Table of Contents
22.4.2 Sealants
Sealants as cured pastes or liquids are especially used for the sealing of flanges from gear boxes and casings, like in fhe front area of the compressor. For the purpose suitableprocessing and application, according the instructions in the specifications and manuals, a sufficient operation safety should be guaranteed. Problematic are rather the little, seemingly harmless deviations. To these belongs the change to a product which is evaluated as comparable.
- The application of a sealing paste in an area where media act, for which the sealant is not explicite specified, can trigger the failure of the seal effect. Not always an unsuitable application is obvious. If we orient ourself at brochures, usually a multitude of media are named against which the sealant is resistant. In contrast we frequently surch in vain informations about media which deteriorate the sealant. For example there is an anaerobic (curing under air exclusion) sealant, which is frequently used in aeroengines (Hylomar®. attention! there are some products with different properties under this label), with an excellent durability against synthetic lubrication oils, water, antifreeze (glycoles) and kerosine. In contrast hydraulic fluids and aviation fuel/avgas can act deteriorating. The probability, that these media can act may be in fact low, however can not ruled out for aeroengines.
Unsuitable sealing gaskets from slicone rubber can swell dangerously in contact with aviation fuel/avgas and form large blisters, which also restrict larger cross sections (Fig. "Inner crack formation at O-rings", Lit. 22.4-22).
- An incompatibility (corrosion) of some sealant pastes during the contact with metals must be expected. This concerns some age hardening under air exclusion, single component silicone rubbers (room temperature vulcanizing = RTV). During age hardening with air humidity, acetic acid develops (feature: smell!). The acetic acid indeed volatises widely after some time, however a little rest (ca. 0,5 %) remains. It can act at some metals corrosive. To these belong coppere, brass, zinc, carbon steels, galvanic plated iron alloys and magnesium alloys. In contrast aluminium alloys seem resistant. In any case the trapped acetic acid between two unequal metals (corrosion cell) may promote corrosion. An other version of these single component silicone rubber releases during curing methyl alcohol. This attacks aluminium alloys. So it can be realised how exact specified product must be kept.
- Limits of application: Single component silicone rubber needs for curing/age hardening oxygen from the air (aerobic behaviour). This starts from the surface. At the beginning, a film forms which by the time becomes thicker. Therefore, for these compounds the width of a sealing area is limited at about 1 cm. In contrast, two component silicone rubbers harden throughout without needing access of the air. So the width of the flange is not limited by the curing. This applies also for anaerobic sealants like Hylomar®, which only harden in contact with air. If there is contact with air, we will wait for the curing/hardening in vain.
Fig. "Correct amount of sealant is important" (Lit. 22.4-13): For the sealing of flanges from casings/boxes in the temperature region of -50 bis +250°C, anaerobc sealing componds (type Hylomar®) proved itself since long time. This is especially true for flanges of oil delivering/wetted casings (sketch above). Surplus sealant is squeezed during the compressing of the flanges out of the sealing gap (detail above right). This material can separate and block important flow cross sections. At the approachable outside, it is no problem to remove the rests, especially when not yet hardened. In contrast, sealant which escapes inwards, often can no more be removed. It hardens and than cen be transported by the oil stream. Larger amounts of these particles clogg filter sieves/screens (Fig. "Correct amount of sealant is important"), pipe lines for fuel drainage (Fig. "Excessive use of sealant") and oil jets/nozzles. A danger at which usually is poited in the manuals, respectively processing instructions. In the oil circuit especially the relative small cross sections of the sieves are concerned (frame below). In Fig. "Excessive use of sealant" the case is described, during which after an aborted start fire of an APU occurred, because the drainage pipe line was blocked. So the fuel could accumulate.
