STL is a tool for end users to deal with spurious trips

Endusers feel the annoyance of spurious trips in their pockets. With STL endusers can specify additional requirements to product suppliers and systems designers. They can then design products and systems that cause less spurious trips. The higher the STL level specified, the less spurious trips will happen.

The Spurious Trip Level

The Spurious Trip Level® or STL is a metric for endusers. The STL concept helps endusers to loose less money due to spurious trips. It helps endusers to specify (or design) processes that are more available.

There are many reasons why processes might trip spuriously. Usually a process consists of many different systems and each system might have many different functions. All these functions can fail. If a function fails that fails in a way that causes a spurious trip will cause the enduser to loose money.

STL applies to functions

The spurious trip level is best applied to support functions, i.e., functions which you do not actually need to carry out the process. Safety functions are probably the most famous support functions that you don’t need to run a process. You only need them to run a process safely. In most cases you can easily produce without safety functions. For example, emergency shutdown functions in the chemical, oil & gas, process, nuclear, or machinery industry. These functions are supposed to bring the process into a safe state (often this means stop the process) in case of an emergency. They should not fail in a way that they do not work when needed, but they should also not fail in a way that they work unintentionally and thus cause a spurious trip of the process.

STL 1, 2, 3, …

The spurious trip level is defined as a discrete level for specifying the spurious trip requirements of any function of a systems that can cause a spurious trip. An STL of 1 means that this function has the highest probability of spurious trips. The higher the STL level the lower the probability of a spurious trip. There is no limit to the number of spurious trip levels. The probability levels are defined as follows:

STL Probability of a spurious trip
etc etc
5 0.00001 – ≤0.000001
4 0.0001 – ≤0.00001
3 0.001 – ≤0.0001
2 0.01 – ≤0.001
1 0.1 – ≤0.01
0 0 – ≤ 0.1

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The spurious trip level represents asset loss due to an internal failure of a function. The more financial damage the function can cause due to a spurious trip the higher the STL level of the function should be.

Each company needs to decide for themselves which level of financial loss they can or are willing to tolerate. Actually, this depends on many different factors including the financial strength of the company, the insurance policy they have, the cost of process shutdown and startup, and so on. All these factors are unique to each company.

The table below shows an example of how a company can calibrate its spurious trip levels. In this example the enduser defined seven STL levels. Your company might have more or less depending on your needs and you you set the levels for monetary loss.

Example of an enduser STL table
STL Description
6 Spurious trip costs over USD 1M
5 Spurious trip costs between USD 500k and USD 1M
4 Spurious trip costs between USD 200k and USD 500k
3 Spurious trip costs between USD 100k and USD 200k
2 Spurious trip costs between USD 50k and USD 100k
1 Spurious trip costs between USD 10k and USD 50k
0 Spurious trip costs between USD 0 and USD 10k


Frequently Asked Questions

Why do I need STL?

If you do not mind spurious trips you don’t need STL. If you don’t like spurious trips then STL gives you a metric to measure them and thus to deal with them. 

What is the difference between STL and SIL?

There is a big difference between STL and SIL. Actually you need both to get the best out of your process.

Property SIL STL
Consequences for: People, Environment, Assets Production Loss, Down Time
Measure for: Safety Availability Process Availability
Levels: 1,2,3,4 Unlimited levels
High level means: Process is more protected Process is more available
Risk based: Yes Yes
Who specifies? Enduser Enduser

Can STL be independently certified?

Yes. Risknowlogy certifies the designs that suppliers give to endusers. With our certification we help the enduser receive better designs.

Does STL work with with safety instrumented functions?

Actually STL was originally invented for safety functions. So yes. But you can actually use it on any type of function that can cause a spurious trip.

How does STL help me with my safety functions?

For end-users there is always a potential conflict between the cost of safety solutions and the loss of profitability caused by spurious trips of these safety solutions. The STL concept helps the endusers to end this conflict in an a way that safety solutions provide both the desired safety and the desired process availability.

Can STL be applied to single products?

Yes. Systems and thus functions carried out be these systems usually consists of multiple products. STL can be applied to single products so that the enduser can see how much contribution this individual product has to the overall function STL. In this way the enduser can select suitable products during the specification or design process.

Can you calibrate our STL table?

Yes. Actually this is the start of each STL study.


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