs for spares. This figure can be as much as 20—26% of the cost of the spare part itself. For old or obsolete pumps, manufacturers don't always keep parts on the shelf; you must either store them on the shelf or wait until they get made.
Flushing/cooling water. Furnishing treated water to a pump for stuffing box flushing or bearing cooling can cost as much as $3/1000 gal. It might not seem like much, but only 1-gpm injected into the stuffing box of a pump operating a single shift, 5 days/wk, 50 wk/yr consumes over 100,000 gal./yr, or $300/yr. Changing to a mechanical seal could reduce this cost about 75%.
Effluent treatment. Treating the leakage from a pump before discharging from the plant can cost $20—25/1000 gal. About 50% of the injected water goes to drain. About 50,000 gal./yr costs at least $1000. Changing to a mechanical seal eliminates this cost.
Pump installation costs. If existing foundations, piping, etc., have to be removed to install a different pump for an application, costs are usually 3—5 times the cost of the new pump, possibly more if work is done by an outside contractor. These expenditures usually fall under a capital budget. This cost may be justifiable if there are energy, maintenance, and operating cost savings over the time period used to amortize the capital expenditure. Note that aging pump foundations can cause a significant increase in MTBR through misalignment and vibration. This point may be the time to replace the foundation and base.
Vendor technical support. This support may be provided by the manufacturer or the distributor, and varies with the technical skills of individual pump sales engineers. Be in continuous contact with two or three different sources of pump knowledge, and they, in turn, should keep you apprised of product enhancements and the potential positive impact on your pump operations.
Commercial considerations. Does the manufacturer/distributor maintain a local inventory to support pumps, or the proposed new pump? Also evaluate local service capability. Some distributors have repair shops that may be able to more effectively and economically make repairs to equipment than the plant shop. Manufacturers make pumps and subassemblies, and the pricing for them can be substantially lower than the sum of the parts that comprise them. When a manufacturer or distributor supplies a subassembly it often carries a warranty, while repairs done in the shop don't.
Energy, $/hp/yr for electricity, steam, diesel fuel, etc.
Water consumption, $/1000 gal.
Other consumables (lubricants, nitrogen purge, etc.), $/gal. or $/lb
Repair parts consumption, $/repair session/ pump, with detailed parts usage history
Waste treatment and disposal, $/1000 gal.
Shop costs, $/hr, including direct labor, supervision, overhead applied to disassembly, cleaning, inspection, administration, and reassembly operations
In-place maintenance costs, $/hr, including adjustments, inspections, lubrication, removal/ reinstallation, alignment, etc.
Inventory-related costs, $/$1000 of inventory, related to capital costs, handling, space, administration, tax, receiving inspections, etc.
Engineering time, $/hr
Purchasing and accounting costs, $/purchase order.
Once the costs relevant to internal operations have been determined, interact with the vendor to analyze each cost element with the intent of reducing or eliminating it.
The plant engineer probably knows more about the operation and maintenance of the pumps in the facility than vendors do. What they have is knowledge of their products and services, as they would be applied to specific applications.
Depending on the time available, bring in one, two, or three vendors. Present the situation to each, letting them know what the evaluation criteria are, such as reduced horsepower consumption, reduced inventory, improved MTBR, better seal reliability, easier/simpler maintenance, or the best replacement for an obsolete pump. Ask for their recommendations and emphasize that they should detail the features, functions, benefits, and costs of their product/service.
— Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135
The author is available to answer questions about pump repair or replacement. He can be reached at 315-539-4127 or email@example.com.
Modify or replace
This decision means changing something in an existing pump to make its operation more economical. As hard as it is to write a specification for a new pump application, it is seldom done entirely right. Learn from operating experience to make improvements, such as upgrading materials to get longer parts life, trimming an impeller to reduce energy consumption without sacrificing functionality, changing from packing to mechanical seal, etc.
If there is erosive or corrosive wear on close-clearance parts, consider upgrading materials to extend MTBR or MTBPM.
Example: If there is erosion on a 316SS ANSI pump impeller, investigate upgrading to CD4Mcu, which is about 12—15% more expensive than 316SS, but can give 1-1/2—2 times the life in erosive services.
