wet leg vs dry leg

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Wet leg vs Dry Leg (level instrument) Appreciate if someone can provide technical expertise on the advantanges and disadvantanges between "Wet leg" and "Dry leg" installation for level measurement. Reply to this post... it is not a case of advantages or disadvantages. it is purely a case of application. Wet legs are used on differential pressure measurement where both sides are filled with liquid. Exception is level measurement in a pressurised vessel (boilers). Dry legs are used where the low side is exposed to atmosphere (using a DP transmitter for level measurement in a open tank. Regards Reply to this post... As both measurements are used for measuring the level in pressurised vessels, it's not so much a question of advantages and disadvantages but of pure need depending on the application. If you have a medium that evaporates you have to use the dry leg method as the liquid in the compensation line will evaporate. In the case where your product condensates you will have to use the wet leg method as the compensation line will end up full of product anyway. The wet leg method is the most difficult one to calibrate as the measurement will indicate 0% when the tank is full and 100% when it's empty. Also after installation, it may take quite a while before the measurement actually indicates the correct level unless you are able to manually fill the compensation line. A good alternative is the use of cappilar level transmitters. These are filled with special oil thus eliminating the need to fill the line or wait until it has filled with condensate. Also they can be used in those cases where the liquid goes trough both the evaporating and condensating phases (e.g. some reactors). Other alternatives like radar should be considered as often the density changes in these applications may be very significant, leading to difficult calculations to transform the measured differential pressure into a valid level indication. Reply to this post...

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Page 1: Wet leg vs Dry Leg

Wet leg vs Dry Leg (level instrument)

Appreciate if someone can provide technical expertise on the advantanges and disadvantanges between "Wet leg" and "Dry leg" installation for level measurement.Reply to this post...

it is not a case of advantages or disadvantages. it is purely a case of application.

Wet legs are used on differential pressure measurement where both sides are filled with liquid. Exception is level measurement in a pressurised vessel (boilers). Dry legs are used where the low side is exposed to atmosphere (using a DP transmitter for level measurement in a open tank.

RegardsReply to this post...

As both measurements are used for measuring the level in pressurised vessels, it's not so much a question of advantages and disadvantages but of pure need depending on the application.

If you have a medium that evaporates you have to use the dry leg method as the liquid in the compensation line will evaporate. In the case where your product condensates you will have to use the wet leg method as the compensation line will end up full of product anyway. The wet leg method is the most difficult one to calibrate as the measurement will indicate 0% when the tank is full and 100% when it's empty. Also after installation, it may take quite a while before the measurement actually indicates the correct level unless you are able to manually fill the compensation line.

A good alternative is the use of cappilar level transmitters. These are filled with special oil thus eliminating the need to fill the line or wait until it has filled with condensate. Also they can be used in those cases where the liquid goes trough both the evaporating and condensating phases (e.g. some reactors). Other alternatives like radar should be considered as often the density changes in these applications may be very significant, leading to difficult calculations to transform the measured differential pressure into a valid level indication.Reply to this post...

The question is basically not for "advantages" or "disadvantages". They are basically used to compensate the pressure change of the gas or vapour above the liquid in a closed tank. If the vapour or gas above the liquid is dry and doesn't condense to liquid, "dry leg" can be used without any problem. However, this is not for most of the cases and then "wet leg" has to be used so that condensate in the impulse line tapped above the liquid level cannot change the reading.Reply to this post...

Most replies are right, but theres a differences. Firstly - if the tank is open, the LP leg will be left open to atmosphere. If it's a closed tank liable to pressurise, both legs will be piped to the tank, the HP at the bottom and the LP at the highest level point.

Now.

If the process fluid will evaporate & condense, usually something like Glycol is used to fill the wet leg.Basically anything stable that wont evaporate or degrade and with a known SG.

Page 2: Wet leg vs Dry Leg

(Process fluid SG also has to be taken into consideration when choosing a wet leg fluid.)

The Level transmitter then has its zero suppressed with a full wet leg, so it reads Zero.Any 'gas pressure' in the closed vessel acts on both sides of the DP cell, wet leg and everything so it shouldnt actually read anything, only once liquid starts to fill the tank will the level start to register.

Capilliary systems remove all these problems, BUT are prone to temperature differential problems in extreme cases.Reply to this post...

All comments given as valid and enough information.It all depends upon the process appln.1.If your process fluid is solid ,semi solid,slurry or density more than water and naturally the tank/vessel will have closed top then understood its under pressure in this case we have to go for remote seal DP txr.its an wet leg.2.If the above scenario is same and the material inside the tank is condensable,and the process operating pr. is vacuum and so we have to go for DP txr.with the LP side as dry leg and will turn automatically to wet as the process changes. Generally in this case the process fluids density will be less than water. 3.Tanks/vessels operating with Atm.pressure , we shall go for dry leg.4.Generally DP application is implemented w.r.t our tank,column,vessel heights.make an total summary of all comments givenReply to this post...

Shoot mail to me if anyone wants the calculation on either dry or wet legs (for calibration purpose)