Utilities - Instrument Air
September 18, 2002
Introduction
The instrument air system
supplies dry and oil free air to the instrument air system of the sulphuric acid
plant. A typical instrument air system consist of the following
components:
- Compressor
- Oil Filter
- Particulate
- Air Receiver
- Air Dryer
- Local Control Panels
The
quality of instrument air is what distinguishes it from a compressed or service
air system.
Air Quality
The
quality of the air is important to ensure that instrumentation will function
properly and reliably. The most important parameters in specifying
air quality are:
- Dew Point
- Oil Content
- Particulate
- Temperature
The Instrument Society of
America sets quality standards for instrument air in ISA S7.3.
Dew Point
The dew point temperature or
saturation temperature can be defined as the temperature at which condensation
or moisture begins when moist air is cooled. This temperature can be
determined by observing, either visually or by a photoelectric cell,
condensation of water vapour on a polished metal surface that is slowly being
chilled. Indirect methods for measuring moisture include wet and dry
bulb psychometry, adiabatic expansion, electrical resistivity and methods that
depend on the hygroscopic properties of various materials.
When an air-water vapour
mixture is compressed:
1. its ability to hold water is decreased;
2. water vapour will start to condense at a higher temperature; and
3. the dew point of the mixture at elevated pressure will be higher than that
of the same mixture at atmospheric pressure.
The dew point required for an instrument air system is generally set by the
minimum ambient temperature to which the instrument air system will be
exposed. The dew point at line pressure shall be at least 10°C (18°F)
below the minimum local recorded ambient temperature.
In cold climates, a -40°C dew point is typically used. In warmer
climates the dew point temperature can be increased appropriately.
Under no circumstances should the dew point at line pressure exceed 2°C
(35°F).
In is important that the dew point temperature be specified at the operating
pressure of the instrument air system and not atmospheric pressure. This
will ensure that no condensation of water will occur anywhere in the system.
Oil Content
Oil free air is generally required for an instrument air system. In an
oil free system, an oil content of less than 0.01 ppm is generally specified.
Where a instrument air consumer requires air with oil for lubrication
purposes, individual oilers are installed specifically for the consumer.
Particulate
Particulate in the air may plug the small passages in some valves and
instruments. Instrument air is generally filtered to remove particulate
matter to a level less than 0.02 mg/m³ (size 100% < 0.01 micron).
Temperature
When any gas is compressed its temperature will increase. For and
instrument air system this increase in temperature is undesirable. After
coolers are generally provided immediately after the compressor to cool the
air and remove the heat of compression. A typical outlet temperature is
a maximum of 40°C.
Control Philosophy
In some systems, two
compressors are provided to ensure greater reliability and availability.
One compressor unit is designated the operating unit while the other is placed
on automatic standby. If the operating unit should fail, the standby unit
comes on line to ensure uninterrupted supply of instrument air.
A typical means of controlling
when a compressor load/unloads is as follows::
Duty
Standby
Compressor Compressor
High/High Pressure
Unloads
-
High Pressure
-
Unloads
Low Pressure
Loads
-
Low/Low Pressure
-
Loads
Compressor
Air compressors generally come as an integrated unit consisting of the following
components:
a)
Compressor
b)
Inlet air filter
c)
Drive motor
d)
Lubrication system
e)
Oil separator
f)
After coolers
g)
Control system and panel
All components are housed in an enclosure to minimize noise.
Compressors must be sized to deliver the maximum
quantity of air at the specified pressure. Various types of compressors
are available, including:
- Reciprocating oiled piston
- Reciprocating oiless piston
- Rotary vane
- Rotary liquid ring
- Diaphragm
- Rotary Screw
- Centrifugal
Compressors of the
non-lubricated type are commonly used to prevent problems with oil or lubricant
contamination of the air. Where lubricated compressors are used, provision
must be made to adequately remove these contaminants from the air.
Inlet Air Filters
Generally, the instrument air system intake should be located outside and in the
coolest area. The lower the air intake temperature, the greater the
compressor efficiency. If contamination exists in the compressor intake
area, the air should be taken from an elevated or remote location free from
contamination.
Inlet air filters are essential to remove grit
and dust that are present in practically all plants. Removal of these
substances is important in order to achieve the desired air quality and to
protect the compressor, since any solids in the air will cause wear on the
moving parts. These filters must be regularly inspected and replaced.
