23:234:2341:2341

23.4.1 Static seals, O-ring seals

This chapter is limited to O-rings. They are the most frequent sealing element. This is not least based on its simplicity in design and assembly. Anyway, they are not unproblematic. This shows already emerged incidents up to flight accidents, which stay in causative connection with the failure of an O ring seal.

O-rings are liked to use there, where the seal must be frequently loosened/disassembled and again fixed/assembled. However this increases the likelihood of a problem. Typical example are sealings of magnetic plugs in the oil system (Ill. 19.2-2.2, Ill. 19.2-2.3 and Ill. 19.2-3.1).

The seemingly simple installation of the O-rings and the positioning between the faces to be sealed can prove difficult. It demands application specific experience, expertise and skill. For example experience is demanded during the pushing together or screwing of the components to be sealed. Thereby is must be payed attention at features like mating forces and clearances. These can indicate a damage of the O ring (Ill. 23.4.1-2 and Ill. 23.4.1-10). Expert knowledge is already demanded when the O-ring must be pretreated for postioning (e.g., greasing as specified). It is especially demanded, if the ring is slided elastically prestressed without damage over edges (e.g., of grooves or thread tip, Ill. 23.4.1-3).

Also in service O-rings can be damaged (Ill. 23.4.1-4). Because this seal element consists of an elastomer, a compatibility with the medium to be sealed is essential (Ill. 23.4.1-12 and Ill. 23.4.1-13). If this medium is changed (e.g., change of the lubrication oil type or the fuel) compatibility must be guaranteed. If necessary, attention must be payed at measures, demanded by the OEM. Additionally temperature and load depending aging effects (embrittlement, plastic deformations, Ill. 23.4.1-4) must not deteriorate inacceptable the sealing effect through the scheduled operation interval. A specified exchange of the O-ring must take place in the specified period of time. Especially after long stand still periods, like a storage respectively `mothballing' of the aeroengine or airplane, the condition of the O rings must be checked. Also attention must be payed at the acceptance of a reuse. In case of doubt usually applies the exchange to „new“.

Concluding, it schould be pointed out that O rings almost `offer' as SUPs (not approved). Therefore also at features like colour, elasticity, production (burrs, joints) must be payed attention before the installation (chapter 20.2.1).

Ill. 23.4.1-1: The mounting and dismounting of components with an O-ring seal (e.g., magnetic plugs) as well as the exchange of an O-ring is a definitely demanding procedure. Typical causes for maintenance required problems are

  • Unfavourable assembly conditions: To these belong bad visibility (Ill. 23.4.1-6) and hindered accessibility, for example at mounted aeroengines (Ill. 19.2-5).
  • Lack of experience and expertise (Ill. 19.2-2.2 and Ill. 19.2-3.2).- Damage during assembly process (Ill. 23.4.1-2 and Ill. 23.4.1-9)
  • Material problems, confusion, SUP (chapter 20.2.1) with early aging. Then typical results are unusual remaining deformations, shrinking and crack formation (Ill. 22.3.1-4).

To these problems belongs also the reassembly of O-rings, which rather should be exchanged against new parts. With this, the scheduled life time could be markedly exceeded and so aging deteriorations can get effective.
Missed exchange, e.g., after an `mothballing' of the aeroengine/aircraft (Ill. 23.4.1-7). This can also apply for the long time storage of a component with O-rings (e.g., gear, control unit) at unfavourable conditions (e.g., too high temperature).

O-ring problems can from experience act dangerously. During leakage of a medium like lubrication oil or hydraulic fluid, important components (e.g., bearings, control units) and the aeroengine can fail. This danger increases, if during the same maitenance procedure, at several aeroengines of the aircraft the problem arose (Ill. 19.2-3.1).
Escaping leakage fluid can ignite and endanger the aircraft. Aggressive media or decomposition products at hot parts (e.g., hydraulic fluid) can trigger dangerous corrosion failures (Ill. 19.2-15).
Also unnoticed scheared parts of an O-ring can block/clogg narrow, close tolerated (e.g., in a control unit) crosssections or oil nozzles.

Ill. 23.4.1-2 (Lit. 23.4.1-5 up to Lit. 23.4.1-10 and Lit. 23.4.1.16): To the layman, the assembly of an O-ring seems simple and unproblematic. This is a mistake. As well the mounting of the O-ring before the joining into a groove, respectively on the seat demands attention and skill. Must the ring slipped over edges of the groove, steps or threads (Ill. 23.4.1-3), it can be damaged by a cutting process. This is promoted by the usual elastic expansion and correspondent high cutting forces.
An especial critical assembly phase is the pushing together of the sealing faces.

