23.5 Pipe lines/tubes

23.5.1 Basics and failure prevention

 Pipe line failures

Pipe lines with the connections have a noticeable failure potential. This depends not least from the flown through medium. Exits fuel or oil, the danger of fire exists. But alsom hot bleed air from the rear compressor areas can trigger extensive secondary failures. Especially the wiring is endangered by overheating. Also, if only `fire alarm' is triggered, at least an inflight shut down must be expected. Tubes/pipes at aeroengines do not only serve the transport of media. They also protect sensitive electronic (e.g., sensor impulses for the contol unit and cockpit displays) as well as electric cables and mechanical feedback cables.

The upper picture provides an impression about the tight and complex pipe laying and cable laying of an aeroenginre. The importance of the accessibility for maintenance and inspections can be observed. Must pipe be lines opened respectively exchanged, the probability of escaping fluid exists. Even small amounts can wet neighbored parts. They can penetrate into porous insulations and stay there over longer periods of time. Develop at high temperatures dangerous decomposition products, exists besides the smell nuisance at specific materials (e.g., titanium alloys), the danger of a deterioration (corrosion, crack formation, embrittlement).

The introduction of tubes from titanium alloys, because of weight reasons, can lead to new problems. Primarily concerned is the susceptibility against fretting and sharp, crack like notches, compared with stainless Cr-Ni steels (Fig. "Risk of titanium pipe lines 1", -7.2). These problems must be seen especially in connecrion with maintenance and assembly work. In some cases these obviously lead to a disassembly.

The most dangerous mechanical load of pipe lines develops through vibrations. These can trigger cracks and fractures. For this several influences can be causative, which can also occur in combination (Fig. "Fatigue cracks at pipe line weldings" and Fig. "Vibration of dinted pipe lines"). To these belong from the clampings/mounts introduced vibrations and pulsations of the transported medium. Further problems develop during loosening and tensioning.

To identify during maintenance problems with pipe lines in time, an experienced expert is necessary. He should know sufficient the funktions of the pipe lines and connected devices.

Fig. "Leakages on pipe lines" (Lit. 23.5.1-7, Lit. 23.5.1-18, Lit. 23.5.1-20c and Lit. 23.5.1-24): Leakages and failures on pipelines of gas turbines can have different causes.

ACracks and fractures: In the most cases may be about vibration fatigue (Fig. "Pipe line crack by tensioning" and ill. 23.5.1-3). Thereby especially the pipe zones near the fixing point (Fig. "Resonance vibration caused by P-clamps" and Ill. 23.5.2-6) are at risk. This may have several causes:

BChafes mean a particular danger (Fig. "Dangerous rubbing contacts of components"). This shows the complex course and the alignment of the lines on an aeroengine (Ill. page 23.5.1-1). With this it becomes understandable, that there is quite a potenzial for the contact of the lines. Naturally the designer will already care that this doesn't happen. The experience shows however, that different effects can lead to breed this.

CFused surface: The danger, that a tube line is fused up to perforation exists under exposure to concentrated flames (volume 2, Ill. 9.3-3) or electric sparks/arcs. Both can happen in connection with pipeline failures. If a pipeline is not sufficient cooled at the inner surface, favors this under strong heat addition the danger of overheating. Problematic is a low flow rate to the empty line. This can be the result of a failure (e.g., malfunction of a pump).
A further danger can be assumed from a titanium fire. So it was observed, that burning titanium drops in the size of a pinhead, are in the position to melt through a flown through fuel pipe line from Cr-Ni steel (volume 2, Ill. 9.1.2-3)

DDamage by fragments: Do rotor fragments escape from the casing (uncontained failure, volume 2, Ill. 8.1-3), then instantaneous exists the danger of a leak from a damaged pipeline. The design engineer tries to minimize this risk, even during the planning of the line (volume 2, Ill. 8.2-3.2).

ELoose connection: It may be that during maintenance/assembly the connection was not enough tightened (Fig. "Result example of an unsufficient maintenance"). In this case, especially the accessibility and the visual check plays a role (human factors, Fig. "Much can be seen during passing by an engine"). Those factors are to consider even at the design phase. There can also be a problem with elastomeric elements like O-rings. Thinkable are damages (Fig. "Technicians unfamiliar with aircraft" and chapter 23.4.1).

