drum level measurement
DESCRIPTION
drum levelTRANSCRIPT
What is boiler ?
What is a boiler ?When water is heated in a boiler, it begins to absorb energy. Depending on the pressure in the boiler, the water will evaporate at a certain temperature to form steam. The steam contains a large quantity of stored energy which will eventuallybe transferred to the process or the space to be heated
Heat exchangerSteam turbine3Drum level measurement
Application overview
• Steam Drum Level is both a critical and difficult measurement to make. Control of the water level in the drum must be precise.
• A water level that is too high can result in water carryover into the steam piping. A level that is too low can expose the generating tubes (down comers), preventing the water in the drum from cooling the furnace tubes, possibly damaging them.
• Several factors make this measurement difficult to obtain. The steam drum itself may not be perfectly level, and even at steadystate conditions, considerable turbulence in the drum can cause the level to fluctuate.
• In addition, a changing rate of water inflow and steam outflow adds to the potential for measurement error.
Drum level measurement 7
Application overview
• Measurement of boiler steam drum level using a differential pressure transmitter must take into account certain physical properties of the fluid.– The Steam drum contains a two-phase mixture of water and steam at
saturation conditions.
– The densities of water and steam vary with saturation temperature or pressure.
– The density of saturated steam above water must be considered, as well as the density of saturated water in the drum.
• This technical note offers a method for calibrating transmittersthat takes into account these factors.
Drum level measurement 8
Water Property
Drum level measurement 9
1 lbm
320F 0.01602 ft3
ρ=
= 62.42 lbm/ft3
1 lbm
0.01602 ft3
ρ =62.42 lbm/ft3
Pressure (constant)
HEAT
Water Property
Drum level measurement 10
1 lbm
1200F 0.01620 ft3
ρ=
= 61.73 lbm/ft3
1 lbm
0.01620 ft3
ρ =61.73 lbm/ft3
Pressure (constant)
HEAT
Water Property
Drum level measurement 11
1 lbm
320F 0.01602 ft3
ρ= 1 lbm0.01672 ft3
1200F 0.01620 ft3ρ =61.73 lbm/ft3
ρ =62.42 lbm/ft3
2120F 0.01672 ft3ρ =59.81 lbm/ft3
Pressure (constant)
= 59.81 lbm/ft3
HEAT
Saturated Water
Drum level measurement 12
1 lbm
2120F 0.01672 ft3Boiling point
HEAT
Boiling
No bubble
Saturated Water
Drum level measurement 13
1 lbm
2120F 0.01672 ft3
swell
Pressure is increased
Water andSteam bubble
Boiling
2120F
BoilingWith
Bubble
Saturated Water
Drum level measurement 14
1 lbm
2120F 0.01672 ft3
more swell
Pressure is increased
Water andSteam bubble
Drum level measurement 16
Drum level measurement
ρWD
ρW
ρS
Pd
0
+ 300
- 300h
H
Saturated water
PH PL
Saturated Steam
Condensing pot
V
Drum level measurement 17
Drum level measurement
Define process variable
• ρS = steam density
• ρWD = drum water density
• ρW = water density
• g = gravity
• PH = Static pressure at the high side of the transmitter
• PL = Static pressure at the low side of the transmitter
Define process variable
• Pd = Static pressure in the steam drum at the top tap
• H = Distance between the high and low drum taps
• h = Drum water level (measured from the bottom tap)
• V =Vertical distance from the bottom tap to the transmitter
Drum level measurement 18
Drum level measurement
Solution
PH = ρw.g.V + ρWD.g.h + ρS.g.(H-h) + Pd
PL = ρw.g.V + ρW.g.H + Pd
∆P = ρw.g.V + ρWD.g.h + ρS.g.(H-h) + Pd - ρw.g.V - ρW.g.H - Pd
∆P = ρWD.g.h + ρS.g.H - ρS.g.h - ρW.g.H
∆P = ρWD.g.h - ρS.g.h - ρW.g.H + ρS.g.H
∆P = g.h(ρWD- ρS) - g.H(ρW - ρS)
Drum level measurement 19
Ex. Drum level measurement
Ex. operating condition 100 °°°°C at pressure 14.696 Psia
Determine hmin and hmax (±300 mm)
From steam table ρWD= 958.098 , ρS= 0.598, ρW(43 °C ) = 990.628
Level max.(100 %) h = H = 0.6 m
∆P(100%)= PH – PL
= g.h(ρWD – ρS) – g.H(ρW – ρS)
= 9.807×0.6×(958.098–0.598)–9.807×0.6×(990.628–0.598)
= 5633.93 – 5825.33
= – 191.40 Pa
Drum level measurement 20
Ex. Drum level measurement
Ex operating condition 100 °°°°C at pressure 14.696 Psia
Level min.(0 %) h = 0 m, H = 0.6 m
∆P(0%) = PH – PL
= g.h(ρWD – ρS) – g.H(ρW – ρS)
= 0 – 9.807 ×0.6 × (990.628 – 0.598)
= 0 – 5825.33
= – 5825.33 Pa
Drum level measurement 21
Calibration range
• Finally, check the zero elevation and span against the transmitter specifications to ensure the selected transmitter can be calibrated to the required values.
