Electro-pneumatic Positioners Economical Handling of Compressed Air Using Intelligent Electro-pneumatic Positioners Increases Energy Efficiency
Particularly in these times of increasing global competition, companies in the process industries are searching intently for ways to optimize their operations and cut their costs. Consumption of compressed air — long neglected in cost considerations — has regained significance in recent years, and with it, the use of modern electro-pneumatic positioners.
Control valves that influence the flow of process media — gases and liquids — for the purpose of regulating process variables such as pressure, temperature or flow, can be found in every process industry. Together with their positioners, which represent the interface to the control system, control valves are elementary components of all automated process plants. They are moved using many different types of actuators: pneumatic, hydraulic, and electric.
Pneumatic actuators are widely used because they are low in cost, fast-acting, and inherently suitable for potentially explosive atmospheres. They require a permanent supply of compressed air at a suitable pressure, quality, and quantity. Air from the surrounding atmosphere must be compressed for this purpose, and then purified of oil and particles to ensure the maximum service life of the components to be supplied. The cost of compressed air is important: the European Union’s Save II study states that around 18 percent of the total energy consumed by industrial electric motors is used to generate compressed air. Energy used by compressors makes up the largest component (65 percent) of the cost of compressed air, with the remainder attributed primarily to maintenance costs.
Compressed Air Is Often Outsourced
In chemical industry parks where several companies share a central infrastructure, the supply of compressed air is often outsourced. The companies can then concentrate on their core business, but pay for this to a greater or lesser extent with higher prices for compressed air. More attention is therefore being paid to this cost factor than in previous times when there seemed to be a surfeit of in-house compressed air available without a separate bill.
To reduce the costs of generating compressed air in their plants, companies have in recent years made a greater effort to optimize their compressors, and selectively renovated their compressed air distribution networks to reduce leakage.
Until now, however, most air users have neglected an equally large source of savings at the point of use: the positioners that monitor and maintain the position of pneumatic actuators. The main guzzler of compressed air is not generally the actuator itself, which typically moves infrequently. Positioners, on the other hand, consume compressed air all the time.
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.
Choosing Positioners Wisely
The good news is that plant operators can dramatically reduce the amount of compressed air consumed by positioners without compromising on other requirements such as control quality, ease of handling, and ruggedness.
Siemens, for example, has the portfolio of positioners with the lowest air consumption. A Sipart PS2 digital positioner has an internal air consumption of just 0.036 Nm3/h, so its yearly figures are: power 33 kWh; costs € 2; CO2 emissions 21 kg. This is a saving of more than 90 percent compared to the average positioner.
Besides its miserly air consumption, the Sipart PS2 offers other advantages including an external non-contact sensor for actuators subject to severe vibration, extensive diagnostic functions, and enormous flexibility when mounting to non-standard actuators.
The latest addition to the Siemens positioner family, the Sitrans VP300, boasts air consumption of 0.095 Nm3/h, an operating temperature range from –40 °C to +85 °C, the simplest possible mounting thanks to an OPOS interface, and a user interface with a graphic display and multiple languages.
In conclusion, modern digital electro-pneumatic positions can provide significant savings on compressed air compared to their traditional counterparts. With the right positioner, cost savings, return on investment, environmental protection, and high-quality control can all be achieved simultaneously.