This is part two of a two-part piece. Part one can be found here.
Droop, Choked Flow & Other Complexities
As mentioned above, it is best to operate along the flattest – or most horizontal – part of a flow curve. Indeed, the ideal flow curve would be a flat line. However, no regulator can produce a perfectly flat line over the full range of pressures because the internal components of the regulator have limitations. In a springloaded regulator, longer springs produce flow curves with broader horizontal sections. Domeloaded regulators, which use a trapped volume of gas instead of a spring, produce even broader horizontal sections. Electrically controlled pneumatic-loaded regulators and domeloaded regulators with external feedback produce the broadest horizontal sections.
Typically, a flow curve consists of three parts: (1) a relatively flat part in the middle; (2) a steep drop on the far left; and (3) a steep drop on the far right. The flat part in the middle is not perfectly flat. Usually, it slopes downward, and this is called droop. As flow increases, pressure will drop some – or a lot depending on the regulator design. While droop is relatively modest along the flat part of the curve, it is quite steep at the far ends of the curve.
Choked flow occurs on the far right of a curve. See the choked-flow area in Figure 2, where pressure begins to droop sharply at 140 std ft3/min (3964 std L/min). Eventually, near 150 std ft3/min (4247 std L/min), pressure drops to zero. At this point, the flow demand has exceeded the pressure-controlling capabilities of the regulator. Here, the regulator is wide open and is no longer regulating pressure. It has essentially changed from a pressure-controlling device to an open orifice. Increasing downstream flow to this point or beyond renders the regulator ineffective. It is not advisable to operate a regulator in the choked-flow area due to the sharp pressure drop.
Figure 2. This typical flow curve for a pressure-reducing regulator demonstrates several phenomena, including the ideal operating area, droop, choked flow, seat load drop or lockup, and flow coefficient (Cv).
Note that Cv is measured at the regulator’s fully choked point, and that’s why it is not an especially useful measurement. It is a poor indicator of a regulator’s overall performance.
Seat load drop occurs on the far left of the regulator curve (Figure 2), where there is initially a steep drop in pressure. If you are reading the curve from left to right, imagine that the system is in a no-flow state. The regulator is set to a certain pressure, but there is no flow. Then, imagine that an operator slowly opens a downstream valve to initiate flow. Right away, there is a sharp drop in pressure because it is difficult for a regulator to maintain pressure at this location. If your regulator is operating along this steep drop in the curve, you may hear chattering or pulsating as the regulator fluctuates between flow and no-flow conditions.
Domeloaded regulators produce flow curves with very broad horizontal sections because they use a trapped volume of gas internally.
Now let’s read the curve from right to left. Imagine that the system is operating along the flat part of the curve. Then, imagine that an operator is slowly closing a downstream valve, reducing flow to near zero. We are moving up the curve. As we near the no-flow state, the regulator has difficulty maintaining the set pressure. Again, you may hear chattering. Eventually, the regulator snaps shut, stopping flow. This is called lock-up.
The terms seat load drop and lock-up are essentially interchangeable. Sometimes, people use lock-up to describe both conditions. It is not advisable to operate a regulator in this area.
Hysteresis
It matters whether you are reading a flow curve from left to right or right to left. When reading left to right, flow is increasing. And the reverse is true when reading right to left. Depending on whether flow is increasing or decreasing, the curve will be slightly different. Outlet pressure will not follow the same “droop line,” nor end at the original set pressure. This phenomenon, which is illustrated in Figure 3, is called hysteresis. The upper line is for decreasing flow, while the bottom line is for increasing flow.
Figure 3. The phenomenon of hysteresis reveals that outlet pressures will be higher at the same flow volume when decreasing flow compared to when increasing flow. (NOTE: Hysteresis shown larger than actual for demonstration purposes.)
Hysteresis, which results from dynamic friction forces within the regulator, is usually not an issue when evaluating the performance of a regulator. However, it can be a point of confusion during system operation. Suppose an operator sets up a system to deliver an outlet pressure of 50 psig (3.4 bar) at 110 ft3/min (3115 L/min). The next day, he notices that the pressure is now 50.5 psig (3.48 bar), but the flow is still 110 ft3/min (3115 L/min). Hysteresis is to blame. It is likely that something in the system temporarily created more flow demand downstream. Moving from left to right on the curve, the temporary flow increase slightly reduced the outlet pressure. Then, as the flow demand returned to 110 ft3/min (3115 L/min), hysteresis caused the outlet pressure to return to a point slightly higher than the initial set point.
A domeloaded regulator with external feedback produces the broadest horizontal sections for a purely mechanical pressure regulator.
This scenario illustrates the need for operators to periodically check systems to ensure regulators are set to the proper outlet pressures. It is recommended to approach your set pressure from a lower pressure. Another best practice is to employ pressure gauges in a system to help fine tune regulator settings to achieve desired operating pressures.
Back-pressure Regulators & Accumulation
Flow curves for back-pressure regulators reveal a phenomenon known as accumulation (see Figure 4), which is the opposite of droop. Because a back-pressure regulator controls inlet pressure, pressure will go up as flow increases instead of down as in a pressure-reducing regulator. This phenomenon occurs because the back-pressure regulator acts as a restriction and essentially limits flow at the inlet. Your objective is the same as it is with pressure-reducing regulators: to operate on the flat part of the curve.
Figure 4. The flow curve for a back-pressure regulator indicates accumulation, which is the opposite of droop.
Conclusion
When selecting a regulator for your application, consult the flow curve, not the Cv. Make sure you are looking at the right curve. Does it reflect the pressure you will set your regulator to (your “set pressure”)? Does it reflect the right inlet pressure range? Are you working with the right units? Do you need to calculate any adjustments if your system media is a gas?
Once these issues have been addressed, you can start reading your curve. On the curve, identify the range of flows you can expect in your system. Given that range, the curve will tell you what pressures you can expect the regulator to maintain. Ideally, you want the regulator to operate along the relatively flat part of the curve. That’s where the regulator will perform most consistently. You want to avoid operating the regulator on the far ends of the curve where undesirable conditions like lock-up and choked flow occur.
If a flow curve is not available, or if you need help selecting a regulator, consult with a knowledgeable sales and service representative who can provide guidance on properly sizing a regulator for your application.
To read part one of this two-part series, please click here. For more information, please email Menz via [email protected] or visit www.swagelok.com.