% Level (m) D/P (Pa) Signal (mA)
100 0.6 -191.40 20
0 0 -5825.33 4
Drum level measurement 22
Calibration range
Level %
DBR(m)
Range (m)
PH (Pa)
PL(Pa)
DP(Pa)
O/P (mA)
Controller(m)
100 0.60 0.30 5633.93 5825.33 -191.40 20.000 0.600
75 0.45 0.15 4225.44 5825.33 -1599.88 16.000 0.450
50 0.30 0.00 2816.96 5825.33 -3008.36 12.000 0.300
25 0.15 -0.15 1408.48 5825.33 -4416.85 8.000 0.150
0 0.00 -0.30 0.00 5825.33 -5825.33 4.000 0.000
Drum level measurement 23
Normalization Unit
Percent (0 - 100%) to level(0 – 0.6 m)
At 50 %, what is Level ?
Solution
sensitivity = span output / span input
= 0.6/100
= 0.006 m / %
At 50 %
level = 0.006 × 50
= 0.3 m
Drum level measurement 24
Normalization Unit
Level 0 – 0.6 m to DP (- 5825.33 – (-191.40))
At 0.3 m, What is differential pressure ?
Solution
sensitivity = span output / span input
= (-191.40 – (- 5825.33)) / (0.6 – 0)
= 9389.88 Pa/m
At 0.3 m
∆P = 9389.88 ×(0.3-0) + (- 5825.33)
= - 3008.36 Pa
Drum level measurement 25
Normalization Unit
DP (-191.40 – (- 5825.33)) to Signal (4-20 mA)
At (- 3008.36 Pa), What is Signal ?
Solution
sensitivity = span output / span input
= (20-4) / (-191.40 – (- 5825.33))
= 2.84 ×10-3 mA/Pa
At (- 3008.36 Pa)
signal = 2.84 ×10-3 × (- 3008.36 - (- 5825.33)) + (4)
= 12 mA
Drum level measurement 26
Level Transmitter
Transmitter Signal 12 mA ,What is water in drum level ?
Solution
sensitivity = span output / span input
= (20-4) / (0.6 – 0)
= 26.67 mA/m
At 12 mA
controller = [(12-4)/26.67] + (0)
= 0.3 m
Drum level measurement 27
Level & DP
Level
∆P
0% 100 %50%25% 75%
0 0.15 0.30 0.45 0.6 m
-191.40
-1599.88
-3008.36
-4416.85
-5825.33
Drum level measurement 28
DP Transmitter signal
-191.40
-1599.88-3008.36
-4416.85
-5825.33
mA %
∆P (Pa) 0% 100 %50%25% 75%
20
16
12
8
4
100%
0%
50%
25%
75%
0.60
0.45
0.30
0.15
0.0
Level (m)
Drum level measurement 29
Drum level measurement
At Operating condition 353 °°°°C at pressure 2498.1 Psia
Level min.(50 %) h = 0.3 m, H = 0.6 m
Solution
∆P(50%)= PH – PL
= g.h(ρWD – ρS) – g.H(ρW – ρS)
= 9.807×0.3×(560.478 – 122.4)– 9.807×0.3 ×(990.628 – 122.4)
= 1288.82 – 5108.65
= – 3819.83 Pa
Drum level measurement
PH PL
0%
50%
100%
PH PL
0%
50%
100%
Operating conditionTemp = 100 °CPressure = 14.696 Psia �P = - 3008.36 Pa Signal = 12 mA
Operating conditionTemp = 353 °CPressure = 2498.1 Psia �P = - 3819.83 Pa Signal = ???? mA
30Drum level measurement
Drum level measurement 31
Normalization Unit
DP (-191.40 – (- 5825.33)) to Signal (4-20 mA)
At (- 3819.83 Pa), What is Signal ?
