Pneumatic silencer: clogging and replacement

A pneumatic silencer is the small porous plug screwed into the exhaust port of an air valve. It makes the exhaust quieter, but every blast of exhaust air must pass through it. When it clogs with oil and dust, the cylinder slows down. The Festo U-1/8 (part 2307) is a common G1/8 example.

This guide is for fleet and workshop managers in Indonesia. It covers what the silencer does, the Festo data, the signs of clogging and how to replace one.

What a pneumatic silencer does

Festo describes its U series silencers as parts for noise reduction at the exhaust ports of pneumatic components (Festo silencers catalogue). The exhaust air passes through a porous insert, which slows it down before it reaches the open air.

The UK Health and Safety Executive explains the trade-off. A silencer works by creating a controlled pressure drop, and that always restricts the airflow to some degree. The guide also says the most usual source of back pressure in a pneumatic system is the progressive blockage of silencers (HSE guidance note PM56).

A silencer is not a filter. SMC states this in the cautions for its silencers (SMC silencer catalogue). Dirty air must be cleaned before it reaches the valve, not at the exhaust.

Reading the Festo U-1/8 data

Festo's data sheet for the U-1/8, part number 2307, lists these values (Festo 2307 data sheet):

• Pneumatic connection: G1/8.
• Operating pressure: 0 to 10 bar.
• Flow rate to atmosphere: 2000 l/min.
• Sound pressure level: 74 dB(A).
• Ambient temperature: -10 to 70 °C.
• Weight: 2.3 g, with a PE cushioning insert.

The Festo catalogue gives the body as POM plastic, an overall length of 35.9 mm and a diameter of 15.2 mm. It also notes that lubricated air is possible. The same U range runs from G1/8 up to G1, with flow rising with size.

FDA lists the Festo 2307U - 1/8 silencer with the same thread, pressure and flow data.

Check the thread before you order

G1/8 is a parallel pipe thread to ISO 228-1, also called BSPP. Power Transmission Engineering explains that G threads do not seal on the thread, so a pressure-tight joint needs an external seal or gasket. It also notes that NPT and ISO pipe threads have different flank angles and are not compatible, even if some sizes can be screwed together (pipe thread compatibility article). If the valve came from an American machine, check the port thread first.

Signs the silencer is clogged

Look for these at the valve and the cylinder:

• The cylinder moves slowly in one direction but normally in the other.
• The cycle time has crept up over weeks.
• The silencer is dark, wet with oil or caked with dust.
• The exhaust sound has changed, or air whistles at the port.

A cylinder that is slow in only one direction points to the exhaust path used on that stroke. Fluidics calls a clogged silencer a common and often overlooked cause of slow cylinders (exhaust silencer guide).

Do not prove the fault by running the machine with the silencer removed. Removing it makes the exhaust louder and can change how fast the machine moves. Fit a new, identical silencer and compare the stroke time instead.

Replace or clean?

SMC says the sound absorbing material in its silencers cannot be replaced, and that a silencer should be replaced with a new one when clogging lowers the exhaust speed and system performance. It also warns that running a clogged silencer can lead to breakage.

HSE says only silencers with large pore membranes improve much from cleaning. It also says porous plastic silencers, particularly sintered polythene, must not be cleaned with chlorinated hydrocarbon solvents. For a small plastic silencer such as the U-1/8, a new part is usually the simple answer.

How to fit a new silencer

1. Isolate the air supply and release the stored air, following the machine procedure.
2. Clean the exhaust port thread. HSE warns that oily exhaust mixed with dust fills the threads with paste, which can damage the new part.
3. Screw a plastic silencer in by hand first. SMC warns that overtightening a resin body, or using a pipe wrench or pliers, can damage it.
4. Mount it horizontally or pointing down where you can. HSE says this helps oil and water drain out, while a silencer pointing up lets condensate run back into the valve.
5. Keep it close to the machine surface. HSE notes that plastic bodies are prone to impact damage.

Fix the cause of fast clogging

If new silencers keep clogging, the problem is the air. Fluidics says filtered, correctly lubricated air keeps silencers clear for longer. On a screw compressor, check the oil filter and separator service as well. Our guide to the screw compressor oil filter explains when to change it.

Related parts and pages

FDA also quotes the SRP 1196335 air silencer. Browse Festo parts on FDA, the fuel and air system category, or search any number on the parts search page.

For air systems on trucks and trailers, see WABCO air brake valves and the trailer and truck spare parts page. Always follow the vehicle maker's manual for brake system work.

What to send for a quotation

To get the right silencer quoted, send:

• The part number or type on the old silencer, such as 2307 or U-1/8.
• The thread size and type, for example G1/8, G1/4 or 1/8 NPT.
• The valve or machine it is fitted to, and the quantity.
• A photo of the silencer and the port, and your delivery location in Indonesia.

Send these through the quotation request form.