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Time based V’s Condition based

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I was reading Ramesh Gulati, AEDC Best Practices Result in Reliability Book and in it he discusses how typically world class, or best in class plants have a ratio of 20:80 of time based to condition based PM’s. 

I then thought about my own 20 year old petrochemical plant and I’d say we’re probably 20:80; but the other way round.

It got me thinking, how do you get to that ratio? 

How do you check if your PM routines add little to no value? 

How do you actually make the switch from time based to CDM?

We have a reliability team but they said until we have a CMMS that can be set up for CDM (Maximo 20 odd year old) we cant simply move over to condition based maintenance. Why not? I didn’t really understand how that can be when you simply generate a Work Order for a CDM maintenance routine.

We routinely have a backlog of PM’s either because they are mostly time based and the plant will not be released to us, and we do not have a lot of redundancy assets.

Anyway, if anyone has made the switch I’d love to hear how you did it, what was the challenges and any advice you have.





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Great topic @Derek Brown and I think one that many older plants do struggle with. 

In my view there is no reason why your CMMS would stop you from moving to condition based maintenance. For example vibration monitoring can be simply set up as a 1-monthly or 3-monthly time based task where someone goes out and does the vibration round. That data is then analysed and used to determine whether condition is deteriorating and make an assessment on when a scheduled restoration task might be required.

Many organisations fall down in this area in that they diligently do the CBM rounds, collect and analyse the CBM data, but then do not take action quick enough to address an impending failure. If this happens you still end up with an unplanned failure that needs urgent attention and you have spent money on CBM, but did not get the benefit (i.e. reducing downtime and potentially collateral damage due to the failure).

It might be that your CMMS team was thinking of a solution where the CMMS actually triggers the work order based on condition data - that is a step further down the implementation curve and one that can be wrought with other issues like false positives. As with many of these things start with small steps.

Regarding the 20:80 ratio of PM:CBM tasks I would be careful in taking that as a hard rule of thumb, instead I would see it more as an aspiration especially in a complex, hazardous plant like a refinery where you will have a lot of time based compliance related maintenance. Also a ratio like that is never really our objective, it is merely a tool to get us to a highly reliable plant that is safe and with minimal cost.

Before you make a big move on CBM I would actually first analyse your corrective workload to determine whether your PM program is effective in mitigating failures. If it is then your journey to CBM is really about efficiency and cost effectiveness. In that case I would start looking at the PMs that are the most costly across say a 5 or 10 year period when accounting for materials, services and internal labour. Take the worst offenders and analyse the failure mode that PM is mitigating and see if it can be done more efficiently by extending intervals or adopting a CBM approach. However, if your PM program is not effective in preventing significant failures I would first put your effort there (and that may or may not include CBM).

Hope that helps.

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  • 1 month later...

Hi Erik,


Thanks for the reply.

We do CBM but we most definitely utilise technology enough whereby we currently have a contractor who comes to the plant every 5 weeks and does their measurements and then leaves to issue a report 5 weeks later. Not ideal as we need continuous monitoring on a lot of equipment in my opinion.

On another note @Erik Hupje, we have a large backlog of PM's we are now only doing (or trying to do) when we have an outage. The problem being that we never fully get through them all and where plant can be released on the run, the Ops/management are too risk averse to release anything for maintenance. That is the strategy they are using at the moment - extremely frustrating. We now have to start looking at using external technicians at outages to get through our PM's, to me, this is ludicrous. I have not had time yet to look at the backlog, but I suspect most if not all will be time based PM's.

Have you had a situation like this where plant wont be released and the backlog is piling up and you cant get enough time at shut-downs to do them all?




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