A boiler is operating at 400 PSI, and the safety valve opens at 410 PSI and closes at 398 PSI. What is the blowdown?

The blowdown of a safety valve is defined as the difference between the pressure at which the safety valve opens and the pressure at which it closes. In this case, the safety valve opens at 410 PSI and closes at 398 PSI.

To calculate the blowdown, you subtract the closing pressure from the opening pressure:

Blowdown = Opening Pressure - Closing Pressure
Blowdown = 410 PSI - 398 PSI
Blowdown = 12 PSI

This indicates that the safety valve maintains a certain margin (blowdown) before it reseals after venting excess pressure. A blowdown of 12 PSI means that the safety valve is designed to open at a pressure of 410 PSI and will remain open until the pressure drops to 398 PSI, thus providing a controlled method to manage pressure within the boiler system safely.