How to flatten regulator flow curves

When an application calls for maintaining consistent downstream pressures at high flows, it will most likely require the use of a domeloaded pressure-reducing regulator.

Even then, you may need to add external components to the regulator to achieve your desired performance. Your goal in doing so is to produce a reasonably flat flow curve for the regulator at a set pressure.

A flow curve for a pressure-reducing regulator illustrates the range of outlet pressures (vertical axis) the regulator will maintain based on various flow rates (horizontal axis) in a system. Ideally, you want to operate the regulator on the flattest – or most horizontal – part of a curve, which is where it will maintain relatively constant pressure even with significant changes in flow.

Unfortunately, every pressure-reducing regulator flow curve will experience some droop. Figure 1 represents four flow curves, and as you read each one from left to right, the downward slope in pressure is referred to as "droop." On the far right of each curve, you can see where pressure drops steeply.

The point from where the pressure begins to decline rapidly to where it approaches zero is called the choked-flow area. You want to avoid operating your regulator in this region because it will result in a relatively large pressure drop downstream.

This is the region where your regulator is not efficient or effective. If your flow rates will take you into this region, you should consider a different or larger size regulator.

However, when operating a regulator in the functional portion of the curve (for example, before the choked-flow area begins), you can reduce droop and achieve a flatter curve across a wider flow range by expanding the regulator's capabilities.

Using a springloaded regulator as a baseline (Figure 2), this article will explore the following system configurations: 

  • Employ an upstream pilot regulator to control dome pressure in a dome-loaded regulator (Figure 3);
  • Employ an upstream pilot regulator and add downstream external feedback to the domeloaded regulator (Figure 4); or
  • Employ an upstream pilot regulator and add downstream external feedback to the pilot regulator (rather than the domeloaded regulator) (Figure 5).

These configurations offer good, better, and best performance in terms of maintaining more consistent pressure – and therefore flatter flow curves – over increasingly broader flow ranges.

Assumptions

For this article, we'll focus our attention on a facility that uses nitrogen from a single source for multiple processes. Assuming that the processes are not all in continuous operation, nitrogen flow demand will vary throughout the day.

FIGURE 1: These four flow curves represent different systems employing a springloaded regulator (curve 1) and the same domeloaded regulator with slight modifications (curves 2, 3 and 4). Each of the three domeloaded regulator systems features a different configuration designed to flatten and extend the flow curve.

Figure 1: These four flow curves represent different systems employing a springloaded regulator (curve 1) and the same domeloaded regulator with slight modifications (curves 2, 3 and 4). Each of the three domeloaded regulator systems features a different configuration designed to flatten and extend the flow curve. 

If the facility were to employ a springloaded regulator to control gas pressure, an increase in downstream flow would cause pressure drops, while a decrease in downstream flow would cause pressure spikes.

Both pressure changes would require frequent manual adjustments to the regulator or additional point of use regulation.

Instead, the facility will use a domeloaded regulator, which enables dynamic pressure control – without manual adjustments – to provide more consistent pressure as flow demands vary.

FIGURE 2: A springloaded regulator, which employs a spring to control flow, provides a baseline for the flow curve comparison.(Pictured alongside) Figure 2: A springloaded regulator, which employs a spring to control flow, provides a baseline for the flow curve comparison. 

In a domeloaded regulator, a volume of pressurised gas in the regulator's dome chamber replaces the role of the spring in a springloaded regulator (Figure 6a).

The dome chamber is pressurised at a level slightly above the required outlet pressure. This constant pressure creates a force on top of the diaphragm.

If that force is higher than the force created by the outlet pressure, the poppet will open. As pressure equalises, the downstream pressure will apply a force upward on the diaphragm to close the poppet (Figures 6b and 6c).

Figure 1 shows four flow curves for regulators that could be used in this system, each based on the same outlet set pressure of 20 bar.

The first curve represents the springloaded regulator as a baseline for comparison.

The remaining three curves represent different system configurations using the same domeloaded regulator.

