## Union-based Gas Flow Measurement

Posted: 3/14/09

Updated: 8/4/09

**Introduction**

Measurement of gas flow rates through a gasifier is critical to better understanding and optimizing the operation of the GEK.

There are many methods available to determine the flow rate of gases. One of the most robust and easy to implement techniques is the orifice flowmeter. An orifice flowmeter is comprised of a flat constriction (orifice) plate placed within a larger pipe. Two taps, upstream and downstream (generally 1 pipe dia. upstream of plate, and 1/2 pipe dia. downstream of plate), are used to measure the pressure difference created by flow through the pipe. The pressure differential follows a simple equation, with dependencies on the fluid density, pipe diameter, opening (orifice) diameter, orifice smoothness/design, and flow rate.

*An early, simple implementation of an orifice flowmeter. A washer welded between a cut pipe.*

*After multiple calibration runs, this flowmeter has remained consistently repeatable and accurate. The pipe is 1" NPT with ~0.5" orifice with 1/8" NPT barbs (avoid screwing the barbs past the interior wall of the pipe)*.

**A quick summary of the math:**

A calculator and description of the math is available here:

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm

Flow can be found with:

Where:

is volumetric flow

is friction coeffient, dependent on design

is orifice area

is pressure differential

is fluid density

There is an Excel spreadsheet below used to fit when calibrating the flowmeter using another accurate flowmeter (like a rotameter).

###

### The new design

By using a union to hold the orifice plate, we can provide a number of interchangable constrictions allowing for measurement across a range of flows, either electronically or with a manometer. The electronic pressure sensors that will ship with the GCU will allow high resolution measurement with very little pressure drop, producing lower losses throughout the system.

*Parts for a union-based orifice flowmeter*

Further testing is required before we can deliver this as a product.

## Update

Testing and calibration of the union-based flowmeter has shown poor repeatability, likely owing to a number of alignment issues introduced by the union. Practically, it is unlikely that the orifice size will need to be changed often and unions introduce a reasonable parts cost. Therefore, we recommend using hard welded flowmeters of the type described at the top of the page.

**Resources:**

Excel spreadsheet for calibrating your own flowmeter:

Orifice Flowmeter Calibration.xls

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