Calculating the PFHD value (Probability of Dangerous Failure per Hour)

Calculating the PFHD value (Probability of a Dangerous Failure per Hour) is an important part of the safety evaluation of control systems. Here are the steps you can follow to determine the PFHD value:

1. Determine the MTTFd (Mean Time To Dangerous Failure)
   • MTTFd is the average time until a dangerous failure occurs. This value can be calculated using the B10d value (the number of operating cycles after which 10 % of the components exhibit a dangerous failure) and the number of cycles per year (Nop).
   • Formula: MTTFd = B10d / (0.1 × Nop)
   • B10d = Number of operating cycles at which 10 % of the components fail.
   • Nop = Number of operating cycles per year.

2. Calculate the PFHD value
   • Once you have the MTTFd, you can compute the PFHD value.
   • Formula: PFHD = 1 / (MTTFd × 8 760)
   • Where 8 760 is the number of hours in one year.

3. Using SISTEMA
   • SISTEMA is a software tool that can help you calculate the PFHD value and other safety parameters. Enter the required data, such as MTTFd, and the tool will determine the PFHD value for you.

Example:

• Suppose a component has a B10d value of 1 000 000 operating cycles and the yearly operating cycles (Nop) are 100 000.
• Calculate MTTFd: MTTFd = 1 000 000 / (0.1 × 100 000) = 100 years
• Calculate PFHD: PFHD = 1 / (100 × 8 760) ≈ 1.14 × 10^–6 per hour

• The B10d value is a measure of the reliability of a component in a safety system. It indicates the number of operating cycles after which 10 % of the tested components experience a dangerous failure.
• This value is essential for calculating the MTTFd (Mean Time To Dangerous Failure) and ultimately the PFHD value (Probability of a Dangerous Failure per Hour).
• The B10d value is commonly used in the context of mechanical, pneumatic and electromechanical components such as relays and valves.
• It helps determine how often a component can be safely used before there is a significant risk of a dangerous failure.