- Oct 06, 2017 -
We use these configurations during the calibration of a Differential Pressure Transmitter (in short DP Cell) in a level calibration of a closed pressurized vessel. This can only be used when you are making use of a Diff Press Transmitter that is piped to the high and low tap off points
on the vessel with stainless steel piping. You cannot use it on any other type of level measurement device, even if it is also a Diff Press Transmitter with capillary tubes and pad cells installed on the H/L tap off points and not stainless steel piping. When you use capillaries you need to do the calibration completely differently from normal, so be careful when using capillaries in level applications. Ok back to wet and dry leg calibrations.
The dry leg is the most common and the easiest to do. This is much the same as the basic open tank level calibration. The transmitter is mounted anywhere below the HP (bottom) tap off point and it's HP leg is connected via S/S tubing to the HP (Bottom) tap off point on the vessel. The LP side of the transmitter is connected to the LP (Top) tap off point on the vessel. The HP side will always be in contact with the liquid in the vessel and the LP side will always be in contact with gas since it's is tapped of from the top of the vessel. You obviously can only achieve this if you have a 5-way manifold (isolation, vent and equalization valve piece) installed on the transmitter.
You will start your calibration by opening up the transmitter to atmosphere and make sure that when equal press is applied to HP and LP side the transmitter shows zero and 4 mA. After this zero check it is a simple matter of measuring where your Zero and 100% positions are on the vessel in relation to the transmitter and multiply these with the density of the liquid you are measuring and and install these Z AND 100% values in the transmitter.
Ok this is very easy so far but what happens when the liquid is hotter than the ambient temperature and it's vapor in the top half of the vessel starts to condense and run into the dry LP leg? In a very short time this dry leg is going to start filling up with condensate and there goes your calibration because the calibrated diff press (your calculated Zero and 100% values) begins to chance. To resolve this problem we fill the LP leg with a buffer solution like diesel, glycerin, glycol or even the same liquid you have in your vessel can work as well, in non critical applications. I prefer glycol since it's density is higher than water so if the gas starts to condensate it will just lie on top of the glycol buffer solution and run back into the vessel from the LP leg and not mix with it. The mixing of the wet leg liquid with the gas condensate could also cause problems and inaccuracies, since this could chance the buffer density over a period of time. To calibrate the transmitter will depend on the type and era of transmitter you are using. The following calibration is for smart transmitters only.
The smart transmitters that we use today can measure in the negative (-1Bar) and you can do your calibration as normal. The final result will be something like this, LRV = -1230mmH2o (4mA), URV = +125mmH2o (20mA). I know it looks a bit strange when you see it for the first time but here is how it works.
Before you can do this calibration you need to know the ATM value for the installation. The atmospheric value (ATM) can be read directly from the transmitter by disconnecting the HP side (Bottom) and open it up to atmosphere, so the only pressure on the transmitter is on the LP side and this will obviously push the transmitter into the negative. Maximum negative differential pressure for a instalation = ATM pressure. Make sure the LP line is filled to the position where it will start to run back into the vessel, then read off the displayed value on the transmitter. This is your ATM value. In this example it might be something like -1350mmH2o. This value is determined by, where you have installed the transmitter and what you use for a buffer solution. To calculate the actual zero and 100% positions on the vessel you do the same as before and just measure from the transmitter to you zero and 100% positions on the vessel, multiply them with the density of the liquid you are measuring and add them to the ATM value. You can then input these values to this transmitter's LRV and URV and the calibration is done.
So assuming you have installed the transmitter slightly below the lower tap off point the above LRV and URV is about right in relation to the ATM value in this example. Be sure to understand the difference between the ATM value and the LRV it will in most cases not be the same. The more accurately you can determine your ATM value the more accurate the calibration will be.
THIS IS THE ANSWER TO YOUR QUESTION
Now the calibration of the 4to20mA and the pneumatic DP transmitters. These transmitters cannot measure in the negative so you need to change the HP and LP sides around so that the HP side goes to the top of the vessel and the LP side goes to the bottom tap off point on the vessel. You now need to do you calibration in the reverse as well. Again find the ATM value first, in other words max positive differential (HP wet leg filled and LP open to atmosphere) on the transmitter will now be your ATM value. Will be say +1350mmH2o. Actual zero will now be 20mA and not 4mA and will be determined by making use of the ATM value minus the actual zero measured value, multiplied by the liquid density. The actual 100% value will be determined by making use of the ATM value minus the actual 100% measured value, multiplied by the density. You should end up with something like this, zero = +1250mmH3o = 20mA and 100% = +150mmH20 = 4mA. Finally the display on you remote level indicator needs to be changed as well otherwise it will read in the reverse. If you use a pneumatic DP Transmitter just substitute 4 and 20 mA with 20 to 100Kps or 3 to 15 Psi the principle stays the same.
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