 Leakages on pipe lines

Further possibilities are the loosening of the nut and/or a damage of the sealing element during operation. A loosening is experienced as likely result of an assembly fault. In such cases vibrations and heat expansion (Fig. "Aged O-rings") can trigger the leak.

During test runs after an assembly such leaks will not always be found. This is especially the case, if not the full engine performance existed. Are the pressures (fuel, oil) not high enough, no observable leak (Fig. "Simultaneous maintenance work") may occur.

FLoose flanges : Studs or flange bolts as well as T-head-bolts (Fig. "Failures by unfavourable assembly conditions") from V-bands (Fig. "Leakage of accessory devices") can come loose or fail (Fig. "Brittle failure modes of bolts and nuts").
A possible cause are deficiencies in repair and/or assembly (Fig. "Assembly caused fuel leak a nd fire").
Bolts made of high-strength steels can fracture due to faults in production like heat treatment (volume 1, Ill. and hydrogen embrittlement (volume 1, Ill. and volume 4, Ill.
A further possibility of a leak in the region of a flange is a failing elastomeric sealing between the seal faces. Also here the whole line-up of causes like material problems (aging, chapter 23.4.1)) and assembly can be relevant.

 Pipe line crack by tensioning

Fig. "Pipe line crack by tensioning" (Lit. 23.5.1-11, Lit. 23.5.1-13, Lit. 23.5.1-14, Lit. 23.5.1-16, and Lit. 23.5.1-19): Fatigue cracks in pipe lines are not seldom attributed to mechanical tensioning. The dynamic load can occur during vibrations and/or low frequent cyclic strains by temperature or changes of the inner pressure.
A tensioning respectively prestressing lowers the fatigue strength of the tube material (diagram above right) because it increases the mean stress. Thereby an excess of the dynamic design conform operation stresses gets more likely.

Causes for the tensioning of a pipe line (sketch above left):
Dimension inaccuracy from the production, can as well at the end connections, as in the region of the clamps, require an elastic bending (Ill. 23.5.2-6). During tightening of the connection, especially in the region of the ends, high stresses must be expected.
The frame shows the sequence of the attachment procedure at a pipe line for an as low as possible controllable tensioning. Generally however count the instructions in the specifications, respectively manuals of the OEM.

Damages before the assembly can result in a similar situation like dimension deviations. Its also possible, that an already assembled line got damaged, i.e., plastically deformed. In this case the spring-back leads to a tensioning.
Of course, the crack position is influenced by notches and deformations like buckles. A fatigue crack can be expected in the region of the end connections on the side of the tube, that is located opposite to the deformation.

Operational tension can be a consequence of different thermal expansions between pipe and attachments. An example is a fuel cooled pipe, which is connected to a hot casing, respectively accessories. Similar conditions develop, if mechanical operation forces deflect connections and/or clamps.
Also a high internal pressure can elastically deform a pipe between the connections and so setup stresses.

Internal stresses promote as tensile stresses the vibration fatigue. Such stresses are inserted by the manufacturing.They are generated during welding, mechanical machining and especially during dressing (Ill-. 23.5.1-4, volume 4, Ill. Thereby process deviations are particularly problematic. As result of a plastic deformation (damage) by spring back develop tensile stresses too. They can, according to the size of the deformated zone, concentrate rather at the deformation or distribute over the whole pipe length.

 Fatigue cracks starting from inside

Fig. "Fatigue cracks starting from inside" (Lit. 23.5.1-13, Lit. 23.5.1-14, and Lit. 23.5.1-16): During bending vibrations and tensionings the highest stresses are to be expected at the outer side of the pipe. However the experience shows, that in some cases fatigue cracks start at the inner side of a pipe wall (volume 4, Ill. This seems to be a contradiction, that will be later discussed.
Cracks starting from the inner side have an especially dangerous property to be identified not before the passage to the surface. Then they may be already relatively large and weaken the cross section up to a spontaneous ripping. So it is no more possible to react at a small leak as advance notice.

Explanations for the start of a fatigue crack at the inner side of the pipe:

Weak points/flaws: Primarily concerned is the root of connection welds in the region of pipe ends (frame above, Fig. "Fatigue cracks at pipe line weldings", Fig. "Resonance vibration caused by P-clamps" and Fig. "Danger by a rubbing safety wire", Lit. 23.5.2-11 and Lit. 23.5.1-27). Basically a weld forms a potential weak point as shape notch and material notch (volume 3, Ill. 13-18). Further notches are scratches/grooves. Also during a bending load at the inner side of a thin walled pipe, a high stress level must be expected. Additionally a hindered visual inspection from the inner of the pipe side promotes undetected notches.