If the O-ring sits already in the key seat („A”), it can be observed in the delicate moment of the pushing together.

Sits the O-ring in a groove of the bore („B“), it can not be observed. Unsuitable handling and/or a too sharp edge can shear the ring unnoticed. To indicate this, experience and sense are needed.

Shifted O-rings („D”) can no more fulfill their sealing function. Thereby the O-ring can be shifted out of the groove as a whole or partly. (Ill. 23.4.1-7). It can even during the assembly shift as far, that its missing will not be noticed (Ill. 19.2-5).

As an earnest problem, hardly to experience, the forgetting of O-rings has proved („E“, Bild 19.2-2.2). Such a situation seems more likely for a sealing with two O-rings (Ill. 19.2-3.1). Not seldom a lacking consideration of the `human factors'(Ill. 19.2-3.1) is also concurrently causative.

Ill. 23.4.1-3 (Lit. 23.4.1-6): Already the preparation of the sealing surfaces of the O-ring like groove or the opposite face (sleeve), is of importance for a continuing safe sealing effect. They must be clean and must not have rough or sharp edged damages like wear spots, scores, notches or burrs.
Before the insertion, all contact surfaces of the O-ring must be suitable lubricated. This is especially important, when not sufficient guaranteed by the medium to be sealed (e.g., aeroengine oil). For example this applies for the whole length of a driving rod. The lubricant must be suitable for the service conditions. It also must not inacceptable influence the medium to be sealed. Therefore, only lubricants corresponding the specifications must be used. In case of doubt the OEM must be consulted.
For movable gas seals (pneumatic) often barium saponified grease/lubricant is used.
During the insertion, the ring shoud not be twisted and not pulled over sharp edges (sketch above left). Dangerous are spline toothings, slots, connection openings and threads. To prevent a damage, dangerous edges can be covered with a plastic cap or a plastic tape (sketch above right). During sliding on, the expansion should not be too large. Till the pushing together, the O-ring must have enough time to contract again. This is due to the fact, that elastomeres do not resile like metal. In the groove, the O-ring should not more expanded than 5% of its inner diameter. During sliding into the `sleeve' rotary motions and back and forth motions are not recommended. With it, the risk of a deterioration rather increases.

Ill. 23.4.1-4 (Lit. 23.4.1-2, Lit. 23.4.1-3 and Lit. 23.4.1-15): To prevent failures at O rings at the beginning or with remedies, it is necessary to identify failure causes and failure mechanisms. Requirement is the evaluation of the failure mode. Specific problems shall discussed in the following more in detail:

Chemical decomposition is primarily determined from the service temperature. It should lay below the longtime temperature for elastomers, mostly between 135 and 150°C (Ill. 23.4.1-10). Thereby it must be suggested, that concerned is an endurance temperature and not a short time maximum temperature (see section „thermal deterioration”). During too high temperature, a change of the molecular strukture (aging) will occur. The thermal stability of an elastomer is also for the same type markedly depending from the individual quality. Thereby contaminations and the interlacing of the molecules play an important role. For example a decomposition can be triggered also with trace metals. This means, that the quality assurance must finally rely on the producer and the supplier/source. This can be important undre the view of SUPs (chapter 20.2.1).
Typical consequences of an aging are:

  • The embrittlement is the main problem and can be seen as the result of a temperature caused aging process. It makes especially prone for failing and crack formation during shock like load. To embrittlement tend „nitriles“.
  • Drop of strength.
  • Material change/decomposition.- Crack formation and crack growth. A special form is inner crack formation, caused by diffused gases into the material (Ill. 23.4.1-5).
  • Blister formation: Also these failures are traced back to gas diffusion (Ill. 23.4.1-5).
  • Lasting deformation, creep. In this connection the drop of resilience/spring back must be seen.
  • Carbon dioxide can be fully dissolved in certain elastomers and gets them to swell with a sponge like structure.


Thermal deterioration is understood as caused by overheating. It develops, when the temperature, during which the elastomer decomposites short-term was exceeded. Such a situation exists when after the shut down of the aeroengine, the cooling by oil and air lacks. This leads to heavy heating by the neighboured hot parts („heat soaking”, Ill. 23.4.1-6).

Compression deformation is a plastisc (enduring) deformation of the O ring under the compressive contact forces. In connection with temperature changes and the so arising volume change of the ring, a leak can develop (Ill. 23.4.1-7). Especially fluorine elastomers (fluororubber) tend to creep deformation at the typical high service temperature.

