Air Mains
The cost of the air mains frequently represents a high proportion of the initial cost of a compressed air installation. Using a reduced diameter pipe might save on capital cost, but the greater pressure drop in the system would result in higher operating costs which would soon exceed the price of a larger diameter piping. Any system, but especially one employing a single compressor, should incorporate a suitable connection point for an emergency compressor so as to allow the normal compressor to be serviced without the need to shut down the whole system. In a new or redesigned plant early consideration should be given to the layout of the compressed air system, otherwise the air mains may have to follow a tortuous path to avoid the existing electrical conduits, and steam, water and other pipes. There are two basic systems for compressed air mains:
- A single (Main) line from the supply with branch line to the point(s) of usage.
- A ring main
For installations where the point of usage and the supply are relatively close, a direct single line should suffice. In this case, the diameter of the pipe must accommodate the maximum flow with no more than the maximum acceptable pressure drop. All piping must be well supported. Pipes must not be allowed to sag or hog especially at threaded or flanged joints since the bending stress could lead to leakage and possibly to pipe fracture. Thermoplastics should not be used for a compressor delivery pipe. When determining the diameter of the pipe, excessive energy loss can be avoided by restricting the velocity to less than 6 m/s. Actual velocity may be calculated by using the following formula: V = 1273Q
(P+1) D2 Where:
V = Flow velocity metres per second
Q = Free air flow rate in litres per second
P = Air pressure in bar (gauge)
D = Pipe internal bore in mm Alternatively if the free air flow is known the minimum internal diameter of the air main to ensure velocities below 6 m/s can be found from the following formula:
For standard installations with a line pressure in the region of 7 bar gauge this can be further simplified to:
For larger installations with numerous take off points a ring main has advantages in that the velocity to only one point of usage is reduced since the air can converge on that point of usage is reduced since the air can converge on that point from two directions: also by correct siting of isolating valves, a section of the system can be shut down for servicing. The air mains can be sited at any level from under floor ducting to overhead, as required. For ease of servicing and draining and for access when connecting to service points, the overhead main has many advantages. The primary consideration is that the main line(s) should cover the working area while being as direct as possible. Ring mains should be formed with long sweep bends, not elbows, in order to minimize pressure losses. The main should have a fall of approximately 1% in the direction of flow, and at suitable intervals should be brought back to its original height by using long sweep bends. By arranging drain legs, preferably fitted with automatic discharge valves, at all these points, it is possible to prevent condensate being carried to the points of use. Such measures may not be necessary if a n air dryer is fitted. Where the air flow is disturbed at a junction or at an isolating valve, the pressure loss is increased; this can be minimized by using long sweep pipe fittings and full flow valves to give a smooth flow; this will also eliminate impingement areas where condensate may "fall out". When pipe mains are subject to variations of temperature, for example where the main runs from one building to another, there is the possibility of a temperature drop. In such instances a separator and drain leg should be fitted at the point where the air flow enters the building, so that any condensed moisture is collected and removed. This condensate shall not be permitted to discharge to ground or sewer. If an automatic drain is used and fitted with a drain line this should comply with the manufacturers requirements in respect of minimum bore, maximum length and maximum lift of the condensate discharge above the drain trap level. To avoid any possibility of freezing, an exposed main should be insulated. If the pipe is embedded in concrete, for example under a roadway, the line should be sloped (not less than 1%) and a drain and access hold fitted at the lowest point. For guidance on installation of air mains see Insulating the Air Main page 75.
Materials for Air Line Pipes and Fittings
Main line pipe systems can be of steel, copper, aluminium or ABS material. Generally copper or thin wall stainless steel is not used for pipe systems over 40mm bore.