Pipe elbows have an especial relation to dynamic loads. The deflection of the flow can produce high loads and transfer to the pipe line (Ill. 23.5.2-6). Additionally they can be highly dynamic loaded. Sometimes from the inside starting axial/<U>longitudinal</U> cracks are observed (frame below), which obviously produce during bending vibrations no dangerous loads. An explanation could be, that during pressure pulsations and pressure schocks in the medium flow, vibrations develop. These compress and extend the pipe elbows elastically. Thereby in the elbows the sides of the pipe walls are oval deformed (sketch below). This cyclic ovalisation can highly bending load the pipe wall at the inner side (details below).
Similar to a pressure tank, the pipe/tube wall is in axial direction (longitudinal direction) especially high loaded at the inside by the internal pressure. A crack can be promoted from production caused longitudinal scratches/marks and a longitudinal orientated material structure (`fibre direction' cross to the load) of a pultruded pipe or from a longitudinal weld.
Therefore for such cracks it must be verifyed, if not unusual pressure changes in the system occur. Thinkable are problems at pumps and control units and/or cavitation.

Straightening processes: During cold dressing or locally heating back spring induce during the straightening process in plastically upsetted compression zones, high internal tensile stresses (Fig. "Fatigue cracks at pipe line weldings" and volume 4, Ill. Increasing the medium stresses they lower the fatigue strength (Fig. "Pipe line crack by tensioning").

Note: Straightening processes must keep exactly at the approves/specified processes and its parameters. In case of doubt the OEM must be consulted.

 Fatigue cracks at pipe line weldings

Fig. "Fatigue cracks at pipe line weldings" ( Lit. 23.5.1-13): In the aeroengine technology, normally thin walled pipes are used. Thereby in some cases, fatigue crack formation in <U>circumferential</U> direction is observed at the inner side of the tube. Not always this can be sufficiently explained alone by usual welding flaws. In such cases there is no noteworthy flaw (volume 4, Ill. Thinkable are high internal tension stresses, induced during welding which cold not be sufficient reduced during a heat treatment (volume 4, Ill.

But also the possibility of dangerous internal tensile stresses from spring back of a straightening process exists (volume 4, Ill. They can be found in the region of the plastically extended zone at the inner side of the wall (sketch above).
A lower yielding strength in then region of a weld seam leads to corresponding high plastic extension (sketch below). Here develop after the release internal compression stresses, which stand in an equilibrium with internal tensile stresses. The level of tensile stresses is very high, because the pronounced compression zone, which must be compensated.

Note: Even `small' changes at pipe lines and its peripheriy can enable dangerous vibrations. In case of doubt the OEM must be consulted.

Ill.23.5.1-5 ( Lit. 23.5.1-9, Lit. 23.5.1-11 and Lit. 23.5.1-13): Pipe lines are prone for vibrations. To guarantee the necessary operation safety, detailed investigations in the phases design, development and testing are required (volume 3, Ill. This concerns especially type, position (Fig. "Resonance vibration caused by P-clamps") and mounting/fixing of the clamps, as well as if necessary, damping measures. A resonance can be prevented with a sufficient distance to a unavoidable, dangerous exitation frequency (e.g., aeroengine rotation speeds, see also volume 3, Ill. Natural frequencies of the pipe are determined from different influences.

  • Stiffness (e.g., material, wall thickness) of the pipe, The end screw connections and the mountings, as well as the clamping conditions and tensioning torques.
  • Masses and mass distribution is determined from properties like size and type of the screwings, material (e.g., Al
  • orTi-alloys, steel) and the arrangement of the mountings (Lit. 23.5.2-11).

The hight of the resonance load depends from damping (volume 3, Ill. of the pipe. Here, heat insulating collars or backings between the clamp and the pipe play a role. In especially problematic cases flexible lines from wire covered elastomers (oil, fuel, hydraulic fluid, sketches below) are used. To these belong also supporting wire bellows at lines of thot pressurised air. These lines, caused from their multilayer, show an excellent damping. The intense friction suppresses dangerous vibrations. Also seemingly small changes compared to the proven and approved version, can cause dangerous vibrations with crack formation.

