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| Regulator
Selection and Performance |
| Evaluating
Regulators for Reliability and Performance |
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The following criteria
are used:
- Pressure
regulation as a function of flow. All regulators have
some delivery pressure drop with increased flow rate. The
smaller the drop as flow is increased, the better the
performance.
- Pressure
regulation as a function of inlet pressure. As the inlet
pressure source is reduced, regulator delivery pressure may
either rise or fall depending upon the regulator design. In
both cases this is known as regulator "droop.'' Two
stage regulators generally provide better regulation under
these circumstances.
- "Lockup'' of
a regulator. This is the difference in pressure between
a flowing and non-flowing condition. If a regulator has its
delivery pressure set while gas is flowing, and flow is
suddenly stopped, a small rise in delivery pressure (lockup)
will occur before the regulator's valve closes fully. The
lower the lockup, the better the performance.
- Seat leakage of
the regulator. This is the tendency of the regulator to
leak across the seat, with the regulator valve knob closed
fully (counter-clockwise) and high pressure on the inlet
side. A low leakage value is preferred.
- Leakage rate
across the diaphragm or fittings on the regulator. This
value is normally measured using helium gas and a mass
spectrometer or other type of helium detector. Regulators
for specialty gas service may have published values of
typical leakage rates either inboard (from the atmosphere
into the regulator) or outboard (from inside the regulator
to the atmosphere). For safety, it is important that this
leak rate value be as low as possible in order to prevent
possible contamination by ambient air and moisture or escape
of hazardous gases.
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| Choosing
a Regulator for a Particular Gas or System |
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Several important
factors must be considered when choosing a regulator for a
particular application:
- Gas pressure.
Some regulators are designed for lower pressures and should
not be connected to sources of pressure greater than they
are rated for. The inlet pressure gauge should be inspected
for suitable range of inlet source pressure.
- Regulator
materials of construction. Consideration should be given
to the gas being used and the requirements of the system.
Brass regulators are typically used with inert gases since
brass is attacked by corrosive gases. Many regulators
designed for controlling gases used in semiconductor
processes are constructed of low tensile strength stainless
steel (usually type 316 or 316L). This material displays
good corrosion resistance and is not prone to hydrogen
embrittlement problems. However, stainless steel can be
corroded when exposed to some of the corrosive gases with
small amounts of water present. For severe corrosive
conditions, Monel® or Hastelloy® material should be used.
Regulators may also be
plated with corrosion resistant coatings when controlling some
gases. Another important factor to consider is the soft plastic
materials, or elastomers, used in regulators for seat and
sealing surfaces. For example, some regulators use an elastomer
called Viton® which is a fluorocarbon. Viton® is compatible
with many gases but gases such as ammonia will cause it to
dissolve. When choosing a regulator for a particular gas, all
internal regulator parts should be compatible with that gas
under normal operating conditions.
- Regulator
cleaning prior to use. Most regulators used in specialty
gas applications are cleaned to very tight specifications to
match the purity needs of the process. If oil or other
combustible residue is left inside the regulator,
spontaneous combustion may result when certain gases, such
as oxygen, are introduced into the regulator.
- Regulators should
be used in a specific gas service, for maximum safety.
Once a regulator is used for one gas it should not be used
for any other gas unless it has been thoroughly cleaned, or
the operator is absolutely sure that using the regulator in
this manner presents no hazard. For example, an explosion
could result if a regulator was used on an oxidizer and then
placed in flammable gas service.
- Always follow the
manufacturer's instructions and warnings when
installing, operating, purging, or shutting down a pressure
regulator.
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| Reading
Flow Curves |
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Flow Performance:
The flow properties of
a pressure regulator are illustrated by the flow curve. The
vertical axis indicates the delivery pressure at which the
regulator is set and the horizontal axis indicates the gas flow
that the regulator passes. The curves are made by setting the
delivery pressure while there is no gas flow and then slowly
opening the outlet valve downstream while measuring both the
flow and the delivery pressure. Typically, as flow increases,
delivery pressure drops. The portion of the curve to the far
left is fairly flat and it is in this range that the regulator
demonstrates a stable pressure regulation even though the flow
is changing. For example, increasing the flow from point
"A'' to point "B'' shows only a slight decrease in
pressure. The portion of the curve to the right shows a rapid
drop in pressure with increasing flow rate, indicating that the
regulator valve seat is almost wide open. If flow is increased
from point "B'' to point "C'', there is a large drop
in pressure that is typical for all regulators.
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