After Coolers
The compression of air results in a rise in the
temperature of the air. After the compressor the air is passed
through an after cooler to reduce its temperature. Upon cooling the air
temperature may be below its dew point, so a mechanical moisture separator is
provided.
After coolers are generally water or air cooled.
Water-cooled after coolers are generally sized to achieve a 5.5°C to 8.5°C (10°F
to 15°F) approach to the cooling water supply temperature. Air-cooled
after coolers are generally sized to achieve a 14°C to 17°C (25°F to 30°F)
approach to the ambient air temperature.
The air receiver is sized to store enough air to handle system demand surges, to
allow time for moisture separation and to provide a reserve for orderly or
emergency shutdown. The receiver pressure is usually used for loading and
unloading the compressor. A receiver that is too small will result in a
compressor that is constantly loaded which will shorten the life of the
equipment.
The receiver ambient temperature is typically
lower that the dew point of the air entering the receiver. This causes
moisture to condense inside the receiver. An automatic liquid drain
device must always be furnished on an air receiver to dispose of any moisture.
Oil Filters
Oil and lubricants are major contaminants in instrument air systems.
They enter the air stream in liquid, aerosol (mist) and vapour form. Oil
and lubricants are most troublesome when they combine with moisture and solid
contaminants to form a sludge that can clog instruments. Oil can also
contaminate desiccants in air dryers.
Oil filters are generally installed after the
air receiver and before the air dryers. These filters will generally be
installed in a duplex fashion allowing one filter to be maintained while the
other is in operation.
Air Dryers
Water in an instrument air system can cause rust in distribution pipes or freeze
in exposed outdoor lines. It is important to dry the air prior to
distribution to prevent problems in the system.
Air drying
is generally done in one of two ways:
a) Heatless
Regenerative Desiccant Dryers
b) Heat Regenerative
Desiccant Dryers
c) Refrigerated Dryers
Heatless Regenerative Desiccant Dryers
Heatless regenerative dryers utilize the principle that expansion of a
compressed gas from a high pressure to a low pressure reduces the partial
pressure of all constituents in a proportion equal to the ratio of absolute
pressure change. Thus, a water saturated adsorbent will give up its
water vapour to a lower vapour pressure environment. In practice this is
achieved by taking a portion of the high pressure dry air and expanding it to
essentially atmospheric pressure and passing it through the desiccant bed to
strip off the water content.
The disadvantage of heatless type regenerative desiccant dryers is the loss of
a portion of dry air used to regenerated the desiccant bed.
The heatless dryer works by passing compressed air through a tower filled with
desiccant. Moisture from the air stream is adsorbed onto the desiccant
thereby producing dry outlet air. While one tower is on line performing
the drying function, the second tower is off line being regenerated by a
portion (approximately 10 to 20%) of the dry discharge air.
This operation is controlled by valves (V1 and V2) which divert the flow of
air to the on-line tower and away from the regenerating tower. At the
same time, a purge valve (V3 or V4) allows the regenerating air flow entering
the off-line tower to escape and thereby purge the moisture adsorbed from the
desiccant to the atmosphere.
Once the regeneration process is complete the off-line tower is re-pressurized
to prevent fluidization of the desiccant bed. The purge valve (V3 or V4)
will close so that the off-line tower is allowed to re-pressurize.
After re-pressurization is complete, the flow valves (V1 and V2) will divert
the drying air flow to the previously off-line tower. Simultaneously, a
purge valve (V3 or V4) will open allowing the previously on-line tower to
de-pressurize and begin regeneration. The sequence is repeated
continuously ensuring a constant supply of dry air to the system.
Heat Regenerative Dryers
Heat regenerative dryers utilize the principle that a hot gas can hold a
larger amount of water vapour than a gas at a lower temperature. Thus, a
water saturated adsorbent will give up its water to a gas at a higher
temperature. Hot air, heated by an electric or steam heater passes
through the desiccant bed removing the water from the adsorbent and
regenerating the bed.
Refrigeration type dryers
utilize the principle that the lower the dry bulb temperature of the air, the
lower the water vapour content. Typical, refrigeration type dryers are
capable of achieving a dew point temperature of -1.7°C.
After Filters
The after filter provides final cleaning of the
compressed air stream by removing particulate matter from the dryer discharge.
After filters are required on all instrument air systems, particularly
desiccant-type dryers to prevent desiccant dust from passing downstream.
These filters will generally be installed in a duplex fashion allowing one
filter to be maintained while the other is in operation.
