Pressure Regulators – Design Principles, Types and applications
A pressure regulator is a device which controls the pressure of liquids or gases (medium) by reducing a high input pressure to a controlled lower output pressure. They also work to maintain a constant output pressure even when there are fluctuations in the inlet pressure.
Pressure regulators, in various forms, are used in many domestic and industrial applications, like regulating propane used in gas grills, to regulate oxygen in healthcare equipment, to supply compressed air in industrial applications, to regulate fuel in automotive engines and aerospace applications. The main aspect that is common across all these applications is pressure control – from a higher source pressure to a lower output pressure.
Example of a pressure regulator for pneumatics.
Basic elements of a pressure regulator
A typical pressure regular consists of the following elements:
- A pressure reducing element such as a poppet valve.
- A loading element to apply necessary force to the reducing element such as a spring, piston actuator or a diaphragm actuator.
- A sensing element such as a diaphragm or a piston.
Fig. 1: Schematic representation of a typical single stage pressure regulator
Pressure reducing element
Spring loaded poppet valves are commonly used as a pressure reducing element. Poppet valves have elastomeric sealing in regular applications and a thermoplastic sealing in high pressure applications. This seals the valve seating against any gas or fluid leakage. The poppet valve is controlled by the spring force to open the valve and let the medium flow from inlet to outlet. As there is a rise in output pressure, the poppet valve closes due to the force generated by the sensing element, which overcomes the spring force.
The loadig element is used to force the sensing element to open a valve. The amount of spring force can be varied which determines the amount of outlet pressure obtained.
The sensing element
Pistons are normally used for high pressures, rugged applications and applications where wider tolerances on the outlet pressure are acceptable. They tend to be sluggish due to friction between the pinion sealing and regulator body.
For higher accuracy, a diaphragm type of sensing element is suitable. They are made of elastomer or a thin disc type of material which is sensitive to changes in pressure. Diaphragms tend to have lower friction than piston type designs. They also provide a higher sensing area, for a given regulator size.
Types of pressure regulators
Pressure regulators can be broadly classified into the following categories:
- Direct Operated or Self-operated
- Pilot operated
Direct operated regulators
They are the simplest form of regulators (Fig. 1). They normally operate at lower set pressures, below 0,07 bar (1 psi) and can have greater accuracy. At higher pressures, up to 35 bar (500 psi), they can have 10-20% accuracy levels.
Direct operated regulators are self-contained, that is, they do not require an external sensing line at the output in order to operate effectively. They consist of a spring actuated valve that is directly controlled by a diaphragm assembly. Energy or pressure from the flowing medium works to activate the diaphragm. The increasing downstream pressure acts on the diaphragm, which closes the valve plug by compressing the spring. This closes the valve. As downstream pressure falls, spring force is now greater than the force of the medium acting on the diaphragm and the valve opens.
Pilot operated regulators
These regulators are ideal for applications with large variation in flow rates, fluctuations in inlet pressure, or decreasing inlet pressure conditions that normally occur with gas supplied in cylinders or small storage tanks. It provides precise pressure control.
This type of regulator is generally a one or two stage device. A single-stage regulator is ideal for relatively small reduction in pressure. They are not suitable where there are large fluctuations in inlet pressure or flow rates.
A double-stage regulator (Fig. 2) is the most commonly used type of pilot operated regulator. The first stage consists of a spring actuated pilot that controls the pressure on the diaphragm of the main regulating valve. As long as the pressure of the medium on the spring actuated pilot is low, there is no flow downstream. As the pressure increases, the spring is compressed and the pilot valve opens, creating a pressure differential between the inlet side of the main regulating valve and the output valve. This pressure differential actuates the main operating valve and flow occurs at a reduced pressure through the outlet valve.
Fig. 2: Schematic representation of a double stage pressure regulator
Double-stage pilot operated regulators provide accurate regulation for a wide range of pressures and capacities. These regulators can be used with only clean fluids or gases as small passages and ports can get clogged. This arrangement results in a stable and sustained outlet pressure from the second stage despite pressure drops in the first stage.
Functions of pressure regulators
Other than reducing input pressures, there are other functions that a pressure regulator can perform:
Backpressure regulators and pressure relief valves
A pressure relief valve is used to limit system pressure to a prescribed maximum by diverting some or all quantity of fluid or gas coming from the pump to the tank, when the designed / set pressure is reached.
A backpressure regulator maintains a desired input pressure by means of varying the flow of fluid or gas in response to a change in input pressure.
Pressure switching valves
These are used in pneumatic logic systems. These valves are either 2/2-way or 3/2-way switching.
They are used to control vacuum. A vacuum regulator maintains a constant vacuum at regulator inlet with a higher vacuum connected to the outlet.
Examples of pressure regulator applications: aerospace, welding, household gas burner and oxygen regulation for medical purposes.
House-hold / Domestic
Gas grills, Gas ovens, pressure cookers and pressure vessels, home heating furnaces
Industrial, commercial and workshops for cleaning, powering air actuated tools, inflating tires, etc.
Propulsion pressurant control, engines and fuel lines.
Welding and cutting
Oxy-acetylene welding to supply gas at required pressure from storage cylinders.
Gas powered vehicles
To deliver pressurized gas to the engine.
Selection criteria for Pressure regulators
Pressure regulators are available in various sizes and constructions, but below are a list of considerations to choose the correct pressure regulator for the application:
- Operating pressure range
- Capacity or flow required
- Nature of the medium (fluid or gas) transmitted
- Operating temperature range
- Material requirements
- Accuracy required
Operating Pressure range
The input and output pressures required in the application determine the type of regulator to be used:
- The supply range of the input pressure that can be handled safely.
- The required values of output pressure.
- The required accuracy of output pressure.
Capacity or flow requirements
The following criteria should be evaluated:
- The maximum flow rate required.
- The variation expected in flow rate.
- Correct selection of pipe size.
Nature of medium (fluid or gas)
Care should be taken about the type of medium to be used in the regulator:
- Liquid / Gas
- Chemical composition
- Flammability / Explosive nature
- Hazardous / Toxic nature
- Corrosive properties
Operating temperature range
The materials used in pressure regulators should be such that they need to be able to perform their function effectively at a certain operating temperature range, without losing their material properties. The elastomers used for regulator sealing can be selected as below:
- Nitrile (NBR) or Neoprene (-40 0C to 82 0C)
- Ethyleneproplene (EPDM) or Perfluoroelastomer (FKM) for higher temperatures
Depending on the medium and operating conditions, various regulator component materials are available such as:
- Brass – Commonly used and economical
- Plastic – Cost effective / disposable application
- Aluminium – Weight considerations
- Stainless steel – Corrosive environments, high cleanliness requirements and high operating temperatures.
The seal used in the pressure regulator should be compatible with the operating temperature and type of medium used.
The size and weight of the pressure regulator are important considerations. The material used, required port size, adjustment requirements and type of mounting should be taken into account to select the appropriate type.
The accuracy of a pressure regulator is indicated by its “Droop” value. Droop can be defined as the amount of drop / reduction of output pressure compared to the original set pressure, with the increase in fluid flow.
For lower accuracy requirements, a relatively higher amount of droop can be acceptable. They tend to be more cost effective. For higher accuracy, the type of construction, optimized valve size and multi-staged design can reduce the amount of droop.
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