Instrument air

The main thing to mention is the amount of gas used; the specifications for the gas required by instruments are generally specified in the relevant automatic control design regulations. I estimate that the gas consumption of control valves is primarily determined by using these valves as a basis for calculation; assuming that each control valve consumes 1–2 Nm3/h of gas, multiplying this value by the number of valves gives the total gas consumption (here only control valves are taken into account; usually, the gas consumption of other devices that use gas also needs to be included to get the total amount). Multiplying this total by 2.3 then yields the designed capacity of the gas supply system for the instruments.

I remember the formula isn’t the air consumption of the control valve multiplied by 2.3; it’s Qs = (3~4)Vm, and also Qs = Qc

Pressure: 0.5–0.6 MPa Temperature: Room temperature Quality: Oil-free, dust-free, with dust particles of <3 μm in size Dew point temperature: ~–40℃

What was said on the 3rd floor is correct; I also use this formula for calculations. Qs = Qc, where Qc represents the total gas consumption of all types of instruments – that is, the sum of the gas consumption of valves and other devices that consume gas. As I mentioned earlier, I consider only the gas consumption of valves for simplicity in explanation. As for that 2.3, I just lazily used the maximum value from the formula. The final value of Qs is the capacity value of the instrument air supply unit. Thanks for pointing it out. :loveliness:

The American standard specifies that the air consumption per control valve is 1 NL/min, and the total air consumption is equal to the number of control valves multiplied by this value. This gas volume refers to the consumption under standard conditions; if the volume under actual operating conditions is required, a conversion must be performed, with Q_standard = 1.557 × Q_operational.

(1) Determine the size of the gas supply station based on the amount of gas required by the engineering equipment. (2) What are the special requirements for the air source, including pressure, temperature, humidity, level of dryness, etc.? (3) What are the requirements for the equipment at the gas supply station? (4) What are the requirements for the gas supply pipeline?

The formula is correct, but does one need to consider a factor of 2.3 when specifying the gas usage conditions? It seems it should be up to the engineers who design the gas supply system to consider this.

In fact, you have confused two different calculation methods. If the calculation is done on the basis of 1–2 Nm3/h per pneumatic valve, then there is no need to multiply by 2 + 0.1–0.3, unless the steady-state air consumption rate provided by the manufacturer for that air user is used in the calculation.

Hehe, I really haven’t studied it carefully; I just learned it.

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 reducers, 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 plant’s instrument air compressor fail.