Compressed Air Transmission and Treatment:Condensate treatment and drain valves

Condensate treatment and drain valves

Water droplets coalesce in receivers, aftercoolers and filters and collect in the lowest part of the system and means must be incorporated to drain off this water. It is far from being clean water. Usually it is mixed with oil from the lubrication system of the compressor and other debris. Most compressor oils contain a detergent which produces an emulsified discharge which does not readily settle out into the constituent parts. This is colloquially known as "mayonnaise" and has that kind of consistency. Unless removed it may adversely affect operation of the circuit valves and other equipment.

The environmental consequences of condensate discharge from compressed air sys­tems is becoming increasingly important, so that instead of merely dealing with the removal of the contaminants that arise from the compression of air and the lubrication of equipment, consideration has to be given to the disposal of those contaminants. It was considered acceptable a few years ago for the contents of the drain traps to be discharged into the sewers with little more than a simple oil trap. Nowadays increasingly stringent regulations are being applied by the Water Authorities and the Environmental Agency (which has taken over the responsibilities of the National Rivers Authority in the U.K.) to limit the amount of oil that is permissible for discharge.

Fortunately industry has made equipment available which can satisfy the most stringent requirements.

Controls on discharge into the environment

The discharge of trade effluent, which includes oil, into the public sewers, soakaways and water courses is governed in the U.K. by:

Public Health Act 1936

The Environmental Protection Act 1990 The Water Resources Act 1991

The Water Industry Act 1991

The Environmental Protection Regulations 1991

In the European Community generally, reference should be made to Directive 76/464/ EEC on pollution caused by dangerous substances discharged into the aquatic environ­ ment.

Consent for discharge will depend on its situation and the nature of the discharge. It is controlled by the Environmental Agency in the U.K. Discharge into the foul sewer finally ends up at treatment works where it is the responsibility of the water authority. Each Consent is decided on an ad hoc basis by the appropriate body. Discharge levels can be between 5 and 500 mgll of hydrocarbons (approximately the same as parts per million). There may be a more general description such as there must be no visible sheen on the water, which occurs at about 5 mg/1. Most Authorities are at present likely to specify the maximum permissible hydrocarbon content of water discharge to be about 20 mg/1, with the possibility that some may require a significantly lower value. It is well within the capability of modern oil separation techniques to produce a level of 5 mg/1, so that it can be expected that more and more authorities will insist on that level.

Typically, compressor condensate contains 5000 mg/1, which is unacceptable for direct discharge. It is not permissible to discharge untreated condensate into surface water sewers or into the ground.

A useful summary of the issues involved can be found in "Compressed Air Condensate"

published by the British Compressed Air Society.

Bio-degradable lubricants

Synthetic compressor lubricants are increasingly being used and so it may be thought that as such a lubricant is bio-degradable, it is permissible to discharge it directly into the sewers.

The view in the U.K. of the Environmental Agency (which has taken over the environmental protection function of the National Rivers Authority and the water authorities) is that the discharge of bio-degradable oils is unacceptable into drains, soakaways and watercourses on the following grounds:

• Oil floats on water, forming a film which can cause environmental damage to wildlife habitat, and although the film will degrade eventually, damage will be done during the time delay.

• A bio-degradable substance in the aquatic environment exerts an oxygen demand on the water reducing the oxygen available for other organisms leading at worst to anaerobic conditions.

• There is a philosophical objection to the discharge of waste material, when recycling is a better environmental option.

Some of the modern compressor lubricants are claimed to be particularly environmen­ tally friendly. One recently introduced synthetic lubricant is claimed to be superior to washing-up liquid in its bio-degradability and therefore permissible for discharge into the drains. Such a claim should be treated with caution and specific advice taken from the appropriate discharge authority. There is also the possibility that there may be toxic elements present in the lubricant which would make it unacceptable for that reason.

Responsibility of the user

It is the duty of the factory operator to ensure that the discharge of compressed air drains does not cause pollution. It should no longer be acceptable for factory drain traps to discharge directly into a foul or surface drain. An automatic drain should always be ducted away to a central station either for separation into its constituents or for industrial disposal, preferably the former. One still sees condensate traps discharging directly over a concrete floor, which is presumably expected to absorb the waste. In fact it will find its way into the underlying soil and eventually into water courses.

It has been estimated that 10% of the air which is wasted in industrial installations is down to blocked or jammed open condensate drains, so it makes economic as well as environmental sense to install efficient separators.

Water drain traps

Manual drain taps can be used provided that care is taken regularly to open the tap and not to leave it open, but a more satisfactory method is the installation of an automatic drain trap, of which several types are available.

