What is a plant-critical spare?

A plant-critical spare is a part held because the consequence of not having it is unacceptable, even if it may never be used.

Protection, not consumption

Most stock is held because it will be used. A plant-critical spare is held because of what would happen if it were needed and not there. A large gearbox, a crusher main shaft or a specialised motor may sit on the shelf for years. That doesn’t make it excess; it protects the operation against a failure that would stop production for weeks.

This is why plant-critical spares cannot be planned like other stock. A forecast based on past usage will usually say “hold nothing”, because the part has rarely or never been used.

Questions to consider

What happens if the part fails and there is none in stock?

How does the lead time compare with the outage the operation can accept?

Is there a substitute, a repair option or stock at another site?

How does the cost of a failure compare with the cost of holding the part?

How these questions are weighed differs between operations, and is usually set out in the operation’s own criticality and stocking rules.

Equipment criticality and material criticality

Equipment criticality describes how important a piece of equipment is to the operation. Material criticality describes how important a specific part is to that equipment. A filter on a critical pump may still be non-critical if it can be bought locally the same day. A bearing on a less important conveyor may be critical if it takes six months to arrive and there is no alternative.

Both views are needed. Linking spares to the equipment they support is what makes this possible.

Common mistakes

Forecasting critical spares

Rare demand produces a forecast of zero, and the part is not restocked after use.

Treating every slow mover as obsolete

No movement is not the same as no requirement.

Making the decision once

Equipment is retired, suppliers change and lead times move. A spare that was critical five years ago may not be today.

Marking everything critical

When everything is critical, nothing is, and working capital grows without reducing risk.

Review critical spares regularly

A yearly review keeps the list honest. Check the installed base, retired or modified equipment, current lead times, alternative suppliers, repair options and the date of last use.

How StoXeo applies it

StoXeo separates the decision to stock from the calculation of how much to stock.

Frequently asked questions

Is a plant-critical spare the same as a slow mover?

Not always. Most plant-critical spares are slow movers, but many slow movers are not critical. The difference is the consequence of not having the part.

How many plant-critical spares should be held?

Often one, sometimes none if there is a reliable alternative such as stock at another site or a fast repair. The quantity follows from the risk assessment, not from a forecast.