Table 1. When deciding whether to stock a part in inventory, consider factors such as the cost of downtime if the part fails, the expected time to replace the part, the expected part life and its shelf life.


In my previous articles, I presented methods to measure and reduce the total cost required to operate your process heating equipment. In this article, I will focus in on a ubiquitous but seldom scrutinized area of equipment maintenance: spare parts. We all have them, we all use them, but are there opportunities to improve the way you specify, inventory and utilize replacement parts? The answer is yes, but only if you work proactively.

By taking a methodical approach, you can reduce the total operating cost associated with your process heating equipment by:

  • Improving equipment uptime by ensuring only the highest performing replacement parts are used in equipment repair.
  • Reducing the time required to repair a piece of equipment.
  • Reducing the overall cost to specify, procure and inventory replacement parts.

    Generally, the most costly replacement part is the one you can't get in a reasonable length of time. The facility loses production, incurring overtime costs to meet shipment commitments. The purchasing depart must scramble to locate the replacement part -- often paying a premium to purchase and ship the parts. The part in question may no longer be manufactured or have an excessive lead time, so plant engineering must often work to apply a part that is far from the original.

    The first question is which components should be stocked? Obvious question, certainly, but difficult to answer. One thing is for certain: You want to minimize the cost of inventory you keep on hand. A better question may be, how can you ensure ready access to the parts you need? In a perfect world, there would be a hardware store next door that stocked every conceivable part; and in this instance, you would not need to stock any parts. You don't live in a perfect world, though, so you need to develop a plan.

    It is also costly to inventory every imaginable part: Some components rarely -- if ever -- fail. Some components are so common they can be purchased in a matter of hours. Inventory costs money to procure, takes up space, and requires labor to maintain.

    Yet, some parts fail frequently or cost very little inventory, or are used in a great many machines. To determine which parts to inventory, there are eight factors to consider.

  • Calculate the cost of equipment downtime. Is the machine or subsystem critical to the operation of the facility, or is a period of downtime acceptable? For example, due to in process inventory levels, it may be acceptable to shut down a washer for up to two days without adversely affecting production. Its downtime cost during this period is very low. By contrast, an oxidizer that serves multiple paint lines may start affecting output in a few hours. It's important to consider the equipment's overall production impact when determining which parts to stock.

  • Determine if the part's failure will cause equipment downtime.

  • Classify parts as consumable or durable. Generally, purchasing and inventorying consumable parts is less risky -- they are going to be needed eventually.

  • Estimate the expected life of each component. This is the most difficult part of the analysis because there is seldom adequate information on which to base the estimate. The recollections of your maintenance people and plant engineers may be all you have to go on.

  • Estimate the approximate cost of each component. Obviously, it's easier to justify stocking a $10 bearing than a $10,000 SCR control.

  • Estimate the delivery of each component. Even if the part is relatively expensive, if it is critical to the operation of the machine and delivery is six weeks, it may be imperative that it is inventoried locally. Is there an alternate distributor for the part? (As more equipment is built abroad, using parts that are uncommon to the facility, this may become more of an issue. Try buying a metric pipe fitting at your local home improvement warehouse Sunday morning.)

  • Determine the shelf life of the part. A component may not have an unlimited shelf life. Metal parts may corrode, or polymers may harden or crack due to age or UV exposure.

  • Judge the quality of the currently deployed component. If a component fails frequently, it may be time to locate a different item or manufacturer.

    A sample analysis is shown in table 1. In this case, the company decided to stock the first item even if its life is unknown because it was relatively inexpensive and required two weeks to obtain. However, the company decided not to stock the valves and bearings because they can be obtained and installed within the two-day target repair interval. The conveyor, as a consumable, will need to be purchased as it reaches the end of its life.

    Not all replacement parts have an unlimited life. Suppose that a company decides to stock every imaginable spare part for its critical oxidizer when the unit is purchased -- just to be on the safe side. However, 15 years pass before any spare parts are needed. The blower motor is replaced with one from inventory, only to fail a few days later. The motor bearings developed flat spots after the motor was stored in the same position for 15 years. A pilot valve is replaced with one from the “just in case” supply, and it leaks excessively because the rubber seats have hardened. The PLC processor is replaced from the spare supply, but it faults unexpectedly -- the firmware needs to be updated. While not common, inventory spoilage should be considered.



    Table 2. Equipment purchased from different vendors will no doubt have different components performing the same function. Evaluate frequently replaced parts on different machines to determine whether you can identify a universal replacement.

    Reducing Your Replacement Part Inventory

    If you, as most people do, operate equipment purchased from different vendors at over the years, no doubt it will have different components performing the same function. On Furnace A, the low combustion air pressure switch is manufactured by Acme and on Dryer B, the low combustion air pressure switch is manufactured by Super. Both switches perform the same function, carry similar agency approvals, have setpoints in the same pressure range and require similar electrical ratings. Should you stock both?

    Any time this is the case, it would be best to standardize on one replacement switch that meets both sets of requirements. Table 2 illustrates an approach to compare switch specifications to determine if a “universal” replacement can be identified. During this process, you can evaluate the overall quality and durability of the components. Consider questions such as what is your history with the device? Is it susceptible to vibration? Does the setpoint drift with time? Also, check to ensure they do not contain any plant-prohibited materials such as silicone or mercury, and whether any modifications are required to apply the universal replacement. As new equipment is purchased, you can use the outcome of these analyses to compile your facilities' standard component list.

    When collecting information, the most important consideration is the application requirements. Take the time to examine the electrical drawings and calculate the current draw across switches. Do not assume the existing component is meeting every application requirement. Pay special care to environmental factors -- temperature and NEMA ratings -- as components frequently are operated outside these specified limits.

    Components that are excellent candidates for this type of standardization include pressure switches, valves, regulators, modulating motors, control instruments, thermocouples, flame safeguards and fuel filters. Consider if any electrical modifications are required to use the “universal replacement” before you make your selection.

    Improving Your Replacement Part Inventory

    Don't settle for the status quo. As you formulate a plan, work to improve the performance of your process equipment through improvements in the performance of individual parts. Ask the OEM for feedback about which components they feel are superior performers and which can be improved. Challenge manufacturers' representatives to suggest parts that will provide better life, improved performance or better efficiency.

    In conclusion, the time to develop a replacement part strategy is before the emergency. Unfortunately, it is one of those areas where success goes unrecognized but failure is obvious, but no matter how you go about it, the key is to be proactive.

    Sidebar:
    What I'd Give for a Union

    Looking at most piping systems, you would swear the life of every component was infinite, or at least that's what the pipe-fitter thought.

    When specifying new equipment, insist that the supplier provides a reasonable means to change components without having to disassemble 20' of pipe with 200 lb of valves and flow meters attached.

    Look to ensure there are unions or other means of disassembling the piping and that there is adequate clearance to “swing” threaded components.

    Inspect electrical connections to ensure that they can be removed with minimal fuss.

    Finally, when servicing equipment, insist the mechanics or pipe-fitters take the extra time to correct existing problems to make the next person's job easier.

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