Choosing How Long to Run Duty and Standby Equipment and When to Swap Needs a Risk Logic and Economics Approach

The strategy to run duty and standby equipment is dependent on the operating risk from the combined configuration. When to swap duty and standby equipment is an economic choice.

Many operating plants have redundant equipment. How long to run the duty asset before running the standby, and when to swap between them, involves risk and economic choices.

 

Once you have redundant equipment, where a standby asset is ready to replace the duty asset, the question of the use-ratio between duty and standby equipment arises, as does the question of changeover frequency.

 


Hi,

I’m a mechanical engineer intern in a chemical plant in Spain. I have been reading the blogs and articles on your website about my problem, but I still having questions. Maybe, I have not found the right article. Could you help me, please?

The issue that I have to investigate is the right strategy to swap between duty and standby equipment. How can I know when is the right time to swap between the equipment. Should the redundant equipment share the operation time 50/50? Or is better to start the standby assets just the 10 % of the time to monitor its conditions? And in that case, how can I now the right periodicity, 10% of the time each three months? 10% of the time each 6 months? (The assets are mainly pumps, compressors, blowers, and pelletizers)

Do you know some references or standards about the topic?

Kind regards, Jose

 

Duty and standby equipment example of process pump installation

 

Hello Jose,

Maybe it’ll be easier to make the choice of when to swap between duty and standby if we take a simpler situation. Say you had two identical cars you could drive. Both cars were always kept in a condition to be available for immediate service.

One is the “duty” car, and the second is the “standby” car. Would you prefer to use each car equally (50-50)? Or would you rather use the duty car 9 times more often than the standby car (90% vs. 10%)? Both cars need to be maintained in a healthy state regardless of the ratio of use.

Operating Risk ($/event) = Consequences ($/event) x [Opportunity to Fail x (1 – Reliability at the Opportunity)] This is the full three-factor risk equation.

In the scenario where the duty car is used 9 times more often than the standby, the duty car accumulates stresses that age and degrade it severely compared to the standby car. A duty car used 90% of the time, has 9 times more opportunity to fail than the standby used only 10% of the time and so carries 9 times more operating risk.

If you aim to minimize the overall operating risk and costs to your company from the redundant equipment swap choice, then 50-50 use of duty and standby equipment is the lowest level of operating risk. The risk logic presumes the duty equipment in not allowed to fail and is refurbished before breakdown.

Once a duty unit fails the operating risk step-changes up to the level it would be as if there was no standby available. Letting duty assets fail and then starting the standby creates the highest risk possible to the organization—it’s the very risk that justified having a redundant asset ready-to-go in the first place. By letting the duty item fail you greatly increase risk to production because the standby must start immediate, and it must run as required until the duty asset is repaired and recommissioned.

With regards the frequency of the swap. It is dependent on the optimal economics—the time period is that at which operating costs plus maintenance costs for both assets together are minimized. If you swap too late the duty equipment operating and maintenance costs are higher because of the greater degradation of more and more components. If you sway too early you paid too much, since the components in the duty item have not worked long enough to return their investment. You need to do a basic spreadsheet analysis of the operating and maintenance economics of using the duty and standby combination for several different time periods. The modelling will identify the time period when the economics is optimal to swap over the duty and standby items.

I hope the above is of use to you.

All the very best to you,

Mike Sondalini
LRS Consultants HQ