SIS Spurious Trip Calculator

In IMS SIS 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 SIS looks at the cost of lost production hours for the unit against the additional investment cost.

Using the Spurious trip calculator in IMS SIS

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.

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.

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

• Lambda S (Spurious Trip Rate on the Equipment Details Page): 0.01/y

• Mean Time To Repair (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:

(Mean Time To Repair * Production Loss per hour) * Lambda S of the non-SFT tag

Yearly cost of the non-SFT SIF design (1oo1) = (100.000 * 4) * 0.01 = 4.000 USD/year

Calculating the yearly cost of the SFT SIF design (2oo2)

Assume:

• 2oo2 Pressure sensor arrangement

• Lambda S sensor 1 (Spurious Trip Rate on the Equipment Details Page): 0.01/y

• Lambda S sensor 2 (Spurious Trip Rate on the Equipment Details Page): 0.01/y

• Mean Time To Repair (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 Beta factor) must always be added because usually

The Beta factors of the Equipment can be found in the FMEA section on the Equipment Details Page. In the IMS Spurious Trip Calculator, conservatively, the Beta factors of both Identical and Diverse devices are averaged and added.

From below example:

Lambda S Identical Devices High and Low added and divided by 2: (2 + 3)/2=2.5%

Lambda S Diverse Devices High and Low added and divided by 2: (1 + 2)/2=1.5%

The Lambda S (β) in the STR formula above used in IMS SIS 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 Lambda S Identical Devices and Lambda S Diverse Devices, conservatively, IMS SIS will use the highest Lambda S of the devices in the ROACE calculations.

As stated before, the common cause failure term (second part of the formula with the Beta 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:

STR = 0.04*(0.01 + 0) = 0.004/y

Lambda dd is considered already included in Lambda s in IMS SIS failure rate data.

The risk of a Spurious Trip cost per year of the SFT design is then calculated as:

(MTTR * Production Loss per hour) * STR of the 2oo2 of the SFT system

Yearly cost of the SFT SIF design (2oo2) = (100.000 * 4) * 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%.