Selecting pressure regulators can be complicated because many different types exist and each has specific functionalities. Presented here is guidance for evaluating pressure-control needs based on process conditions, and for selecting the best regulator
Delivering consistently on-specification products is a core priority for chemical processors. Doing so requires that chemical handling and transfer systems consistently operate as intended and under carefully controlled processing conditions. Many critical components play key roles in maintaining those process conditions. Tube fittings, valves, gages and other items all contribute to optimal pressure, temperature and flow control inside your systems. Pressure control is especially critical — unexpected pressure drops can lead to system inefficiencies or other process problems that may interfere with the fluids inside. Elsewhere, an unwanted increase in pressure can potentially damage sensitive analytical equipment or, in the worst case, create a safety hazard for facility personnel.
Thus, pressure regulators play an important role in chemical processing systems, helping control or maintain desired pressure in response to system changes. It is important to select and install pressure regulators that can meet the specific requirements of your application — but making the right choice isn’t always easy.
Regulators come in a variety of different types, each with their own specific functionality to meet the needs of a given application. This article explores how to evaluate pressure needs based upon your application conditions, how regulators work and their typical behaviors, and the different varieties available to professionals today.
Understanding process conditions
Choosing a suitable regulator for a particular process system begins with understanding the pressure, temperature and flow requirements of the process, as well as the material compatibility of your chosen regulator with system media. The following are a few key factors to consider.
Pressure. The different behaviors exhibited by gases and liquids impact regulator selection, so it is important to understand the composition of the process fluid before selecting a pressure regulator. For example, pressure regulators can handle more flow with a low-density gas than they can with a high-density gas. The details of characteristics like this will dictate adjustments to regulator sizing (Figure 1).
FIGURE 1. The graph shows pressure control ranges for regulator sizing for nitrogen, and the table indicates correction factors for other gases, compared to nitrogen
It is also important to understand the anticipated pressure requirements of the processes. Pressure regulators are typically rated for maximum, minimum and regularly anticipated operating system pressures, and the chosen regulator must be aligned with these expectations.
Pressure control ranges, indicated by their corresponding flow curves, are typically featured in regulator product specifications. Two important questions to ask before making your selection include: “How is the outlet pressure related to the anticipated flow?” and “Is the expectation that outlet pressure is the same at minimal, normal and maximum flow?”
Temperature. Temperature is another factor that can influence regulator selection. Some system media can dramatically fluctuate in temperature as the pressure changes. This is often caused by a phenomenon called the Joule-Thomson effect (Figure 2).
FIGURE 2. The Joule-Thomson effect occurs when certain system media experience rapid temperature change as a result of changing pressure levels
For example, compressed natural gas may cool as much as 45°C as pressure drops, which could cause the regulator to freeze — compromising functionality — unless the proper accommodations have been made within the fluid system. Tools are available to calculate Joule-Thomson effects within process systems, and engineers can often work with regulator suppliers to predict the potential effects.
Process media. It is critical to ensure that all parts of the pressure regulator are compatible with the chemicals being handled by the system. All wetted components of a regulator can be negatively affected by system media. This includes not only metal alloy components, but elastomeric and polymeric materials used in seats and seals. Chemical and physical incompatibility could contribute to accelerated deterioration, particle generation and premature regulator failure, resulting in excessive system downtime. Regulator suppliers can help determine which materials of construction will be chemically compatible with your system media, and which ones will also provide the additional performance characteristics required by the process, such as extreme temperature resistance.
Pressure control requirements
Once process conditions have been considered, the next step should be determining what the requirements for the pressure regulator are. Regulator functionality falls into one of two distinct categories, and the choice depends on process requirements (Figure 3). These general categories are pressure reduction and backpressure control.
FIGURE 3. Back-pressure regulators and pressure-reducing regulators each can play an important role in system pressure control
Controlling back-pressure. Back-pressure regulators sense the inlet pressure and control pressure from upstream. They can help control and maintain upstream pressure by releasing excess pressure if system conditions cause levels to become higher than desired.
