How to calculate the amount of compressed air required by the instrument system

The calculation of the amount of compressed air required by the instrument system requires comprehensive consideration of factors such as instrument type, quantity, working mode, system loss and standby requirements. The following are detailed calculation steps and examples:

1. calculation steps

1. Determine the instrument type and air consumption

the air consumption of different instruments is significantly different, which needs classification statistics:

• pneumatic control valve:
  • single-seat valve: 0.3~0.5 Nm & sup3;/h (standard cubic meters per hour)
  • double-seat valve: 0.5~0.8 Nm & sup3;/h
  • angle valve: 0.4~0.6 Nm & sup3;/h
• pneumatic actuator:
  • film type: 0.2~0.4 Nm & sup3;/h
  • piston type: 0.5~1.0 Nm & sup3;/h
• pneumatic controller:
  • locator: 0.1~0.2 Nm & sup3;/h
  • filter/relief valve: 0.05~0.1 Nm & sup3;/h
• other Pneumatic Equipment:
  • air motor: according to power calculation (for example, 1kW needs about 0.3 Nm & sup3;/min)
  • cylinder: calculated according to stroke and cylinder diameter (for example, for a cylinder with a cylinder diameter of 50mm and a stroke of 100mm, the air consumption for a single action is about 0.25L, and the frequency needs to be converted to Nm & sup3;/h)

example:
if the system includes 10 pneumatic control valves (single-seat valves, 0.4 Nm & sup3;/h/set), 5 diaphragm actuators (0.3 Nm & sup3;/h/set) and 20 positioners (0.15 Nm & sup3;/h/set), the basic air consumption is:
10×0.4+5×0.3+20×0.15=4+1.5+3=8.5Nm³/h

2. Statistical instrument quantity and working time

• continuous operation instrument: such as regulating valve, positioner, calculated by 100% of the time.
• Intermittent operation instrument: For pneumatic on-off valve, it is necessary to estimate the action frequency (for example, it takes 5 actions per hour and consumes 0.1 Nm & sup3 each time;, then the gas consumption per hour is 5 × 0.1=0.5Nm & sup3;/h).
• Standby instrument: It is usually reserved by 10% ~ 20% of the actual operation quantity.

Example:
if the system has 5 pneumatic on-off valves with intermittent operation (3 times per hour, 0.1 Nm & sup3; each time), the intermittent air consumption is:
5×3×0.1=1.5Nm³/h
the total basic gas consumption is updated to: 8.5+1.5 = 10Nm & sup3;/h

3. Consider system losses

there is leakage and pressure loss of compressed air during transmission, and the loss coefficient needs to be increased:

• leakage rate: Usually 10% ~ 30% (older systems may be higher).
• pressure loss: If the system pressure is higher than the instrument demand (such as air compressor output 0.8MPa, instrument demand 0.6MPa), it needs to be adjusted through the pressure reducing valve, but the air consumption remains unchanged.

Example:
taking the leakage rate of 20%, the corrected air consumption is:
10×(1+20%)=12Nm³/h

4. Reserve spare capacity

in order to cope with future expansion or sudden demand, it is recommended to reserve 10% ~ 20% of the margin:

• spare capacity:12×15%=1.8Nm³/h
• final demand:12+1.8=13.8Nm³/h

2. key influencing factors

1. instrument working mode:
   • the gas consumption of continuous operation instrument is stable, and the operation frequency of intermittent operation instrument shall be calculated.
   • Example: If the pneumatic motor runs continuously, the gas consumption shall be converted according to the power (e.g. 1kW & asynmp; 0.3 Nm & sup3;/min, I .e. 18 Nm & sup3;/h); If the operation is intermittent, it shall be converted according to the actual operation time.
2. System pressure:
   • instrument air consumption is usually expressed in volume at standard atmospheric pressure (1bar,0.1MPa). If the system pressure is higher than the standard, it shall be corrected according to the actual pressure (for example, under 0.8MPa, the actual gas consumption = standard gas consumption × 0.8/0.1=8 times, but the pressure is usually considered in the instrument demand and no additional correction is required).
3. Ambient temperature:
   • temperature affects the air density, but the air consumption of the instrument is usually based on the standard temperature (20 ℃), and the temperature effect can be ignored in actual calculation.
4. Air compressor efficiency:
   • the actual gas production of the air compressor may be lower than the rated value (e. g. rated 10 Nm & sup3;/min, actual 8 Nm & sup3;/min), which shall be adjusted according to the performance curve of the air compressor.

Summary of 3. calculation formulas

Q total = (I = 1 & sum;n Qi ×ti )×(1 + L)×(1 + R)

• qi: Gas consumption of a single instrument of category I (Nm & sup3;/h).
• ti: percentage of operating time of category I instrument (1 for continuous operation, converted by frequency for intermittent operation).
• L: System leakage rate (10% ~ 30%).
• R: Spare capacity (10% ~ 20%).

Simplified version(mainly continuous operation instrument):

Q_{text {total}} = Q_{text {base}} times 1.2 , text{(leak 20%)} times 1.15 , text{(spare 15%)}

4. practical application recommendations

1. classification statistics: Establish a table according to the instrument type to specify the gas consumption, quantity and operation mode of a single unit.
2. Dynamic adjustment: The gas consumption is corrected in real time according to the change of production load (such as the shutdown of some instruments).
3. Type selection matching when selecting air compressor, the rated gas production should be slightly higher than the calculated value (for example, 13.8 Nm & sup3;/h, 15 Nm & sup3;/h model is optional).
4. energy saving optimization: Reduce actual gas consumption through frequency conversion control, leak detection and other measures.

Sample table:

type of instrument	gas consumption per unit (Nm & sup3;/h)	quantity	operation mode	basic air consumption (Nm & sup3;/h)
pneumatic control valve	0.4	10	continuous	4.0
film actuator	0.3	5	continuous	1.5
locator	0.15	20	continuous	3.0
pneumatic switch valve	0.1 (each time)	5	3 times per hour	1.5
total	-	-	-	10.0

final demand:10×1.2×1.15=13.8Nm³/h