Steel Pipes
Steel pipes to ISO 65 should be used; they are available in black or galvanised form; the latter, being less liable to corrode, is recommended. This type of pipe can be screwed to accept the range of several proprietary fittings (See BS 143 & 1256 and BS EN 10241); threading should be done to BS21. For air mains over 65 mm nominal bore (76mm OD), welded fittings to BS 1965 are recommended. As an alternative to threading ISO 65 pipe, especially in sizes above 65mm nominal bore, or for use with metal pipes that are not intended for threading, special coupling joints sets are available. They cover a range of sizes from 15mm to 220mm outside diameter for working pressures up to 16 bar. They offer an alternative method to welded or flanged joints and in addition to gripping the pipe ends have internal seals to ensure a leak free joint. They can also be supplied in a design suitable for making the transition between steel and rigid plastic pipe runs.
Stainless Steel Pipe
The increased use of compressed air in industries requiring ultra-clean air eg. Pharmaceutical and electronic has led to a requirement for stainless steel pipes as main lines. Stainless steel pipe to ISO 7598 can be threaded to BS21 and used with stainless fittings to ISO4144. Thin wall stainless tubing to BS4127 can be used with compression or capillary fittings, or soldered, welded or cold adhesive bonded.
ABS Pipe
ABS is a non-corrodible an non toxic polymeric material which exhibits extreme toughness and ductility and is particularly suitable for conveying compressed air. A suitable product is available self-coloured light blue in accordance with BS1710, "Identification of Pipelines", for compressed air services. As with all thermoplastics, the working temperature range is limited, the supplier should therefore be consulted before use. Other thermoplastic piping should not be used in compressed air systems without consultation with the supplier Available in outside diameter of 16mm to 110mm corresponding to 10mm to 100mm nominal bore pipe to ISO 65. ABS pipes for compressed air should not be threaded, connections to metal components being achieved through adaptors or composite unions. Only jointing compounds which are compatible with ABS should be used. The ABS system has a pressure rating of 12.5 bar at 20°C reducing to 8 bar at 50°C. At temperatures between 20°C and 50°C the pressure rating should be in accordance with the suppliers recommendations. ABS should not be used below -20°C. Most synthetic oils and a few mineral oils will degrade thermoplastics and elastomers, therefore oil suitability must be checked withthe pipe suppliers.
Copper Pipe
Copper tubing may be used for air mains with a nominal diameter of up to 40mm. The tubing should conform to BS EN 12449. The tubing and fittings chosen must be checked to ensure that they are suitable for the maximum working pressure of the system. The fittings for this type of tubing are of the compression type. Copper tubing is easily worked and, with the use of bending equipment, the quantity of fittings can be reduced. When making comparisons with steel pipe the initial cost and the labour charges for installation should be taken into account. Copper tubing with its compression fittings could prove expensive for main line installation except where small flow rates are envisaged. Pure copper tubing is recommended for medical compressed gas supplied and medical vacuum because pure copper tends to inhibit the growth of micro organisims.
Aluminium Piping
A recent development is the use of extruded aluminium hollow bar having a square cross section with a hollow round interior. Bore diameters up to 63mm are available and branch lines can be attached at any point by means of special connectors. Precaustions are needed if used in a flammable atmosphere in order to reduce the possible risk of thermite sparking. Consultation with the suppliers is recommended in order to verify the degree of protection provided against corrosion.
Threaded Fittings
Malleable iron and copper alloy fittings should conform to BS 143 & 1256. Wrought steel fittings should conform to BS EN 10241.
ABS Fittings
A range of ABS pipe and fittings are available in sizes from 16mm to 110mm outside diameter joints are made by solvent fusion.
Copper and Stainless Steel Pipe Compression Fittings
When installing compression fittings, it is advisable to grease the threades of the connections before assembling so as to reduce friction. After the joint has been tightened, it should be broken to ensure that the furrule is correctly located on the pipe, after which the joint can be fully tightened.
Threaded Joints
Care must be taken to ensure that pipe threads and resultant threaded joints are only made by experienced personnel.
Cleanliness
Due to he possibility of air becoming contaminated, clean air installations, such as those in hospitals, have to be given special consideration. Reference should be made to the DHSS for the latest hospital requirements HTM 2022, and the National Health Service Engineering Specification for Medical Gases C11. Pipes should be capped at both ends when delivered to site and stored under cover. Before installation each section of pipe should be blown through with compressed air to remove dirt and swarf.
