Economical Handling of Compressed Air Using Intelligent Electro-pneumatic Positioners Increases Energy Efficiency
The High Cost of Old Technology
Companies spend a lot of money to generate compressed air and maintain their compressed air systems, and then squander this valuable resource in positioners that consume unnecessarily large volumes of compressed air in the settled state. Although all positioners use compressed air, the problem relates mainly to old pneumatic positioners based on the flapper-nozzle principle.
That so many old pneumatic positioners are still in use is understandable. Many older plants use pneumatic positioners because they have not been modernized since they were built in the days before fieldbus and Hart communications. When plants are upgraded, large sections of older equipment frequently remain untouched.
The average air consumption of a pneumatic positioner, weighted between the various types according to their market share, is around 1.18 Nm3/h at 90 psig. Analog electro-pneumatic positioners use much less: around 0.69 Nm3/h at 90 psig.
Digital electro-pneumatic positioners can be even more economical. There are huge differences between different models: even among the latest generation of positioners, air consumption ranges from 0.02 to 1.5 Nm3/h. Weighted by market volume, however, the average digital positioner consumes around 0.45 Nm3/h at 90 psig.
Across all types of positioner, the average air consumption weighted by global installed base is 0.6 Nm3/h at 90 psig. We can also assume that the average compressor requires 0.11 kWh to produce 1 Nm3 of compressed air, that power stations emit 0.63 kg CO2/kWh, and that electricity costs € 0.06 /kWh. From these figures, an average positioner in one year:
- consumes 528 kWh of electricity for compressed air generation alone;
- costs € 32 in compressed air alone; and
- is responsible for 333 kg of CO2 emitted.
This calculation refers to a notional average positioner, including modern digital types. For pneumatic or conventional analog positioners, still used in many plants, the values are many times higher.
And, of course, positioners are not used in ones and twos. Over a decade, a large plant with 1,000 positioners will be responsible for 3,500 t of CO2 for the positioners alone.