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by James Dickie on Thu, Oct 24, 2024 @ 12:10 PM
I'll try here to give a short, clear explanation of what a flow curve is, why it matters, and how to interpret it. I'll touch on common issues like droop and choked flow, providing practical examples and offering tips for selecting the right regulator based on flow curves.
A flow curve shows how well a regulator maintains downstream pressure as flow rate changes. On the vertical axis, you have downstream pressure, and on the horizontal axis, flow rate. Each regulator’s curve will be unique based on its set pressure and system conditions.
The ideal flow curve would be flat, meaning pressure stays constant regardless of the flow rate. However, real-world regulators have limitations. As flow increases, you’ll notice a drop in pressure on the right side of the curve—this is “droop.”
Droop indicates a decreasing ability to maintain pressure as flow rises. Further and you’ll reach “choked flow,” where the regulator is wide open and can no longer control pressure.
On the left side of the curve, at lower flow rates, the curve also dips. This is called “seat-load drop” or “lockup,” where the regulator struggles to keep pressure steady at low flow.
The goal is to choose a regulator with a relatively flat curve within your operating range. If your conditions fall in areas with steep drops, a different model might be better.
To learn more about flow curves, get Swagelok's Flow Curves Technical Bulletin. We also invite you to visit our local Swagelok Regulators website page. It offers additional downloads and provides information on design features, comparing value, customer stories, technical tips videos, and a regulators FAQ.
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To flatten the flow curve and improve performance, you can use an upstream pilot regulator with a dome-loaded regulator, which helps maintain pressure over a broader flow range. Adding external feedback tubing to the system and regulator can further stabilize performance.
For the flattest curve, feedback tubing should be added to both the dome-loaded regulator and the pilot regulator. Our team can work alongside yours to identify the best setup for your system and ensure optimal regulator performance.
Ensure that the units of measurement across your system match. Pressure is typically measured in psig or bar. Liquid flow is in gallons per minute or liters per minute, while gas flow is in cubic feet per minute or standard liters per minute.
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