Solution
sensitivity = span output / span input
= (20-4) / (-191.40 – (- 5825.33))
= 2.84 ×10-3 mA/Pa
At (- 3819.83 Pa)
Signal = 2.84 ×10-3 × (- 3819.83 - (- 5825.33)) + (4)
= 9.69 mA
Drum level measurement 32
Level Transmitter
At Transmitter Signal 9.69 mA ,What is water in drum level ?
Solution
Sensitivity = span output / span input
= (20-4) / (0.6 – 0)
= 26.67 mA/m
At 12 mA
Controller = (9.69-4)/26.67 + (0)
= 0.214 m
Drum level measurement 33
Drum level measurement
• Error = 0.214 - 0.3 = - 0.086 m
• Water Level at 35% from 50 % or reduce 15%
Condition % Level
(m)
DP
(Pa)
Signal
(mA)
Controller
(m)
100 °C
14.696 Psia
50 0.3 - 3008.36 12 0.3
353 °C
2498.1 Psia
50 0.3 - 3819.83 9.69 0.214
Drum level measurement 34
DP Transmitter signal
-191.40-3008.36-5825.33
mA %
∆P (Pa) 0% 100 %50%25% 75%
20
16
12
8
4
100%
0%
50%
25%
75%
9.69
-3819.83
0.214
0.60
0.45
0.30
0.15
0.0
Level (m)
Drum level measurement 35
Drum level measurement
Ex Operating condition 353 °°°°C at pressure 2498.1 Psia
Determine hmin and hmax (±300 mm)
From steam table ρWD= 560.478 , ρS= 122.400, ρW(43 °C ) = 990.628
Level max.(100 %) h = H = 0.6 m
Solution
∆P = PH – PL
= g.h(ρWD – ρS) – g.H(ρW – ρS)
= 9.807×0.6×(560.478 – 122.4)–9.807×0.6×(990.628 – 122.4)
= 2577.65 – 5108.65
= – 2531 Pa
Drum level measurement 36
Drum level measurement
Ex Operating condition 353 °°°°C at pressure 2498.1 Psia
Level min.(0 %) h = 0 m, H = 0.6 m
Solution
∆P = PH – PL
= g.h(ρWD – ρS) – g.H(ρW – ρS)
= 0 – 9.807 ×0.6 × (990.628 – 122.4)
= 0 – 5108.65
= – 5108.65 Pa
Drum level measurement 37
Calibration range
Level %
DBR(m)
range (m)
PH (Pa)
PL(Pa)
DP(Pa)
Output (mA)
Controller(m)
100 0.60 0.30 2577.65 5108.65 -2531.00 13.356 0.351
75 0.45 0.15 1933.23 5108.65 -3175.41 11.526 0.282
50 0.30 0.00 1288.82 5108.65 -3819.82 9.696 0.214
25 0.15 -0.15 644.41 5108.65 -4464.23 7.865 0.145
0 0.00 -0.30 0.00 5108.65 -5108.65 6.035 0.076
Drum level measurement 38
Density compensate
From differential pressure formula
∆P = g.h(ρWD- ρS) - g.H(ρW - ρS)
Liquid level in boiler drum
∆P + gH(ρw-ρs)
g(ρWD-ρs)h =
Drum level measurement 39
Controller or DCS Card
Density compensate
4-20mA to steam table (ρWD,ρS)
4-20mA to ∆P
∆P + gH(ρw-ρs)
g(ρWD-ρs)h =
ρWD
ρS
signal
signal
Exercise
Drum level measurement 40
ρWD
ρW
ρS
Pd
0h
H
Saturated water
PH PL
Saturated Steam
Condensing pot
V
What is calibration range of D/P transmitter at operating condition 200 °C and wet leg temperature 43.333 °C WhereH = 1000 mmh = 700mmV = 150 mm