Jon Kestner, Product Manager, Regulators and Valves, Swagelok Company on Why the Flow Coefficient Isn’t Very Helpful
A regulator’s main purpose is to maintain steady pressure in a fluid system application – preferably across the application’s full range of anticipated flow rates. To help confirm that your regulator is sized appropriately, consult its flow curve, which represents the range of pressures the regulator will maintain, given certain system flow rates. The regulator’s flow curve is much more helpful than its flow coefficient (Cv), as the Cv represents the regulator’s maximum flow capacity, at which point it can no longer control pressure. If system flow rates are expected to reach the regulator’s Cv, you will likely need a larger size regulator.
Understanding the Basics
A flow curve plots a regulator’s outlet pressure (Y axis) and flow rate (X axis) and shows how the regulator will respond as system flow demand changes. As flow demand increases, the regulator will try to maintain its original set pressure, but the outlet pressure will decrease as flow increases. This behavior is called droop.
Typically, a flow curve for a pressure-reducing regulator consists of three parts: (1) a steep drop on the far left; (2) a relatively flat part in the middle; and (3) a steep drop on the far right. Ideally, you want to operate the regulator on the flattest part of the curve, where it will maintain relatively constant pressures, even with significant changes in flow.
Reading the curve in Figure 1 from left to right, it starts at 6.3 bar – its original set pressure – and then drops quickly before leveling off. The initial steep pressure drop at the far left is called seat load drop. It occurs when an operator initiates flow through the regulator by opening a downstream valve. The pressure drops as the poppet unloads from the seat. It is difficult for a regulator to maintain pressure until the poppet moves far enough from the seat surface. This distance is typically very small, but it does result in a sharp pressure drop as flow starts through the regulator.
The curve then levels off slightly but is not perfectly flat, with a continued droop downward. As flow increases, pressure will continue to decrease. While droop is relatively modest along the flat part of the curve, it is quite steep at the far ends of the curve where seat load drop begins (far left) and choked flow conditions occur (far right).
The choked flow area in Figure 1 begins where pressure starts to droop sharply around 3900 std L/min. Eventually, near 4200 std L/min, pressure drops to zero. At this point, the regulator is wide open and is no longer regulating pressure. Increasing downstream flow to this point or beyond renders the regulator ineffective. Note that Cv is measured at the regulator’s fully choked point, which is why Cv is a poor indicator of a regulator’s overall performance.
Another phenomenon – lock-up – occurs when slowly stopping system flow. You’ll therefore read the curve in Figure 1 from right to left. Just the opposite of seat load drop, when flow nears zero, the regulator has difficulty maintaining a set pressure as the poppet nears the seat. As the poppet is pushed against the seat, creating a bubble-tight seal, there will be a sharp rise in pressure, which mirrors the sharp decrease seen during seat load drop.
Selecting a Regulator
Because it’s best to operate a regulator along the flattest – or most horizontal – part of its flow curve, the ideal flow curve would be a flat line. However, this is not possible, as droop will always occur. Therefore, consult a regulator’s flow curve – not its Cv – to ensure it will maintain the expected system pressures based on the anticipated range of flows within your system. For the most consistent performance, the regulator will ideally operate along the relatively flat part of its curve. Avoid operating the regulator on the far ends of the curve where undesirable conditions like seat load drop, lock-up, and choked flow occur.
One final note: It is possible to achieve very flat flow curves over a wide range of flows by using a dome-loaded regulator coupled with additional components. Learn how by reading “Three Ways to Reduce Droop in High-Flow Systems.”
This article was written by Jon Kestner, product manager of regulators and valves at Swagelok. An article on reducing droop in high-flow systems by Kestner appears in the January/February 2019 issue of Fluid Handling International.