A ball float valve, Figure I, is one of the simplest and most common types of drain; it maintains a positive shut-off, opening only when enough water has collected to lift the float. Another version is shown in Figure 2; the trap must be installed with the inlet (6) at the top so that the float can rise and fall vertically. After the water has discharged, the float

drops and shuts off the valve, so no air is wasted; a strainer (3) is incorporated for safety; the drain can (and in most cases should) be piped away. A simple version of this type of valve can be fitted into the base of a filter bowl of an FRL (filter-regulator-lubricator) assembly, where it is not likely to have to discharge much water because most of it will have been previously removed. Where there is the possibility of a large quantity of water to be discharged from a ball trap, an air lock may occur, which can be cured by fitting a balance pipe, as shown in Figure 3. It is always safer to include a balance pipe, even if the volume of water is considered to be small.

When the moisture collects in the form of an emulsion, this can result in sluggish

operation or may block the trap altogether. Regular cleaning out of the trap is essential and should always be done if it appears to have ceased to function. One way ofhandling heavily contaminated water is to use an aerodynamic type of drain, Figure 4. This blasts open when a specific quantity has collected and this action means it can handle heavily contaminated

water . The disc (3) across which the air passes can be adjusted according to the degree of contamination; this disc and its seating face require examination and can be serviced by lapping the mating surfaces. Note the screen (5) which should be inspected and cleaned regularly . It is usually fitted horizontally in one of the distribution mains . No trap can handle more than a limited amount of contamination, and so the cause ofthe contamination should be sought and remedied before considering the use of one of these traps. It may be that a trap in an air receiver, for example, is not working correctly.

Another type of drain incorporates an electronic timer which opens the drain for an adjustable period of time at an adjustable interval. The chosen times can be set by knobs on the face of the trap, see Figure 5 . A disadvantage of this type of drain is that the times have to be set by experiment; also there is a periodic discharge of air whether or not any condensate has collected .

Another type of electronically operated drain is illustrated in Figure 7. This incorporates additional safety features which cater for an occasional malfunction. In operation, the condensate collects in the drain through the inlet (I) and accumulates in the tank (2). The diaphragm valve (4) is kept closed by system pressure until the condensate reaches the upper sensor. The solenoid is energised which pulls up the diaphragm valve venting the volume above it. The condensate is driven out by the system pressure until the level reaches the lower sensor, which causes the solenoid to de-energise returning the valve to the closed position. If the unit malfunctions, such as in freezing conditions, a self monitoring program identifies the problem on an LED display, while it continues to function with set timing intervals so as to limit air loss. These units can be equipped with an optional heater if freezing conditions are likely to be experienced.

Oil/water separators

As indicated above, the Environmental or other Agency will probably require that the condensate be treated to separate the oil and water so that the water may be discharged in

the normal way and the oil sent for disposal or recycling. One other option that might be considered is for the whole of the condensate to be collected by a disposal company (at say lOp per litre), but this is likely to prove more expensive than installing a separator.

Even allowing the condensate to settle for hours in a settling tank does not guarantee that the water underneath the oil is safe for discharge. This is because modern compressor oils have a tendency to emulsify and settlement alone is insufficient to produce a liquid free from contamination.

In these circumstances an oil/water separator, should be installed. All the automatic condensate drains in the system, from the air receiver, dryers and filter can be ducted to one separator as shown in the system of Figure 9. Separation is achieved in three stages as illustrated in Figure I 0: the first stage relies on gravity separation; the oil forms a layer

on the surface of the settlement tank to be collected in a separate oil tank; the water beneath the oil is then filtered to remove solid impurities; and finally passes through an activated carbon filter to remove traces of oil remaining. It should then be pure enough for discharge into the main drainage system.

The oil can be periodically drained off when the container is full, and the filters can be replaced without shut-down of the system. The oil can be sent for processing.

Different separation techniques apply according to whether the oil is present as an emulsion or as separable mixture. A convenient way of deciding this is to take a sample of the condensate in a jar and leave it overnight. If the oil is clearly sitting on top of the water and the water is comparatively clear, the condensate is separable. Ifit remains mixed and there is little separation, it has to be treated as stable emulsion. An emulsion separator incorporates a membrane filter which has to be replaced at regular service intervals, although some separators include an automatic back-flushing cycle to extend the service intervals. The appropriate oil separator should be chosen in consultation with the separator manufacturer.

Examples of the two types are shown in Figures 11 and 12.

For efficient working the separator must be correctly rated to the compressor capacity and the moisture content of the air. A higher rated separator is required if a dryer is installed.