Reliability Insights

Hidden Failures in Maintenance

Sometimes, we have hidden failures in our organisation. And if they’re not discovered, you will find yourself dealing with a pile of problems later on. To prevent hidden failures from happening, you must first understand what they are. 

Let me explain what a hidden failure is in more detail.

A hidden failure is a functional failure that is NOT evident under normal operating circumstances. Often they occur on their own, and are only revealed after another failure or event occurs. You normally see hidden failures as a result of standby functions and protective functions—both of which aren’t used under normal conditions. To illustrate them better, here are some simple examples: 

Hidden failures from standby functions 

First, let’s have a look at two common pump configurations.

Configuration one is a standalone setup where we have pump A, and that’s it.  So when pump A fails, it’s pretty obvious because we’re going to have no flow.  This is an evident failure. And it’s very easy to notice. The other configuration is a duty-standby setup.  

Pump B is our duty pump, and runs during normal operating conditions. And the standby pump, pump C, only runs when pump B is down (failed or is undergoing maintenance). Now… 

Failure of pump B would be evident because pump C would have started, right? So we’ll know immediately. However…  

If our standby pump, pump C, has failed, we wouldn’t know. Because, under normal operating circumstances, we would have our duty pump, pump B, running. And pump C could be sitting there in a failed state for a long time. We would only find out if pump B failed and pump C didn’t start on demand…. 

…or if we started pump C as part of a routine maintenance task. So, when pump C is unable to start, that is a hidden failure

Hidden failures from protective functions 

Let’s have a look at a second example. Here, we have three types of valves:  

The control valve is used to regulate a downstream pressure. If that control valve fails to control the pressure, the failure will be evident because the pressure is going to rise. However, if the solenoid in the emergency shutdown valve has seized, we would not know about that during normal operation. We would only find out if the control valve failed to control pressure.  

Our pressure exceeds the trip limit, and then the emergency shutdown valve is expected to close, but it doesn’t. So that is clearly a hidden failure. 

Now, when that emergency shutdown valve fails to close, the pressure continues to increase, because our control valve is still not controlling the pressure, and the shutdown valve hasn’t locked in the pressure. So now we need to rely on our pressure relief valve to relieve the excess pressure to make sure our pipe doesn’t rupture. But what if the pressure relief valve is stuck because it’s actually a very fouling operating environment? 

We would not know about that either until we expected the pressure relief valve to operate, but it doesn’t.  If left alone, you’ll have a ruptured pipe on your hands. Now, you can see how important it is to prevent hidden failures. Now the question is… 

How do you deal with hidden failures? Here’s the good news… 

There’s an easy way to handle hidden failures. It’s a type of Preventive Maintenance, called Failure Finding Maintenance. 

Failure Finding Maintenance 

A failure finding task is really just a functional test, like starting that standby pump, or stroking that emergency shutdown valve, or testing any pressure trip transmitter to see whether it’s actually going to function when required. And if you can’t find an effective failure finding tasks, then you basically need to consider redesign.  

It’s something I talk about more inside my new course: PM100: Developing and Improving Preventive Maintenance Programs, where we touch on RCM and preventive maintenance optimization. 

If you want to learn how to develop and improve your Preventive Maintenance Programs so you have a system in place to deal with these hidden failures, then this course is for you. Inside the course, we will teach you HOW to prevent hidden failures from occurring using failure finding maintenance. 

Not only that, the course focuses on teaching you the principles and practices you need to achieve higher reliability and availability whilst doing LESS maintenance. Poor PM programs waste time and money, do not effectively address failures, and drive our organisations into a reactive, fire-fighting culture.  

In the meantime, you can learn more about preventive maintenance by reading this article on the
9 Principles of a modern preventive maintenance program. 


Do you have a system in place for dealing with hidden failures?

Share it with us in the comments.

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