Firstly we should be clear that this article relates to precision instrumentation pressure reducing regulators with a poppet style control valve. Whilst cylinder regulators fall within this description other industrial styles such as globe valves or sliding gate valves do not.
A good starting point is a schematic of the “internals” of a regulator.
The important components that we need to consider for a Pressure Regulator are:
As can be seen in the functional schematic above, the gas enters the regulator via the inlet on the right hand side. With the control knob backed off, a zero pressure set-point, the poppet spring pushes the poppet upwards such that the chamfered shoulder seals against the soft seat. The regulator is shut and the inlet pressure (coloured orange) is held.
When the operator turns the control knob to apply a positive pressure set-point the load spring is compressed. This applies a force to the diaphragm (coloured blue) which is pushed slightly downwards and, in turn, applies a force to the shaft of the poppet. As the poppet moves downwards the orifice is opened and flow/pressure allowed to pass.
The action of the poppet in the orifice controls the amount of pressure being sensed by the diaphragm. When the sensed pressure (coloured yellow) is the same as the force exerted by the load spring the regulator is in steady state. The sensed pressure is therefore stable and “regulated”.
Within the body of the regulator, ports can be connected to either the inlet or the outlet chambers. Pressure gauges can be threaded into these ports so that the operator can see both the supply pressure and the controlled pressure being applied by turning the knob.
A practical issue arises with the arrangement of these ports. If your gas cylinder is to the right of the operator and the experiment is to the left, then the port configuration (as seen below) would be Type “C”. This gives a right-to-left flow path. If the lab configuration is the other way around then using Type C configuration would leave the pressure gauges upside down ! Fortunately a left-to-right configuration is available with Type “M”. Other port configurations are available to allow for valves, transmitters, switches, etc.
A second practical issue arises with the choice of orifice size. Choose too small an orifice and the down-stream process will be flow starved. Choose too large and you risk instability. The Orifice Size (Cv) is essentially the size of the hole within the pressure reducing element through which fluid passes. The larger the size, the more flow the regulator can pass. This orifice size needs to be calculated as over-sizing can result excessive pressure variation with only a slight turn of the handle (sensitivity) or may cause excessive droop.
This needs to be completed with separate formulae for gas and liquid. As a general rule of thumb, select a regulator orifice size with double the calculated Cv. The definitions used are as follows:
Cv Gas - The flow of air at standard conditions in SCFM for each psig of inlet pressure.
Cv Liquid - The flow of water at 16 C in US Gallons per minute at a pressure drop of 1 psig
SL - Specific Gravity of liquids relative to water at 16 C. Sg water = 1.0.
Sg - Specific Gravity of a gas relative to air. This equals the ratio of the molecular weight of the gas to that of air. Sg Air = 1.0 at 16 C.
P - Line pressure (psig).
P1 - Inlet pressure (psig).
P2 - Outlet pressure (psig).
ΔP - Differential pressure (P1 – P2 in psig).
psia - Absolute pressure. Gauge pressure plus 14.7 (atmospheric pressure).
QL - Liquid flow in US gallons per minute (GPM).
Qg - Gas flow in SCFM.
when P1 equals or is greater than 2 x P2
when P1 is less than 2 x P2 or P2 is greater than 50% of inlet pressure
Low Pressure - Pressure Reducing Regulator. Max inlet pressure 3000 psi (210 Bar), Max outlet pressure 500 psi (35 Bar).
High Pressure - Pressure Reducing Regulator. Max inlet pressure 6000 psi (420 Bar), Max outlet pressure 4500 psi (310 Bar).
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