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
Measure First

How to find the range of DP cell Transmitter for measurement of level in closed vessel.

3 Answers   Rosemount,

---

Advantages of servers over normal desktop in PLC SCADA system

1 Answers   InfoMedia,

---

How an rtd work, and what volts/current goes to a rtd to measure the resistance?

0 Answers  

---

Who to difference between analog in & analog out in dcs?

0 Answers  

---

how radio frequency level probes works with its controller ? what is its principle ? pls give me brief explanation?

0 Answers  

---

how many maximum enter 2mm Dia solid cylindrical pipe into 8mm Dia hollow cylindrical pipe. give answer with calculation.

0 Answers  

---

difference between null and deflecting instrument.

5 Answers   ONGC,

---

what is the difference between 2 wire and 3 wire RTD- application wise,cost effectivity?

2 Answers  

---

HOW TO CALIBRATE A PRESSURE SWITCH?

7 Answers  

---

why we are maintain the gap voltage in vibration probe when installing time can explain me?

1 Answers   CIL,

---

why we use negative 24vdc to a proximeter

4 Answers   KNPC,

---

how u calibate pr tx without master tx?

2 Answers  

---