Safety and savings01 February 2006
Compressed air continues to offer many advantages in process industry applications, especially in hazardous areas, but often continues to be used inefficiently. Help - much of it free - is readily available to help reduce energy costs and technical solutions often have surprisingly short payback times, which are likely to become even shorter as energy prices rise.
"Pneumatic components offer some very significant advantages for processing applications compared with electrical drives," says Dave Bolwell, Festo's process industry consultant. "They are much lower cost to both purchase and run and they offer the highly desirable attribute of overload tolerance. If a valve becomes stuck, no motor will burn out. They really represent an ultimate in reliability, and merit serious consideration, especially wherever there's new plant construction or major upgrades."
The company has recently launched three new families of ATEX-certified pneumatic cylinders, suitable for driving butterfly, ball and gate valves. Configuration choices include single or double acting, fixed quarter-turn actuators with torque ratings up to 8,800 Nm. Each actuator can be set for clockwise or anticlockwise rotation and is suitable for local, on the spot or automatic operation. When used with an electro-pneumatic controller, speed and position can also be controlled. A wide range of ATEX-compliant accessories, such as position feedback sensors are available and may be attached quickly and easily.
Parker can make its ATEX-compliant solenoid valves with plastic instead of stainless steel connection boxes. This is achieved by making the ignition volume in the connection box sufficiently small, by choosing suitable plastics, by carefully designing the path to atmosphere and by encapsulating electronics and coil.
Rob Bentley, area sales manager for Parker Hannifin Fluid Controls, explains that propagation of a flame or explosion from within the terminal box is not possible because its volume is 9cc. At less than 10cc, there is not enough space to allow a proper explosion. Even so, regulations require that the enclosure must be able to resist 13bar pressure without leakage and it does this, being able to withstand 20bar. Furthermore, the joint under the lid and that between the cable and lead-in are made so that they have a safe gap of 0.1mm and a flameway of 7mm, which is 1mm more than the 6mm minimum.
Cover strength
The cover of the terminal box is made out of polyarylamide (Solvay's IXEF), which offers the necessary strength and rigidity to ensure the dimensional stability of the safe gap. The coil and electronics are encapsulated in polypropylene sulphide, or PPS ('Ryton' made by Chevron Phillips Chemical Co). The coil is ATEX-certified EEx dm IIC T4, T5 and T6. The devices are further protected by incorporating fuses to prevent any possibility of coils overheating, and varistor surge suppressors. Power levels of coils are 'low' 2W to 2.5W, or 'standard' 8W, suitable for a range of Lucifer valves with 1/4-inch and 1/8-inch ports and operating pressures up to 10bar.
The downside of using pneumatic actuation is the overall energy inefficiency of many systems. These can, however, be greatly improved, usually without too much difficulty.
For example, Festo offers an energy-saving service undertaken by skilled engineers equipped with calibrated flowmeters to measure machine air consumption in both static and dynamic situations. Consequently, detailed reports can be produced on the location and size of air losses, the costs associated, recommendations for improvements and calculated payback times. The company reckons that losses can account for 30% of total air use, or even more in machines over four years old. Most improvement work has a payback time of between six months and two years. Savings can be benchmarked against original machine performance.
What the Festo engineers do, which compressed air users can, of course, do for themselves if they care to and have the required equipment and expertise, is to make measurements of static leakage, with machines at standstill, and consumption measurement during machine operation. What hampers many ordinary users, however, is a lack of skills and appropriate technologies to make accurate air consumption measurements that can then be used to generate performance data and subsequent cost analysis.
In addition, the teams use inspection to pinpoint leaks, looking at tubing and connectors. Tubing and connectors may then be upgraded and defective components repaired or replaced.
Another way to reduce energy consumption in all compressed air systems is to switch from using on-off fixed speed compressors to machines driven by variable speed drives.
Those doubting the efficacy of such a strategy can consult the free online ABB Energy Saving Calculator (see www.armitage-comms.co.uk/energy/esc). Potential savings come from two sources: reducing energy losses in the motor, and reducing compressor losses. These are especially significant in the case of centrifugal compressors, where torque varies directly with the speed squared, while power varies directly with the speed cubed. Even on a reciprocating compressor, where torque is relatively constant, payback times are likely to be between one and two years.
Speedy return
A payback time of 18 months is estimated for the recent installation of an ABB variable speed drive on a compressed air system at Riverside Bakery in Nottingham, owned by Northern Foods.
The 45kW drive was supplied by Inverter Drives Systems. James Whieldon, maintenance analyst for Northern Foods says: "Our monitoring and targeting software gives us profiles of the compressors and indicated that there were definite opportunities to improve the amount of energy they were using. We had used Inverter Drive Systems before and know them well, so we were happy to use them again."
IDS confirmed the energy use figures recorded by Northern Foods and recommended the use of a 45kW ABB standard drive.
There are three compressors in total: two 30kW fixed-speed assist compressors and a 45kW duty compressor. The drive was fitted on this compressor, which is in operation all the time. One of the fixed speed 30kW compressors is used to top up the pressure when demand increases, while the other is on standby in case one of the others breaks down.
Zac Dorn of IDS says: "There are always two out of three compressors running, se we had to ensure that the fixed-speed assist compressor only came online when the duty compressor with variable speed drive could provide no more air - that is, when it is running at full speed." This was achieved by the installation of a pressure sensor to provide system pressure feedback for the drive's built-in PID controller.
James Whieldon adds: "The drive is now operational and we calculate the actual payback time of the project is 1.5 years, achieving an annual saving of £2,450, but with energy prices rising, the monetary savings will increase. We knew that IDS could complete the job quickly and they were the only company we asked to quote. They perform accurate energy monitoring and investigate the installation thoroughly. They are timely, give you the information you require and deliver good savings."
ATEX explained
ATEX is derived from the French translation for Explosive Atmosphere (ATmosphere EXplosible).
Two ATEX Directives have been issued by the European Community: one to promote free trade of goods (Directive 100a: 94/9/EC); and the other to increase the safety of workers (Directive 137: 1999/92/EC).
Both have now been implemented throughout the European Community. In the UK, they have been implemented as the EPS and DSEAR Regulations.
EPS: UK Transposed standard of ATEX 100a. Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996.
DSEAR: UK Transposed standard of ATEX 137. Dangerous Substances and Explosive Atmospheres Regulations 2002.
Source: 4 Square Engineering Consultancy, www.4sq.co.uk
SOE
Related Websites
www.4sq.co.uk
This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies
contact the sales team.