What happens when the valve caught fire?
When a fire breaks out, the pneumatic actuator’s fail-safe position will be adversely affected. The high temperatures result in the springs losing their strength.
As the temperature increases, the spring keep reduces its strength and no longer keep the valve in its fail-safe position. In the case of fail-safe action Fail-close, increasing leakage cannot be avoided.
How to make the valve safe when the fire occurs?
There are two methods to make the control valve fail-safe in case fire:
- Safety cartridge
- Reversible operating direction
This cartridge consists of two cylinders sliding freely within each other as shown in the figure.
The cartridge is made up of a two-piece cylindrical case. The enclosed cylindrical chamber contains the intumescent material and composed in a way that material expands when it detects temperature within the range. The increase in volume causes the cylinders to move in opposing directions.
This safety cartridge is placed between the diaphragm and seal. When overheat occurs the cartridge expand and ensure decreasing spring force compensation.
Installation of the safety cartridge must be at the centre of the actuator springs. The pneumatic actuator’s mounting position is not important, in the event of a fire, the cartridge can always perform its task of closing the valve.
Reversible operating direction:
This method includes pre-loaded spring disks that act as opposed to standard actuator springs. A solder strip binds the spring disks.
The solder type determines the temperature at which the spring disks are released to act as an additional safety device. The standard fail-safe action is reversed when the spring disks are released. Under practice, the solder joint is extremely stable