What is the best method for steam drum level measurement?

The guided-wave radar technology is a preferred choice since it ensures higher plant availability and these transmitters provide more level detection capabilities even under harsh conditions

Diffential pressure transmitter

Wet leg DP Pressure measurement.

• DP level measurement is great once a system is running, but the fluctuations in density during start-up and shut-down can cause a loss of visibility to the actual level.
• Hydrastep is great for visual indication as it can actually "see" where the water and steam are.
• Guided wave radar needs to do compensation for the fluctuations in steam dielectric which can give incorrect readings if not properly compensated for. The vapor dielctric starts to affect the accuracy at 145 psia (10 bar). Emerson has Dymnamic Vapor Compensation which calculates the dielctric of the steam and uses it to compensate the level measurement.
• Using a mix of technologies can be a good idea instead of just relying on a single one, since steam drums are critical level measurements.

1.  Control and Instrumentation
2. 2. Steam drum level measurement with a differential pressure transmitter can be a tricky business when the pressure is higher than for "low" pressure steam. What happens is that as the temperature rises, the density of water drops while at the same time that of steam rises. To compound the problem, the wet leg temperature is not well defined and its density is a third variable. A technical way around the wet leg problem is to use the following level capture apparatus. fig.. The constant condensation in the top connection maintains a constant influx of hot water at equilibrium with the steam. This maintains the heat and ensures both wet leg and measurement sections are at the same temperature (that of the water in the steam drum), below the apparatus, the two impulse lines are in close contact and therefore at the same temperature. Whatever the density of the water is, it is the same in both legs and cancels out in the differential measurement WHYTHE LEVEL MEASURENT IS COMPLEX
3. 3. Within the apparatus itself, the dilemma remains.The simplest way is to use a correlation to provide the steam and the water densities as a function of the absolute pressure in the boiler.
4. 4. Looking at figure, we see that the differential pressure, P2 - P1 is: ∆P=g.(pw –ps). (H-X) where g is the gravitational acceleration constant: 9.807 m/s2, and pw =Is The Water Density at Operating Condition. ps =Is The Steam Density at Operating Condition. Furthermore we assume the transmitter is calibrated with cold water which has a density of Pc=998.2 Kg/m3 We note that when the level is 100%, the differential pressure is 0, regardless of whether the water is hot or cold. When the level is 0%, the differential pressure is maximum although this maximum value depends on the pressure in the steam drum. Now, assuming the transmitter has a reversed output, Lraw, calibrated with cold water, so that when the level increases so does the analog signal, then it is possible to express the level, L, in % as a function of the densities of the water (both hot and cold) and of the steam: L=100-(100-Lraw)/K , Where K= (pw –ps)/ Pc Than L= 100-(100-Lraw)/ (pw –ps) ,Where Assuming Pc=1 g/cc Formula Derive
5. 5. Lvl = (dRefCol – dDrmStm – LvlIn * dTransWtr) / (dDrmWtr – dDrmStm) Lvl = the percentage of the distance between the level measurement taps of the water level above the lower level measurement tap – expressed as 0 – 1.0. dRefCol is the density of the reference column water dDrmWtr is the density of the drum water dDrmStm is the density of the drum steam dTransWtr is the density of the transmitter calibration water Lvl= (dRefCol – dDrmStm – LvlIn) / (dDrmWtr – dDrmStm) Where dTransWtr= 1 g/cc Formula in Our DCS
6. 6. Lvl= 100(pa-ps)-(100-Dp%) (pw-ps) pw =Is The Water Density at Operating Condition. ps =Is The Steam Density at Operating Condition. pa=Is The wet Density. Dp %= Raw value of Transmitter out put. We are assuming here the drum pressure is 120 Kg/Cm2 and Dp % is 30 % Than pa-ps=0.936 and pw-ps=0.583 Lvl={100x0.936-(100-30)}/0.583 Lvl=40.48 % How calculate the Level in DCS
7. 7. L= 100-(100-Lraw) (pw –ps) We are assuming here the drum pressure is 120 Kg/Cm2 and Lraw is 30 % Than at 120 kg/cm2 ,pw-ps=0.583 L= 100-(100-30) (0.583) L=51.45 % How to calculate the Level if wet leg Density not Compensated
8. 8. Calculation Of Level By IJT Formula LEVEL SIGNAL AFTER PRESSURE COMPENSATION = {DP +(Pa-Ps)}/ (Pw-Ps) PW = Water Density in gm/cc PS = Steam Density in gm/cc DP= LEVEL IN DP TRANSMITTER Pa = Wet Leg Density in gm/cc We are assuming here the drum pressure is 120 Kg/Cm2 and Lraw is 30 % Than at 120 kg/cm2 ,pw-ps=0.583, pa- ps=0.936 L =(30+0.936)/0.583 L=53.06
9. 9. Drum press At 50deg C Pa Pw Ps Pa – Ps Pw – Ps In kg/sq.cm (g/cc) (g/cc) (g/cc) (g/cc) (g/cc ) 00.99940.95880.00060.99880.9582100.99990.88370.00550.99440.8782201.00040.84820.01030.99010.8379301.00080.82110.01520.98560.8059401.00130.7980.02020.98110.7778501.00180.77750.02540.97640.7521601.00230.75860.03070.97160.7279701.00280.74070.03680.9660.7039801.00320.72360.04220.9610.6814901.00370.70690.04840.95530.65851001.00420.69040.05490.94930.63551101.00470.6740.06180.94290.61221201.00520.65750.06920.9360.5831301.00570.63820.0780.92770.56031401.00620.62080.0870.91920.53781501.00670.60290.09670.910.5062 Drum pressureVs.Water and Steam Density

An insrument called the Hydrastep.

I works by sending electric current through water and steam at the same time and gives you an indication proprtional to the resistance of steam and water.

A differential pressure transmitter would be sufficient

we use it in our steam heater.

The Guided wave radar transmitter is the best method of level measurement in boiler steam drum.

• The guided-wave radar technology ensures higher plant availability as these transmitters provide more level-detection capabilities even under harsh conditions. Completely independent of density changes - no algorithms needed to be programmed in the control system

• You save time and money, and reach a stable level signal

• Fit for applications with temperatures up to 450˚C (842 ˚F) /400 bar (5801 psi); if more, traditional dp level is used and density correction calculation are offered

The drum level is measured using a differential pressure transmitter. The output of the instrument increases as the differential pressure decreases. A typical range is 30 inches of water column. A condensing reservoir is installed to allow the high side of the D/P to measure the steam pressure plus the hydraulic pressure in the reference leg. The low pressure side senses the boiler drum pressure, the weight of the water above the low pressure tap and the weight of a column of saturated steam from the high pressure tap to the water level. Being a differential pressure device, the boiler drum pressure is canceled out of the measurement, leaving only the water column pressure difference.

The level measurement is accurate only at a single drum pressure. If needed, a pressure measurement can compensate for varying drum pressures by applying a gain and bias to the drum level signal.