Typical exitation causes for vibrations of pipe lines are:

Vibrations at the mountings can be feeded into the pipe line. Usually rotation speeds of the main shafts are concerned. They are transferred through bearings and casings to the accessory devices and pipe clamps. Also typical combustion oscillations of the combustion chamber (rumble, buzz, volume 3, Ill. can be dangreous intense (Fig. "Resonance vibration caused by P-clamps").

Shocks and pulsations in a transported medium: The deflection in elbows of a pipe (see sketches) produce mass forces. During pressure oszillations, corresponding pulsating pressure and friction forces add.
The higher the pressures in the system, the more intense the exitation potential. This can mean, that modern aeroengine types react more sensible than elder at deviations and damages of pipe lines. Experiences or indications for elder types therefore can not be transferred uncritical. System pressures depend from the pressure level, especially the compressor exit pressure. This can at modern aeroengines types be quite above 40 bar (newest development up to 70 bar). For example the fuel pressure at the injection nozzles must be sufficient high for the needed fine atomisation. The oil pressure in the spray nozzle must overcome the air pressure in the bearing chamber. Also the cooling air pressure for the high pressure turbine area rises in the supply line with the compressor exit pressure. All this leads at modern aeroengines to higher demands for the pipe lines.

Pressures in hydraulic pipe lines lay markedly higher with up to some hundred bar.
Especially endangered are cranked nozzles form oil and fuel (sketches above) and bended lines. Single pressure shocks can occur during switching on and off (Ill. 23.5.2-6). Pulsating pressures develop in the flow behind pumps.

A special cause is cavitation (Fig. "Vibration of dinted pipe lines"). It occurs, if bubbles of vapuor or air implode in the flowing liquid in the region of rising pressure. Thereby high frequency pressure oscillations develop.

 Causes of pipe line vibration

 Vibration of dinted pipe lines

Fig. "Vibration of dinted pipe lines": By mistake deformed/dented pipe lines are exposed to an increased risk of fatigue cracking. These cracks can occur directly at the dint (crack position „1“) but also at other pipe locations (crack positions „2” and „3“). Not always a causative connection with the dint is easy to identify.
For fatigue cracks by damages, a cooperation of different effects is responsible.

Deterioration in the deformed area: To this belong internal tension stresses, shape notches and material deteriorations. Are these influences high enough, a fatigue crack can develop, even at normal, otherwise tolerable vibration load. Deterioration outside of deformed regions: The deformation can be accompanied by a tensioning of the pipe line between the flanges From this, especially flange transitions may be loaded (crack position „2”).

Exitation of vibration: Whirl formation, flow separations and cavitation at the dint are in the position to exite the pipe line to unusual vibrations. This can lead to a failing at all three preferred crack positions, depending from the hight of the dynamic load.

Note: At dinted pipe lines an increased failure risk must be expected. Only in the limits of the manual and the specifications, deformations of pipe lines are acceptable.

 Risk of titanium pipe lines 1

Fig. "Risk of titanium pipe lines 1", -7.2 (Lit. 23.5.1-7 and Lit. 23.5.1-10): Titanium alloys virtually offer itself for pipe lines in aeroengines. Convincing arguments are:

  • High specific strength (high strength related to low specific weight). Is this advantage fully used with corresponding thin wall cross sections, however high load/stresses can turn to a disadvantage (see section `fracture mechanic behaviour').
  • Low modulus of elasticity (low stiffness) drops the stresses during elastic deformations (e.g., thermal stresses). This also facilitates the compensation of warpage. This means low medium stress with increased fatigue strength (Bild 23.5.1-2).

 Risk of titanium pipe lines 2

  • Good formability and weldability with suitatcan ble adjusted production processes.
  • Proven in aeroengine technology for high loaded components.
  • Availability as tubes/pipes with suitable quality.

Anyway it was reported of catastrophic failure cases of pipe lines from titanium alloys. These pointed at unfavourable properties for this application, respectively have been traced to these (Ill. 23.5.2-1). Obviously, therefore at least in particular cases, it was changed again to CrNi steels (type 18/9). This permits the conclusion at serious disadvantages ot titanium alloys. Seemingly these have been estimated as material typic and inevitable. Unfortunately, available papers describe the problems and failure mechanisms insufficient.

