Up Time Magazine- April/May 2008 - (Page 50) precision maintenance upload When is Critical Really Critical? Turning a Critical Eye to Your Inventory by Phillip Slater T here is an old saying that goes something like ‘a maintenance engineer never met a spare he didn’t like’. However, perhaps the maintenance engineer should not like his meta-spares quite so much. What’s a meta-spare? I’ll get to that. In my work helping companies optimize their spare parts inventory management, I am almost always greeted by a new client with the exclamation ‘you can’t do anything about that spare, it is critical’. Critical spares are held in high regard and treated as untouchables. However, whether or not a spare is critical is not the point when it comes to inventory review, because you can still hold too much of a critical item. In my opinion, the comments I hear on critical spares are usually more emotional than scientific. Let me explain. were never needed. If there was a failure between planned valve maintenance activities, the pins, plates and springs would be used, not the valves. Based on the way these parts were used, it could be argued that the pins, plates and springs were critical, even though they were not classified as such. Perhaps what this really means is that the definition of critical needs to be expanded to include the words ‘for which there is no viable alternative’. Therefore, our definition of critical is now ‘a component that if unavailable would prevent the plant from operating and for which there is no viable alternative’. No matter what definition is used, classifying an item as critical tells us that we need to have ready access to the item when it is required. It doesn’t tell us how many need to be carried, nor does it tell us whether they need to be owned prior to needing the item. Let’s examine those statements for a moment. The goal of any maintenance program is to minimize the frequency and consequence of equipment failure. Spare parts availability plays an important role in minimizing the consequences of failure by reducing the downtime associated with the failure. Of course, spares do nothing to impact the actual occurrence of the failure. (Yes, we could have a discussion about the quality of spares used so let’s just assume that here we are discussing parts of suitable quality.) A critical spare can be defined as a component that ‘if unavailable would prevent the plant from operating’1. So, if the absence of that item would prevent the plant from operating, then the item is considered critical. In most plants, however, this definition covers a wider number of items than those defined as critical within the plant’s inventory system. So, my first point is that truly critical spares are likely to be more widespread than usually defined. Engineers will happily review items that are not classified as critical, but which actually are critical just not classified as such. Yet they will shy away from reviewing items that are officially defined as critical. In this case any review is, therefore, driven by the classification rather than the opportunity or cost benefit. Nobody would use this approach in reviewing any other capital investment. My second point is that not all items classified as critical are genuinely critical. In a recent example, a company had 24 compressor valves defined as critical items. And, while it was true that the compressor would not run without these valves, the company separately held pins, plates and springs for these valves in inventory. So, in practice, this meant that the valves ‘Ready access’ - means that it is available when we need it. It doesn’t mean that it must be in our store or on our site but that we must be able to readily access the part when needed, in a time frame that is acceptable. ‘It doesn’t tell us how many we need…’ - all classifying the items as critical tells us is that we will need the item, whether we stock more than the minimum required is dependent upon the usual usage, ordering and supply issues. ‘Nor does it tell us whether they need to be owned by us…’ - classifying an item as critical says nothing about ownership, why can’t someone else own the item just as long as we control the item? 50 april/may 2008
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