In IMS the Spurious Trip Calculator can be used to determine whether it makes sense, from an investment cost point of view, to add additional hardware to the SIF design to make the SIF design more robust, or as it is also called “Safe Fault Tolerant”.
Safe Fault tolerant design means that on a failure of a single component of the SIF (e.g., a pressure sensor), the SIF will not go to the trip state thereby tripping the process unit but keeps on performing its safety mission with a redundant device. Examples of Safe Fault Tolerant SIF designs are 2oo2, 2oo3, 2oo4 etc.
Safe Fault Tolerant SIF designs will lead to less process upsets or unit trips as a result of a faulty SIF component. Less process upsets or unit trips due to spurious trips means more uptime and more profitability of that unit.
Therefore, when determining whether it makes sense from an investment cost point of view to add additional hardware to the SIF design to make the SIF design Safe Fault Tolerant, the Spurious Trip Calculator in IMS looks at the cost of lost production hours for the unit against the additional investment cost.
Important Notice
The spurious trip calculator in IMS can only be used from the equipment level to research different Safe Fault Tolerant designs (e.g. 2oo2, 2oo3 etc.) against a NON Safe Fault Tolerant design (e.g. 1oo1, 1oo2 etc.) per subsystem such as a Sensor or Final Element subsystem.
The resulting outcome of whether the inclusion of additional hardware to the original subsystem hardware design into a Safe Fault Tolerance design is “Justified“ as per a 15% ROACE cut-off is only indicative to the user. It will not automatically update the SIF design page that the equipment is used in with a Safe Fault Tolerant design and will not appear in any SIF report or SRS printout. It is designed to support the SIF designer and the company project organization to make informed decisions on their (project) SIF designs.
Most of the time, the spurious trip calculator will be used on sensors as sensor configurations are relatively cheap to upgrade to a dangerous AND safe fault tolerant design (e.g. from 1oo1 to 2oo3 or from 1oo2 to 2oo3) thereby assuring a low sensor subsystem spurious trip rate. As the new safe fault tolerant design still has to achieve the SIF Target PFD, often a 2oo3 sensor configuration is chosen as opposed to 2oo2 to maintain a low sensor subsystem PFD.
For valves, the spurious trip calculator will be used less as dangerous AND safe fault tolerant valve configurations are relatively expensive and complex. This is especially so when upgrading to a dangerous AND safe fault tolerant valve design. For example, from 1oo1 valve configuration to a dangerous AND safe fault tolerant valve design, 3 additional valves would have to be installed in a 2oo4 valve configuration.
Using the Spurious trip calculator in IMS
To use the Spurious trip calculator:
Go to the Equipment Details Page.
Click on the Spurious trip calculator button (to the right of the Equipment name).
A new screen opens where you need to fill in the below data before the ROACE calculation can be performed:
Economic Consequence of a Spurious trip
In this section you need to fill in the economic cost of a spurious trip when this piece of Equipment (sensor or valve) spuriously fails to the safe position. This economic cost is typically due to the time to repair the Equipment and starting up the process unit again.
Click Edit in the Consequence of spurious trip section.
Select an economic consequence of spurious trip via the drop-down box.
Alternatively you can use the Production Loss Equation (PLE) option to set the economic consequence of a spurious trip:
Click the PLE button to open a new window.
Select which unit PLE to use, what the estimated downtime of the spurious trip is, etc.
Click Save when done.
Take Note
If a PLE is defined on the Unit the Equipment resides in for the period it takes to repair the Equipment, the equivalent economic consequence will be calculated by multiplying the downtime in above screenshot by the PLE economic loss per hour. If a fixed value is selected from the Asset consequence severity dropdown, the value in the Equivalent economic consequences field is used as a fixed economic consequence per spurious trip.
Device like
In this section select the Equipment type that will be added to the sensor or valve configuration, which will make the design Safe Fault Tolerant. You can choose to add a Master Equipment or an existing Equipment. Choosing what Equipment to add depends on the existing configuration and the future configuration as explained below:
If you want to check a future configuration of 2oo2 against a current configuration of 1oo1, you only add a Master Equipment that is of the type to be added in the future configuration.
If you want to check a future configuration of 2oo3 against a current configuration of 1oo1, you add twice the Master Equipment that is of the type to be added in the future configuration.
If you want to check a future configuration of 2oo3 against a current configuration of 1oo2, you add a Master Equipment that is of the type to be added in the future configuration AND you add the Equipment that is already there in the “1oo2“ subsystem.
Success Criteria Configuration
Here click the Edit buttons to set the Success Criteria of the current configuration and the future proposed Safe Fault Tolerant configuration.
Calculator
Once all the data has been entered click the Calculate button in the right of the screen. IMS will calculate whether the additional investment cost of the proposed Safe Fault Tolerant design is justified against the ROACE cut-off point of 15%.
