Most manufacturing plants using compressed air apply quick-connect couplings. Proper selection and sizing of these inexpensive, ubiquitous devices are usually not given the consideration they deserve.

By Ed Kent

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Most manufacturing plants using compressed air apply quick-connect couplings. Proper selection and sizing of these inexpensive, ubiquitous devices are usually not given the consideration they deserve. Although not a major maintenance expense, the effect of poorly chosen couplings can create unexpected problems with productivity, energy conservation, and safety.

Couplings for compressed air service are available in several connection configurations and materials of construction. The choice depends on service requirements. Industrial interchange couplings (Mil~-C-4109 & ISO 6150 Series B) are the most common for industrial applications. They are offered in the widest size and configuration selection.

Selection factors

The first decision in the proper selection of a quick-connect coupling is choosing an appropriate style for the application involved. There are four important factors to consider: environment, frequency of use, location, and safety.

Environment

Couplings used outdoors must be resistant to weather. Those made from copper alloys or malleable iron last the longest; plated steel rusts. If the application involves physical abuse or a dirty environment, such as a foundry, the proper choice would be quick couplers with no moving parts, because they are not affected by dirt as much as couplers with movable sleeves.

Frequency of use

How frequently a quick-connect coupling is used has a direct bearing on the type selected. Axial connect couplings are usually valved and can simply be disconnected. A rotary connect coupling has no valving mechanism. When the need for disconnect arises, the air must be shut off at the source.

The most common wear points on valved quick-connect couplings are the valve, springs, and seals. For heavy-duty use, couplings without spring-loaded, poppet-type valves and with O-ring seals instead of flat washers should be considered.

Location

Position or location of the coupling affects the selection process. If the coupling is at the end of a loose hose, most styles will do. When a coupling is located on a pipe or other fixed location, an automatic latch or zero-pressure type is convenient to use, because only one hand is required to operate it. When a coupling is located at the end of an overhead hose drop, a zero-pressure type is best because of the lack of engagement effort and ease of disengagement without hose whip.

Safety

Since air is compressible, release of pressure upon disconnect is accompanied by a rapid expansion of air from the unvalved half of the coupling. This release, similar to the action of a rocket engine, causes the hose, if unrestrained, to flail about with the potential for injury. The larger the air volume contained in the hose, the more violent and sustained the action.

There are two ways to eliminate the problem of hose whip. The downstream coupling half -- the plug -- can be equipped with a ball check valve. When air is released at disconnect, the ball prevents the air from escaping rapidly. A disadvantage of this design is restricted airflow during use.

An alternative is to select a zero-pressure, quick-connect coupling. This design automatically depressurizes the downstream air volume as it shuts off the upstream supply. By the time the coupling is disconnected, there is no residual air pressure remaining to cause hose whip.

Sizing

Quick-connect couplings are sized according to two factors; connection size determined by the hose or pipe fitting to which the coupling is connected, and body size determined by the required airflow.

When using couplings with cleaning or dusting devices, the flow rate is not critical. However, the majority of couplings are used with air tools that require 90 psi at the inlet. Since typical plant compressor output is between 100 and 125 psi, the amount of pressure drop allowed at the coupling is usually quite small.

The following rule-of-thumb, based on 90-psi inlet and 2-psi pressure drop, helps determine coupling body size based on flow requirements. (Refer to the manufacturer's catalog to determine actual pressure drop.)

Pipe size, in. Consumption, scfm

1/4 15

3/8 30

1/2 50

Tips

Minimize the effect of vibration or shock from tools which can deform the plug by impacting the latching balls or pins or elongate ball bodies in sockets. This action can be done by using a 2-ft whip hose between the tool and coupling.

Check couplings periodically for leaks in the coupled and uncoupled position. Compressed air leaks are expensive in terms of energy cost and excessive compressor capacity. Selection of O-ring seals instead of washer seals provides a product that lasts longer and is leak free.

Replace couplings that are not performing properly. Although repairable, their initial cost does warrant the cost of labor to repair.

-- Edited by Joseph L. Foszcz, Senior Editor, 847-390-2699, j.foszcz@cahners.com

More info

The author is available to answer questions regarding the selection of quick connect compressed air couplings. He can be reached at 314-629-3700.

Key concepts

Rotary couplers are better suited to harsh environments than axial types.

Frequently used couplings should have a built-in shutoff valve.

Airflow determines the coupling body size.

Types of couplings

Rotary Axial

Glad hand Ball latch

Universal claw Pin latch

Quick lock Bar latch