This method uses normally a
differential pressure measuring system with "wet leg", that is, the
impulse pipes to the positive measuring point below the lowest water level and
the impulse pipe to the negative measuring point above the highest water level,
are both filled with water. The differential pressure signal is normally
elevated so the output signal corresponds to the level in the boiler. 0% of
water in the level glass correspond to 4 mA (3 psi) and 100% corresponds to 20
mA (15 psi). The measuring result is insensitive to foam in the boiler drum
since the measuring method is based on the weight of the water.
If you connect the transmitter this way, with the high pressure leg to the
boiler's lower measuring point, then the output signal will be direct, i.e.,
raising water level gives an increasing output signal, and hence a signal cable
failure will cause a burner-trip due to low water level. That is, if you use
the level-signal to activate the emergency low level trip of the burners.
This method uses normally a
differential pressure measuring system with "wet leg", that is, the
impulse pipes to the positive measuring point below the lowest water level and
the impulse pipe to the negative measuring point above the highest water level,
are both filled with water. The differential pressure signal is normally
elevated so the output signal corresponds to the level in the boiler. 0% of
water in the level glass correspond to 4 mA (3 psi) and 100% corresponds to 20
mA (15 psi). The measuring result is insensitive to foam in the boiler drum
since the measuring method is based on the weight of the water.