Explosive pressure drop: Into the O ring elastomer diffused medium, especially gas, gathers at flaws below the surface, like micro pores. During a sudden pressure drop, the gas epands or it comes to vapour formation. The result are deteriorations like blisters, cracks and holes.

Extrusion traces back to an overload of the O ring by the pressure deviation in the sealing gap (Ill.23.4.1-7).

Assembly damages: These occur in different types. Most frequently are cuts and sheared areas, which develop during carelessness during pushing together of the seal faces (Ill. 23.4.1-2 and Ill. 23.4.1-3).
To these count so called „spiral damages“. They stay in connection with a twisting of the O ring. It occurs during pushing together with a rotative moverment.

Outgassing and/or dissolving from components, shows as shrinking because of the volume loss. During high temperatures it can come to a chemical decomposition process. It makes components of the seal material liquid or gaseous. Especially thermoplastic urethans are susceptible for such a deterioration.
Gets water vapour or overheated water into the pores which form during outgassing, a sponge like structure develops. Is the O-ring not sufficiently supported, a fast seal failure must be expected.

Ill. 23.4.1-5 ( Lit. 23.4.1-12): The development of blisters in innner cracks, is traced back at a diffusion of the medium to be sealed. This diffusion proceeds to flaws like tiny pores. These (about 3 Vol %) must be expected in elastomers. The there accumulated medium leads to pressure rise and to tensile stresses vertical to the surface (not net modal flow). Then the crack development takes place parallel to the surface. A widening of the cracks forms a blister at the surface.
Susceptible for blistering are especially soft duromers (low shear modulus respectively low hardness). The propensity to blistering can be markedly reduced in a tight temperature range with a filler.

Ill. 23.4.1-6 Ill. 23.4.1-6 (Lit. 23.4.1-5): During shut down of an aeroengine, also the oil flow and the cooling air come to an stand still. With this, the heat from the hot parts can advance to the seals (heat soaking). Are the temperaturees too high for the used seal elastomer, it comes to a permanent deformation and/or deteriorations like embrittlement and crack formation (Ill. 23.4.1-4). At pumps for shaft seals, usually a minimum leakage of about 1 drop per minute (correlates 3 ml/h) is tolerated. Also this will be prevented if possible, because painting/lacquerung and coatings can be damaged (especially annoying because visible from the outside!). However thereby it should be considered, that an aging of the O ring leads fast to an unacceptable leakage of several drops per minute.

Ill. 23.4.1-7 (Lit. 23.4.1-18): O-rings are from a further mechanism mechanical loaded, besides the load, due to the pressure difference in the medium.
Usually an O-ring is embetted in a circumferential groove. It is elastically compressed by the sealing face and contacts this as well as at the groove face („A”).
Does it come during service to a heating, the elastomer expands markedly more (about a magnitude) than the metallic components. This volume expansion leads to a rise of the compression of the ring („B1“). Under this compression the ring forms plastically (creep). Does now the region of the seal cool down, for example duriing shut down of the engine, the ring volume shrinks correspondingly. Is the plastic deformation so large, that the elastic spring back of the ring material is no more sufficient to shut the gap, a leak will occur („C1”).
This process can also lead to a further failure mode, shear. In this case the seal material is extruded through the sealing gap into the groove edge at the low pressure side („B2“). This is promoted from an aging caused loss of strength. Thereby the ring can at least locally sheared off and a leak form („C2”).

Ill. 23.4.1-8 (Lit. 23.4.1-14): Failure relevant O-rings usually are positioned inside badly or not visible cavities. During the disassembly, the danger of a damage or a change of failure explaining position features exist (Ill. 23.4.1-9). This suggests wrong conclusions. Therefore it is of high importance, to identify and document the unchanged position of the O-ring in the as assembled condition. Against expectation, the practice showed, that this is possible with X-ray, at least in casings from light metals. The O ring material (possibly depending from the filler) weakens the suitable adjusted X-ray sufficient, to produce enough contrast in the pictures.

Note: Not before the X-ray documentation the disassembly of this component should begin.