As we'll review, adding various external components, and internal design changes, to the domeloaded regulator enables pressure in the dome to be adjusted dynamically to improve the regulator's performance capabilities.

'Good' domeloaded regulator

In what we're calling the "good" regulator configuration, we'll employ a domeloaded regulator that responds to pressure fluctuations by enabling pressure in the regulator's dome chamber to remain constant over a wider system flow range.

FIGURE 3: 'Good' - adding a pilot regulator and outlet loop to a domeloaded regulator enables dynamic control of dome pressure to enhance the domeloaded regulator's performance.

Figure 3: 'Good' – adding a pilot regulator and outlet loop to a domeloaded regulator enables dynamic control of dome pressure to enhance the domeloaded regulator's performance. 

To allow this, we'll add a pilot regulator to deliver pressurised gas to the domeloaded regulator's dome chamber, as well as an outlet loop to relieve excess dome pressure (Figure 3). This setup provides dynamic dome pressure control to help the domeloaded regulator maintain its initial set pressure.

The pilot regulator's gas source originates from the system media (nitrogen) itself. Connected with the outlet pressure through the bleed line, the pressure inside the dome will stay constant at 20 bar.

When the downstream pressure drops below 20 bar, the dome pressure will also drop; the pilot regulator will then compensate by increasing the dome pressure to the initial set pressure.

When downstream pressure rises, the domeloaded regulator will compensate by closing its orifice. This will cause the dome pressure to increase, which, in turn, will reduce the orifice of the pilot regulator and bleed the excess dome pressure into the downstream process line. These actions bring both regulators back to the initial set pressure.

The second flow curve in Figure 1 illustrates the extended capabilities the domeloaded and pilot regulator configuration enables compared to the system using a springloaded regulator (curve 1). The set spring in a springloaded regulator loses force when it gets longer when pushing the poppet open.

This causes the droop (drop in outlet pressure). With dynamic dome pressure control provided by the pilot regulator, the useable area of the flow curve is greater compared to the springloaded regulator.

Depending on the specifics of the application, the "good" configuration will allow for use on systems with increased flow without worry of experiencing significant outlet pressure drops. But, we can do better.

'Better' domeloaded regulator

In our second – or "better" – configuration, we'll add a feedback line to enable the domeloaded regulator to compensate for downstream pressure drops.

Using the same pilot regulator and outlet loop setup as above, we'll incorporate a downstream tubing run that delivers external feedback to the sensing area of the domeloaded regulator (Figure 4). This configuration flattens the curve.

FIGURE 4: 'Better' - employing a tubing run to deliver downstream pressure feedback to the sensing area of the domeloaded regulator allows this system to compensate for downstream pressure drops.

Figure 4: 'Better' – employing a tubing run to deliver downstream pressure feedback to the sensing area of the domeloaded regulator allows this system to compensate for downstream pressure drops. 

On the downstream side of our system, pressure will drop slightly (commonly referred to as "recovery") after the system media exits the domeloaded regulator. For example, if the outlet gas pressure was initially 20 bar, it may be only 19 bar a short distance downstream.

An external feedback line, located a short distance downstream (usually five to 10 times the distance of the tubing outside diameter) directs the lower pressure gas back to the sensing area of the domeloaded regulator.

Here, the regulator's diaphragm detects the lower 19 bar pressure and opens the regulator's poppet further to increase outlet pressure.

As a result, the regulator responds dynamically and more accurately to the less turbulent downstream pressure rather than a static setting.

Looking at the third flow curve in Figure 1, you'll notice that the curve is flatter for this "better" configuration. Remember, we're using the same domeloaded regulator, with modifications to the sensing area, set to the same outlet pressure for the second, third and fourth flow curves.

In addition, you'll see that the operating flow rate of the regulator has expanded before reaching a choked-flow state. However, we can still do better.

'Best' domeloaded regulator

The third system configuration – our "best" scenario – enables the pilot regulator to make highly accurate adjustments to pressure in the domeloaded regulator's dome chamber based on the actual downstream system pressure.

