In Part VII of our natural gas flow meter types’ series, here I discuss the orifice plate, a differential pressure meter commonly used to measure natural gas flow when high accuracy is not required.
Differential Pressure Natural Gas Flow Meter
Differential pressure (DP) flowmeters determine flow by measuring pressure drop over an obstacle inserted in the flow pathway. Common types of these flow devices are orifice plates, averaging pitot tube, venturi tubes and flow nozzles.
The orifice plate (orifice meter) is a differential pressure meter frequently used in natural gas measurement. It measures volumetric flow, though can also calculate mass flow depending on the calculation associated with the device. It uses the same principle as used in a Venturi-style meter (Bernoulli’s principle), suggesting that there is a relationship between the velocity and pressure of the fluid. That relationship is – as the velocity increases the pressure decreases. With an orifice meter, the flow is determined through the difference in pressure between the upstream and the downstream side of a partly impeded pipe.
Orifice plates have a long history of use; there is wide industry acceptance, and they are well documented in standards. They are simple and inexpensive. The orifice meter has no moving parts and is mechanically stable. Dry calibration is acceptable, and there are no limits on temperature, pressure or size.
On the down side, orifice meters have low accuracy at low flow conditions. They have low turndown/rangeability and a high-pressure loss which can impact operating cost. Additionally, orifice plates are flow-profile sensitive and require a long meter tube or flow conditioner; they are not self-cleaning and can be damaged or clogged by high flow rates. Also, to calculate mass flow temperature and pressure correction is required.
Orifice Plate Manufacturers
There are many differential pressure orifice plate manufacturers, too many to list in this post.
If you are interested in natural gas flow measurement, you may want to review other meter styles, including ultrasonic meters, diaphragm meters, rotary meter, turbine, Coriolis, and of course the thermal mass flow meters.