Orifice flowmeter

Orifice flow meters measure differential pressure across a restriction and calculate flow from it. The oldest technology in the catalogue, and still the most common, because a plate costs almost nothing and fits any pipe size.

Three types in this range

• Standard orifice plate. A single bore in a plate. Cheap, well understood, and covered by every standard from ISO 5167 onward. Needs long straight run, typically 20D to 40D upstream depending on the beta ratio.
• Balanced orifice. A multi-hole plate that conditions the flow as it measures. Straight run drops to roughly 5D to 10D, which is the reason to specify it on a retrofit where the pipe cannot be re-routed. Accuracy improves to about plus or minus 0.5 percent.
• Restriction orifice. Not a measurement device at all. It exists to drop pressure or limit flow, and should not be confused with the metering plates.

What decides the model

Available straight run is the first question. If you have 30D of clean pipe, a standard plate is the cheapest correct answer. If you have 6D on a skid, the balanced plate pays for itself by making the installation possible at all.

Then permanent pressure loss. An orifice plate costs pressure, permanently, and on a large water main that energy bill runs for twenty years. Check it against the pump cost before defaulting to the cheap option.

Then the impulse lines. Differential pressure measurement is only as good as its tapings. Condensing legs, blocked impulse lines and air pockets in liquid service cause more wrong readings than the plate itself ever does.

Where they are the wrong answer

Low flows in large pipes, dirty service that plugs the taps, and anywhere the permanent pressure loss is unacceptable. For those, look at electromagnetic or vortex.