Answer / carl_ellis

Water vapor in a vessel (like a boiler drum) will condense
in piping/tubing attached to the upper tap. The condensed
water collects in the piping/impulse tubing connected to the
transmitters low side port.

The water exerts a pressure on the transmitter's low side
port. If the liquid columns stays at the same height, then
it can assumed to be a constant which can be used as part of
the level calculation.

So boilers use a 'wet leg' where the low side leg is
deliberately filled with fluid and is expected to remain at
a constant level.

A differential pressure level transmitter works by
subtracting the low side from the high side.
DP = high side - low side

In this case, the low side liquid column of water exerts a
pressure of 450 mm wc. wc = water column

When the drum is empty,
the high side measures 0 pressure, 0 mm wc.
the wet leg measures 450 mm wc.
DP = high side minus low side
DP = 0 mm - 450 mm = -450 mm wc

When the drum is full, the high side pressure will be (steam
pressure + 450 mm wc) where the 450 mm wc is the water
column in the drum.

The low side pressure will be (steam pressure + 450 mm wc)
where the 450 mm wc is the water column in the low side
impulse tubing/piping.

DP = high side minus low side
DP = (steam pressure + 450 mm wc) - (steam pressure + 450 mm
wc) = 0 mm wc

So empty = -450 mm wc, full = 0 mm wc.

The transmitter's range is -450 to 0 mm wc.

Carl Ellis
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