The Calculations Explained
IMS uses the following formula to determine whether it makes sense from an investment cost point of view to add additional hardware to the SIF design to make the SIF design Safe Fault Tolerant:
ROACE: Return On Average Capital Employed (Note: Cut off point in IMS is 15%.)
Depreciation: The depreciation cost of the investment per year (USD/y)
Test Cost: The additional Testing Costs of the investment per year (USD/y)
Repair Cost: The additional Repair Costs of the investment per year (USD/y)
Benefits: The difference in economic risk of the non-SFT and SFT design (USD/y)
If the IMS Spurious Trip Calculator returns a value >15%, the additional investment cost is considered “Justified” against the cost of a spurious trip for a non-SFT SIF design.
Note: Safety considerations are rarely included in the Spurious Trip Calculation as a spurious trip of a SIF puts the process into a safe state.
Take Note
If a Safe Fault Tolerant design is “Justified” according to the Spurious Trip Calculator, before implementing that design, the SIF designer needs to verify whether the SIF target PFD, as set by the SIL of the SIF, can still be achieved with this Safe Fault Tolerant design also.
Determining the Benefits factor in the ROACE formula
The Benefits factor in the ROACE formula is determined by calculating the yearly cost of the non-SFT SIF design and subtracting the calculated value of the yearly cost of the SFT SIF design.
Calculating the yearly cost of the non-SFT SIF design (1oo1)
Assume:
1oo1 Pressure sensor arrangement
λs (Spurious Trip Rate on the Equipment Details Page, Details Section): 0.01/y
MTTR (Mean Time To Repair on Equipment Details Page, Repair Section): 4 hours
Production loss of the unit on spurious trip: 100.000 USD/hr
The risk of a Spurious Trip cost per year of the non-SFT design is then calculated as:
Yearly cost of the non-SFT SIF design (1oo1) = (4 x 100.000) x 0.01 = 4.000 USD/year
Calculating the yearly cost of the SFT SIF design (2oo2)
Assume:
2oo2 Pressure sensor arrangement
λs sensor 1 (Spurious Trip Rate on the Equipment Details Page): 0.01/y
λs sensor 2 (Spurious Trip Rate on the Equipment Details Page): 0.01/y
MTTR (Mean Time To Repair on Equipment Details Page, Repair Section): 4 hours
Production loss of the unit on spurious trip: 100.000 USD/hr
The formula used to determine the STR (Spurious Trip Rate) of a 2ooN is as follows:
The common cause failure term (second part of the formula with the β-factor) must always be added because usually:
The β-factors of the Equipment can be found in the FMEA section on the Equipment Details Page. In the IMS Spurious Trip Calculator, conservatively, the β-factors of both Identical and Diverse devices are averaged and added.
From below example:
λs Identical Devices High and Low added and divided by 2: (2 + 3)/2=2.5%
λs Diverse Devices High and Low added and divided by 2: (1 + 2)/2=1.5%
The β in the STR formula above used in IMS is then: 2.5% + 1.5% =4%
Note: if there are 2 or more devices for the Safe Fault Tolerant design with different values in the FMEA section for λs Identical Devices and λs Diverse Devices, conservatively, IMS will use the highest λs of the devices in the ROACE calculations.
As stated before, the common cause failure term (second part of the formula with the β-factor) of the STR formula of a 2ooN system is much larger than the first term (because MTTR 4 hrs/year is so small). Therefore, in this example, for calculating the yearly cost of the SFT SIF design (2oo2) only the common cause failure term of the STR formula of a 2ooN system is used:
Take Note
The above statement is equally true for any MooN configuration as:
STR2oo2 = 0.04 x (0.01 + 0) = 0.004/y
λDD is considered already included in λs in IMS failure rate data.
The risk of a Spurious Trip cost per year of the SFT design is then calculated as:
Yearly cost of the SFT SIF design (2oo2) = (4 x 100.000) x 0.004 = 1.600 USD/year
Calculating the Benefits
The Benefits from the ROACE formula are thus: 4.000 – 1.600 = 2.400 USD
Determining the Depreciation factor in the ROACE formula
IMS calculates Depreciation based on the Investment Cost divided by 10 years. The Investment Cost can be changed in IMS on the Equipment Details Page in the Repair Section.
Determining the Test Cost factor in the ROACE formula
IMS calculates Test Cost based on the yearly test cost of the additional hardware installed to achieve a Safe Fault Tolerant design (e.g., 1 additional transmitter for a 2oo2 design).
Determining the Repair Cost factor in the ROACE formula
IMS calculates Repair Cost based as the average repair cost per year, based on the average safe and dangerous failures of the Equipment Tag. The repair cost can be changed in IMS on the Equipment Details Page in the Repair Section.
Determining the Investment Cost factor in the ROACE formula
The Investment Cost can be found in the in IMS on the Equipment Details Page in the Repair Section.
Result
When we fill in the (above example) data for Depreciation, Test Cost, Repair Cost and Investment Cost in the ROACE formula we get a ROACE result of >15% and is therefore Justified for sites that use a ROACE cut-off point of 15%.