You need to consider -

a) can you design and install a system that "fails to safety"

b) carry out regular maintenance and functional checks of your safety devices

c) check your wiring for these devices

d) have duplicate devices

e) have monitors to check warning indicators

f) include motor power monitoring (not current monitoring) in your system

But most of all, get a robust system designed and installed and regularly manually check it simulating failures. ■

It might not be expedient to test the interlocks etc. because of the required availability of the equipment. We just buy the best and hope for the best as designer implies.

If there are pull wires on both sides of the conveyor you can make provision to bridge out one side and test the operation without stopping the conveyor. You must have an operative on the side where the active in line chord is being relied on.

Of the remaining items only the blocked chute probe might be passive; but perhaps there is an active mechanical operated device for that nowadays. Otherwise one of E&H's live devices would be better for a blocked chute probe & even better if it was duplicated. ■

Dear Mr GAnesh

If FAIL SAFE CONCEPT IS IMPLEMENTED EFFECTIVELY I DONT THINK YOU WILL HAVE ANY PROBLEM WHICH YOU ARE MENTIONING.

iN FACT MANY TIMES reliabilitY TERM IS NEGLECTED CONSIDERING price AS IMPORTANT TERM

YOU CAN CALL US FOR ANY FURTHER REQUIREMENTS WITH THIS RELAIBILITY AS THE CONCEPT ■

Originally Posted by sganesh

Dear all,

I have ensured reliability of hooters ( Sirens ). i.e., if they are not working, conveyor will not start. Please refer the other thread started by me for detail. I feel that an important safety requirement is fulfilled.

Regards,

From a Process Hazard Audit/Assessment which you are doing, you have to examine things and decide the severity of a failure. If the hazard is too great, then you have to build redundancy into it. There is no other way.

For example, if you rely on an audible signal, you need to provide two sounders so that if one fails, the other will work. The same for a visual indicator. If one unit fails, the other will provide an indication.

On things like safety switches, you need to double up if necessary.

I have just been through this exercise on a herbicide production facility. There is no alternative. You assess each element and decide whether it is PSM (Process Safety Management) Critical. If the element is PSM Critical, then there has to be redundancy. For example, if you have a potential dust explosion hazard in a machine, you need redundancy. You must provide at least two mechanisms to ensure that the system will be safe. You can have an oxygen analyser that will render things safe and a rupture disc to vent any possible explosion should the single oxygen analyser fail, or you can provide two oxygen analysers where both must provide a safe signal for the process to continue.

If a machine must have a lid that is known to be closed, you need at least two limit/proximity switches where both must indicate closed for the process to continue.

On a conveyor, you have a different severity of actions, but the process of determining the criticality of the elements is the same. ■

Interesting point. Did you know that microswitches have a very poor reliability index. I researched the availability index for Kennecott Copper in 1986-1987. I developed a MTBF and MTTR table for all equipment. The worst offenders were the instruments for all mechanical and electrical components. You can obtain many MTBF and MTTR from the IEEE codes and tables for all generic instrumentation. ■