Pressure regulator


A pressure regulator is a valve that controls the pressure of a fluid to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.
Two types are found: the pressure reduction regulator and the back-pressure regulator.
  • A pressure reducing regulator is a control valve that reduces the input pressure of a fluid to a desired value at its output. It is a normally-open valve and is installed upstream of pressure-sensitive equipment.
  • A back-pressure regulator, back-pressure valve, pressure-sustaining valve or pressure-sustaining regulator is a control valve that maintains the set pressure at its inlet side by opening to allow flow when the inlet pressure exceeds the set value. It differs from an over-pressure relief valve in that the over-pressure valve is only intended to open when the contained pressure is excessive and is not required to keep upstream pressure constant. They differ from pressure-reducing regulators in that the pressure-reducing regulator controls downstream pressure and is insensitive to upstream pressure. It is a normally-closed valve which may be installed in parallel with sensitive equipment or after the sensitive equipment to provide an obstruction to flow and thereby maintain upstream pressure.
Both types of regulator use feedback of the regulated pressure as input to the control mechanism and are commonly actuated by a spring-loaded diaphragm or piston reacting to changes in the feedback pressure to control the valve opening. In both cases, the valve should be opened only enough to maintain the set regulated pressure. The actual mechanism may be very similar in all respects except the placing of the feedback pressure tap. As in other feedback control mechanisms, the level of damping is important to achieve a balance between fast response to a change in the measured pressure and stability of output. Insufficient damping may lead to hunting oscillation of the controlled pressure, while excessive friction of moving parts may cause hysteresis.

Pressure reducing regulator

Operation

A pressure-reducing regulator's primary function is to match the flow of gas through the regulator to the demand for fluid placed upon it, whilst maintaining a sufficiently constant output pressure. If the load flow decreases, then the regulator flow must decrease as well. If the load flow increases, then the regulator flow must increase in order to keep the controlled pressure from decreasing because of a shortage of fluid in the pressure system. It is desirable that the controlled pressure does not vary greatly from the set point for a wide range of flow rates, but it is also desirable that flow through the regulator is stable and the regulated pressure is not subject to excessive oscillation.
A pressure regulator includes a restricting element, a loading element, and a measuring element:
  • The restricting element is a valve that can provide a variable restriction to the flow, such as a globe valve, butterfly valve, poppet valve, etc.
  • The loading element is a part that can apply the needed force to the restricting element. This loading can be provided by a weight, a spring, a piston actuator, or the diaphragm actuator in combination with a spring.
  • The measuring element functions to determine when the inlet flow is equal to the outlet flow. The diaphragm itself is often used as a measuring element; it can serve as a combined element.
In the pictured single-stage regulator, a force balance is used on the diaphragm to control a poppet valve in order to regulate pressure. With no inlet pressure, the spring above the diaphragm pushes it down on the poppet valve, holding it open. Once inlet pressure is introduced, the open poppet allows flow to the diaphragm and pressure in the upper chamber increases, until the diaphragm is pushed upward against the spring, causing the poppet to reduce flow, finally stopping further increase of pressure. By adjusting the top screw, the downward pressure on the diaphragm can be increased, requiring more pressure in the upper chamber to maintain equilibrium. In this way, the outlet pressure of the regulator is controlled.
This operation is described by the following equation:
where

Single-stage regulator

High-pressure gas from the supply enters the regulator through the inlet port. The inlet pressure gauge indicates this pressure. The gas then passes through the normally open pressure-control valve orifice, and the downstream pressure rises until the valve-actuating diaphragm is deflected sufficiently to close the valve, preventing any more gas from entering the low-pressure side until the pressure drops again. The outlet pressure gauge indicates this pressure.
The outlet pressure on the diaphragm and the inlet pressure and poppet spring force on the upstream part of the valve hold the diaphragm/poppet assembly in the closed position against the force of the diaphragm-loading spring. If the supply pressure falls, the closing force due to supply pressure is reduced, and downstream pressure rises slightly to compensate. Thus, if the supply pressure falls, the outlet pressure increases, provided the outlet pressure remains below the falling supply pressure. This is the cause of end-of-tank dump where the supply is provided by a pressurized gas tank. The operator can compensate for this effect by adjusting the spring load by turning the knob to restore outlet pressure to the desired level. With a single-stage regulator, when the supply pressure gets low, the lower inlet pressure causes the outlet pressure to climb. If the diaphragm-loading spring compression is not adjusted to compensate, the poppet can remain open and allow the tank to rapidly dump its remaining contents.

Double-stage regulator

Two-stage regulators are two regulators in series in the same housing that operate to reduce the pressure progressively in two steps instead of one. The first stage, which is preset, reduces the pressure of the supply gas to an intermediate stage; gas at that pressure passes into the second stage. The gas emerges from the second stage at a pressure set by user by adjusting the pressure-control knob at the diaphragm-loading spring. Two-stage regulators may have two safety valves, so that if there is any excess pressure between stages due to a leak at the first stage valve seat, the rising pressure will not overload the structure and cause an explosion.
An unbalanced single-stage regulator may need frequent adjustment. As the supply pressure falls, the outlet pressure may change, necessitating adjustment. In the two-stage regulator, there is improved compensation for any drop in the supply pressure.

Applications

Pressure-reducing regulators

Air compressors

s are used in industrial, commercial, and home workshop environments to perform an assortment of jobs, including blowing things clean; running air-powered tools, and inflating things like tires, balls, etc. Regulators are often used to adjust the pressure coming out of an air receiver to match what is needed for the task. Often, when one large compressor is used to supply compressed air for multiple uses, additional regulators are used to ensure that each separate tool or function receives the pressure it needs. This is important because some air tools, or uses for compressed air, require pressures that may cause damage to other tools or materials.

Aircraft

Pressure regulators are found in aircraft cabin pressurization, canopy seal pressure control, potable water systems, and waveguide pressurization.

Aerospace

Aerospace pressure regulators have applications in propulsion pressurant control for reaction control systems and attitude control systems, where high vibration, large temperature extremes and corrosive fluids are present.

Cooking

Pressurized vessels can be used to cook food much more rapidly than at atmospheric pressure, as the higher pressure raises the boiling point of the contents. All modern pressure cookers have a pressure regulator valve and a pressure relief valve as a safety mechanism to prevent explosion in the event that the pressure regulator valve fails to adequately release pressure. Some older models lack a safety release valve. Most home cooking models are built to maintain a low and high pressure setting. These settings are usually. Almost all home cooking units employ a very simple single-stage pressure regulator. Older models simply use a small weight on top of an opening that is lifted by excessive pressure to allow excess steam to escape. Newer models usually incorporate a spring-loaded valve that lifts and allows pressure to escape as pressure in the vessel rises. Some pressure cookers have a quick-release setting on the pressure regulator valve that essentially lowers the spring tension to allow the pressure to escape at a quick but still safe rate. Commercial kitchens also use pressure cookers, in some cases using oil-based pressure cookers to quickly deep-fry fast food. Pressure vessels of this sort can also be used as autoclaves to sterilize small batches of equipment and in home canning operations.

Water pressure reduction

A water-pressure regulating valve limits inflow by dynamically changing the valve opening so that when less pressure is on the outside, the valve opens up fully, and too much pressure on the outside causes the valve to shut. In a no-pressure situation, where water could flow backwards, it is not impeded. A water-pressure regulating valve does not function as a check valve.
They are used to avoid damage to appliances or pipes in applications where the water pressure at the end of the line is too high.