In the following should be tried to highlight the problems of pipe lines from titanium alloys more in detail.

Fracture mechanical behavior (Fig. "Risk of titanium pipe lines 1"): The damage of a pipe during assembly, respectively maintenance work, can not be ruled out, although if very seldom. It can happen unnoticed on site, especially under hindered conditions, with the sharp edges of a tool (e.g., screw driver). Also especially hard contaminant particles between P-clamps have proven as dangerous (Lit. 23.5.1-9, Fig. "Resonance vibration caused by P-clamps").
As long the damage has a sufficient large notch radius, even an only slighter drop of the fatigue strength, than for CrNi-steels can be supposed. This quite positive behaviour is due to the low notch sensitivity of the titanium alloy. In the following it is represented by the factor ßk, which lays below αk. With this the notch effect will be lessened.

However, if we deal with sharp notches (scores, scratches) or cracks, the so called fracture toughness is the characteristic feature of a deterioration. In this case, a titanium alloy with typical low fracture toughness is disadvantageous (Fig. "Risk of titanium pipe lines 1", diagram below left). It triggers already at small damages crack growth and accelerates it. The low fracture toughness enables already at a lo `critical crack length' (volume 3, Ill. 12.2-3) the forced fracture, i.e. the catastrophic failing (Fig. "Risk of titanium pipe lines 1". diagram below right). This means, that already relatively small, less striking scratches and scores can get danngerous.
Under these influences the time period up to the failing is relatively short. So there is little chance, to identify and to intercept the failure at a small leak.

Strain hardening behaviour: An accidentally inserted notch, which occurred not as a cut but as an indentation, shows certain features (Fig. "Risk of titanium pipe lines 2", sketch below right). It has a notch root with a bigger, plastically deformed radius. Unfortunately the comparable favorable, i.e. low notch sensitivity of the titanium alloys, has no effect like at steels. This is due to the low cold work hardening of the titanium alloy (Fig. "Risk of titanium pipe lines 2", diagram below left). But a strain hardening in the noth root, together with protective internal compression stresses can act as a protection against vibration fatigue.

Behaviour during fretting: This means for metallic materials besides the removal (notch, reduction of a bearing cross section), also a material deterioration (volume 2 , Ill. 6.1-3 and Ill. 6.1-4). Thereby is the fatigue strength of steels and Ni alloys markedly less concerned (about 10 %) as from titanium alloys. Their fatigue strength drops to about 70%, i.e. at 30 % (volume 2, Ill. 6.1-8). This phenomenon causes since the introduction of titanium alloys quite a big headache for the designers. For example the contact faces, respectively the roots of compressor blades from titanium alloys must be markedly overdimensioned. Only so the load can be lowered to an unproblematic stress niveau (volume 2, Ill. 6.2-3).

Do titanium pipes touch other components, the always in aeroengine acting vibrations trigger fretting and drop of the fatigue strength. This is for pipes from titanium alloys already dangerous during normal vibration load.
In this connection the P-clamps have a special importance. A loosened bolting and/or unsuitable interlayer/pad can also lead to the failing of a Cr-Ni pipe line (Fig. "Pipe line crack by tensioning").

Note: Pipe lines from titanium alloys require especially cautious handling during maintenance and assembly. Already small sharp edged notches, scratches and fretting spots can trigger fatigue cracks.

Note: The contact surfaces of clamps and mountings must be clean. Hard particles can leave dangerously sharp indentations and scratches at which fatigue cracks can develop.

 Leakage of accessory devices

Fig. "Leakage of accessory devices" (Bild 23.5.1-22): Compilation of potential connection and tightness problems in accessory devices:

Typical failure causes are:

  • Fracture of the tensioning bolt (T-bolt) because of production faults (Fig. "Production caused bolt head failing" and volume 1, Ill. ).
  • Fatigue fracture in the region of spot welds at V-band profile and draw shackle.
  • Fracture of the shackle for the bolt grommet at the V-band after a change of the material structure due to too high long time temperature influence (volume 1, Ill.

A further danger exists, when instead of silver plated tension bolts/nuts misleadingly cadmium plated are used (Lit 23.5.1-25). At temperatures above 200°C the danger of cracks exists. This is true for steels and titanium alloys even during contact with not molten cadmium (SMIE, volume 4, Ill. Thereby it can come to the fracture of the bolt or the burst of the nut. Especially on pipe lines for bleed air, critical temperatures must be expected.

