Level measurement

Hi, In my opinion the transmitter won't be damaged if it is connected backwards, on wet leg and remote seal applications the low side is exposed to higher pressure than the high side with no level in the vessel so again in my opinion the transmitter should be ok.The high side needs to be connected to the bottom of the vessel and the low side to the top.If you connected backwards it will still work but the calibration will be different it is not a good practice to do it that way.

Hello guys,

i wanted to know what will be effect on Output value if

1)the LP port if left open to atmosphere,

2) and if the LP port is connected to top of the tank, where tank is closed.

I guess In both the cases the pressure acting on LP is not same.

Thanks

Hi, the effect on the out put to the valve depends if the valve is in the inlet or outlet to the tank.If the tank is closed the low side needs to be connected to the top of the tank because between the top of the liquid level an the roof of the tank there is vapor pressure. The high side is exposed to the head pressure of the liquid PLUS the vapor pressure by connecting the low side to the top of the tank you will compensate for that vapor pressure.If the low side is left open to atmosphere the transmitter is going to read a level higher than what it actually is.

As tanks get deeper, the pressure at the bottom gets higher. The pressure at the top is usually atmospheric and is the low pressure datum.

The effect on the valve should have no relation to how the dP cell is connected. It would be controlled based on the output of a controller that would base its action on the measured variable. The measured variable, on the other hand, is completely dependent on how the dP cell is connected. It may seem to be an arbitrary distinction to separate the input and the output, but you need to realize they are independent to a point. If the LP is left open to atmosphere on an open tank, there would be no difference in the measured variable and the actual level. If it is a closed tank, the difference between atmospheric pressure and tank head pressure would need to be compensated for somehow. The most accurate way is to connect the LP port to the top of the tank. Another way is to use a separate pressure transmitter to compensate. This is very inaccurate for short tanks since the scale of the differential pressure is in inches of water column and the pressure of the tank may be in pounds per square inch. The uncertainty of the pressure transmitter may be plus or minus more than the actual measurement. Without some way to compensate accurately for the head pressure, the dP reading is meaningless.

The bottom line is that your controller would see a higher than actual reading without the LP connected for a higher than atmospheric head pressure and a lower than actual reading for a less than atmospheric head pressure and act accordingly. Either way you are acting on a bad reading and will likely break equipment and may injure or kill someone.

The high side must always be connected to the bottom of the tank, since the bottom of the tank must always head pressure plus vapor pressure. If connected where the low pressure side is connected to the bottom of the tank, there would likely be no damage unless the differential pressure exceeds the rated pressure of the dP cell. The meter would likely read zero (or negative) for all levels though.

Now, the low side does not need to be connected to the top of the tank at all unless it is a closed tank and the tank would operate at other than atmospheric.

so what is the thumb rule for dp type level measurement, whether the LP side should be open to atmosphere or should it be connected to top fo tank i.e subjected to vapour pressure of tank.. which one would give correct value. We actually have a Hot water tank of 8000 Litres, temp in it goes upto 70deg c..

Thanks and regards

Really the main factor is if the tank is open to atmosphere. If the hot water tank has an open vent to the roof for example, you don't need to hook up the low side. However, if the vent has a rupture disk then one is required since air and vapor can and will be trapped as the tank is filled from an empty state. Also, if the level drops on a closed tank, the pressure of the vapor will drop causing the reading to show less water in the tank than there actually is.

andrew_ward is correct, the high side is connected to the bottom of the tank because it is the highest pressure of the differential due to the head pressure of the liquid you are measuring. Hooking it up backwards will give you a backwards reading and your DCS will read zero or negative. Atmospheric on the low leg is OK provided that the tank is at atmospheric on the top.

Dear Guest,

Many d/p transmitters that manufactured by serious brands have solid-supports inside it, formed same as the diaphram, against inversed pressure inputs except low pressure transmitters (likely smaller than +/-1 bar).

In situations that the pressure is applied inversly, you can obtain only a constant current output, say 3mA, but not a current output proportional with the pressure. As a result the transmitter will not damage but you can get an output of "0".

The true connection of pressure inlets is the one lp side to upside of tank so as many guys commented.

Best regards.

Reverse connection of d/p level transmitter where H-port connect to Top side of the tank and L-port connect to bottom side, has a purpose?

i.e on condensate pot.

Hi guys,

Thank you very much for the quick response.