When To Replace Pneumatic Filters
More than 90% of the world's manufacturing facilities use compressed air in their processes, but even air can become contaminated during its passage through a system. Installing compressed air filters is therefore common practice, forming part of an overall purification system for pneumatic machinery. Regular replacement of the filters will ensure that the level of contamination is reduced, enabling the system to operate more safely, more efficiently, and in the long run, more cost-effectively. Manufacturer's recommendations must always be followed so as not to invalidate any guarantees, but as a general rule, most filters should be replaced every year, or immediately on detection of any odour or leak.
There are ten main contaminants commonly found in compressed air systems, which come from four main sources: ambient air entering by the compressor's intake port; the fabric of the compressor itself; the air receiver for the pneumatic system; and the air distribution piping. Efficient filtration can dispose of nine of the ten contaminants, many of which are found in air, water, oil and dirt. Water in vapour or aerosol form, or condensed water after cooling, will account for 99.9% of contaminants in liquid form. Oil can also contaminate a system in liquid, aerosol or vapour form, containing unburned hydrocarbon particles that can clog up a system. The remaining particle groups are formed of solids, including atmospheric dirt, micro-organisms, pipe scale and rust, all of which are regulated by the International Standards Office.
The six air and water contaminants can be substantially reduced by coalescing filters, and these are therefore the most important part of any purification system, usually supported by additional filters for the removal of oil and dry particulates. All these types of filter have replaceable elements or filter cartridges, which require periodic replacement in order to maintain a continuous supply of quality compressed air. This also helps to keep down energy costs, as inefficient filters will use more power to function as effectively. There is, however, still a lot of confusion about why, when, and how often filters should be replaced.
Why Replace A Filter?
Compressed air filters are one of the hardest working components in a pneumatic system, often operating in extreme or arduous conditions. They are constantly subjected to:
- variations in pressure
- variations in temperature – summer to winter and night to day
- variations in humidity – from extremely low dew points to 100% saturated air
- pulsing air demand
- chemical contamination from lubricant additives and acidic oily condensate
All of these factors can degrade and weaken the filter elements, as constant bombardment of the filtration media by high velocity dirt particles will eventually cause the filter media to fail. The most minute hole or damage in the filtration media can lead to disastrous consequences. Any substance operating under pressure will always seek out the path of least resistance, so if any damage occurs to the filter media, the compressed air will use that weakness to force its way through. This will cause the whole element to rupture and tear, resulting in all the contaminants in the air getting carried downstream. Ruptures can also occur instantaneously as a result of significant pressure differentials ('pressure spikes'), so isolation valves should always be opened slowly when (re)pressurising the system, in order to prevent any damage.
When Filters Should Be Replaced
ISO standards govern air quality to specific degrees of purity, which must be maintained for the proper operation of pneumatic systems. The maintenance periods recommended by filter manufacturers ensure that compressed air filter elements are changed in accordance with these standards, so that machines provide optimum performance. It may also be necessary to change a filter element if the differential pressure is affected by a premature blockage that reduces downstream pressure.
Changes in differential pressure are commonly shown on a DP Indicator, Gauge or Monitor which may be fitted to the filter housing. A pop-up indicator, moving needle or digital display registers changes in DP, but such devices are generally only ±25% accurate. They may look genuine, with displays that imitate a real DP gauge, but are imprecise and uncalibrated, and are only useful to indicate a premature blockage in the filter unit. It is a myth that DP reflects the air quality or that filter elements only need to be replaced when there is a noticeable rise in differential pressure. Although increased DP should not be considered the primary reason for replacing a filter element, higher DP will inevitably result in increased consumption of power. So if a system is experiencing a larger power drain through pressure loss, replacing the filter may well improve the situation.
Changing Filter Elements Annually
While users may take into consideration the expense of regular maintenance and purchasing replacement parts, this is insignificant compared to what might result in terms of downtime and spoiled product if a filter element should fail. The benefits of regularly changed filters include:
- fully protected adsorption dryer beds
- fully protected downstream equipment, processes and personnel
- guaranteed top quality compressed air
- reduced operating costs
- increased operational security
- increased productivity and overall profitability
Can filter elements be washed?
Contaminants cannot be washed out of filter media with soap and water or solvent, and attempting to clean the filter in this way will damage the elements.
The filter element still looks clean: does it really need to be changed?
The part of the filter element that is visible is only the drainage layer, which prevents any coalesced liquids being carried downstream. The actual filtration media is found beneath this layer and can't be seen unless the filter element is dismantled. Furthermore, dry particulate and coalescing filters deal with aerosols and minute sub-micron particles that are invisible to the human eye, so it is physically impossible for a human to detect any blockage or filter damage.
The filter doesn't look damaged enough to need replacing and has worked well for years – why change it?
As above, only the drainage layer is visible on the outside of the unit, while the actual filtration media is inside two support cylinders. Although the drainage layer may appear to be intact, the filter media underneath it may still be damaged, allowing contamination to pass downstream.
Does the float drain also need to be changed when the filter element is replaced?
Yes. The float drain is as vulnerable to wear and tear as the rest of the unit, and should be replaced annually when changing the filter element, in order to maintain high quality air flow. Electronic energy efficient drains (Zero Loss Drains) do not add substantially to the benefits of regularly replacing the float drain, and are significantly more expensive to service.
Do the same principles apply to in-line adsorption (activated carbon) filters ?
This type of filter often looks identical to a coalescing or dry particulate filter on the outside, and uses the same housing, but they function differently. Adsorption filters utilise a layer of activated carbon to attract the gaseous oil vapours out of the compressed air, and form a film on the AC bed. The activated carbon has only a limited adsorption capacity for the oil vapour, so their cartridges or adsorption beds must be replaced once this capacity is reached. Smaller, in-line AC filter elements are affected by many variants, including the presence and temperature of liquid oil, the oil vapour inlet concentration, air pressure and dew point, so they need to be replaced frequently throughout the year to maintain a supply of oil-free compressed air.
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