An instrument air system is a critical part of any Chemical Plant. It provides compressed air to a variety of instruments and equipment, including control valves, safety systems, and process monitors. The instrument air must be clean and dry to ensure the accuracy and reliability of these instruments.
An instrument air system typically consists of the following components:
Instrument Air Compressor
- Instrument Air Compressor: This is the heart of the system. It compresses ambient air to the desired pressure. The air compressor is typically driven by an electric motor or a gas turbine. There are many types of compressors that can be used in instrument air service. The most common types are:
- Reciprocating compressors: Reciprocating compressors use pistons to compress the air. They are the most common type of compressor used in instrument air service because they are relatively inexpensive and easy to maintain.
- Screw compressors: Screw compressors use two helical screws to compress the air. They are more efficient than reciprocating compressors and can produce higher pressures.
- Centrifugal compressors: Centrifugal compressors use centrifugal force to compress the air. They are the most efficient type of compressor, but they are also the most expensive.
Instrument Air dryer
- Instrument Air dryer: This removes moisture from the compressed air. If the dew point of the instrument air is too high, it can be reduced by using a dryer. There are a variety of dryers available & they are
- Refrigerated dryers – Refrigerated dryers work by passing the compressed air through a series of coils that are cooled by a refrigerant. The cold coils cause the water vapor in the air to condense and collect in a drain pan. The dry air is then delivered to the instruments. Refrigerated dryers are available in a variety of sizes and capacities to meet the needs of different applications. They are typically used in applications where the dew point of the air must be below -40°F (-40°C).
- Desiccant dryers – Dessicant dryers are a type of dryer that uses a desiccant material to remove moisture from air. Desiccant materials are hygroscopic, meaning they have a strong affinity for water. When water vapor comes into contact with a desiccant material, it is absorbed by the material. There are many different types of desiccant materials that can be used in dessicant dryers. Dessicant dryers can be either single-tower or dual-tower dryers. Single-tower dryers have a single bed of desiccant material. As the air passes through the bed, the moisture is absorbed by the desiccant. Once the desiccant becomes saturated, it must be regenerated. Regeneration is the process of removing the moisture from the desiccant. This can be done by heating the desiccant or by passing a stream of dry air through the bed. Dual-tower dryers have two beds of desiccant material. One bed is in use while the other bed is being regenerated. This allows the dryer to provide a continuous supply of dry air. Some of the most common types include:
- Silica gel: Silica gel is a type of porous material that is made from silicon dioxide. It is a very effective desiccant and is relatively inexpensive. Silica gel can be regenerated at temperatures between 120 and 200 degrees Celsius.
- Activated alumina: Activated alumina is a type of porous material that is made from aluminum oxide. It is also a very effective desiccant and can withstand higher temperatures than silica gel. Regeneration temperature of alumina ranges from 180 to 350 degrees Celsius.
- Molecular sieves: Molecular sieves are a type of synthetic desiccant material that is made from aluminosilicates. They are very effective at removing water vapor from air and can produce very low dew points. 3A molecular sieves have a regeneration temperature of 175-260°C, while 4A molecular sieves have a regeneration temperature of 200-315°C. It is important to note that the regeneration temperature of molecular sieves can vary depending on the moisture content of the air. For example, if the air is very humid, the regeneration temperature may need to be increased.
- Heatless dryers – Heatless dryers are a type of compressed air dryer that uses a desiccant to remove moisture from the air. The desiccant is a material that has a strong affinity for water, and it absorbs the moisture from the air as it passes through the dryer. Heatless dryers are a good choice for instrument air drying because they do not require any heat. This makes them a good option for applications where heat is not available, such as in outdoor locations or in cold climates
Instrument Air Filters
- This removes contaminants from the compressed air. There are two types of air filters used in instrument air systems: pre-filters and post-filters.
- Prefilters – Ceramic candle filters are a type of air filter that is used as prefilter in instrument air systems. They are made of a porous ceramic material that traps contaminants as the air passes through it. Ceramic candle filters are effective at removing a wide range of contaminants, including dust, dirt, oil, and water. They are also resistant to corrosion and can withstand high temperatures.
- Post-filters are the final line of defense against contaminants. They used to prevent carry over of desiccants to instrument air.
