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The Advantages and Disadvantages of Traditional Flow Measurement Devices

When using traditional landfill gas flow measurement devices it can be difficult to obtain accurate flow readings. Inaccurate flow measurements can lead to an improperly balanced wellfield, resulting in numerous problems including regulatory exceedances.

Flow measurements can be taken with various devices. Three of the more commonly used flow technologies are listed below. Each of these devices will generate a differential pressure that can be used to calculate a flow rate.

Pitot tubes work by comparing the pressure in a small diameter tube (impact pressure) to the static pressure around the tube. In a landfill gas application, they can clog from mineralization and harsh, binding fluids yielding false readings; and they tend to have poor accuracy for flows under 20 scfm.

Another way to measure flow is with a Venturi. Pressure measurements are taken before and at the mid-point of a constriction, then the flow rate is calculated from the difference in these two measurements. Venturis are fixed in place and can have a significant pressure drop at high flows and an insufficient press drop at flows under 25 scfm, resulting in unreliable flow measurements.

Orifice plates are a well-proven flow measurement technology. Because orifice plates are interchangeable, a technician can change the plate size to achieve desired pressure differential with changing flow rates. Changing an orifice plate has traditionally been time consuming and cumbersome, and it required shutting off the gas flow and breaking pipe connections. This often requires two people, and the technicians cannot see if a plate is installed or what size it is without opening the union or flange assembly.

MoldedValveCloseupAll of these issues can be avoided by using QED’s innovative Quick-Change™ Orifice Plates. Quick-Change Plates utilize proven orifice plate technology, but changing these plates is as easy and 1, 2, 3 and can be done without tools, in seconds, by just one person. No time is wasted shutting down the control valve, breaking pipe connections, or rebalancing the well. Using the most appropriate size orifice plate and achieving a pressure drop of 0.5” - 5.0” water column is essential to generating stable flow readings with the best resolution. And, the Quick-Change Wellhead’s accessible plate housing allows for easy confirmation of plate size and placement giving you the highest confidence in your flow reading.

Learn more about QED's Precision Quick-Change Orifice Plate Wellhead.