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, electrically
or pneumatically, so the output signal corresponds to the level in the boiler.
0% of water in the level glass corresponds 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.
Differential pressure transmitter is a very common method for measuring the
level in a boiler.
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, electrically
or pneumatically, so the output signal corresponds to the level in the boiler.
0% of water in the level glass corresponds to 4 mA (3 psi) and 100% corresponds
to 20 mA (15 psi). The measuring result is insensitive to