from the CS department...
Orifice Plate straight run requirements
 Posted by arch15 on 14 October, 2007 - 6:10 pm
Why does straight run requirement increase as beta ratio increases for a particular piping geometry? In other words, why is straight run requirement dependent on beta ratio ?

Is it because as beta ratio increases, the dp reduces and we need even better flow profile to achieve less uncertainty at lower DPs?

 Posted by John Catch on 20 October, 2007 - 6:46 pm
A discussion on this topic could make a nice extended magazine article, or a chapter in an engineering reference. I suggest you review "Flow Measurement Engineering Handbook" by R.W. Miller (McGraw-Hill). Miller has a number of charts, graphs, and explanations for all of this.

John Catch
www.inflowinc.com

 Posted by Tushar on 9 September, 2011 - 7:26 am
Is straight run required for Restriction Orifice?

 Posted by Roy Matson on 10 September, 2011 - 1:56 am
> Is straight run required for Restriction Orifice?

NO

 Posted by Dick Caro on 10 September, 2011 - 12:21 pm
Come on Roy. Yes, the answer is NO, but you pay for not using a straight run of piping before and after the orifice. The typical rule-of-thumb as I recall, is 10 pipe diameters upstream (before the orifice) and 3 pipe diameters downstream. This is to allow all of the known empirical equations relating flow to the pressure drop across the orifice at a specific beta ratio (orifice diameter to pipe ID.) See this reference:

http://en.wikipedia.org/wiki/Orifice_plate

If you do not have the recommended straight run before and after the orifice, you will still get a pressure drop across the orifice proportional to the square root of the flow rate, but you cannot use the typical empirical equations to determine an accurate flow rate. You may, however, calibrate the differential pressure to flow measured another way, or perhaps your need for accuracy is not essential. For example, in order to control flow to a constant value, accuracy may not be required. Once a flow is established, a control loop can be configured to hold that flow at a constant value represented by a constant differential pressure across the orifice.

Dick Caro
Certified Automation Professional
CEO, CMC Associates

 Posted by David on 21 September, 2012 - 7:57 pm
The rule of thumb is 10 pipe diameters before and 5 pipe diameters after. However, if you want ot be conservative, checkout the Spiraxsarco website and they give correct dimension for most applications where restrictions, joints or bends can affect the flow.

 Posted by Roy Matson on 23 September, 2012 - 5:53 am
Dick,

In my industry most restriction orifices are used in pump kickbacks to provide minimum flow and protect the pump, who cares if you can't calculate the flow to decimal places in that situation? We don't even have taps to measure it with.

Another use for restriction orifices is to create a little bit of back-pressure to encourage flow past an analyzer probe in a side-stream, it would look ridiculous if we had to space the branches so far apart.

Roy

 Posted by a s reddy on 4 October, 2012 - 1:43 am
The issue of straight run requirements of orifice comes into effect depend on the application. if the application is Custody transfer metering of natural gas, the the straight runs are required and must meet the installation standards of AGA-3 instalation reports.
As per AGA -3 minimum required upstream length is 10D and 5 D for down stream. these lengths are valid when a Flow Straightener or Flow profiler are installed in the Up Stream of Orifice and the installation of Straightener/Profiler is defined in AGA standards,

with out Flow straightener the up stream straight run will go up to 145 D depending on the Beta ratio.

it is better to refer to AGA-3.

for other applications straight run really doesn't matter as there is no issue of metering accuracy.

hope it clarifies.....

 Posted by d- on 10 September, 2011 - 2:46 pm
Normally the run requirements for R.O.'s are ignored, but there are two issues to consider:

the coefficient is affected by "swirl" such as occurs past an elbow, the coefficient can be reduced by 10-20%, downstream you must consider the high velocities of orifice discharge as this can lead to erosion and or cavitation damage in liquids.

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