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Participant

Join Date: Apr 2012
Posts: 3

### Level Transmitter Zero Suppression

04/16/2012 2:29 PM

Level transmitter with seal fluid in capillary connections for interface level measurements. (Oil and water)

1. how can I set the zero.

2. Hi and low connections are 2 meter apart, what will be the set range . How can I calculate it.

SG of fluid/oil/water are 0.93/0.87/0.1025

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Power-User

Join Date: Jul 2010
Posts: 400
#1

### Re: Level Transmitter Zero Suppression

04/16/2012 7:17 PM

1) How will you detect an interface level with only a head pressure measurement?

2) Is the SG of the fluid the fill fluid in the capillary?

Power-User

Join Date: Jul 2010
Posts: 400
#2

### Re: Level Transmitter Zero Suppression

04/16/2012 7:46 PM

1) To calculate zero and span for head pressure, the elevation of the transmitter's High and low ports with respect to the center line of the lower tank port needs to be established:

above by x mm, or

below by x mm, or

or even elevation with the lower port

2) what is the span elevation between the lower tank port center line and the maximum expected upper liquid level?

Participant

Join Date: Sep 2011
Posts: 4
#3

### Re: Level Transmitter Zero Suppression

04/17/2012 2:54 AM

You state the connections are 2000 mm apart - the SG of process is oil/water = 0.8 and 1 respectivitly. The range of the level is the difference of the process S.G. 1.0 minus 0.8 = 0.2 multiplied by the height of connections 2000 mm = 400 mm. The range of transmitter is 400 mm. When installing the DP transmitter, ensure that the HP side is connected to low connection of the vessel. The tubing on both connections is filled totally water. Now enter the URV as -400mm and the LRV as 0mm. To check the 100% condition , one simply has to isolatate one leg of the 3 way isolation manifold and open the equalise valve 100% condition will exist. To check 0% - one has to isolate both takeoffs and vent them, then apply 1600 mm H2O pressure to the HP leg - this will indicate 0% condition. When these calculations are used - there is no need to check 0% condition as the 100% condition tell you that the transmitter is responding, range figures must be entered correctly.

 Level Calculation Tag No 42LDC1 42LDC1 SG of Upper Liquid SG of Lower Liquid Takeoff Height 0.465 1.00 920 TX 0%= -492.2 mmWG 0.465 SG of 100% Liquid 920 Height in mm 1.00 SG of Zero liquid 100%= 0 mmWG
Power-User

Join Date: Jul 2010
Posts: 400
#4

### Re: Level Transmitter Zero Suppression

04/17/2012 8:40 AM

Mismatch:

rammohan2000: >Level transmitter with seal fluid in capillary connections

Garry: >The tubing on both connections is filled totally water.

Participant

Join Date: Sep 2011
Posts: 4
#5

### Re: Level Transmitter Zero Suppression

04/17/2012 9:26 AM

Both connector tubing must be totally filled - Sorry the range input to transmitter should have stated LRV (-400 mm H2O) and URV - (0 mmH2O).

Iris for a interface to be measured, the lighter liquid must be above the top takeoff being measured.

2
Power-User

Join Date: Jul 2010
Posts: 400
#6

### Re: Level Transmitter Zero Suppression

04/17/2012 9:49 PM

Garry, welcome to the forum.

>You state the connections are 2000 mm apart - the SG of process is oil/water = 0.8 and 1 respectively.

OP: >SG of fluid/oil/water are 0.93/0.87/0.1025

An SG of 0.87 makes a substantial difference.

>The range of the level is the difference of the process S.G. 1.0 minus 0.8 = 0.2 multiplied by the height of connections 2000 mm = 400 mm. The range of transmitter is 400 mm.

An SG of 0.87 gives a range of 260mm: 1.0 minus 0.87 = 0.130 * 2000 = 260mm

But you're thinking wet leg and water filled impulse tubing/pipes. OP says capillary connections; probably something like this:

The capillary fill exerts a head pressure of its own that must be taken into account because it directly affects the LRV and URV.

Either Syltherm or DC-200 fluid fill is close to an SG of 0.93, so the OP's fluid 0.93 SG is likely the fill fluid in the capillaries.

A capillary extending 2000mm above the transmitter will exert a pressure on the low side port of 2000 mm * 0.93 = 1860 mm w.c.

The high side subtracts the low side pressure, so empty, the LRV will be -1860 mm w.c. (that's negative 1860 mm w.c.)

When the tank is 100% full of oil, 2000mm of oil exerts a head pressure of 2000*0.87, or 1740 mm w.c. head pressure, or a span from the LRV of -1860 up to -120mm (w.c.) [-1860 + 1740 = -120].

When the tank is 100% full of water, 2000 mm of water exerts a head pressure of 2000 mm w.c. or a span from the LRV of -1860 to +140mm w.c.

The span from -120 mm to +140 mm (as the interface level varies from all oil to all water) is the calculated 260 mm span difference between an interface level at the bottom port (100% oil above the port) and an interface level at the top port (100% water above the lower port). All other interface levels between 100% oil and 100% water will be proportional across that span.

If the zero is elevated or suppressed (I can't recall which is which at the moment, I'm sure 17 people will respond with the answer) so that 1740mm w.c. is the 4.0 mA 'zero' output and the 20.0mA is scaled 260 mm above the zero point, then

- when the tank is 100% full of oil, the interface level is at the bottom of the tank at the lower port elevation, the output will be 4.0 mA

- when the tank is 100% full of water, the interface level will be at the top or upper port and the output will be 20.0mA

My original question to the OP about getting interface level was rhetorical, I was hoping to prompt a response, because I've found many people think the transmitter should somehow reflect the interface level directly, when the idea is to span the transmitter's output as above, 0 to 260 mm w.c. (or 1740 to 2000 mm w.c.) which can be scaled in a receiver (meter, DCS, PLC, PAC, indicator, recorder, whatever) to represent the interface level somewhere between the 0 to 2000 mm distance between the ports or 0 to 100%.

Participant

Join Date: Apr 2012
Posts: 3
#7

### Re: Level Transmitter Zero Suppression

04/18/2012 1:30 AM

Thanks for both of you Iris and Garry for such a good discussion.

Participant

Join Date: Sep 2011
Posts: 4
#8

### Re: Level Transmitter Zero Suppression

04/18/2012 1:59 AM

As per your drawing-it shows the usage of capilaries - In my experience capilaries are effected by thermo changes, the process will perform better without the use of capilariers hence I say retube LDT using a standard DP level hookup. I have used capaliries to measure the level of LPG bullets, temperature varience made the levels in accurate - used kerosene as the seal liquid in a standard DP hook up - no inaccuracies. I have a exel sheet that calculates all level DP conditions - email me at garry.pitman@engenoil.com

Participant

Join Date: Apr 2012
Posts: 3
#9

### Re: Level Transmitter Zero Suppression

04/18/2012 5:42 PM

Thank you. Will post a mail to you.

Rammohan

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