Ill. 23.4.1-9 Ill. 23.4.1-9 (Lit. 23.4.1-14): During former flights, the crew noted fuel/oil smoke in the cockpit during flight. This lead about 1 week before the flight accident to the exchange of the Hydromechanical Metering Unit (= HMU) at the side of the aeroengine. The work was carried out from the operator. The dismouting took place as specified by a mechanic. The work was controlled and signed by an inspector. After this, an other mechanic mounted the change component. To this work belonged the installing of the cable to the bypass switch at the cover. To the HMU belongs the fuel filter. Also this was signed by an inspector. After this followed a test run of the aeroengine with a fuel leak clontrol. Two inspectors monitored the run and signed it. A mechanic cleared the aeroengine for flight. Obviously, at this time no noticeable fuel leak has occurred.

The HMU of the suspect right accident aeroengine was dismounted for a test run. Already at below 10% of the operation rotation speed of the fuel pump an extreme leakage occurred at the casing of the fuel filter. After this the test was cancelled.
Already during the first inspection of the aeroengine on the wreckage, an almost 0,3 mm wide gap between the filter cover and the contact face of the casing was noticed (sketch left). After about one day, fuel leaked further out of the gap.

Before the removing of the filter cover, the filter was X-rayed. This was carried out although it was not certain if the O ring material weakens sufficiently the X-rays in the metallic casing for an analysis. However the O ring was perfectly visible in its position. It could be seen, that the O-ring was partly pressed out of the groove to the cover (sketch below right). This enabled the exit of the fuel at the aeration bores and the drainage bores as well as the gap at the cover. It can be supposed that the usual pressure fluctuations in the high pressure fuel system loosened the unsecured filter cover and caused the dislocation of the O-ring. At the opinion of the investigator the cover was already not mounted as specified at the producer supplied component before the assembly.

After X-ray the filter cover was unscrewed. Now the O ring was again in the designated groove (!). Obviously the O-ring had displaced back. Merely in the before displaced region, small traces of abrasion and a little cut could be seen. This shows the high valuue of the X-ray indication without which inportant findings would have lacked. The cover of the brushes from the starter generators of both aeroengines have not been fixed as specified in the manual. These are belts which are laid around the generator. The corresponding instructions could neither found in the work handouts of the operator, nor in the maintenance instructions of the airplane manufacturer. The gaps at the fasteners have been unfortunately so distorted, that they have been positioned over the casing openings to the brushes. So during a fuel leak, here an ignition could occurred.

Conclusion of the investigating authority (analogous): Cause for the flight accident was probably at the OEM the faulty installation of the cover from the fuel filter. Thereby it came to a gap between cover and supporting face. The operator personnel did not notice this obvious failure. So the large fuel leak was possible. The fuel could ignite at the incorrect bare generator brushes. The consequence was an explosion inside the aeroengine nacelle. This brought the fire extinguish device out of function. So the fire was possible.

Note: After long storage periods in the as assembled condition, O rings must be checked for aging damages like shrinking and flattening. More safe is a generally exchange.

Ill. 23.4.1-10 (Lit. 23.4.1-9): Three years after the already about thirty years old aircraft has been „mothballed“ three years and the rebuild to a fire extinguishing airplane, it came again into service. Before it underwent an extensive inspection. Thereby no alarming changes have been determined. Particularly during the examination of the fuel system about three months before the accident, no fuel leaks have been observed.

Anyway it was known, that it can come after a three years storage to „drying-out and shrinking of the O-rings. However the O-rings are `on condition' parts, these are not life time limited. In this case an exchange applies for flattened and shrinked O rings (Ill. 23.4.1-4). The producer of the aeroengine indicated, that a used O ring must no more incorporated again. Already at other airplanes of the same type, during military operation fuel leaks occurred as result of O-ring failures. However, a fire in connection with a leak was not observed. Anyway there was a warning of the military operator in case, that the fuel leak develops near an aeroengine.The hot part of the primary failed aeroengine is positioned exactly below the nacelle region (dry bay). In it, leakage fuel can be gathered and flow on the hot aeroengine. The used fuel (Jet-A1) has a relatively low ignition temperature (flash point, ignition of the fuel vapour with a flame, Ill. 22.2.1-2) of about 40 °C.

These failures have been traced back to bending or thermal expansions of fuel pipe lines. They occurred more frequently at screw connections (couplings) as at a fuel valve.
The leaks had different intensity. Over a longer period of time `bleedings' or drops developed. The end phase is a sudden failing of the seal with a spray cone.

Ill. 23.4.1-11 (Lit. 23.4.1-9): Through thermal expansion differences between pipe lines and its screw connections, markedly movements can occur. Thereby bending and/or axial displacement is possible. Are so the sealing O rings deformed in the screw connections/couplings and/or underlie wear leaks can develop. In such application specific areas especially at the O-ring condition must be payed attention.