Example: If wearing ring clearances on a double suction pump deteriorate because of erosion or contact, consider upgrading from bronze to 11—13% chrome.
Component or subassembly replacement
Clearances increase every time a bearing frame assembly or power end is repaired. Consider buying a new, completely assembled bearing frame (Fig. 2).
Cost of a new assembly might be less than the cost of the replacement parts, and the cost for shop time and may include some type of warranty. It may be a good way to get rid of those lip seals that were furnished on most ANSI pumps 10-yr ago, greatly enhancing MTBR due to bearing/lubricant failures inherent with lip-sealed designs.
When changing from packing to mechanical seals, also change the stuffing box cover to a seal chamber design. Pump manufacturers have made several design changes over the past 10 yr to improve the operating conditions and service life of mechanical seals. These are often supplied in kit form for convenience. Depending on the pump design, the kit may also include the mechanical seal. If the application requires a specific seal, consider a cartridge seal. Prices for cartridge seals have dropped dramatically in recent years and although not foolproof, they provide a tremendous advantage in simplicity of installation and reliability.
If repairs include frame parts, impeller, and stuffing box cover, consider purchasing a back pull-out assembly (except the casing) (Fig. 3).
Many manufacturers price this assembly as a percentage of a new pump price, which can be much more economical than the sum of the parts plus shop costs. Include upgrades for performance improvement and get what is, essentially, an entirely new pump.
If bearing/seal life is being reduced by throttling a pump on discharge or wasting horsepower by bypassing capacity to achieve a low flow, continuous operating condition, consider retrofitting a low flow pump wet end.
Vertical turbine pumps are a special case. Basic components are the bowl assembly, column assembly, discharge head, and driver. Almost any manufacturer's bowl assembly can be replaced by another manufacturer in a process called rebowling. Depending on the size of the pump, expertise of the maintenance staff, degree of required repairs, etc., the pump can be reassembled inhouse or at the manufacturer's or distributor's service facility. Or, the pump can be reassembled inhouse under supervision of a manufacturer's service engineer or technician.
Entire pump assembly replacement
All too often, when a pump really needs to be replaced, internal accounting practices dictate buying parts, since a pump is usually considered a capital expenditure and subject to complicated approval processes. An easier route is to spend more maintenance dollars because the expense comes under a different portion of the budget. A total pumping unit (pump, baseplate, coupling, driver, installation, piping, controls, etc.) is definitely a capital item. The pump portion of the total pumping unit is debatable as a capital or expense item. If most pump parts must be replaced, especially a casing, it may be advantageous to buy a new pump, warranted by the manufacturer.
ANSI pump design requires piping, foundation, and coupling dimensional interface interchangeability between manufacturers on a generic size-for-size basis. Internal designs and hydraulic performance vary. There have been many improvements in ANSI pump designs, including magnetic drive versions. When replacing an ANSI pump, explore alternatives. Watch any impact on inhouse spares inventory levels. They could go up or down, depending on the distributor's inventory.
End-suction water pumps and double suction pumps are not interchangeable between manufacturers. There may have been hydraulic and/or mechanical improvements made on a particular manufacturer's offering that still provides piping and foundation interchangeability. It's worth checking, especially for modifications to improve performance, and/or for older pumps that are ready for retirement.
Total pumping unit replacement
Sometimes the entire pump installation must be replaced. This action could be in a reactive mode, due to an improperly applied pump, change in service conditions beyond the capabilities of the existing unit, a pump that's become obsolete, or the result of changes in air or water quality regulations. It can also be done proactively when the financial advantages of replacing a total pumping unit exceed the costs of a new installation.
Pump performance improvement
Always look for ways to improve performance. These improvements can be expressed in technical terms, such as improved MTBF or MTBPM, increased operating efficiency, or downtime/uptime. But that, in itself, is not enough. It has to be put into a dollars-and-cents definition, understood by everyone in the organization.
Establish an inhouse value system
Like it or not, many people get involved in one way or another in the decision-making process, from maintenance and engineering to purchasing, stores, and accounting. Develop a shared understanding of the costs and their derivations:
Vendor support and commercial considerations should be evaluated.
Internal technical and cost accounting
Numerous factors require evaluation in this area.
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