The basics of the direct acting gas pressure regulator

Purpose: reduce the inlet pressure before the regulator to a lower pressure after the regulator. The pressure needs to be constant with certain limits, independent of changes in the inlet pressure and consumed volume.

The simplest gas pressure regulator is the direct acting. There are 2 design principles; weight-loaded and spring loaded. The spring loaded has been the favorite for many years.
A spring-loaded regulator consists of the spring/diaphragm system in the actuator that provides the regulating action.

 

Stable situation
The outlet pressure comes under the measuring diaphragm through the line from the measuring point.
The pressure under the diaphragm causes a force that pushes the diaphragm up. As a result the stem and valve plate that are connected to the diaphragm move up.
The setpoint spring presses its force on top of the measuring diaphragm and tries to push it down.
Both forces are in balance if there is a stable condition in the outlet pipe: the set pressure is reached. There will be no more movement of the measuring diaphragm.

What causes changes in the stable condition and requires a change of the valve plate.

Change in gas consumption:
When gas is consumed there will be more gas exiting the outlet pipe that there can be added through the opening between valve plate and seatthat exists at that time. This causes the pressure to drop at the outlet and as a result the pressure under the diaphragm also decreases. The force of the spring is now bigger than the one below the diaphragm and a downward motion starts. The opening between the valve plate and seat increases and more gas flows through the bigger opening. This compensates the gas volume at the outlet and the pressure rises. As the pressure rises the situation with which we started exists again: balance, de set pressure is reached again.

The inlet pressure changes:
The volume of gas going through the opening between plate and seat is depending on the pressure difference before and after the opening.
If the pressure before the opening changes, the flow through the opening changes.
If the inlet pressure goes up more gas comes at the outlet side and the pressure rises.
The diaphragm gets pushed up and the valve plate goes up, the opening between the plate and seat gets smaller: the volume going through the opening gets less.
If the inlet pressure goes down the reverse will happen and the opening will get bigger.

With the above described situations we have ignored the fact that the valve plate has a surface and also applies a force and as such has an influence on the movement of the plate.

The inlet pressure triest push the plate to the closed position, the lower outlet pressure works against the inlet pressure. Dependant on the difference between inlet and outlet pressue is the force that triest o close the valve plate bigger or smaller and works against the force of the set spring. If the inlet pressure would be constant the forces would be balanced, but with changing inlet pressure the outlet pressure will fluctuate.

To compensate these forces a compensation diaphragm is used.
For this to be effective a change in the flow direction needs to be made and the body needs to be changed. Instead of flow under it is changed to flow over and the result is that the inlet pressure will be on top of the valve plate.

The active surface of the compensation diaphragm and valve plate are identical, as the inlet pressure is higher than the outlet pressure there will be a force on the stem that will try to pull it apart but is has no influence on the movement of the plate.
A change in inlet pressure now has no influence on the outlet pressure.

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