Like the "better" configuration, this design employs a pilot regulator, an outlet loop to relieve excess dome pressure, and an external feedback line.

In this setup, however, the downstream tubing run delivers external feedback directly to the pilot regulator (Figure 5 below).

FIGURE 5: 'Best' - a tubing run in this system configuration delivers downstream pressure feedback to the pilot regulator, enabling the domeloaded regulator to compensate by changing its outlet pressure. The result is a very flat flow curve over a very broad flow range.

Figure 5: 'Best' – a tubing run in this system configuration delivers downstream pressure feedback to the pilot regulator, enabling the domeloaded regulator to compensate by changing its outlet pressure. The result is a very flat flow curve over a very broad flow range. 

With adjustments made at the primary pressure control source – the pilot regulator – this configuration maintains very precise control of downstream pressure and yields a very flat flow curve over a very broad flow range.

Consider this example. The outlet pressure for the pilot regulator is initially set to 20 bar, which means pressure in the domeloaded regulator's dome chamber is slightly higher. On the downstream side of the domeloaded regulator, system pressure drops to 19 bar.

This lower pressure is directed back to the pilot regulator through the feedback line. In response, the pilot regulator increases pressure in the dome of the domeloaded regulator, resulting in the needed correction to downstream pressure.

Both regulators – the pilot and the domeloaded – adjust dynamically to enable pressure at the downstream feedback loop to maintain the desired 20 bar. This system configuration creates a loop that enables constant, automatic adjustments to stabilise the system at the desired set pressure for optimum performance.

The results are evident in the fourth flow curve in Figure 1, which is very flat – with almost no droop – over an extremely broad flow range. This system will not experience choked flow until flow climbs even higher.

The only way to produce an even flatter – almost perfectly flat – flow curve is to replace the manual pilot regulator in this configuration with an electronically-controlled pilot regulator.

An electronic sensor attached to the regulator would make multiple pressure adjustments per second, yielding an extremely flat flow curve.

However, electronic controls are often not desirable due to power requirements and safety considerations. A system employing a manual pilot regulator with feedback directly to the pilot regulator provides very similar results without the added electronic components, wiring, and power source.

High-flow applications typically require the use of regulators with very flat flow curves over broad flow ranges.

Variety of components

At times, a simple domeloaded regulator will deliver the required parameters for an application.

FIGURE 6: Inside a springloaded regulator (6a), a spring applies force (Fs) to a diaphragm to open and close the regulator's poppet. In a domeloaded regulator (6b and 6c), pressurised gas in a dome chamber replaces the role of the spring, providing force (Fd). For example, with no downstream flow demand, the poppet remains closed (6b). As flow is initiated downstream, the outlet pressure drops, causing the diaphragm to open the poppet to the point at which the dome pressure force (Fd) and system pressure force (F) equalise (6c).

Figure 6: Inside a springloaded regulator (6a), a spring applies force (Fs) to a diaphragm to open and close the regulator's poppet. In a domeloaded regulator (6b and 6c), pressurised gas in a dome chamber replaces the role of the spring, providing force (Fd). For example, with no downstream flow demand, the poppet remains closed (6b). As flow is initiated downstream, the outlet pressure drops, causing the diaphragm to open the poppet to the point at which the dome pressure force (Fd) and system pressure force (F) equalise (6c).

However, it may be necessary to add a variety of components to a system to expand the capabilities of the regulator. Systems that deliver downstream pressure feedback to an upstream pilot regulator typically offer the best performance.

While one can never achieve the ideal of a perfectly flat flow curve over an infinite flow range, it's good to know there are ways to enhance a domeloaded regulator's capabilities to realise very flat flow curves over very broad flow ranges.

[Michael D. Adkins is Manager, Field Engineering and Pressure Regulators, Swagelok Company. Adkins joined Swagelok in 1994. In his current role he oversees the Swagelok field engineering team, engineers dedicated to providing direct technical support to customers in the field. He also manages the regulator product line for Swagelok, where he assesses market needs and develops product strategy, positioning, and pricing.]