Several questions regarding instrument air and plant air

The main difference between instrument air and process air is that instrument air needs to be dried, whereas process air does not.

I only know about gas supply for instruments, so I will only answer regarding the instrument part； 1. The air source used for instrument gas supply must be purified. Through the purification unit, at the filter outlet, the instrumentation air is required to have dust particle sizes not exceeding 3 μm. The dust content should be less than 1 mg/m3. The oil content in the instrument air supplied by the air supply unit should be less than 10 mg/m3 (8 ppm(W)). 2. The material of the main gas supply pipe is available in stainless steel and brass ； For the gas supply piping behind the panel, copper tubes or nylon pipes are recommended ； For the main pipes and trunk piping of the gas supply system, galvanized steel pipes are recommended. For the piping on the downstream side of the air filter pressure reducing valve, stainless steel pipes or copper pipes are recommended. (Possible consideration is atmospheric corrosion resistance) 3. Personally, I recommend using vortex flow meters

Answer: 1. Instrument air is part of the plant air. Instrument air is high-pressure air compressed by air compressors, with an outlet pressure typically ranging from 0.7 MPa to 0.8 MPa; after oil and water removal, it serves as the air source for the instruments in a factory’s control system. Plant air includes instrument air, low-pressure air, and specialty gases. Low-pressure air refers to air that has been filtered and compressed for use in industrial processes; its outlet pressure is determined based on the requirements of the factory, typically ranging from 0.2 to 0.3 MPa. Special gases are composed of other gases that have been compressed or not, including nitrogen, natural gas, etc. 2. Since instrument air is supplied for use in instruments, and to prevent instrument failures, it must undergo dehydration, degreasing, coarse filtering, and fine filtering before reaching the instruments. As a result, the air purity standard is quite high; galvanized steel pipes are used to prevent the air from becoming contaminated. The factory air has no special requirements; only coarse filtration is needed. 3. Vortex flowmeters are generally used for measuring instrument air, while for factory air, both vortex flowmeters and orifice plates are employed; the choice depends mainly on the pipe diameter, as using vortex flowmeters for large-diameter pipes is not cost-effective.

The required pressure of the instrument air is generally 0.4–0.7 MPa (gauge pressure), and the static air consumption of pneumatic valves for instrument air is 20 L/h. 　　The main air consumption of a pneumatic control valve is not in the valve itself, but in the positioner. The main air consumption of the cut-off valve is for the venting of the solenoid valve. Generally speaking, the air consumption of a control valve (including the positioner) is approximately 1 m3/h. The frequency of opening and closing the valve is closely related to this; under normal conditions, it can also be calculated at 1 m3/h. If you are performing calculations for utility consumption, it is recommended to use 2 m3/h as the gas consumption per valve. 　　Different pressures relate to the size of the instrument air buffer tank. The pressure required to actually operate the valve stem of a control valve, that is, to activate the positioner, is much lower than the pressure in the main instrument air supply (usually not exceeding 0.35 MPag). Moreover, the pressure required for control valves under different operating conditions varies. To ensure that the necessary pressure is available, control valves are equipped with their own instrument air pressure regulators, which adjust the instrument air to the desired pressure level. Additionally, the instrument air pressure is set at around 0.7 MPag, for two reasons: one is to ensure that there is sufficient pressure for the operation of any valves, and the other is to guarantee a supply of air in emergency situations should the factory’s instrument air compressor fail.

1. What is the difference between instrument air and plant air? This includes its uses and composition. Instrument air needs to be cleaner than factory air; it should be dust-free and dry. 2. Why is galvanized carbon steel pipe used for instrument air, while carbon steel pipe is sufficient for general process air? When selecting the material for the instrument air pipelines, avoid using materials that are prone to rusting. 3. What type of flow meter is generally suitable for measuring the flow rate of air in measuring instruments and in the factory? Orifice plate or vortex street? Flow meters using uniform-flow tubes are preferable, as they have low pressure loss and are easy to maintain

Instrument air is dried and filtered.

Instrument air should be clean and dry; in flammable and explosive environments, it is better to use nitrogen, which does not burn nor support combustion, as it offers greater safety. Factory air mainly needs to be clean. The choice of pipes depends on the characteristics of the instruments involved. If the air is not clean, our instruments can become contaminated, posing safety risks to control systems. Galvanized pipes and stainless steel pipes help to avoid this problem to a large extent