Duty versus standby

Sept. 26, 2006
Maintenance is about cost-effectively managing assets for a given level of performance and risk. Learn to avoid the mistake many plants make - using faulty logic to set up start-up intervals.

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Maintenance is about cost-effectively managing assets for a given level of performance and risk. Part of this equation is defining failure management policies for equipment that provide the best value for each operational maintenance dollar, a reduction of the risk of critical failures to a tolerable level, and sustainable levels of production or operation.

If only it were that simple. The fact is, too often the failures maintenance personnel are required to manage are not under their control.

Fundamentally, there are three ways that equipment fails (discounting failures caused by human error):

  1. Failures caused by component wear-out or degradation due to use, time or the environment. Corrosion, erosion, wear, chemical breakdown and structural degradation are among the causes of these types of equipment stoppages.
  2. Failures caused by operating equipment out of limits, in a way that increases the probability of failure, or with incorrect feed materials. Equipment overload, lack of knowledge of operational parameters, and changes in equipment demands are the primary reasons for failures in this area.
  3. Failures caused by inadequate or unsuitable components. These are caused by assets being not fit-for-purpose. Poor purchasing practices, inadequate asset selection practices, and poor initial design are the primary reasons for these types of breakdowns.

It is the first category that maintenance professionals are most able to directly influence, but they also are required to manage and react to the failures caused by operations, purchasing and/or asset design. This reminds me of a favorite comment by my friend Steve Turner: “Maintenance is a process, not a department.” Maintenance managers need to be the plant diplomats, stitching together agreements on operational and purchasing strategies to ensure cost-effective performance.

One area where there is some opportunity to reduce operationally induced failures is rotation of redundant equipment. Duty and stand-by have essentially become interchangeable terms in many plants, and have been replaced by a 50/50 split in operational running. It is most common with pumping systems, but can affect any duty/stand-by asset configuration.

This type of thinking appears sound on the surface. The common belief is that by regularly changing the pump (or whatever) that is operational at any one time, the company is gaining two advantages:

  1. It is ensuring that the stand-by pump is going to be ready for operation if called upon.
  2. The company will have a longer mean time to repair (MTTR) by interchanging the two pumps rather than running only one pump, and thus gain the combined life out of the installation.

These are intoxicating arguments, and they do appear sound on the surface. However, they are both wrong and misleading.

The most rigorous and stress-inducing time for a pump is at start-up. This places significant strain on the pump in various areas. Most notably, seals and bearings are prone to failures due to frequent starts. Therefore, stops and starts increase the risk of failure.

With a true duty/stand-by arrangement, the pumps can run continuously for long periods of time without having to go through the start-up process. In shorter runs, just one pump at a time is exposed to the regular stresses of start-up, while the other remains dormant and tested infrequently (but regularly) to ensure reliability.

Higher reliability through frequent starts is not a valid argument. What about the second argument – that interchanging the pumps increases time between failures of the process?
Pumps contain components that wear out at a rate dependant on the type of pump material, the rate required, and the design of the pump impellers and housing. If identical pumps are dispensing the same media, at the same head pressure and rate, then they will wear out at roughly the same time. If we discount the effects of frequent start-up, swapping pumps would extend the overall time between failures.

But then what? The fundamental reason for having a standby unit is to eliminate the risk of losing the function of the process. Stand-by units are effectively an insurance policy against the failure of the prime or duty unit. This strategy does not decrease the risk of failure. In fact, it increases the likelihood that both pumps will fail at approximately the same time. The need for a standby pump becomes questionable -- it would be cheaper to have just one and to lose the function when it fails.

In the vast majority of cases, it is hard to justify anything less than a 90/10 split of operational time. In fact, even this could be reduced without any adverse effects.
The limiting factor is avoiding a failure of the standby asset due to, for example flat spots in bearings, hardening of the pumped material, or some other reason.

A recent case I reviewed involved lime dosing pumps in a water treatment plant. The configuration was three duty units and one standby pumping unit. One of the factors we needed to take into account was the fact that lime hardens over time, so if the standby pump was left for too long it would never start. As the cost of failure was too high, the frequency of the functional test was based on not allowing the lime to harden.

Other similar cases include a factory that pumped milk and a gold processing plant.  The milk system needed a clean-out on changeover. In the gold processing plant, gold would settle out of the slurry, representing a small loss of profit.

The goal is to prove that equipment will work when needed. Where safety is not an issue, testing frequency is determined by the cost of a failure and the likelihood of that failure occurring.  These costs have to be balanced against the cost of doing the task itself. Because the consequences of failure are purely economic, there will come a point where the risk of failure is less than the cost of doing the task at the required frequency.

PlantServices.com Reliability Expert Daryl Mather works with Knowledge Based Management, London, and  has assisted companies to increase the profitability of their physical asset bases in more than 23 countries including the United States, Europe, Asia and Latin America. Author of the book, “The Maintenance Scorecard” and publisher of the Modern Asset Management blog. Mather can be reached at [email protected]

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