NEBOSH International Diploma Dip loma in Occupational Health & Safety Please be advised that the course material is regularly reviewed and updated on the eLearning platform. SHEilds would like to inform students downloading these printable notes and using these from which to study that we cannot ensure the accuracy subsequent to the date of printing. It is therefore important to access the eLearning environment regularly to ensure we can track your progress and to ensure you have the most up to date materials. Version 2.0 (21/08/2014)
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On completion of this element, candidates should be able to demonstrate understanding of the content through the application of knowledge to familiar and unfamiliar situations and the critical analysis and evaluation of information presented in both quantitative and qualitative forms.
1. Outline the principles of operation of liquefied gas storage, refrigeration systems and heating systems. 2. Outline key features and safety requirements for 'simple' unfired pressure syste ms. 3. Outline the key features and safety requirements for process pressure systems. 4. Outline the likely causes of the failure of pressure systems, and the testing and prevention strategies that can be used. . o
Safety of pressure systems, L122 HSE Books.
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Written schemes of examination, INDG178, HSE. http://www.hse.gov.uk/pubns/indg178.pdf.
. Steam is the utility most observed by people passing by refineries and chemical plants. White clouds of escaping steam are seen all over a plant because steam can be used for so many things. Steam is used to purge air from vessels and lines prior to startup of a unit however visible plume of steam at the purge vent is not a reliable sign that a vessel has been thoroughly purged of air. After vessels and lines that operate at atmospheric pressure or above have been purged with steam prior to startup, fuel gas or any other suitable gas must be pumped into the vessel. This is done to displace th e steam, which if left inside, will condense and form a vacuum. If the vessel is left full of steam with valves closed, condensation can produce a vacuum great enough to collapse the vessel. Since valves frequently do not close tightly, the vacuum caused by condensing steam may draw in a ir. This creates a flammability hazard when hydrocarbons are introduced into the equipment. . A steam explosion is a violent boiling of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it or th e interaction of molten metals. The water c hanges from a liquid to a gas with extreme speed, increasing dramatically in volume. A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions, cr eating a danger of scalding and burning. However if the rapid expansion is constrained by some form of containment there may be such a pressure rise caused by the steam that the containment then over-pressurises over -pressurises and fails violently. The following case study outlines an example of a steam explosion.
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At the premises of Corus UK Ltd, Port Talbot, No. 5 Blast Furnace exploded at approximately 17.13 pm on 8 November 2001. The entire furnace, which w ith its contents weighed approximately 5000 tonnes, lifted bodily at the lap joint, rising some 0.75 m from it s supporting structures, leading to the explosive release of hot materials (an estimated 200 tonnes in total, comprising largely solids and semi solids, with a little molten metal) and gases into the cast house. Three e mployees died: Andrew Hutin, Stephen Galsworthy and Len Radford. A further 12 employees and contractors sustained severe injuries. Many more suffered minor injuries and shock. The outcome of the explosion was unprecedented in the steel-making industry, but was the result of many failings in safety management by the company over an extended period. The explosion occurred after a prolonged attempt - over two days - to recover the furnace from a chilled-hearth situation caused by cooling water ingress. The immediate cause was the mixing of wa ter and hot materials within the lo wer part of the furnace; the precise mechanism remains a matter th at is not fully resolved. The event attracted considerable public attention locally, nationally, and internationally within the w ider steel-making industry. The company was subsequently prosecuted under sections 2(1) and 3(1) of the Health and Safety at Work etc. Act 1974 and was fined £1.33 million in the Crown Court, with £1.74 million costs also being awarded. The immediate cause of the explosion was water and hot molten materials mixing within the lower part of the furnace vessel. The water had entered the furnace from its cooling system following a chain of events initiated by the failure of safety-critical water cooling systems. At the time of th e explosion, attempts were continuing to rectify the abnormal operating conditions that this had created and to recover the furnace. The precursors to the explosion were a combination of significant failures in health and safety management extending over many years. These failures were not confined solely to the blast furnace plant; they extended elsewhere within the company, and in particular to the Energy Department which supplied essential cooling water for the furnace. A failure to carry out suitable and sufficient risk assessments for blast furnace operations resulted in the failure to implement robust technical and procedural controls. There was insufficient redundancy and security of cooling water supplies, and overall cooling system reliability showed a downward and deteriorating trend over several months.
