Pressure regulator curve for nitrogen to hydrogen

Choosing the right regulator for your needs involves closely reviewing its flow curve, typically available from the manufacturer. The term “flow curve” might seem a bit counterintuitive because the primary function of a regulator is to manage pressure rather than flow. It might be more aptly described as a “pressure curve.” This curve outlines the various pressures a regulator can sustain across different flow rates within a system.

What is the Flow Curve of a Pressure Regulator?

The flow curve of a pressure regulator, often referred to as its flow characteristic curve, graphically represents the relationship between the flow rate of the gas or liquid through the regulator and the downstream (outlet) pressure. This curve is crucial for understanding how the regulator will perform under varying flow conditions and is essential for selecting the right regulator for a specific application.

How to Read a Pressure Regulator Curve?

A typical pressure regulator curve will have the flow rate on the horizontal axis (usually in units like standard cubic feet per hour, SCFH, for gases or gallons per minute, GPM, for liquids) and the outlet pressure on the vertical axis (in units like pounds per square inch, PSI). To read this curve:

1. Identify the inlet pressure: The curve might be presented for a specific, constant inlet pressure or multiple curves for different inlet pressures.
2. Locate the desired outlet pressure on the vertical axis.
3. Move horizontally from the outlet pressure point until you intersect the curve.
4. Move down vertically to the horizontal axis from the intersection point to find the flow rate corresponding to that outlet pressure.

The curve allows you to predict how the outlet pressure will change as the flow rate increases or decreases and is vital for ensuring the regulator can maintain a desired pressure under expected operating conditions.

Difference Between Droop and Lockup

• Droop (also known as regulation or deviation) refers to the decrease in outlet pressure as the flow rate increases through the regulator. It occurs because as the demand (flow rate) increases, the regulator may not be able to fully compensate for the increased flow, leading to a drop in the controlled pressure. The droop characteristic is a key aspect of the flow curve and is more pronounced in regulators not well-suited for the flow rates being demanded.
• Lockup refers to the slight increase in outlet pressure observed when the flow is stopped (e.g., when a valve downstream of the regulator is closed). It is the pressure at which the regulator starts to pass flow again after having been closed. This phenomenon occurs because the regulator needs to build up a small amount of pressure “overhead” to overcome internal friction and spring forces to reopen and allow flow again.

What is the CV of a Regulator?

The CV (Flow Coefficient) of a regulator is a measure of its efficiency at allowing fluid flow. It’s defined as the number of US gallons of water at 60°F that can flow through the regulator per minute with a pressure drop of 1 PSI. A higher CV indicates a higher capacity for flow. The CV is a crucial parameter when selecting a regulator, as it helps ensure that the regulator can handle the required flow rates for a particular application without introducing significant pressure drops.

CV Calculations

The pressure regulator curve for nitrogen, like for any gas, illustrates how the outlet pressure of the regulator varies with the flow rate of nitrogen passing through it. These curves are specific to each regulator model and are provided by manufacturers to help users understand how a particular regulator will perform under different conditions. To interpret such a curve for nitrogen, you would typically look for the following:

• Inlet Pressure: The curve should indicate the inlet pressure for which it is rated. This is the pressure of the nitrogen before it enters the regulator.
• Outlet Pressure vs. Flow Rate: The curve will show how the outlet pressure changes as the flow rate of nitrogen increases. This is crucial for applications requiring a constant pressure despite changes in flow demand.
• Droop: As the flow rate increases, the curve will typically show a decrease in outlet pressure, known as “droop.” This is due to the regulator’s inability to maintain the set outlet pressure under higher flow conditions.
• Lockup Pressure: Some curves may also indicate the lockup pressure, which is the pressure at which the regulator begins to allow flow after being in a no-flow condition. This is slightly higher than the set pressure.

To read such a curve:

1. Identify the Set Outlet Pressure: Start by determining the desired outlet pressure for your application.
2. Trace the Flow Rate: From your set outlet pressure, move horizontally across the curve until you intersect the plotted line. This intersection represents the regulator’s behavior at your set pressure.
3. Interpret the Performance: At the intersection, trace downward to determine the flow rate. This tells you how much nitrogen (in terms of flow rate) can be passed through the regulator at your desired outlet pressure.

Understanding these curves is essential for ensuring that the chosen regulator can provide the necessary pressure control for your application with nitrogen. If you have a specific regulator model in mind or need further assistance, providing the model details can help in giving a more targeted explanation.

A pressure regulator curve for hydrogen will depict how the regulator performs specifically with hydrogen gas under various conditions. Given the unique properties of hydrogen, such as its low density and high diffusivity, regulators used for hydrogen need to be carefully selected and understood. Here’s a general guide on what to look for in a pressure regulator curve for hydrogen and how to interpret it:

Understanding the Curve

• Inlet Pressure: This is the pressure of the hydrogen gas before it enters the regulator. The curve might be given for a specific inlet pressure or multiple curves for different inlet pressures.
• Outlet Pressure vs. Flow Rate: The curve shows the relationship between the flow rate of hydrogen through the regulator and the outlet pressure. As the flow rate increases, the outlet pressure typically decreases due to the regulator’s inability to maintain a constant pressure at higher flows.
• Droop: This is the decline in outlet pressure as the flow rate increases, noticeable in the curve’s downward slope as you move from left to right. Droop is a critical factor to consider for applications requiring a stable outlet pressure.
• Lockup Pressure: Some curves might also show the lockup pressure, which is the pressure at which the regulator closes to prevent flow. This is usually a bit higher than the set pressure and is crucial for safety and efficiency in hydrogen systems.

Reading the Curve

1. Select the Desired Outlet Pressure: Determine what outlet pressure you need for your hydrogen application.
2. Trace Along the Desired Pressure: Find your selected pressure on the vertical axis and trace horizontally across to where it intersects with the curve.
3. Determine the Flow Rate: At the intersection, trace down to the horizontal axis to find the corresponding flow rate. This value indicates how much hydrogen gas (in terms of flow rate) the regulator can handle at your selected pressure.

Special Considerations for Hydrogen

• Material Compatibility: Ensure the regulator is made from materials compatible with hydrogen to prevent embrittlement or other forms of degradation.
• Safety Features: Due to hydrogen’s flammability and potential for high-pressure applications, safety features like relief valves or burst disks are crucial.
• Precision and Stability: Given hydrogen’s use in sensitive applications (like fuel cells), regulators need to provide precise and stable control over pressure.

When selecting a pressure regulator for hydrogen, it’s essential to consult with the manufacturer or a specialist to ensure the chosen regulator meets the specific requirements of your application, considering factors like flow rate, pressure, material compatibility, and safety.