Discussion on the consumption of instrument air
Thread Content
Everyone, there’s a question that has been bothering me lately, and I’d like to discuss it with you. According to the SHT 3020-2013 Standard for the Design of Gas Supply Systems for Petrochemical Instruments, there are three formulas for calculating the gas consumption of instruments. The design capacity of the instrument gas supply system depends on the total gas consumption of the instruments. The gas consumption of instruments can be divided into calculated consumption, continuous consumption, and startup consumption. a) The gas consumption is calculated as follows: qv = (qv1 + qv2) + (qv1 + qv2) × 20% + (qv1 + qv2) × (1 + 20%) × 10% = 1.32(qv1 + qv2). Here, qv represents the gas consumption of the instruments under standard conditions, in m3/h; qv1 represents the total gas consumption of continuously operating equipment under standard conditions, also in m3/h; qv2 represents the total gas consumption of intermittently operating equipment under standard conditions, in m3/h. qv1 + qv2 represents the actual gas consumption, while (qv1 + qv2) × 20% represents the reserve gas volume, and (qv1 + qv2) × (1 + 20%) × 10% represents the gas lost due to leaks.b) For continuous gas consumption, the formula is: qv = (qv1 + 10%qv2) + (qv1 + 10%qv2) × 20% + (qv1 + qv2) × (1 + 20%) × 10% = 1.32qv1 + 0.24qv2. Here, (qv1 + 10%qv2) represents the actual gas consumption, and (qv1 + 10%qv2) × 20% represents the reserve gas volume.
c) For gas consumption during operation, the formula is: qv = (qv1 + 25%qv2) + (qv1 + 25%qv2) × 20% + (qv1 + qv2) × (1 + 20%) × 10% = 1.32qv1 + 0.42qv2. Here, (qv1 + 25%qv2) represents the actual gas consumption, and (qv1 + 25%qv2) × 20% represents the reserve gas volume.
When calculating continuous gas consumption, the total gas consumption of intermittently operating equipment is taken as 10% of the total. When calculating gas consumption during operation, this value is taken as 25%. As designers, which formula do you usually use when determining the amount of instrument air required for a process?
Take a look at clause explanation 4.2.2
It must be quite difficult to calculate this accurately; most of the time it’s just an estimation, as there is a huge difference in the valve operation frequency when the device is working properly and when it isn’t.
4.2.2 Doesn’t it provide an estimated value for the gas consumption of a single device? I’m not aware of the relationship between these three formulas or how to use them; it seems that the standards don’t explain this clearly
The latest HGT20510 specification specifies 0.7~1.5 NM3/H for control valves; other pneumatic instruments are considered at 1 NM3/H, while cut-off valves are estimated based on the size of the cylinder and the number of operations per hour. Multiplying the resulting value by X(2+0.3) gives the design capacity, which is 0. 3 is the leakage amount. I think it’s simpler to use 1.5 NM3/H per unit
For a pneumatic valve, calculations are based on 1–2 Nm3/min; for analyzers and similar devices, different formulas are used – but all of these are theoretical values.
I have the same doubt; what is the difference?