Flashcards in 11.4 Failure of pressure vessels Deck (21)
the most common causes of failure of pressure systems, especially boilers
Over temperature Over pressure
Outline FOUR examples of the mechanisms of mechanical failure in pressure systems. 8 marks
(b) Examples of the mechanisms of mechanical failure in pressure systems include: excessive external stress overheating ductile failure mechanical fatigue thermal fatigue brittle fracture creep hydrogen embrittlement at welding repairs corrosion with internal fluids water/steam hammer and caustic embrittlement. Note: an outline of the 4 chosen mechanisms should also be given.
Excessive stress is stress beyond the capability of the vessel or system. This could arise because of
poor design, because of stresses arising during manufacture of components or assembly of the system, the application of other external stresses (see below), or weakening of the system due to ‘wear and tear’
Abnormal external loading may occur from
vehicular impacts, explosions from other vessels, or ladders being supported by pipework.
Mechanical fatigue is
When the positive pressure within a pressure system causes tensile stresses in all directions. If pressure levels vary within a system the stresses also fluctuate. These cyclical fluctuations can give rise to fatigue failures.
Mechanical shock is
a sudden acceleration or deceleration of an object, typically induced by an impact. Mechanical shocks can cause brittle materials to fracture and ductile materials to stretch or bend.
Thermally induced stress is an obvious concern in steam boilers and hot water heating systems. Fatigue failures are caused by
thermally induced stress cycling, which in a boiler occur during every firing cycle of the burner (burner on / burner off).
Several conditions can contribute to boiler stress and eventual cracking. All involve
introducing excessively low temperature water, or cool water at high flow rates into a hot boiler.
Thermal shock occurs when
a thermal gradient causes different parts of an object to expand by different amounts. Rapid and uneven expansion and contraction of a boiler’s structure can result in catastrophic failure.
Brittle materials tend to fracture suddenly without any signs of plastic deformation. Brittle fractures are more likely:
if materials are under high tensile stress at low temperatures as a consequence of an impact. Thick steel plates and welded joints are vulnerable to brittle failure.
Creep (a progressive plastic deformation) is observed in all material types. It becomes an issue with metals at temperatures
>40% of their melting point in Kelvin (0.4TmoK)
The stress (force applied) giving rise to the deformation may be due to
gravity, centrifugal force or positive pressure in a pressure system.
Since around 1960, hydrogen attack, or embrittlement, has been encountered with increasing frequency in high-pressure, high-purity boiler systems. It is not encountered in the average industrial plant. It occurs only when
a hard, dense scale is present on the tube surfaces, permitting hydrogen to concentrate under the deposit and permeate the metal
Vessel corrosion can result in a range of conditions which may result in catastrophic failure of the system and an explosion. These are: 4
Wastage - loss of metal thickness and strength. Grooving - mechanical corrosion, due to expansion and contraction which is accelerated by a build-up of solids. Distortion - a situation where excessive scale is allowed to build up on surfaces. Sooting - gives rise to sulphuric acid when wet.
Small, simple systems may need little more than the establishment of the maximum pressure for safe operation. Complex, larger systems are likely to need a wide range of conditions specified such as: 5
Maximum and minimum temperatures. Maximum and minimum pressures. Nature, volumes and flow rates of contents. Operating times. Heat input or coolant flow.
Before a pressure system is operated the user/owner must ensure that
a written scheme of examination has been prepared and drawn up by a competent person
The manufacturer/supplier’s maintenance instructions should form the basis of the maintenance programme. The type and frequency of maintenance tasks (inspections, replacement of parts, etc.) should be decided for all those parts which, through failure or malfunction, would affect the safe operation of the system. A suitable programme should take account of: 9
the age of the system the operating/process conditions the working environment the manufacturer’s/supplier’s instructions any previous maintenance history reports of examinations carried out under the written scheme of examination by the competent person the results of other relevant inspections (for example: for maintenance or operational purposes) repairs or modifications to the system the risks to health and safety from failure or deterioration.
Records retained should assist the competent person in the examination under the written scheme, the purpose being to assess whether the system is safe for continued use, and/or if any planned repairs or modifications can be carried out safely. The user/owner should keep the following documents readily available: 5
Any designer’s/manufacturer’s/supplier’s documents relating to parts of the system included in the written scheme. Any documents required to be kept by the Pressure Equipment Regulations 1999. The most recent examination report produced by the competent person, under the written scheme of examination. Any agreement or notification relating to postponement of the most recent examination under the written scheme. All other reports which contain information relevant to the assessment of matters of safety.
Outline the technical AND procedural measures to minimise the likelihood of failures in pressure systems. 8 marks
(c) Technical and procedural measures that should be taken to minimise the likelihood of failures in pressure systems include: a correct design specification ensuring the system was fit for purpose fitting specific safety features such as pressure relief valves and level sensors ensuring quality control during the manufacture the introduction of inspection and maintenance procedures including statutory examination with the scheme of examination being prepared by a competent person the role of non-destructive testing ensuring the system operates within its design parameters and, in the case of boilers, the filtering and treatment of water.
Pressure Systems Safety Regulations 2000 (PSSR) List of Regulations
Regulation 2: Interpretation Regulation 3: Application and duties Regulation 4: Design and construction Regulation 5: Provision of information and marking Regulation 6: Installation Regulation 7: Safe operating limits Regulation 8: Written scheme of examination Regulation 9: Examination in accordance with the written scheme Regulation 10: Action in case of imminent danger Regulation 11: Operation Regulation 12: Maintenance Regulation 13: Modification and repair Regulation 14: Keeping of records, etc. Regulation 15: Precautions to prevent pressurisation of certain vessels Regulation 16: Defence (act or default of another person / due diligence)