I need to measure the level in an open tank. I am planning to use a flanged dP level transmitter.

The height of my tank is 2000mm. I need to measure the low alarm at 300mm and high alarm at 1800mm, both from the bottom of the tank.

The transmitter will be installed to a nozzle at elevation of 150mm from the bottom of the tank. However, the manufacturer of the tank has recommended to install the nozzle at the bottom (200mm below) of the tank. In this way, the full range of the tank can be measured.

If we do it this way, please advise what I need to consider in my level calculations other than density x gravity x height?

 

It would certainly work 200 below the tank, but do you have access? Do you plan on drawing down to empty? There is no point having the transmitter range outside your working range.

I usually have the C/L of the transmitter at top of pump suction because at that point the pump is sucking air and I consider the tank empty.

Hope this helps.

Roy

 

I agree with Roy. There's little point in putting the transmitter all the way at the bottom of the tank unless you plan to empty it, and pump suction centerline is a good rule of thumb. Depending on what's in the tank, you're also inviting more sludge accumulation that could adversely affect the reading when it's at or near the bottom.

Another consideration is accessiblity for maintenance, which is often overlooked. Being in maintenance, I loathe installations where you've got to build scaffolding or snake a ladder into some tight spot to get to it. These instruments tend to suffer from lack of maintenance because nobody wants the hassel, or in some cases, risk bodily injury to access them.

You also mentioned a DP transmitter was to be used on this open tank. DP's are more expensive than GP's, and since you're referencing atmospheric pressure anyway, why pay the higher price?

 

1) considerations: sludge, build-up.
Will problems associated with sludge build-up on the bottom be avoided by a level transmitter installed at 200mm elevation above the bottom ?

Conversely, will sludge or build-up adversely affect the measurement or require maintenance when the level transmitter is below the bottom of the tank?

2) > density * gravity * height

H=P/S.G.

Density is specific gravity (S.G.) of the medium.

Transmitter typically measure in water column (w.c.) units of head pressure.

actual level (height) = measured w.c. pressure / S.G.

3) All industrial flange mount pressure transmitters have an adjustment to correct the output which 'ignores' the head elevation below the bottom of the tank.

David

 

Thanks for your reply.

There will be no problem with sludge built-up as the medium is liquid. Maintenance is still OK as the transmitter can be accessed. However, one consideration I need to consider is the transmitter will be installed with a diaphragm seal which may be difficult to drain if the valve is too low.

Can you tell me more or guide me to a web site about transmitters that have an adjustment to correct the output which 'ignores'
the head elevation below the bottom of the tank. How does it do it?

Is it automatic or do we have to account for it thru calculation when calibrating the transmitter?

 

Nearly every transmitter has the ability to elevate or suppress the zero point. In your case the zero would be set at +200mm WC and the span at +2,200.

Bear in mind that these figures are for water, so you need to correct for the SG of your liquid.

Roy