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Associate

Join Date: Jan 2013
Posts: 41

# Zero Supression

11/07/2013 12:20 AM

The idea of supression and elevation is not very clear to me. Lets look at supression first.

When the transmitter is below the zero datum both its legs are filled with process fluid/ fill fluid in case of zero level as well. So the length below the datum point should not affect the delta P value.

for example i have a DP level inst. installed on a service with say SG=1 and C to C for HP and LP nozzles to be 50" i feed the following in the instrument LRV=-50"WC and URV=0"WC this should give me 4mA for -50 and 20mA for 0. The transmitter is installed 5" below the HP nozzle. So the LP and HP legs are filled with 5" of water at all times.

I will get the following readings: delta P = HP-LP

full level delta P = (55)-(55) = 0 =20mA

Half Level delta P = (30)-(55) = -25 = 12mA

Empty delta P = (5)-(55) = -50 =4mA

So why do we need a supression??

P.S: i posted this question on an old thread but it does not show up in recent posts, so i got no answers there.

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Guru

Join Date: Mar 2010
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Posts: 3020
#1

### Re: Zero Supression

11/07/2013 1:42 AM

What is your thought on noise? Sensitivity? In some processes a zero might be floating a little.

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Associate

Join Date: Jan 2013
Posts: 41
#3

### Re: Zero Supression

11/07/2013 5:33 AM

maybe you are right but i just didnt get the concept, from my search i found that zero supression is done when we want to eliminate the the effect of pressure imparted to the element by the impulse line below the HP nozzle. But my question is when we hav a differential instument why do we need to supress, that extra bit of pressure automatically subtracts from both legs.

Here is the post from four years ago that has brought the question to my mind

"on the reverse, if the txr is placed below the zero datum line of the tank, the the HP side of the txr will be filled with more amount of process fluid. hece txr will show high reading than the acutal reading, in such cases the value has to supressed to zero, this is known as zero supression."

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#5

### Re: Zero Supression

11/07/2013 8:14 PM

Iris gives you a practical example of a tank measurement. It is as clear that it doesn't need further explananation. G.A.

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#2

### Re: Zero Supression

11/07/2013 3:46 AM

...and if zero is 4mA, <4mA can be used by the system to which the transmitter is connected to indicate a fault, as can >20mA. Useful?

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Guru

Join Date: Jul 2010
Posts: 655
#4

### Re: Zero Supression

11/07/2013 10:05 AM

The datum is the level at the high side port, HP. The true level is 'b' in the diagram.

A is a pressure contribution to the DP measurement which is eliminated by subtracting the low side measurement from the high side measurement.

The pressure created by the liquid over the elevation 'c' is 'seen' and measured by the transmitter which is located below the high side port as shown in the illustration.

The differential pressure measurement includes the pressure contributed by the elevation distance, c, as shown below.

As the arithmetic shows, DP = b + c.

C is the distance eliminated by a zero suppression calculation.

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#6

### Re: Zero Supression

11/08/2013 7:13 AM

See Iris' good response...the zero elevation is used when the opposite is true, ie when the DP cell is mounted above the datum line.

Associate

Join Date: Jan 2013
Posts: 41
#7

### Re: Zero Supression

11/08/2013 10:24 AM

but are you guys not missing the condensate pot?

Iris's diagram is very clear but what if the vapour condenses in the LP leg then we will have a false reading unless the LP leg is drained.

I use a condensate pot with the LP leg completely filled of water at all times (hence called the wet leg) which (in my case) ensures that the element is not exposed to high temperatures also makes the LP leg the reference from which the level is compared.

So in my case

HP = a+b+c
LP = a+c+d
DP = (a+b+c) - (a+c+d)

so DP = b-d

where d is known to us and constant and the level will depend on the value "b". Notice that "c" is not present in the final equation and the range fed into the transmitter will be from negative inches of WC to zero(the max value).

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Join Date: Sep 2011
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#8

### Re: Zero Supression

11/10/2013 11:39 AM

One detail you may need to consider is the difference in liquid densities between the hot process vessel (dimension "B") and the cold "wet" leg (dimension "D"). The differential pressure seen by the transmitter then becomes (B)(specific gravity of B) - (D)(specific gravity of D).

If the application in question is high-pressure steam (e.g. a power boiler in a generating plant), the density of the steam is also significant, and so dimension "A" becomes relevant as well.

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#9

### Re: Zero Supression

11/11/2013 1:28 AM

Your question didn't include a condensate pot, and made no reference to vapour or temperature.

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