11.4 Failure of pressure vessels

Question 1

the most common causes of failure of pressure systems, especially boilers

Answer 1

Over temperature Over pressure

Question 2

Outline FOUR examples of the mechanisms of mechanical failure in pressure systems. 8 marks

Answer 2

(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.

Question 3

Excessive stress is stress beyond the capability of the vessel or system. This could arise because of

Answer 3

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’

Question 4

Abnormal external loading may occur from

Answer 4

vehicular impacts, explosions from other vessels, or ladders being supported by pipework.

Question 5

Mechanical fatigue is

Answer 5

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.

Question 6

Mechanical shock is

Answer 6

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.

Question 7

Thermally induced stress is an obvious concern in steam boilers and hot water heating systems. Fatigue failures are caused by

Answer 7

thermally induced stress cycling, which in a boiler occur during every firing cycle of the burner (burner on / burner off).

Question 8

Several conditions can contribute to boiler stress and eventual cracking. All involve

Answer 8

introducing excessively low temperature water, or cool water at high flow rates into a hot boiler.

Question 9

Thermal shock occurs when

Answer 9

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.

Question 10

Brittle materials tend to fracture suddenly without any signs of plastic deformation. Brittle fractures are more likely:

Answer 10

 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.

Question 11

Creep (a progressive plastic deformation) is observed in all material types. It becomes an issue with metals at temperatures

Answer 11

> 40% of their melting point in Kelvin (0.4TmoK)

Question 12

The stress (force applied) giving rise to the deformation may be due to

Answer 12

gravity, centrifugal force or positive pressure in a pressure system.

Question 13

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

Answer 13

a hard, dense scale is present on the tube surfaces, permitting hydrogen to concentrate under the deposit and permeate the metal

Question 14

Vessel corrosion can result in a range of conditions which may result in catastrophic failure of the system and an explosion. These are: 4

Answer 14

 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.

Question 15

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

Answer 15

 Maximum and minimum temperatures.  Maximum and minimum pressures.  Nature, volumes and flow rates of contents.  Operating times.  Heat input or coolant flow.

Question 16

Before a pressure system is operated the user/owner must ensure that

Answer 16

a written scheme of examination has been prepared and drawn up by a competent person

Question 17

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

Answer 17

 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.

Question 18

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

Answer 18

 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.

Question 19

Outline the technical AND procedural measures to minimise the likelihood of failures in pressure systems. 8 marks

Answer 19

(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.

Question 20

Pressure Systems Safety Regulations 2000 (PSSR) List of Regulations

Answer 20

 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)

Question 21

A new, self-contained air compressor is to be installed in a workshop. Identify the information that must be displayed on the air receiver in order to comply with EU requirements for pressure vessels. 4 marks

Answer 21

(b) Information that must be displayed includes:  the CE marking with the last two digits of the year in which it was affixed  the maximum and minimum safe working pressure and temperature (in ºC)  the capacity of the vessel in litres  the name or mark of the manufacturer  the type and serial or batch identification  a reference to the relevant EN standard. The information must be displayed in easily legible and indelible form either on the vessel itself or on a data plate that cannot be removed.