This month's instalment discusses the laboratory deployment of gas cylinders with appropriate pressure regulators, gas filtration,
and gas fittings, as well as the question of cylinder retirement.
A previous "GC Connections" instalment (1) discussed the intake and disposition of compressed gas cylinders, including verifying
gas cylinders and their content upon receipt, safely moving and securing them in place, and organizing them by their current
use. After cylinders are in place in the laboratory, the next steps are to attach appropriate gas regulators, tubing, filters,
and fittings to bring the high-purity gas supplies to one or more gas chromatographs.
Pressure regulators are unsung heroes in the laboratory. They accomplish the expansion of high-pressure cylinder gases from
dangerous levels to what can reasonably be handled by laboratory instruments while maintaining nearly constant output pressures
and — if correctly matched to the application — not contaminating the gas stream. Of course, pressure regulator selection,
installation, and maintenance are essential for obtaining the best performance. But before discussing the whys and whats of
regulators, here's a brief description of how they work.
In Control: Laboratory pressure regulators are designed to expand gases from cylinder pressures as high as 3000 psig (20.7 MPa) down
to operating pressures as low as 10–20 psig (150–275 kPa). Gas-tank regulators are mechanical devices with springs, screws,
and a counterbalancing diaphragm. Electronic pressure control (EPC) has not yet found its way into the tank realm in any significant
way, although some laboratory hydrogen generators incorporate EPC. Electronic pressure control of in-instrument gases will
be covered in a future "GC Connections" instalment.
Figure 1 shows a simplified diagram of a typical single-stage mechanical gas-pressure regulator such as what is found at the
gas tank or sometimes in-line as part of a manifolded gas distribution setup. The regulator consists of a two-piece sealed
housing with the internal volumes separated by a flexible diaphragm. The outlet gas pressure exerts force on one side of the
diaphragm and a spring pushes in the opposite direction. The amount of force exerted by the tensioning spring on the diaphragm
determines the output pressure level. A pressure adjustment knob turns a screw that increases or decreases the force on the
spring and diaphragm.
Figure 1: Mechanical gas-pressure regulator: (a) The adjustment knob is fully withdrawn and no pressure is applied to the
tensioning spring — the poppet valve is closed and no gas flows. (b) The adjustment knob is turned clockwise and advanced
into the regulator. There is tension on the spring so that the poppet valve is contacted and gas flows to the outlet. 1 =
inlet connection, 2 = outlet connection, 3 = poppet valve, 4 = flexible diaphragm, 5 = tensioning spring, 6 = adjustment knob.
The inlet gas chamber is coloured red; the output chamber with pressure-controlled gas is shown in green.
In Figure 1(a), the adjustment knob is fully turned out and the spring exerts no force on the diaphragm: The pressure on the
outside of the diaphragm is equal to ambient pressure. Gas at the tank pressure has entered the regulator at the inlet connection,
and the poppet valve below the diaphragm is fully seated so that no gas flows through it. The upper shaft of the poppet is
not in contact with the actuating seat that is attached to the diaphragm.
In Figure 1(b), the adjustment knob has been turned clockwise and its shaft has moved downwards to exert force on the spring
and diaphragm. This movement has caused the actuating seat under the diaphragm to contact the upper shaft of the poppet valve
and move it downwards, which in turn unseats the poppet and allows gas to flow into the outlet chamber. As pressure builds
up in the outlet chamber, the diaphragm is forced upwards against the tensioning spring. The regulator settles into a constant
outlet pressure when the spring force and the pneumatic force are balanced as gas flows out of the regulator.
As the output gas flow increases, the poppet valve opens up to sustain the output pressure balance against the tensioning
spring. Conversely, as the output gas flow decreases the poppet valve moves upwards and restricts gas flow, again so that
the output pressure is maintained. If the outlet flow goes to zero, the poppet will seal off the inlet.