If Liquid Petroleum Gas (LPG) is stored and used in installations compliant with relevant health and safety legislation and industry codes of practice it is a safe fuel. LPG (propane or butane) is a colour-less liquid, which readily evaporates into a gas. It has no smell, although it will normally have an odour added to help detect leaks. When mixed with air, the gas can burn or explode when it meets a source of ignition. It is heavier than air, SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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so it tends to sink towards the ground. LPG can flow for long distances along the ground and can collect in drains, gullies and cellars. LPG is stored in pressurised tanks to keep it liquefied. The tanks can be installed above or below ground. They are strong and not easily damaged, but liquid or gas leaks can occur from valves and pipe connections. The liquid can cause cold burns to the skin.
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Obtained during the processing of crude oil, or direct from the North Sea.
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Colourless and odourless. An odourising agent is added before distribution to give its characteristic smell.
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Half as heavy as water when in liquid form. It will float on water before vapourising.
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Easily liquefied by pressure, taking up only around 1/250th of its gaseous volume.
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Predominantly an indoor fuel. Predominantly an outdoor fuel.
Propane has a lower boiling point and hence a higher storage pressure.
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A vessel that has contained LPG is nominally empty but may still contain LPG vapour and be potentially dangerous. Therefore treat all LPG vessels as if they w ere full.
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At very high concentrations when mixed with air, vapour is an anaesthetic and subsequently an asphyxiant by diluting the available oxygen.
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LPG forms a flammable mixture with air in concentrations of between 2% and 10%. It can, therefore, be a fire and explosion hazard if stored or used incorrectly.
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LPG in liquid form can cause severe cold burns to the skin owing to its rapid vapourisation.
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LPG is approximately twice as heavy as air when in gas form and will tend to sink to the lowest possible level and may accumulate in cellars, pits, drains etc.
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Vapour / air mixtures arising from leakages may be ignited some distance from the point of escape and the flame can travel back to the source of the leak.
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Vapourisation can cool equipment so that it may be cold enough to cause cold burns.
Is the Following Statement True or False. LPG (propane or butane) is a colour-less liquid, which readily evaporates into a gas.
Nine people died and 33 were injured after a catastrophic explosion at ICL Plastics, Glasgow in 2004. The SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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explosion was caused by corroded pipework that led to a build up of liquefied petroleum gas (LPG) in the building's basement and caused the four-storey factory to collapse. A propane gas tank and pipework were installed at the factory in or around 1969.Part of the pipework came vertically out of the ground before entering the building, but five years later, around 1974, the yard outside the building was raised, burying this exposed section of pipework. The court heard that the buried pipework deteriorated and corroded, as did a bend joining two sections of the pipework, leading to an escape of gas, which accumulated in the basement area then exploded, causing the building to collapse. Two hundred firefighters searched through the rubble to find survivors and rescue teams, who normally search for earthquake survivors around the world, were brought in. Sev en people were rescued alive from the rubble.
LPG's and similar flammable gases can be stored in either pressurised spheres or Torpedo's. Theoretically pressure vessels may be constructed in any shape, but most commonly are spheres, cylinders and torpedoes. To understand why this is so, consider any type of Hot Water tank or Boiler. As pressure inside a square or rectangular shape is increased it will try and reform itself into a rounded shape, just like a Balloon does. This would result in failure at any corner, so the design solution is to have no corners. A sphere is a very strong structure. The e ven distribution of stresses on the sphere's surfaces, both internally and externally, generally means that there are no weak points. Large volumes of Liquid Natural Gas will often be stored in spheres. Spheres are used to store low temperature liquids and have the advantage of offering a reduced exterior surface for a given storage volume compared to all other possible shapes. However they are expensive to build and cannot be used near or at cryogenic temper atures (-50° C is the lowest temperature). Bulk LPG is often stored in Torpedoes, which are cylindrical shapes with en d caps. They are cheap to build and suitable for low temperature storage, but are not as structurally strong and can fail under pressure causing the welds of the end caps to fail and thus ejecting them under pressure. It' s for this reason that the torpedo should never be sited with the e nd caps facing any occupied buildings. Theoretically, a sphere would be the best shape for a pressure vessel. However a spherical shape is tough to manufacture, therefore more expensive, so most pressure vessels are cylindrical with 2:1 semi -elliptical heads or end caps on each end making them Torpedoes.
