# Reversing the Range for Level Transmitter While Measuring Level

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#### Sakthi Venkatesh

I've calibrate a Level Transmitter in the range 0 - 2500 mm H2O. After installing the Transmitter in Boiler Drum, My senior Engineer told me to Reverse the Range ie.
Calibrated Range = 4mA -> 0 mmH2O ; 20mA-> 2500mmH2O

Reversed Range = 4mA -> -2500 mmH2O ; 20mA->0 mmH2O

I want to know why we are doing this?

kindly provide information

J

#### jay

the DP cell is put in reverse to calibration (L side of meter is on bottom tap and H side is on top tap) or it has a filled leg going to top tap that applies equal pressure as the span and seems likely since you are measuring steam drum level.

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#### Curt Wuollet

Because your senior engineer told you to:^)

Regards
cww

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#### LItto

or you may do one more thing, consider the range from -2500 to 0. that is what we usually do in our plants

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#### Sam

In the old days when we used electronic and pneumatic DP cells we had to swop the HP and LP legs around for the reason that a electronic and pneumatic DP transmitter could only measure in the positive. Swopping the legs around solved this problem. Now if you are instructed by your supervisor to do this, first have a look what type of transmitter you are working with. If it is a SMART transmitter there is no need to swop the legs around since a SMART transmitter can measure in the positive as well as the negative. So with a SMART transmitter you will have a calibration of about -2500mmH2O to +0mmH2O or close to this depending on what the transmitter first read when you install it and your LP leg which is connected to the top is filled with water. Taking a direct measurement on the DP transmitter is always the most accurate way to determine your zero but you can calculate it as well if it is difficult to use direct measurements.

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#### mahmoudfawzy3

The drum of boiler is closed vessel has pressure in side. to measure the level of water inside we use Dp transmitter and the equation used inside the transmitter is Dp = PH -PL. if high pressure side of Dp transmitter is connected to low tapping point and low pressure side is connected to high tapping point through impulse tube and condensate pot, and the distance between the two tapping point is H" in your case is 2500 mmh2o"

So the equation of transmitter will be
Dp = heed of water inside the drum + pressure inside the drum -(pressure inside the drum + head of water in side the tube in the distance between the two tapping point at low pressure side "2500 mmh2o")

Let speak by number better!

If there is no water inside the drum
Dp = 0 -2500 = -2500 mmh2o---> 4 mA

If the drum full of water
Dp = 2500 - 2500 = 0 mmh2o ---> 20 mA

According to that calculation the correct range must be -2500 to 0.
The reading of Dp transmitter goes to DCS and there are some calculation to get the correct reading of the drum level around the NWL of the drum. try to find this calculation. it will help you

Best regards

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#### Sam

"Dp = heed of water inside the drum + pressure inside the drum -(pressure inside the drum + head of water in side the tube in the distance between the two tapping point at low pressure side "2500 mmh2o")"

No where is the pressure inside a closed pressurized vessel ever considered in a level calculation.
Also please write down the procedure, as you have it now, for us if the zero point is 50mm above the bottom tapping point and the 100% is marked as 70 mm below the top tapping point, with the distance between the tapping points still at 2500mm. How will you calculate the URV and LRV then?

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#### d

In the case of steam drums at high pressures, the density of the steam vapor is pressure dependent and must be included in the transmitter span calculations.

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#### mahmoud fawzy

>No where is the pressure inside a closed pressurized vessel ever considered in a level calculation.

as you see in equation the pressure inside the drum effect removed by the DP transmitter

>Also please write down the procedure, as you have it now, for us if the zero point is 50mm above the bottom tapping point and the 100% is marked as 70 mm below the top tapping point, with the distance between the tapping points still at 2500mm. How will you calculate the URV and LRV then?

as i understand from your description the total length between the two tapping point is 50+2500+70 mm.
first of all the effect of the 50 mm will remove by DP transmitter, still we have the effect of the 70 mm. as you will see
at o level of water inside the drum
DP = 0 - 2570 mm = -25700 mm.---> lower than 4 mA
at 100% level of water inside the drum
DP = 2500 - 2570 mm = -70 mm ---> lower than 20 mA
to remove the effect of the 70 mm your URV & LRV will be

URV = -70 mm.
LRV = -2570 mm.

best regards

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#### Sam

Actually the calibration for the example as I have described is: LRV = -2450mmH2O and the URV = -70mmH2O. That is if the DPT is installed at the lower tapping point which in most applications are not the case. Bad practice to do an installation that way.

The pressure inside a close vessel have no effect on the calibration of the Level DPT since whenever there is a change in pressure inside the vessel, regardless the reason, this pressure change take place all over and will be identical at the LP as well as the HP side of the transmitter. The increase or decrease in vessel pressure will therefore always be at zero deferential across the transmitter and is therefore not taken into account, ever.

In a steam drum application, only very high temperatures and pressures will change the density of the steam vapour but will have little to no effect on the liquid density inside the drum. Evaporation and condensation of the steam at very high temperatures and pressures takes place all the time inside the drum and tends to cancel out the effect that the vapour has on the liquid.

The only thing that will influence a DPT level application is when the actual composition of your product keeps on changing in which case a different non-density dependant type of instrument is used.

D

#### d

grab your steam tables and run a test calc. for a HP boiler with the wet leg temps at ambient temperature.

Some powerhouses want a differential measurement with Normal Condentate Level marked as zero. All of a sudden the 2-3% difference in indicated level when the steam density is included, can be a serious issue. It is more of a concern for higher pressure boilers.

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#### otised

The main reason is so 4 ma is lowest level and 20 ma is highest level. This practice goes back to the old days (pre DCS days). With steam drums, too high a level is just as bad (or worse) as too low a level, so it is difficult to choose what you want to read on a broken wire. Tradition is difficult to change.

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#### Sam

There still seems to be confusion with regards to the pressure and temperature that will influence the density of the drum level. I say again in no application do you add the process pressure to the level calculation in a closed pressurized vessel.

If you use old pneumatic or electronic transmitter you need to swop the legs around but SMART and multivariable transmitters there is no need to do that since they can measure in the positive as well as the negative.

As for adding the pressure and temperature to the level to compensate for density changes this is done by installing separate pressure and temperature devices in the drum and then take these readings to the control system and mathematically draw up formulas or tables to do the compensation before the level output is given. Either that or multivariable transmitters are used to give a direct compensated level output.

This pressure and temperature compensated output is then used as the input to a feed water flow controller. In other words in a cascade control loop, with the flow controller as the slave. To further increase response time of the control system during ups and downs, you can also use the steam outlet flow signal in a feed forward control configuration with the feed water controller as the master and the steam flow controller as the slave, in a new cascade setup to do the final control of the feed water valve.

All these first, second and third level control systems can and are normally done on most boilers but the bottom line is you will never add the vessel pressure to your level transmitter calibration calculation directly.

What we have done in the past on small low pressure boilers was to get the boiler to working pressure and temperature and then only do the level transmitter calibration calculation. This will work for as long as the system stays online and stable. Any upset will change everything and the system will trip after the slightest upset, since the calibration calculation was done for only this one particular pressure and temperature state.