Orifice Flow Meter Practical Straight Run Requirement

[From:来自网络] [Author:admin] [Date:12-07-18] [Hit:]

The requirement of straight length from the ISO 5167-2 is a theoretical requirement since the test was performed based on very long straight run of the pipe upstream of the fitting being tested to ensure a swirl-free flow just right before it entering the fitting. Even though like this, the ISO 5167-2 give guidance on how to implement that theoretical requirement in practical way. See below an example of how to implement the theoretical requirement of ISO 5167-2, the example is based on a normal installation guide dictated in the ISO 5167-2.

See Orifice Flow Meter Theoretical Straight Run Requirement for more detail regarding the theoretical way to implement the orifice straigth run requirement.

Let’s consider below orifice flow meter will be installed in a gas compressor suction with the following arrangement:

Line ID = 13.94 inch

Line ID before reducer = 15.69 inch

Reducer Length = 22.52 inch

Orifice Beta Ratio = 0.6

See below general arrangement of the orifice plate above with its related upstream fitting.

From the above general arrangement, then from table 3 ISO 5167 column 9 we get L9 = 9*13.94 = 125.46 inch

From table 3 ISO 5167 column 2 we get Lext = 0.5*15.69*44 = 345.18 inch

Lt = L9 + Lreducer + Lext = 493.16 inch

Lmin for orifice plate and 90 degree single bend = 44*13.94 = 613.36 inch

Since Lt < Lmin then we need to put some additional length in the upstream of reducer or downstream of reducer so that the Lt is >= Lmin.


The additional length will be Lmin – Lt = 613.36 inch – 493.16 inch = 120.2 inch

Let’s say we put that additional length in the downstream of reducer. Then the overall arrangement will be as below:

The above arrangement (with total required length is 613.36 inch) is a very long straight length requirement. We can utilize a flow conditioner to reduce some of the required straight length as follow:

From the above general arrangement, then from table 4 ISO 5167 we get Lf = 30*13.94 = 418.2 inch

From table 4 ISO 5167 column 9 we get L2 = 12*13.94 = 167.28 inch

Practical length between reducer and 90 degree bend = 34.48 inch

Lt = Lf + Lbetween reducer & bend + Lreducer = 475.2 inch

The overall arrangement will be as below:

What if the straight length is shorter than ISO 5167-2 requirement?

If we install the orifice plate with the straight length usptream or downstream of the orifice plate is shorter than ISO 5167-2 requirement, then we wouldn’t able to predict how much is the uncertainty of the orifice flow measurement reading. By installing the upstream and downstream straight length per ISO requirement, we can get zero additional uncertainty or 0.5% additional uncertainty, depends on our applications. But the ISO standard didn’t give any information to predict the uncertainty of such shorter installation. We can say that the additional uncertainty is unknown, it can be 1%, 2%, 5%, 10%, 20%, etc, we don’t know. That’s why the instrument engineer should avoid any installation that didn’t conform to the ISO 5167-2 to avoid any problem with the measurement uncertainty that can upset the process control.

In our example above, the orifice flow meter is serving a critical application which is a gas turbine compressor. The orifice flow meter is supplying flow process variable in the gas compressor suction as a data to predict and avoid the surge phenomenon. If we install an orifice flow meter that didn’t conform to the ISO 5167-2 requirement, then just be ready to have a lot of problem with the gas turbine compressor due to surge phenomenon, shutdown upset, etc.


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