A critical element are obviously O-ring seals (Fig. "Technicians unfamiliar with aircraft"). Thereby again and again emerge assembly problems, together with flange connections (Fig. "Technicians unfamiliar with aircraft"). To these belong:

Failing of the bolting by hydrogen embrittlement and stress corrosion cracking (SCC, Fig. "Brittle failure modes of bolts and nuts"),
Pull out of threads in casings (Fig. "Problems with additional bolt locking" and Fig. "Bolt connections in light metal castings") or assembly problems concerning securing/locking and tensioning torque.

  • Plugs to be screwed („E“) for the dumping of oil and/or for the chip control of the oil (Fig. "Simultaneous maintenance work"). Here leaks an especially to mention in connection with O-ring seals.
  • Ducts for shafts („F”). Leaking of sliding seals (chapter 23.4.2).

Note: Free line ends and connection openings must be always suitable covered, respectively shut, to avoid certain the intrusion of foreigen objects.

 Blocking danger at open pipe lines

Fig. "Blocking danger at open pipe lines" (Lit. 23.5.1-3): Also biological foreign objects (e.g., insects) must be expected. Under this point of view, only tight fitting protection caps offer sufficient warranty that nothing intrudes. The best ist, that these are firmly attached or screwed.

 Failures by plastic protection sleeves

Fig. "Failures by plastic protection sleeves" (Lit. 23.5.1-5 and Lit. 23.5.1-6): Bandages, washers, pads, interlayers and collars are used at pipe connections as friction protection. These guard against contact touch with other components, but also in P-clamps. In further applications, they are used as thermal insulation and for vibration damping. With this olso risks are combined.

Aggressive components and/or contaminations can dangerously damage pipe walls. For example under operation conditions (e.g., sufficient high temperatures) or during stand still (humidity), halogens (chlorine, fluor) can be emitted. These can trigger material specific pitting corrosion (volume 1, Ill. und/or corrosion cracking (volume 1, Ill. and Ill. Thermal decomposition of a part from a bandage like polyvinylchlorid (= PVC) in neoprene sleeves or fiber glass tapes (Lit. 23.5.1-6), can form salts with absorbed humidity. These are in the position, to attack pipe walls from CrNi steels. Also residues of chlorine containing packing/wrapping material can get dangerous for pipe lines during service.
For titanium pipes, deteriorating reactions have been already observed at 150°C.

Known is, that halogens trigger cracks in titanium alloys at temperatures above 450°C (volume 1, Ill. These can come from sea salt or thermal desintegrated media/materials. So there a warning is given of bindings from PVC which releases chlorine. Act simultaneously sufficient high tensile stresses, the danger of corrosion cracking exists (volume 1, Ill. is the case during tensioning of the line, or service pressures. Sufficient high temperatures for a deterioration are also possible through a heating by thermal conduction or radiation during stand still (heat soak).
Exists the suspicion, that bindings absorbed spilled media or from leakage during service, they must be cleaned and in case of doubt exchanged.
To critical media/materials, which can set free at service temperature dangerous decomposition products belong:

  • Hydraulic fluid, which releases chlorine at high service temperature (Fig. "Contaminated Hydraulic fluid").
  • Chlorine containing cleaningagents and degreasings like „Tri“ and „Per” from maintenance and overhaul (volume 4, Ill.
  • Chlorine containing rust remover e.g., as spray.

Note: For pipe lines of aeroengines, basecally the attachments and bandages specified from the OEM must be used. If there is suspicion of material deviation, the harmlessness must be approved.

 Vibrations by cracks in a fuel line

Example 23.5.1-1 (Lit. 23.5.1-27): In several cases, at the fuel pump, a line to the pressure sensor for the cockpit display broke. Vibrations produced cracks in the line. Within from 30 days after the publication of an airworthiness directive (=AD), work for remedy must be carried out. Concerned is an additional clamp as support.

Comment: Obviously it is a part of the fuel supply from the fuselage. It can be supposed, that the high vibration load was exited by the fuel pump.