Please Select the Missing Word to Complete the Following Sentence. Bulk LPG is often stored in ________ which are cylindrical shapes with end caps.
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ICL Plastics, Glasgow http://www.theiclinquiry.org/ Explosion of No. 5 Blast Furnace, Corus UK Ltd, Port Talbot http://www.hse.gov.uk/pubns/web34.pdf
. The vapour-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. The diagram below provides a schematic diagram of the components of a typical vapour-compression refrigeration system.
Figure 1. Single Stage Vapour Compression Refrigeration. The vapour-compression cycle uses a circulating liquid refrigerant as the medium that absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere.
1. Compressor. 2. Condenser. 3. Thermal expansion valve. 4. Evaporator. Circulating refrigerant enters the compressor in the ther modynamic state known as a saturated vapour and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapour is then in the ther modynamic state known as a superheated vapour and it is at a temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapour is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case). The condensed liquid refrigerant, in the thermodynamic state known a s a saturated liquid, is next routed SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the li quid refrigerant. The auto-refrigeration e ffect of the adiabatic flash evaporation lowers the temperature of the liquid and vapour refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapour mixture. That warm air evaporates the liquid part of the cold r efrigerant mixture. At the same time, t he circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat that is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser. To complete the refrigeration cycle, the refrigerant vapour from the evaporator is again a saturated vapour and is routed back into the compressor.
There are four processes in the Rankine cycle, each changing the state of the working fluid. These are identified by numbers in the diagram below.
Figure 1. Steam Powered Turbine. o
The working fluid is pumped from low to high pressure. As the fluid is a liquid at t his stage the pump requires little input energy.
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The high pressure liquid enters a boiler where it is heated at constant pressure by an
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external heat source to become a dry saturated vapour. The dry saturated vapour expands through a turbine generating power. This decreases
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the temperature and pressure of the vapour and some condensation may occur. The wet vapour then enters a condenser where it is condensed at a constant pressure
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and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change.
A "pressure system" is defined by the Pressure Systems Safety Regulations 2000 as a system comprising one or more pressure vessels of rigid construction, any associated pipework and protective devices. It is a closed container designed to hold gases or liquids at a pressure substantially different from th e atmospheric pressure. . o
A system comprising a pressure vessel, its associated pipe work and protective devices. It is necessary for there to be a pressure vessel in the system for the Regulations to apply under this definition.
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Pipe work with its protective devices to which a transportable pressure receptacle is, or intended to be, connected. Pipe work containing any fluid or mixture of fluids that is at a greater pressure than 0.5 bar above atmospheric and which is a gas or a liquid which would have a vapour pressure greater than 0.5 bar above atmospheric.
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A pipeline with its protective devices.
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The Simple Pressure Vessels (Safety) Regulations 1991 entered into force on 31 December 1991.
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Simple unfired pressure vessels designed to contain air or nitrogen - Part 1: Design, manufacture and testing.
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Simple unfired pressure vessels designed to contain air or nitrogen - Part 2: Pressure vessels designed to contain compressed air for air braking and auxiliary systems for motor vehicles and their trailers.
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Simple unfired pressure vessels designed to contain air or nitrogen - Pa rt 3: Steel pressure vessels designed for air- braking equipment and auxiliary pneumatic equipment for railway rolling stock.
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Simple unfired pressure vessels designed to contain air or nitrogen - Part 4:
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Aluminium alloy pressure vessels designed for air-braking equipment and auxiliary pneumatic equipment for railway rolling.
The components and assemblies contributing to the strength of the vessel under pressure are made either of non-alloy quality steel, or of non-alloy aluminium, or of non-age hardening aluminium alloy. The vessel consists either: Of a cylindrical component with circular cross-section, closed at each end, each end being either outwardly dished or flat and being also co-axial with the cylindrical component, or Of two co-axial outwardly dished ends. The maximum working pressure (PS) is not more than 30 bar, and PS.V (being the product of PS and the vessel's capacity expressed in litres) is not more than 10,000 bar. litres. The minimum working temperature is not lower than minus 50° C, and the maximum working temperature is not higher than: o
300° C in the case of steel vessels.
