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andybule
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Join Date: Feb 2009
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Orifice Flow Element Carrier - Pairs of Pressure Traps

06/05/2009 3:34 AM

Hi,

We have an orifice flow element carrier with double pair of pressure taps(symetrically hooked up, + & - 45° from the bottom of the pipe) in order to install two dp transmitters.

During operation one of the transmitters is always reading less than the correct reading)

Any idea why and if two transmitters can be installed on the same orifice on different pressure taps?
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PWSlack
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#1

Re: Orifice carrier double pair of pressure taps

06/05/2009 3:39 AM

Condensate in the downstream sensor line?

A leak in the upstream sensor line?

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andybule
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#2
In reply to #1

Re: Orifice carrier double pair of pressure taps

06/05/2009 3:41 AM

What you mean by condensate in the downstream line?
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PWSlack
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#3
In reply to #2

Re: Orifice carrier double pair of pressure taps

06/05/2009 3:42 AM

Water.

H2O

Dihydrogen monoxide.

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andybule
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#4
In reply to #3

Re: Orifice carrier double pair of pressure taps

06/05/2009 7:35 AM

Thanks

I forgot to mention that my orifice is measuring condensate return flow so impulse lines will be filled with water.
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PWSlack
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#5
In reply to #4

Re: Orifice carrier double pair of pressure taps

06/05/2009 7:41 AM

If they are both filled, then there shouldn't be a problem.

If the liquid is at or very near its boiling point then all sorts of strange things could be going on in the downstream line as a result of the pressure drop introduced by the orifice; if that is the case, then the downstream leg could be reporting a false low signal.

Consider using a vortex-shedding flowmeter instead of the orifice/differential pressure [DP] arrangement. DP for condensate flowrate is a bit '1970s' these days.

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andybule
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#6
In reply to #5

Re: Orifice carrier double pair of pressure taps

06/05/2009 8:28 AM

Is it common to have double pair of pressure taps?

Why one of the Dp is correct and the other decreasing?

Thanks
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PWSlack
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#7
In reply to #6

Re: Orifice carrier double pair of pressure taps

06/05/2009 9:24 AM

<....Is it common to have double pair of pressure taps?....>

No, in the same way as two clocks will rarely tell the same time.....

<...Why one of the Dp is correct and the other decreasing?...>

One that is decreasing while the other one is steady would indicate a faulty instrument - the one that is decreasing.

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Anonymous Poster
#8

Re: Orifice Flow Element Carrier - Pairs of Pressure Traps

06/06/2009 8:32 PM

Why 2 sets of taps? Why not use the output of a single transmitter to drive more than one output device? Have you performed an orifice plate calculation to determine what the expected flow and DP should be? DP is a square root function of flow, so a little error goes a long way.eg:50 percent DP = 70.0% flow.Alternatively, use the same tap for both transmitters.There is no real flow through the transmitters,so adding a second one in parallel will not affect accuracy.However, both transmitters must be precisely calibrated from same source.

HTRN
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Judge5139
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#9

Re: Orifice Flow Element Carrier - Pairs of Pressure Traps

06/10/2009 11:34 PM

I believed you are using 2 transmitters to seperate the control and safety loops as specified by IEC61508. It should be no problem installing 2 transmitter on the same orifice and the problem that you are having might be caused by the distorted or swirl velocity profile. Therefore please look at the upstream and downstream of the orifice whether it meets the 10D & 5 D minimum straight length for Orifice installation. Valves, elbows possiby caused the insymetrical profile and you might get different reading plus overall those reading might be inaccurate.
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Anonymous Poster
#10
In reply to #9

Re: Orifice Flow Element Carrier - Pairs of Pressure Traps

08/20/2010 12:10 PM

Pressure taps location at side top or bottom. Is taht important?
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Users who posted comments:

andybule (3); Anonymous Poster (2); Judge5139 (1); PWSlack (4)

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