c11_pressure_system_hazards_and_controls_COMPLETE

Question 1

Saturation temperature is…

Answer 1

When the number of molecules leaving the liquid surface is more than those re-entering, the water freely evaporates. At this point it has reached boiling point, or its saturation temperature, as it is saturated with heat energy.

Question 2

Enthalpy is…

Answer 2

the amount of heat content used or released in a system at constant pressure.

Question 3

A steam explosion is…

Answer 3

a violent boiling or flashing of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it, or heated by interaction with molten metals.

Question 4

Condensation induced water hammer occurs when…

Answer 4

large quantities of condensate lying inside a pipe are pushed along the pipe at steam velocities. This can result in damage when the water impacts with an obstruction in the pipe, for example a control valve.

Question 5

Mounded pressurised torpedoes offer a safer option and eliminate the possibility of

Answer 5

BLEVE

Question 6

Refrigeration systems are designed around two key principles:

Answer 6

(1) Liquids absorb heat when changed from liquid to gas. (2) Gases give off heat when changed from gas to liquid.

Question 7

A new, self-contained air compressor is to be installed in a workshop. Identify THREE protective devices that may be necessary to control the risk of over-pressurisation AND for EACH device outline its purpose. 6 marks

Answer 7

(a) Protective devices include:  a pressure gauge to identify the receiver pressure  a safety valve which would relieve excess pressure when the maximum safe working pressure of the receiver is attained  a pressure cut-out or unloading device which cuts off the compressor when the working pressure is reached  a fusible metal plug which prevents over-heating and in turn over pressurisation if external heat sources are applied by melting and thus again relieves the pressure.

Question 8

PER divides pressure equipment into four major categories on the basis of two fundamental hazards:

Answer 8

(1) The degree of danger from failure of the equipment (expressed by the product of the pressure of the fluid and the volume of the equipment or DN value for piping products). (2) The degree of danger from the release of the fluid contained (based on classifications adopted in the directives on dangerous substances).

Question 9

As per the Simple Pressure Vessels (Safety) Regulations 1991 (SPV), simple pressure vessels have the following characteristics or limitations: 5

Answer 9

 Intended to contain air or nitrogen at a gauge pressure >0.5 bar, but less than or equal to 30 bar.  Not intended to be exposed to flame (i.e. ‘unfired’).  Manufactured in series, i.e. more than one vessel of the same type is manufactured during a given period by the same continuous manufacturing processes, in accordance with a common design of welded non-alloy steel, or non-alloy aluminium construction, or non-age hardening aluminium alloy.  A maximum working pressure (PS) of not more than 30 bar, and a PS.V (the product of PS and the vessel’s capacity expressed in litres) of not more than 10,000 bar litres.  A minimum working temperature of not lower than -50°C, and a maximum working temperature of not >300°C for steel vessels, and not >100°C for aluminium or aluminium alloy vessels.

Question 10

Schedule 1 of SPV details the essential safety requirements that qualifying vessels must satisfy. It also gives details of how the vessels should be categorised, the technical requirements to be satisfied, and the conformity assessment procedures to be followed. The ESR’s are presented in four parts:

Answer 10

Part 1: Materials Part 2: Vessel design Part 3 Manufacturing Process Part 4: Definitions and symbols

Question 11

The air receiver should either be constructed to withstand the compressor’s maximum pressure safely, or

Answer 11

be fitted with a safety valve and a pressure-reducing valve to prevent the receiver’s safe working pressure from being exceeded.

Question 12

Since air receivers are typically constructed out of carbon steel, they are subject to internal corrosion from water, which has condensed from the compressed air. Consequently air receivers are fitted with

Answer 12

automatic condensate drains or a manually operated drain valve to eliminate accumulated moisture.

Question 13

With reference to the Pressure Systems Safety Regulations 2000, explain what is meant by a ‘pressure system’. 4 marks

Answer 13

(a) A ‘pressure system’ is a system comprising one or more pressure vessels of rigid construction, any associated pipe work and protective devices; the pipe work with its protective devices to which a transportable pressure receptacle is, or is intended to be connected; or a pipeline and its protective devices which contains or is liable to contain a relevant fluid, but does not include a transportable pressure receptacle. Examples of a ‘relevant fluid’ include: steam; a liquid which has a pressure 0.5 bar above atmospheric at the temperature of the liquid or at 17.5º C; a fluid or mixture of fluids which is at a pressure greater than 0.5 bar above atmospheric pressure when in equilibrium with its vapour at either the actual temperature of the liquid or at 17.5 degrees Celsius; or a gas dissolved under pressure in solvent contained in a porous substance at ambient temperature and which could be released from the solvent without the application of heat.

Question 14

The Pressure Equipment Regulations 1999 (PER) apply to

Answer 14

the design, manufacture and conformity assessment of pressure equipment, and assemblies of pressure equipment with a maximum allowable pressure greater than 0.5 bar.

Question 15

The PER do not apply to excluded pressure equipment and assemblies. Excluded equipment and assemblies are listed at Schedule 1 of the Regulations (Annex A of the Directive). The list is extensive and includes: 4

Answer 15

 Simple pressure vessels  Aerosol dispensers  Blast furnaces  Radiators and pipes in warm water heating systems.

Question 16

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

Answer 16

Over temperature Over pressure

Question 17

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

Answer 17

(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 18

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

Answer 18

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 19

Abnormal external loading may occur from

Answer 19

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

Question 20

Mechanical fatigue is

Answer 20

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 21

Mechanical shock is

Answer 21

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 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

Thermal shock occurs when

Answer 24

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 25

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

Answer 25

 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 26

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

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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 28

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

Question 29

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 29

 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 30

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 30

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

Question 31

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

Answer 31

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

Question 32

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 32

 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 33

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 33

 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 34

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

Answer 34

(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 35

Pressure Systems Safety Regulations 2000 (PSSR) List of Regulations

Answer 35

 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 36

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 36

(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.