 P-clamp failure of a fuel line

Example 23.5.1-2 (Lit. 23.5.1-24): This failure developed in the aeroengine of a helicopter. The pipe line transfers the compressor pressure to the fuel control unit.
A fracture lets the aeroengine suddenly stop. The upper part of the two-piece plastic collar was ground. So the P-clamp from CrNi steel had direct contact to the pipe. With this, under normal vibrations, fretting and crack formation occurred. The deterioration eluded because of the bad insight during the maintenance, even with the use of a mirror, as visual control. This was complicated additonally from the seemingly undeteriorated condition of the controllable underside. The failure is traced back at the loosened fixing bolt. This could be explained with the crushed plastic collar from fiber reinforced silicone rubber.

Comment: The picture shows the clamp after the demounting, which slided out of the contact zone. For this obviously the still present lower plastic collar was removed. It seems no single case. So in the future, further cases can not be ruled out if there are no measures like a hint from the OEM

 Importance of P-clamps

Ill. 23.5.2-11 (Lit. 23.5.1-5, 23.5.1-6 and Lit. 23.5.1-26): P-clamps serve frequently the fixing, respctively mounting of pipes. They influence exceedingly the safe operation behaviour. Shortcomings of such a fixation can act application specific deteriorating in many ways. Fixiation determines the position of the line. It cares, that no contact with other components occur (example 23.5.2-2 and example23.5.2-3). The effect is dictated of the clamping force, the elasticity of the clamp and interlayers (sleeves/collars). These properties are influenced from the service temperature. A loosening of the clamping effect and/or der the bolting can trigger vibrations with dangerous fatigue loads and wear.

Preventing vibrations (Fig. "Causes of pipe line vibration"): The position of the clamps (Ill. 25.5.2-5) influences the vibrating mass and the stiffness (shorter free pipe pieces) and with this the natural frequency of the pipe. Additionally acts the inner damping and the friction of the plastic collars. In this manner, already during design, the vibration loads can be limited at a allowable measure. On the other hand, even seemingly little changes of the clamp position and/or of the damping effect, can cause dangerous resonance vibrations (Fig. "Resonance vibration caused by P-clamps").

Prevention of dangerous tensioning: The right position and an adjusted elasticity should avoid a dangerous tensioning of the pipe line (Fig. "Pipe line crack by tensioning"), as well during the assembly, as also during operation.

No alarming wear at the clamping area: The intermediate layers/collars between clamp and pipe must be selected, that no wear (fretting) develops. The vibration load and/or the fatigue strength may not be influenced dangerously (Fig. "Risk of titanium pipe lines 2", example 23.5.2-1).

No corrosion at the pipeline: Micro movements between pipe and interlayer remove protecting oxide layers (e.g., at titanium alloys and CrNi steels). This promotes chemical processes like corrosion. Does the intermediate layer absorb corrosive media, or penetrate these by capillarity into gaps between then contact faces, an intensified corrosion load occurs (Fig. "Failures by plastic protection sleeves"). This can be prevented with a correct material selection. Without the approval of the OEM, a change of the interlayer material may not occur. Such a situation can arise, when caused by environment protection no clamps with asbestos containing shims/interlayers may be used and alternatives must be found.

Easy assembly: P-clamps are a very simple element. However mistakes are possible:

  • Too low clamping force.
  • Damage of the interlayer (example 23.5.1-2).
  • Use of an unsuitable clamp because ofconfusion or changes.
  • Abrasive acting contaminations of the contactfaces.

Easy control during maintenance: At sufficient accessibility, a check by an experienced technician from the outside for loosening, offset and damages of the interlayers should be possible. Not always these preconditions are given. This can lead to dangerous situations (example 23.5.1-2).


23.5.1-1 National Transportation Safety Board (NTSB), „Brief of Incident, In Flight Engine Fire and Emergency Landing of American Airlines Flight 574, Airbus Industries A300B4-605R, San Juan, Puerto Rico, July 9,1998“, Public Meeting of November 16, 1999, Seite 1 und 2. (3821.1)53 D/E, „Dichtungshandbuch/Sealing Handbook”, März 1999, page 1-126.

23.5.1-2 Transportation Safety Board of Canada, Aviation Investigation Report A03P0332, „Maintenance Error-In Flight Fuel Leak, Air Canada, Airbus A330-300 C-GHKX, Vancouver International Airport, British Columbia, 06 November 2003“, , May 9, 1983, page 1-9.