Receivers in Instrument Air System
- Wet air receivers: Wet air receivers are located before the air dryer. The hot, wet air from the compressor enters the receiver and cools down. As it cools, the water vapor in the air condenses and falls to the bottom of the receiver. The dry air then flows out of the receiver and into the air dryer. It smooths out the flow of air from the compressor, which helps to prevent pressure fluctuations. It provides a buffer for the air dryer, which helps to extend the life of the dryer. It allows the water vapor in the air to condense and be removed.
- Dry air receivers/Buffer receiver: Dry air receivers are located after the air dryer. The dry, compressed air from the air dryer enters the receiver and is stored until it is needed. The air receiver stores the compressed air and provides a buffer between the compressor and the instruments.
Here are some of the common methods for sizing buffer receivers:
- Rule-of-thumb method: This method uses a simple formula to determine the required size of the buffer receiver. The formula is:
Buffer receiver size (gallons) = Compressor output (SCFM) x Compressor duty cycle (hours/day) x 24 hours/day
- Capacity method: This method uses the capacity of the buffer receiver to determine the required size. The capacity of the buffer receiver is determined by the following formula:
Buffer receiver capacity (gallons) = Compressor output (SCFM) x Maximum pressure (PSI) x Minimum pressure (PSI) / (Maximum pressure – Minimum pressure)
- Simulation method: This method uses a computer simulation to determine the required size of the buffer receiver. The simulation takes into account the specific requirements of the instrument air system, such as the compressor output, air dryer capacity, and flow rate.
Instrument Air Distribution system: This delivers the compressed air to the instruments.
The instrument air system must be properly designed and maintained to ensure the accuracy and reliability of the instruments. The air quality must be monitored regularly to ensure that it meets the requirements of the instruments. The system must also be inspected and tested regularly to identify and correct any problems.
A properly designed and maintained instrument air system is essential for the safe and efficient operation of any plant.
Quality Parameters of Instrument Air
The quality parameters that need to be monitored for instrument air systems include:
- Pressure: The pressure of the air must be within the specified range to ensure the accuracy and reliability of the instruments. The instrument air system pressure in any chemical plant typically ranges from 7 to 12 bar (101 to 174 psi). The exact pressure required will vary depending on the type of instruments and equipment that are using the air. For example, control valves typically require a higher pressure than safety systems.
- Humidity/Dew point: The humidity of the air must be low to prevent condensation and corrosion. The recommended dew point of instrument air is typically -40°F (-40°C). This ensures that the air is dry enough to prevent condensation and corrosion, which can damage the instruments.
- Oil content: The oil content of the air must be low to prevent contamination of the instruments.
- Particle content: The particle content of the air must be low to prevent clogging of the instruments.
Here are some of the benefits of having a good instrument air system in a Chemical Plant:
- Improved safety: A good instrument air system can help to prevent accidents by providing clean and dry air to critical safety systems.
- Increased efficiency: A good instrument air system can help to improve the efficiency of chemical plant operations by providing accurate and reliable data to operators.
- Reduced costs: A good instrument air system can help to reduce costs by reducing the need for maintenance and repairs to instruments.
Application of Instrument Air
- Pneumatic control: Instrument air is used to operate pneumatic control valves and dampers, which are used to regulate the flow of fluids in chemical plants.
- Valve actuation: Instrument air is also used to actuate valves, which are used to control the flow of fluids in chemical plants.
- Instrument calibration: Instrument air is used to calibrate instruments, such as control valves, flow meters and pressure gauges and switches.
- Purge: Instrument air is used to purge lines and equipment, which helps to prevent the buildup of chemicals
- Sampling: Instrument air is used to sample fluids, which helps to ensure the quality of the products being produced.
- Fire protection: Instrument air can be used to operate fire suppression systems, which helps to protect chemical plants from fires.
- Cleaning: Instrument air can be used to clean equipment and surfaces, which helps to prevent the spread of contamination.
- Drying: Instrument air can be used to dry equipment and surfaces, which helps to prevent the growth of mold and bacteria.
- Sealing : Instrument air can be used as seal air in turbines to prevent the steam contaminating the lube oil in bearing housing. It can also be used for sealing the damper seating area to prevent leakage of process fluid and to keep the area cool.
- Cooling: Instrument air can be used to cool equipment and surfaces, which helps to prevent the overheating of equipment. For Eg Flame Scanners, View glass in Boilers etc
Overall, a good instrument air system is a valuable investment for any Chemical Plant. It can help to improve safety, efficiency, and reduce costs.