Reducing pressure. Pressure-reducing regulators manage flow by monitoring outlet conditions and maintaining consistent downstream pressure. If there is a need to lower the force from a high-pressure source before system media reaches the main process, a pressure-reducing regulator will be the most appropriate choice.
After the type of functionality has been determined, it is helpful to understand the different elements that make up a regulator and how they work together to provide desired pressure control. Often, the functions of these elements will help further determine the specific choice of regulator. These elements include the following:
Loading element. The loading element applies a downward, balancing force on top of the sensing element. It is typically a spring or a dome, depending on the needs of the application (more information on this is contained in the next section).
Sensing element. The sensing element allows the poppet to rise and fall in the seat, controlling inlet or outlet pressure. This is typically a diaphragm or a piston.
Control element. This includes a seat and a poppet. The seat contains pressure and prevents fluid movement from the opposite side of the regulator when flow is closed. The poppet completes the sealing process while fluid is flowing through a system.
Pressure-reducing regulators use these elements to balance four different forces, as seen in Figure 4. These include loading force (F1), inlet spring force (F2), outlet pressure force (F3) and inlet pressure force (F4). Total loading force must equal the combination of inlet spring force, outlet pressure force and inlet pressure force.
FIGURE 4. Loading, sensing and control elements come together to provide pressure control. A pressure-reducing regulator is seen here, creating a balance of forces by sensing inlet pressure
Back-pressure regulators function by balancing the spring force (F1), inlet pressure force (F2), and outlet pressure force (F3), as shown in Figure 5. The spring force must be equal to the combined force of the inlet pressure force and the outlet pressure force.
FIGURE 5. A back-pressure regulator creates a balance of forces
Factors for regulator behavior
Once a regulator has been installed, it is important to understand how its behavior can be influenced by real-world process factors. There are several natural behaviors engineers should be knowledgeable about to select regulators effectively, including the following:
Flow curve. A regulator’s flow curve illustrates the regulator’s performance under system parameters (Figure 6). The vertical axis shows outlet pressure; the horizontal axis indicates downstream flowrate. The flattest part of the curve indicates where a regulator will maintain consistent pressure, even if flow changes considerably. The far right of the curve indicates where the regulator will be fully open and not able to preserve a consistent pressure. In this area, the poppet reaches the limit of its stroke and is unable to maintain the desired pressure.
FIGURE 6. Flow curves, such as those shown here, indicate a regulator’s ability to control pressure under different process conditions
Droop. Droop naturally occurs when flow requirements cause the regulator’s poppet to open wider. The loading element gradually loses force, which leads to pressure loss (droop). While droop is natural for every regulator, maintaining a flow curve that is as flat as possible before the pressure drops off is ideal. Droop can be minimized by selecting a regulator that is appropriate for your application’s requirements.
Lock-up. Lock-up is the initial drop from a specific pressure just above the set point, necessary to completely stop flow. When flow is turned on, the regulator’s flow curve will drop slightly to the set point at the very start of the flow curve. Lock-up is natural regulator behavior — however, well-designed regulators are able to minimize the effect.
Supply-pressure effect. Supply-pressure effect (SPE) is the change in outlet pressure caused by a change in inlet pressure, where inlet and outlet pressure changes are inversely proportional to each other. If the inlet pressure decreases, there will be a corresponding outlet pressure increase. Conversely, if the inlet pressure increases, the outlet pressure decreases.
