1.6.1   Filtration

 

Filtration is an essential part of any fluid supply system and the standards required are set by the smallest clearances in the system — usually the servo-valve. The purpose of the filters is to remove debris from the system. It is quite normal for debris to be continuously generated within the system, some from wear processes by the pumps and other moving components, some introduced externally via air entering through the reservoir breather or in the valve relays. Debris will be in the form of small particles suspended in the fluid and will be irregular in size, shape and chemical composition.

Filters are usually given 'nominal' and 'absolute' ratings, suggesting that no particles larger than the absolute rating are passed. In fact this is not the case. Because of lack of uniformity in the size of passages in filters, a whole spectrum of particles will be passed, although the general characteristics ensure that there is an increasing probability that the larger the particle, the more likely it is to be 'captured' by the filter.

Plot of particle size against total number of particles/100ml sample

If a sample of fluid from the filter outlet is examined, its contamination level is described by the number of particles greater than a given size encountered in 100 ml of fluid. With a 5 micron absolute filter in circuit, typical sizes of interest would be 1 micron, 5 microns, 10 microns, 15 microns, 25 microns and 100 microns. The total number of particles greater than each of these sizes would be recorded and if plotted on a log/(log)2 plot (as shown in Fig 2.29), would give a nearly linear form. Using these techniques, various contamination classes can be defined. A number of standards are in existence; the one gaining some international recognition is the American NAS 1638 Standard, based on measurements taken with an electronic particle detector, using a light-blocking principle. Another standard in widespread use in the UK is one published by a commercial organisation, Thermal Control Company Ltd of Brighton, England. This standard is based on extracting a 100 ml sample of fluid from the system, passing it through a filter membrane, and then comparing the sample under a microscope with a series of reference slides representing different classes.

Because of the different measurement techniques and ways of defining the 'size' of an irregularly-shaped particle, the standards lay down different acceptability criteria (in terms of numbers of particles) for systems incorporating fine clearance servo-valves. The system user is advised to opt for one of the available techniques and use it on a regular basis. Results taken from a normal system then provide a reference point for what is abnormal, which therefore requires investigation.

In a system incorporating components with clearances of 2 microns it would be normal practice to fit 0.5 micron nominal filters in the system to give typically NAS Class 7 or Thermal Controls Class 3 conditions.

Differential pressure drops across such filters are normally alarmed and provision is made for on-line filter replacement. Prompt maintenance in these circumstances enables the fluid cleanliness to be maintained and helps to ensure trouble-free service from the system components.

As well as particulate contamination, which is readily visible — albeit under a microscope, contamination by water or chemical means is also possible. The former is likely to cause an increase in fluid acidity and possible formation of gels or salts, which cause components to stick. Chemical contamination can result in severe electrokinetic erosion of fine clearances, especially if a source of free chlorine ions is introduced to the system. For further information on these effects refer to Nelson and Waterman [3].

The likely effects on turbine-generator operation, if contamination is allowed to persist, would be high leakage flows through the hydraulic system — eventually giving a low system pressure and a unit trip. Those associated with maintaining and operating such systems should avoid any possible introduction of water, especially seawater. Chlorinated solvents should never be used for cleaning reservoirs, pipework or components. Nevertheless, some water will be present in the system due to contact of the fluid with air in the reservoir and in the valve relays and drain lines. To maintain it at an acceptably low level (<2000 PPM), a vacuum dehydration unit is normally incorporated in the FRF pumping system. This usually works on a separate sub-loop, taking fluid from the reservoir and then returning the conditioned fluid to the reservoir (Fig 2.30).

Fire-resistant fluid conditioner

Fluid is pumped from the main reservoir into the conditioner reservoir, where it is heated to 80°C and then passed through a coalescer unit into a vacuum chamber where the water is drawn off. The processed fluid is then often passed through a fuller's earth filter which further reduces the water content and also reduces its acidity. A trap filter normally follows the earth filter so that possible escape of the earth due to accidental rupture of this filter will not affect the main system.

Fuller's earth filtration is sometimes carried out in a separate sub-loop supplied from, and returning to, the main reservoir. Since the phosphate-ester fluid is delivered in 40 gallon drums by the oil supplier, a transfer pump is normally provided with each system to permit the initial filling and topping-up of the main tank.

The main features of the pumping unit have now been described. All these features are combined into a single baseplate-mounted unit which is factory assembled and shipped to site as a complete package. Pipework joints are kept to a minimum and are welded wherever possible. All pipework is fabricated in stainless steel to avoid long-term problems of corrosion and the generation of rust particles. A typical packaged unit is shown in Fig 2.31. The system also includes local instrumentation and facilities for the connection of remote instruments and the motor starter unit. Interlocks are built into the system to shut down a pumping line in the event of low pump suction or outlet temperature, and alarms of abnormal conditions are given.

Fire-resistant fluid packaged unit

 

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