Improving compressed air distributions systems - things to consider
Published: 13 November, 2019
BCAS looks at what to consider when improving the compressed air distribution system.
A compressed air distribution system will be installed to deliver compressed air from the compressor house to the point or points of use. At the time of original design a number of factors will be taken into account including the total flow capacity of the system, operating pressures at points of use and exceptional flow demands. These factors will be used to determine the type of system, for example, a ring main, a combination of ring mains with sub headers or a simple single line system.
In the same way that air receivers are a source of stored energy, so is compressed air piping. Therefore it is essential that:
• The system is designed properly to be safe and energy efficient.
• The system is installed correctly.
• Proper procedures are followed during commissioning to ensure the integrity of all the connections within the system.
• Correctly sizing the pipework at this stage is critical to ensuring the pressure drop across the system from the discharge at the compressor house to the points of use is no more than 0.2 bar, generally accepted as best practice.
This is not a difficult process for an experienced compressed air engineer; however, as time goes by requirements often change driven by, new processes, updated equipment and variations to production patterns. As these factors are difficult to accommodate in the initial design their impact on the performance of the compressed air distribution system must be understood and monitored to ensure that compressed air continues to be generated and distributed.
Managing expectations
There are several key points when looking at pipework installation or modification it is key that both you and your contractor are working from the same understanding.
The following are minimum requirements before embarking on a project:
• Agree an acceptable leakage rate at the contract stage
• The concept of ‘zero leakage’ is unacceptable unless related to a time factor and the accuracy of the test method
• The pressure at which the integrity test is conducted and its duration should be agreed
• When agreeing the test conditions the test pressure chosen should be related to;
i. Design Pressure;
ii. Actual operating pressure or;
iii. PS’ which is identified as the ‘maximum allowable pressure’.
• Prior to any work a risk assessment method statement should be drawn up regarding safety of personnel.
• Set the criteria for the leakage test & the pressure (integrity) test both will be required.
• Who will be responsible for the follow up check.
• Carry out a risk assessment procedure on the system to establish which areas would be dangerous to personnel should there be a major leak or system failure.
Jointing techniques
In jointing many new modular piping may be similar in appearance, but the jointing procedure can be quite different. If not correctly carried out this presents a real risk of failure and consequent injury to personnel.
Jointing must be carried out employing the process described by the manufacturer of the system or relevant British Standard. Your supplier should be able to advise you in accordance with your applications requirements and also set the testing appropriately at the quotation stage of your project. We have listed some detail for you below but for full detail please download the BCAS pressure and leak test guide from: www.bcas.org.uk
Push-fit Joints
This type of joint is made by mating a soft seal in some form of housing against either the inner or outer surface of the piping system. Box section modular aluminium pipes are connected to each other and fittings employing a double ‘O’ ring seal and dovetail clamping and jointing system.
Screwed Joints
Where pressure-tight joints are made on the threads – the BCAS publication “Pipe Joint Guide” identifies the thread types used for this type of jointing technique and the practices to adopt to ensure a satisfactory air-tight joint.
Welded Joints
The integrity of this type of joint rests solely with the quality of the welded joint. Therefore, a qualified and certificated welder in accordance with BS EN ISO 9606 should perform the welding procedure.
‘Press-fit’ Joints
This is a mechanical pressed joint using an O-ring to make it fluid tight. The joint can be made by either hand force crimping tools or powered hand tools. Where flexibility is required, other systems may be more suitable.
Hydraulic Friction Joints
These are popular for pressures above 20 bar and extend into regions of 450 bar. The use of these fittings to make a joint with a pipe should follow the joint manufacturers’ instructions in every detail.
Flanged Joints
This type of joint relies upon a sealing material being adequately clamped between faces whose surface is suitably prepared to prevent leakage. The flanges are secured to the pipes by threaded or welded joints and the flanges are bolted together with a sealing gasket between them.
Rigid Plastic Joints
The integrity of this type of joint, usually fusion bonded, also rests solely on the quality of the jointing procedure.
Air main location
This is another area that causes confusion and dispute. The air mains can be sited at any level from under-floor ducting to overhead. For ease of servicing and draining and for access when connecting to service points, the overhead main is often used. Some general guidance:
• Pipes must not be allowed to sag or hog, especially at threaded or flanged joints, since the bending stress could result in leakage and possibly to pipe fracture. Follow manufacturer’s instructions for support spacing.
• The air main should be supported throughout its length so that the correct fall to the drain points is maintained and condensate cannot collect in the pipe itself.
• Allowance must be made for thermal expansion and contraction. This allowance will vary with the material used.
• Compressed air piping should not be attached to (i.e. hung from), other utilities such as gas mains.
Your contractor
Under Regulation 6, The Pressure Systems Safety Regulations 2000 Approved Code of Practice, the owner/user of the system must ensure that those doing the installation have the required training, skills and experience.
It is important that the user selects a supplier that can demonstrate competence through experience and evidenced training in pipe installation and jointing. The user should also check that the supplier:
• Is conversant with the safety issues surrounding compressed air.
• Will carry out the work in a safe manner.
• Holds the appropriate insurance.
The Pressure Systems Safety Regulations 2000 do not specify piping types or specific installation procedures. Like other health and safety legislation such detailed recommendations are beyond its scope. Therefore to demonstrate compliance with the law, users (and their contractors) should follow that “good industry practice” during the installation process.
Examples of “good industry practice” are trade bodies’ codes of practice, for example the BCAS installation Guide, and BCAS Pressure & Leak Testing Best Practice Guide (BPG 103).
For full detail these two publications cover the selection of equipment, correct installation and commissioning prior to use and are available to download at: www.bcas.org.uk