Manufacturers typically provide a regulator’s anticipated SPE figure, which is provided as a ratio or percentage. If a regulator is described as having a 1:100, or 1%, SPE, for example, this means that for every 100 PSI drop in inlet pressure, the outlet pressure will increase by 1 PSI. Outlet pressure variation for a regulator can be estimated with the formula in Equation (1):
ΔP(outlet) = ΔP(inlet) × SPE (1)
Generally, it is desirable to minimize SPE, especially in higher-flow applications where regulator poppets are larger. Here, a balanced poppet design can be helpful. Elsewhere, a dual-stage regulator system setup can help minimize SPE. In this system configuration, the first regulator that system media reaches reduces higher-than-ideal inlet pressure, allowing minimal pressure drop for the second regulator. Dual-stage regulators can also conduct two-stage pressure reduction within one regulator body. The specific needs for controlling SPE will depend on the application. Pressure regulator suppliers should be able to help determine which measures are necessary.
Loading elements
A significant variable to consider when choosing a regulator is the loading element. The choice of a spring-loaded or dome-loaded regulator will depend on the specific pressure-control requirements of a process.
Spring-loaded regulators are more common, effective for general-purpose applications, and typically are more familiar to professionals working in the chemical process industries (CPI). In these regulators, a spring applies force on the sensing element, moving the poppet closer or further from the orifice, thereby controlling downstream pressure.
Spring-loaded regulators are available for both pressure-reducing and back-pressure control needs. A pressure-reducing, spring-loaded regulator controls pressure to the process by sensing outlet pressure and controlling downstream pressure. A back-pressure, spring-loaded regulator controls pressure from the process by sensing inlet pressure and controlling upstream pressure, and can help control and maintain upstream pressure by releasing excess pressure.
Dome-loaded regulators are more specialized, providing more dynamic and accurate pressure control as demand varies, as well as lower droop and SPE. The loading force is controlled by a pressurized gas housed in a dome chamber. The gas flexes a diaphragm, moving the poppet from the orifice. Dome-loaded regulators are typically chosen to improve precision in sensitive applications and can be incorporated into various configurations to maintain a very flat flow curve. They can also be coupled with pilot regulators and external feedback lines to achieve highly accurate adjustments when the application calls for it.
Like spring-loaded regulators, dome-loaded options are available for both pressure-reducing and back-pressure control purposes. A pressure-reducing, dome-loaded regulator senses outlet pressure and controls downstream pressure, which helps to reduce pressure from a high-pressure source. A back-pressure, dome-loaded regulator senses and releases excess inlet pressure, which helps to maintain upstream pressure.
Regulator setup best practices
Once the ideal regulator for the needs of the application has been identified, it is important to follow maintenance best practices throughout its lifespan, because like any other piece of equipment, regulators experience wear and tear that can lead to compromised performance.
For example, one of the most common issues with a poorly maintained regulator is called creep, which occurs when a contaminant creates a fine gap between the regulator’s seat and poppet (Figure 7).
FIGURE 7. Creep occurs when contaminants create a fine gap between the regulator’s seat and poppet
The gap allows system media to unintentionally flow across the seat and can lead to unwanted pressure increases downstream — a situation that can become problematic, dangerous or both.
Installing a good filter upstream of the regulator can help remove contaminants before they reach the regulator, preventing creep. Importantly, in order to be effective, filters should be regularly cleaned and replaced as needed. If creep does occur, installing a downstream relief valve can act as a failsafe, relieving higher-than-ideal pressure before it causes system damage or influences system media. Maintaining spare part kits for your regulator can enable you to fix any issue quickly.
Reliable pressure control plays an important role in maintaining the quality of products and the safe operation of critical fluid systems. Armed with the right information, engineers can make informed choices about the best pressure regulator options for a particular application (Figure 8). When in doubt, consult with regulator suppliers, who may be able to assist in evaluating your needs and can guide you toward an effective choice.
FIGURE 8. Suppliers of pressure regulators can help evaluate needs and guide choices
Edited by Scott Jenkins
Acknowledgement
All graphics included in the article were supplied by Swagelok.
Author
Paul Kobasuk is Swagelok’s (29500 Solon Road, Solon, OH 44139; Email: [email protected]; Phone: +1-440-248-4600; Website: www.swagelok.com) global product manager for general industrial pressure regulators and their integration with pressure control solutions. He has worked to create customer value in the industry for two decades.