23.5.1-4 J.Escobar, „Fuel Contamination: Increasing awareness on factors that lead to jet fuel contamination”, Zeitschrift „Aircraft Maintenance Technology“,April 2002, www.amtonlone.com, Seite 1-5.

23.5.1-5 „Turbine Engine Hazard - Incompatibility of Chloride Based Materials and Stainless Steel or NK Titanium Based Components”, www.casa.gov.au, AAC/Part-1/1-013.HTM, page 1.

23.5.1-6 Zeitschrift “Aviator Aviation”, Volume 4 Issue 4, April 2001, page 1-5.

23.5.1-7 “Wachsende Probleme für die Osprey”, Zeitschrift, Flug Revue“ Juni 2001, page 68.

23.5.1-8 “Anziehdrehmomente für Schrauben und Muttern aus A2/A4 Festigkeitsklasse 70/80”, www.va-schrauben.de, page 1 and 2.

23.5.1-9 J.Neff, “Bad tubing grounds Osprey - Problems with Titanium hydraulic lines have plagued the innovative aircraft for years”, www.Newsobserver.com, November 29, 2003, page 1 -3.

23.5.1-10 M.R.Brendt, “Briefing on V-22 Accident by Maj.Gen. Berndt”, www.defenselink.mil., April5, 2001, page 1 -33.

23.5.1-11 “F-16 / 101 Fuel System Modified”, Zeitschrift „Aviation Week & Space Technology”, February 23, 1981, page 24.

23.5.1-12 D.M.North, “Pilot Selection Process Arduous”, Zeitschrift „Aviation Week & Space Technology“, April 12, 1999, page 65.

23.5.1-13 „Australian Transport Safety Bureau (ATSB), „Fractured Fuel Line”, Safety Brief 200006273, Accident & Incident Report, 2002, page 1 and 2.

23.5.1-14 NTSB Report, Identification LAX94FAer3, „Accident Aug-13-94 at Pearblossom, CA, Aircraft: Lockheed C-130A, registration: N135FF“, Update May 31st 2000, page 1-6.

23.5.1-15 Transports Canada, Zeitschrift, „feedback, Canadian Aviation Service Difficulty Reports”, Issue 1/2003, page 25-30.

23.5.1-16 „Engine Plenum Fitting Weld Cracked“, Transports Canada, Zeitschrift „„feedback, Canadian Aviation Service Difficulty Reports”, SDR # 200401 1 5002, Issue 3/2004, page 25-30.

23.5.1-17 R.Meyer, „Fire detection at its most sophisticated“, Zeitschrift „Aircraft Technology & Maintenance - Engine Yearbook”, 1999, page 108-111.

23.5.1-18 „Grumman F-14 Tomcat“, Zeitschrift „Air International”, January 1982, page 28.

23.5.1-19 NTSB Identification: ATL94LA180, „Bell UH-1B, Accident Sept-29-94“ , page 1.

23.5.1-20 NTSB Identification LAX96IA087, „Boeing 747-251B, Incident Jan-05-96”, page 1 and 2.

23.5.1-21 M.Johnson, “Solid Film Lubricants: A Practical Guide”, Zeitschrift „Machinery Lubrication Magazine“, March 2006, 4 pages.

23.5.1-22 AAIU Synoptic Report No: 2006-006, AAIU File No: 2004/0029, Published: 24/4/96. “Airbus A330-301, Incident 4 June 2004”, 5 pages.

23.5.1-23 NTSB Identification DCA01WA053, „Boeing B-777, Incident June 06, 2001”, page 1.

23.5.1-24 “Fuel storage system contaminated”, Ref. 510003474, Flight Safety Australia November-December 2006“, Selected Service Difficulty Reports ”, page 52.

23.5.1-25 U.S.Air Force, Safety Agency, “Safety sage: warning against cadmium plated hardware on bleed air straps”, Zeitschrift „Flying Safety“, June 2003, 1 page.

23.5.1-26 Australian Transport Safety Bureau (ATSB). Investigation Report No. 200505952 vom November 2006, „ In-flight engine fuel leak…Boeing Company 747-438…18November 2005”, page 1-7.

23.5.1-27 Australian Civil Aviation Safety Authority, Airworthiness Directive AD/F2000/17, „Fuel Line Cracking“, 8/2006, page 1.

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