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Norgen air fuses prevent dangerous whiplash

Original article date: April 1998

A new series of air fuses from Norgren has spurred the launch of a companion CD-Rom, to help select which model from the 24 now available. But why an air fuse?

Anyone who has experienced the wild whiplash created by the free end of a severed tube or hose will appreciate the reasoning behind fitting an air fuse. Fitting this simple product between fixed or rigid pipework and flexible tube prevents the dangerous effects created by the loose end of an accidentally severed or uncoupled tube.

Air fuses work by shutting off the air supply when excessive flow occurs. During normal operation, air pressure on each side of the valve is about equal. The valve seat is held open by a spring, allowing unrestricted flow.

When a hose breaks, the air surge closes the valve. A small amount of air seeps through the side bleed hole. The valve remains closed as long as pressure on the upstream side is greater than pressure on the downstream side.

Unlike an electrical fuse, air fuses can be reset after the repair is made. The bleed allows air pressure to equalise on each side of the valve. When this occurs, the spring opens the valve and full flow is automatically restored.

Easy to fit to existing pneumatic systems, air fuses can make a major contribution towards a safer working environment, as well as helping to ensure that equipment complies with the Machinery Directive and PUWER (the Provision and Use of Work Equipment Regulations).

The air fuse should be installed directly between pipework, whether fixed or rigid, and flexible tube. Only tubing after the air fuse is protected. Failure to install the air fuse in the correct orientation will render it ineffective.

When a shut-off valve is located upstream of the fuse, the valve must be opened slowly in order to control the initial air flow, else the fuse could trip. For each port size, there are two types: a low shut-off flow type (0.14 bar drop pressure) and a high shut-off flow type (0.3 bar drop pressure). The port size should be nominally equal to that of the supply line: for example a 0.5in fuse should be used with a 0.5in ID hose. If there is sufficient system pressure for the length of hose to be protected, the high flow model should always be selected. If not, or where long hose lengths are to be protected, the low flow model is used. Each valve must be tested after installation to check it is functioning correctly.

Norgren’s new T60 series, a range of compact, tamper-proof air fuses for use in pneumatic systems, is available in a range of port sizes with pre-set shut-off flow rates, preventing anyone from tampering with the setting once it is fitted. It is suitable for use in industrial hand tool applications, such as blow guns.

However, with application variables such as inlet pressure, flow, length and diameter of tubing affecting which one of 24 different types of air fuse to choose, it is not surprising that some people find air fuse selection a confusing subject.

To help simplify the process, Norgren has developed an electronic package, in a spreadsheet format, which allows an engineer to input the specific criteria of a pneumatic system. It will then provide the optimum air fuse for that application. The software is available free in CD or floppy disc format.

Features of the T60 air fuse
tamper-proof design
compact and safe construction
low pressure drop
port sizes G1/4 to G1 half, available in high flow and low flow settings
high air pressure rating (maximum pressure 16 bar, depending on tubing specification)
automatic reset after failure correction – unlike an electrical fuse, it does not need to be replaced
high corrosion resistance

April 1998