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MTBF/Failure Frequency /MTTR/Failure Severity

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We will explain indicators that quantitatively indicate the reliability and maintainability of a plant and their meanings.

Beginners in maintenance tend to be desperate to deal with problems at the site, but this is a way of thinking that you should know when improving your skills.

While it is important to focus on safe operation at the site, the aspects related to plant planning and management, such as data preservation and the plant life cycle, will become increasingly important in the future.

I would like to understand it as a basic concept.

Equipment reliability and maintainability

What is meant by equipment reliability and maintainability?

The person in charge of maintenance must understand this meaning.

Devices will always break at some point . It’s natural.

However, chemical plant equipment is treated a little differently.

Devices are broken. I can’t help it. let’s fix

In order to use equipment in a chemical plant in a stable manner, we cannot say such carefree things.

Fix it before it breaks, fix it immediately if it breaks.

An aggressive stance is required.

This is exactly what a security guard needs to do.

In order to maintain good health, people try to avoid going to the hospital by having a medical checkup, paying attention to what they eat, and exercising. The original idea is to do the same thing with chemical plant equipment.


Mean time between failures and failure frequency rate are measures of facility reliability.

mean time between failures (MTBF)

The mean time between failures is called MTBF (Mean Time Between Failures).

Equipment operating time from the occurrence of a failure to the occurrence of the next failure.

MTBF = operating time / number of failure stops

is the definition.

A higher value indicates that the equipment is operating soundly.

If there is no equipment failure, it is ∞.

Let’s take a look at a specific example.


This shows the failure time after 8000 hours of operation per year.

Since the operating time is 8000 hours and the number of failures is 2, MTBF is

MTBF = 8000/2 = 4000 (hour/time)

That’s the calculation.

This is data for predicting that after one breakdown and repair, it will take 4000 hours before the next breakdown.

It will be used as a criterion for determining whether the maintenance of the equipment is 3999 hours or 3000 hours.

Failure frequency rate is the reciprocal of MTBF

The failure frequency rate can be thought of as the reciprocal of MTBF.

Failure frequency rate = number of failure stoppages / load time

  • The value is zero if there is no failure, and the higher the failure, the higher the value.
  • It is necessary to work on maintenance for facilities with higher values.

It has this meaning.

In the above example, the failure frequency rate is

Failure frequency rate = 2/(8000/8000) = 2.0 (times/year)

As a result, the data itself that it breaks twice a year is obtained.

Since it is the number of failures itself , it is very general data .


Mean repair time and failure severity rate are measures of equipment maintainability.

Mean time to repair (MTTR)

The mean time to repair is called MTTR (Mean Time To Repair).

MTTR = failure outage time / number of failures

That’s the definition.

In the example above

MTTR = (8+32)/2 = 20 hours/time

It means that it takes 20 hours of repair time per time.

If the failure stop time is used as a parameter, there is a possibility that people will rush to fix it by dashing when it breaks down.

There are many companies that do this successfully.

If it breaks, I will try my best to fix it.

As a result, it is easy to predict that many Japanese will make excessive efforts.

Failure severity rate

The failure frequency rate is

Failure frequency rate = Failure stop time / load time

Define in

This is also a useful indicator.

In the example above

Failure frequency rate = (8+32)/8000 = 0.005

This is the result.

It’s simple and important data in terms of lost production opportunities.

Reliability integrity as data integrity

In recent years, the concept of data security has become a hot topic.

When you say maintenance, do you have an image of a person who repairs on-site when trouble occurs?

Many people are not good at office work.

Not a very good image.

There is an established image that it is a job performed by people who do not have a high academic background, such as high school graduates.

However, maintenance is a data-intensive, intelligent business .

Apart from the person doing the repair work, the person making the maintenance plan is very important.

In order to handle data, it is advantageous for those who are properly studying.

Such a job should be sought for maintenance workers who have graduated from college or graduate school.

When maintenance personnel try to express their opinions within the factory, they tend to express the actual situation at the site emotionally without data.

Or they don’t know how to make a point and tend to work passively.

If maintenance staff can use data to make persuasive arguments within the factory, the factory will change.

As one of the data for that purpose, reliability integrity will be important.

For further understanding

If you are thinking about the reliability and maintainability of equipment for chemical plants, why not try the following books?

It’s never easy.

A chemical plant is in a fight against deterioration as soon as it is built.

Here, indicators of equipment reliability and maintainability are indispensable.


We introduced MTBF, failure severity rate, MTTR, and failure frequency rate as indicators of equipment reliability and maintainability.

MTTR is not sensory data.

The manufacturing department uses the failure frequency rate, and the equipment department uses the MTBF and failure severity rate.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*I will read all the comments and answer them seriously.