COMPRESSED AIR PIPING-ECONOMIC PIPING

 

Introduction

 
The purpose of the compressed air piping system is to deliver compressed air to the points of usage.

The compressed air needs to be delivered with enough volume, appropriate quality, and pressure to properly power the components that use the compressed air. Compressed air is costly to manufacture. A poorly designed compressed air system can increase energy costs, promote equipment failure, reduce production efficiencies, and increase maintenance requirements. It is generally considered true that any additional costs spent improving the compressed air piping system will pay for them many times over the life of the system.


 

Piping materials

 
Common piping materials used in a compressed air system include copper, aluminum, stainless steel and carbon steel. Compressed air piping systems that are 2" or smaller utilize copper, aluminum or stainless steel. Pipe and fitting connections are typically threaded. Piping systems that are 4" or larger utilize carbon or stainless steel with flanged pipe and fittings. Plastic piping may be used on compressed air systems; however caution must used since many plastic materials are not compatible with all compressor lubricants. Ultraviolet light (sun light) may also reduce the useful service life of some plastic materials. Installation must follow the manufacturer's instructions. Corrosion-resistant piping should be used with any compressed air piping system using oil-free compressors. A non-lubricated system will experience corrosion from the moisture in the warm air, contaminating products and control systems, if this type of piping is not used. It is always better to oversize the compressed air piping system you choose to install. This reduces pressure drop, which will pay for itself, and it allows for expansion of the system.


 

Compressor Discharge Piping

 
The discharge piping from the compressor should be at least as large as compressor discharge connection and it should run directly to the after cooler. Discharge piping from a compressor without an integral after cooler can have very high temperatures. The pipe that is installed here must be able to handle these temperatures. The high temperatures can also cause thermal expansion of the pipe, which can add stress to the pipe. Check the compressor manufacturer's recommendations on discharge piping. Install a liquid filled pressure gauge, a thermometer, and a thermo well in the discharge airline before the after cooler. Proper support and/or flexible discharge pipe can eliminate strain.


 
1. The main header pipe in the system should be sloped downward in the direction of the compressed air flow. A general rule of thumb is 1" per 10 feet of pipe. The reason for the slope is to direct the condensation to a low point in the compressed air piping system where it can be collected and removed.


 
2. Make sure that the piping following the after cooler slopes downward into the bottom connection of the air receiver. This helps with the condensate drainage, as well as if the water-cooled after cooler develops a water leak internally. It would drain toward the receiver and not the compressor.


 
3. Normally, the velocity of compressed air should not be allowed to exceed 6 m/s; lower velocities are recommended for long lines. Higher air velocities (up to 20 m/s) are acceptable where the distribution pipe-work does not exceed 8 meters in length. This would be the case where dedicated compressors are installed near to an associated large end user.


 
4. The air distribution should be designed with liberal pipe sizes so that the frictional pressure losses are very low; larger pipe sizes also help in facilitating system expansion at a later stage without changing header sizes or laying parallel headers.


 

Pressure Drop

 
Pressure drop in a compressed air system is a critical factor. Pressure drop is caused by friction of the compressed air flowing against the inside of the pipe and through valves, tees, elbows and other components that make up a complete compressed air piping system. Pressure drop can be affected by pipe size, type of pipes used, the number and type of valves, couplings, and bends in the system. Each header or main should be furnished with outlets as close as possible to the point of application. This avoids significant pressure drops through the hose and allows shorter hose lengths to be used. To avoid carryover of condensed moisture to tools, outlets should be taken from the top of the pipeline. Larger pipe sizes, shorter pipe and hose lengths, smooth wall pipe, long radius swept tees, and long radius elbows all help reduce pressure drop within a compressed air piping system.

The discharge pressure of the compressor is determined by the maximum pressure loss plus operating pressure value so that air is delivered at right pressure to the farthest equipment. For example, a 90 psig air grinder installed in the farthest drop from the compressor may require 92 psig in the branch line 93 psig in the sub-header and 94 psig at the main header. With a 6 psi drop in the filter/dryer, the discharge pressure at the after cooler should be 100 psig.




Piping system Design

There are two basic systems for distribution system.

1. A single line from the supply to the point(s) of usage, also known as radial system

2. Ring main system, where supply to the end use is taken from a closed loop header. The loop

design allows airflow in two directions to a point of use. This can cut the overall pipe length to

a point in half that reduces pressure drop. It also means that a large volume user of

compressed air in a system may not starve users downstream since they can draw air from

another direction. In many cases a balance line is also recommended which provides another

source of air. Reducing the velocity of the airflow through the compressed air piping system is

another benefit of the loop design. This reduces the velocity, which reduces the friction

against the pipe walls and reduces pressure drop.

Compressed Air leakage

Leaks can be a significant source of wasted energy in an industrial compressed air system and may

be costing you much more than you think. Audits typically find that leaks can be responsible for

between 20-50% of a compressor’s output making them the largest single waste of energy. In

addition to being a source of wasted energy, leaks can also contribute to other operating losses:

• Leaks cause a drop in system pressure. This can decrease the efficiency of air tools

and adversely affect production

• Leaks can force the equipment to cycle more frequently, shortening the life of almost all

system equipment (including the compressor package itself)

• Leaks can increase running time that can lead to additional maintenance requirements

and increased unscheduled downtime

• Leaks can lead to adding unnecessary compressor capacity

Observing the average compressor loading and unloading time, when there is no legitimate use of

compressed air on the shop floor, can estimate the leakage level. In continuous process plants, this test

can be conducted during the shutdown or during unexpected production stoppages.

Air Leakage =                           On load time

                                     Q x --------------------------------------

                                               On load time + Off load time

Where Q = compressor capacity

Leakage reduction

Leakage tests can be conducted easily, but identifying leakage points and plugging them is laborious

work; obvious leakage points can be identified from audible sound; for small leakage, ultrasonic

leakage detectors can be used; soap solution can also be used to detect small leakage in accessible

lines.

When looking for leaks you should investigate the following:

CONDENSATE TRAPS -Check if automatic traps are operating correctly and avoid bypassing.

PIPE WORK - Ageing or corroded pipe work.

FITTINGS AND FLANGES - Check joints and supports are adequate. Check for twisting.

MANIFOLDS - Check for worn connectors and poorly jointed pipe work.

FLEXIBLE HOSES - Check that the hose is moving freely and clear of abrasive surfaces. Check for

deterioration and that the hose has a suitable coating for the environment e.g. oily conditions. Is the

hose damaged due to being too long or too short?

INSTRUMENTATION - Check connections to pneumatic instruments such as regulators, lubricators,

valve blocks and sensors. Check for worn diaphragms.

PNEUMATIC CYLINDERS Check for worn internal air seals.

FILTERS Check drainage points and contaminated bowls.

TOOLS Check hose connections and speed control valve. Check air tools are always switched off

when not in use.

The following points can help reduce compressed air leakage:

• Reduce the line pressure to the minimum acceptable; this can be done by reducing the discharge

pressure settings or by use of pressure regulators on major branch lines.

• Selection of good quality pipe fittings.

• Provide welded joints in place of threaded joints.

• Sealing of unused branch lines or tapings.

• Provide ball valves (for isolation) at the main branches at accessible points, so that these can be

closed when air is not required in the entire section. Similarly, ball valves may be provided at all end

use points for firm closure when pneumatic equipment is not in use.

• Install flow meters on major lines; abnormal increase in airflow may be an indicator of increased

leakage or wastage.

• Avoid installation of underground pipelines; pipelines should be overhead or in trenches (which can

be opened for inspection). Corroded underground lines can be a major source of leakage.