Newsletter -Electrical Troubleshooting and Computer Based Training

Reliable Process Control And Instrumentation - Use Redundancy.

This months tip comes from Mike Sondalini of www.lifetime-reliability.com

Each month you get at least one tip to improve the reliability of your control and instrumentation systems.

Redundancy (duplication) is a key factor to reliable, long-life, failure-free process control. The low cost of today's basic process sensory elements means that more than one item of instrumentation and circuitry can be installed for most applications.

A control circuit with duplicated temperature transmitters, duplicated level transmitters, duplicate flow transmitters, etc, and their associated electronics, monitoring and self-checking devices would have a high probability of delivering outstanding reliability.

The cost of duplicated instrumentation and control circuitry is negligible in situations where equipment failure will take life or livelihood.

There are very few situation in continuously operating plant, and even in batch plants, where the total production costs of a minor downtime incident would not easily pay for more highly reliable process control and instrumentation equipment.

If you're a plant or equipment designer, or a corporate accountant, let me advise you (as a person who has to live with the final consequences of production plant failures) that the few extra dollars it costs to provide control system redundancy and duplication will pay for itself many times over in the operating life of the business.

A simple example was a water head tank high up on a building, which provided constant pressure to a production process. The level control was a simple mercury float switch who's position opened and closed a solenoid valve on the water inlet pipe. One night the float switch leaked and sank to the bottom of the tank and did not turn off the water supply. The tank continued to overflow all night till someone saw a pool of water the next day.

During the repair a second float switch was installed into the circuit in parallel with the first. With two floats now monitoring the water level, the likelihood of both failing at the same time and not closing the solenoid was reduced to extremely low statistical levels.

The extra cost for a second mercury level switch, and its installation and wiring, was $200. The future costs saved from wasted water, breakdown maintenance, unplanned disruption to production and lost supervisory and management time, easily paid for the duplication and redundancy.

Do you see what happens when you use a duplication philosophy? You take the failure from a breakdown nightmare to a simple preventative maintenance routine. Suddenly it doesn't matter if you lose an instrument, the other is still operational. A repair is now something you can plan-in and do for little cost compared to a breakdown.

The inspection of the instrument's condition now becomes an operator daily shift routine. If a problem is seen a work order is raised and scheduled into a planned outage.

Don Fitchett of Feed Forward Publications goes one step further and suggests :

"You could wire both sensors into separate PLC inputs or relay, so if you don’t see both sensor inputs at the same time, a ‘sensor 1 or 2 out’ lamp lights-up.

This would save even more time by having one less stop on the operator’s daily inspection shift routine. With automated detection of sensor failure, a daily routing could now be moved to once a month on the PM schedule. Cost to add this to an existing PLC, zero. The cost to wire it up with relays would be about a couple hundred dollars."

By duplicating control and instrumentation equipment you have in fact saved the company a lot of money in the future!

Regards,

Mike S.

Please see my complete Newsletter

free email newsletter - the maintenance war.

Featured Articles :

World Industry News:

RE: Electrical Troubleshooting

Management Help:

Human Resource:

Outsourcing pro and cons - Contract Maintenance or not?