Determination of Pipe Size
The size of pipe used for air and branch mains is determined by the permissible air velocity and the pressure drop caused by the frictional resistance of the pipe. The possiblity of future extensions and possible addition to the compressor capacity should be taken into account when determining the size of the mains. Increasing the pipe size by one step in nominal bore in many cases doubles the flow capacity with only a relatively small increase in total installation costs. In addition to excessive pressure loss, particulary where long pipe lines are involved, a high velocity will cause any moisture to be carried along the main with the flow of air, thus by-passing the drain legs. In all installations it is important that the pressure drop is kept to a minimum. It should be noted that steel pipes are designated by nominal bore, whereas ABS pipes are designated by outside diameter. Most air mains have some bends and valves which cause additional friction. This can be expressed as an additional length of pipe when calculating the overall pressure loss. Whilst the air velocity in main lines should never exceed 6 m/s this can be increased to 15 m/s in those branch lines which, including the down drops do not exceed 15 metres in length. Guidance on maximum permissible flow rates in branch lines at various pressures is given in the Table on page 102 or the maximum permissible flow rate may be calculated from the following formula: Q = (P+1)D2
85 D = Internal bore of pipe in mm
Q = Air flow rate L/s Free Air Flow
P = Air Pressure in bar (gauge)
Reserve Air capacity
In some cases it is necessary to provide a large volume of air for a short period. The equipment using this air may be some distance from the compressor and the abnormally large pipe, required to carry this air with the minimum of pressure drop, can be avoided by fitting an auxiliary air receiver close to the equipment in question. The size of any reserve air capacity is dependent upon the amount of air used per operation and the pressure drop which can be tolerated; it can be calculated as follows
| Required receiver volume = | Demand per operation (litres free air)
(litres) pressure drop (bar) |
It is important to check that the compressor is large enough to recharge the air receiver up to the original pressure before the next period of high demand.
Main Line Accessories
Air lines must be drained at regular interval. This can be achieved by automatic drain traps or manually operated valves. Neglect can result in condensate reaching the point of usage to the detriment of the pneumatic equipment. It should be borne in mind when designing a main that the occasion may arise when it is necessary to isolate a section with shut-off valves, for example when plant has to be moved from one location to another, or when energy can be saved by isloating sections not in use. Pressure gauges or measuring points may be advisable at critcal points and at final service line take-offs; these will also serve as a warning to maintenance engineers of pressure in the line which could cause a hazard. For a warning on the need for pressure relief valves see Isolating Valve.
Installing the Air Main
The air main should be supported throughout its length so that the correct fall to the drain points is maintained. Allowance must be made for thermal expansion and contraction which will vary with the material used. It is recommended that the spacing between supports should be in accordance with those shown in the table below.
| Pipe Nom. bore mm | Max intervals for verti. runs m | Max intervals for horiz. runs m |
|---|
| 8 | 1.25 | 1.0 |
| 10 | 1.25 | 1.0 |
| 15 | 1.75 | 1.25 |
| 20 | 2.50 | 1.75 |
| 25 | 2.70 | 1.75 |
| 32 | 3.00 | 2.50 |
| 40 | 3.00 | 2.50 |
| 50 | 3.00 | 2.75 |
| 65 | 3.50 | 3.00 |
| 100 | 3.50 | 3.00 |
| 150 | 4.25 | 3.50 |
| 200 | 4.50 | 3.50 |
| 250 | 5.18 | 4.25 |
| 300 | 5.48 | 4.87 |
| over 300 | 5.48 | 4.87 |
It is an advantage to mark the air lines with a recognized colour code (BS 1710) in order to identify the various services, and a high and low pressure mains.
Testing an Air System
After installation of main and branch air lines are before connection of down drops and compressor or air receiver, it is strongly recommended that the sytem is tested.