Ill. 23.4.1-12 (Lit. 23.4.1-2 and Lit. 23.4.1-4): O-rings can be produced from many different elastomers (cross column). These materials can get specifically deteriorated by certain media (vertical column). Thereby it must be considered, that besides the material to be sealed, like aeroengine oil, hydraulic fluid or fuel further come into consideration. Naturally the corrosion protection oil must not be critical for a long time storage. During maintenance and overhaul rather short time contace must be expected. Here media like rust remover, degreasing, solvents and lubricants come into question. Also these must be used only correlating the specifications. In case of doubt the OEM should be contacted.

Ill. 23.4.1-13 (Lit. 23.4.1-2): The identification of the O ring material, at least a assignment to larger `families' can be of interest.
For this, the shown sinple device is suitable. It consists of a tube in which a weight drops at the specimen from a specified height. The height of the rebound will be assigned the calibrated range.

The following gradiation with decreasing rebound height is material typic:

  • EP (Ethylene propylene),
  • Nitrile (Buna-N),
  • Perfluor rubber/elastomer (similar PTFE),
  • Fluor rubber, Fluor carbon rubber (Viton ®).

References

23.4.1-1 Fa. Parker Hannifin GmbH, Katalog 3353 D/E, „Dichtungshandbuch/Sealing Handbook”, März 1999, page 1-126.

23.4.1-2 „O-Ring Failure Analysis”, www.allorings.com, page 1-5.

23.4.1-3 „O-Ring Fluid Compatibility Guide“, www.allorings.com, page 1-5.

23.4.1-4 Fa. Ralicks Industrie- und Umwelttechnik, „ Werkstoff-Übersicht und Eigenschaften”, www.ralicks.de, page 1-4.

23.4.1-5 B.Wiebusch, , „ Aerospace seal prevents leakage“, Zeitschrift „ Design News”, June 18, 2001, www.designnews.com, page 1 and 2.

23.4.1-6 U.S. Air Force, Safety Agency, „ Maintenance matters - military airplanes mishaps“, www.findarticles.com, 2002, page 1 and 2.

23.4.1-7 Transportation Safety Board of Canada (TSB), Aviation Investigation Report Number A00P0210, „Loss of power and Collision with Water”, West Coast Air, de Havilland DHC-6 (Twin Otter) C-GGAW, Vancouver Harbour, British Columbia, 1 November 2000, page 1-8.

23.4.1-8 B.J.Crotty, „Unplugged, a maintenance error caused oil to be lost from all four engines…“ Zeitschrift „Flight Safety Australia”, Nov.-Dec. 1999, page 43-44.

23.4.1-9 NTSB Report, Identification LAX94FAer3, „Accident Aug-13-94 at Pearblossom, CA, Aircraft: Lockheed C-130A, registration: N135FF“, Update May 31st 2000, page 1-6.

23.4.1-10 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.

23.4.1-11 „ Why Seals Fail“, www.maintenanceresources.com, TWI Press, Inc. 2000, page 1-8.

23.4.1-12 D.L.Hertz, Jr, „ The Hidden Cause of Seal Failure”, Zeitschrift „Machine Design“, April 9, 1981, page 209-212.

23.4.1-13 Fa. Eagle Engineering Aerospace, „ Aero Engine Applications”, 2000, page 1.

23.4.1-14 National Transportation Safety Board (NTSB), Aircraft Accident Report NTSB/AAR-89/02, „De Havilland DHC-8, Accident April 15, 1988“, page 1-66.

23.4.1-15 S.Helduser, Versuch 2, „Übertragungseigenschaften des ventilgesteuerten Zylinderantriebs”, Antriebstechnik, Aktorik, Arbeitsblätter zur Vorlesung, 2005, sheet 1-6.

23.4.1-16 National Transportation Safety Board (NTSB), Aircraft Accident Report NTSB/AAR-84/04 (PB84-910404), „Eastern Air Lines, Inc., Lockheed L-1011, N334EA Miami, Florida, May 5, 1983“, page 1-46.

23.4.1-17 Flugunfallkommission (Büro Wien), GZ. 84.341/2-FUK/99, Gutachten und Vorschläge vom 29. März 1999 zum „Flugunfall mit Hubschrauber Typ Hughes 500 D, am 21.April 1992”, page 1-31.

23.4.1-18 R.C.Beercheck, „Putting the Heat on Seal Materials“, Zeitschrift „Machine Design”, 25.10.1979, page 124-127.

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