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100° C in the case of aluminium or aluminium alloy vessels and, means the maximum gauge pressure which may be exerted under normal
conditions of use. means the lowest stabilised temperature in the wall of the vessel under normal conditions of use. means the highest stabilised temperature in the wall of the vessel under normal conditions of use. Therefore a 'vessel' is defined as a housing designed and built to contain fluids under pressure. An 'unfired pressure vessel' is a pressure vessel that is not in direct contact with a heating flame. . Means the document by which the producer of the materials certifies that the materials delivered to the manufacturer meet the requirements set by the manufacturer, an d in which the producer sets out the results of the routine inspection tests carried out during the production of those materials (or of materials produced by the same process but not being the materials delivered to the manufacturer) in particular as to their chemical composition and mechanical properties Means the highest stabilised temperature in the wall of the vessel under normal conditions of use Means the maximum gauge pressure which may be exerted under normal conditions of use means: o
The upper yield point for a material with both a lower and an upper yield point.
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The proof stress at 0.2%.
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o
Or the proof stress at 1.0% in the case of non-alloy aluminium.
If pressure equipment fails in use, it can seriously injure or kill people nearby and cause serious damage to property. Each year in Great Britain, there are about 150 dangerous occurrences involving such unintentional releases. Around six of these result in fatal or serious injuries.
Is the Following Statement True or False? 'Vessel' means a simple pressure vessel being a welded vessel intended t o contain air or nitrogen at a gauge pressure greater than 0.5 bar, not intended for exposure to flame.
Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2004 require anyone carrying dangerous goods by road or rail to protect the people involved in handling and carrying the goods, members of the emergency services and the pu blic, as well as both property and the environment, from the potential dangers of such activities. The Regulations also contain special provisions for transportable pressure receptacles, covering their design, manufacture, supply, modification, repair, approval and certification, marking, filling and record keeping.
1. Be of welded construction. 2. Be made of a material and design approved for the time being by the Secretary of State. 3. Have been subjected to a process of heat treatmen t approved, as at the time when the process was carried out, by the Secretary of State. 4. Be capable of withstanding, with a permanent stretch not greater than 10% of the total temporary expansion at test pressure, an internal hydraulic pressure of 51.72 bar (seven hundred and fifty pounds per square inch). The transportable pressure receptacle shall be fitted as completely as possible with a porous substance approved for the time being by the Secretary of State. There shall be no deleterious reaction between th e porous mass, the acetylene, the solvent and any parts of the transportable pressure receptacle in contact with the acetylene; but the quantity of acetone shall be such that, when the transportable pressure receptacle is fully charged with acetylene, the liquid content shall not completely fill the available space within the tran sportable pressure receptacle at an ambient temperature of 650° C. The pressure in the transportable pressure receptacle, when the latter has attained pressure and temperature equilibrium at an ambient temperature of 1 50° C, shall not exceed 17.65 bar (two hundred and fifty six pounds per square inch).
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The transportable pressure receptacle, anything fitted thereto and the porous substance contained therein shall be free from any defect calculated to render the transportable pressure receptacle unsafe. On the outside of the transportable pressure receptacle, as near as possible to the valve or valves, there shall be durably and legibly marked.
Article 100A Directives set out 'essential health and safety requirements' that must be satisfied before products may be sold in the European Economic Area. Products that comply with the Directives must be given free circulation within the European Economic Area. These Directives also apply to equipment made and put into service in-house. Suppliers must ensure that thei r products, when placed on the market, comply with the legal requirements implementing the Directives applicable to their product. It is a common feature of these Directives that compliance is claimed by the manufacturer affixing a mark, 'CE Marking', to the equipment.
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Design for adequate strength.
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Provisions to ensure safe handling and operation.
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Means of examination.
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Means of draining and ventilating.
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Corrosion or other chemical attack.
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Wear.
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Assemblies.
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Provisions for filling and discharge.
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Protection against exceeding the allowable limits of pressure equipment.
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Safety accessories.
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Be non-effervescent and be supplied after normalisation treatment, or in a n equivalent state.
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Have a content of carbon less than 0.25% and of sulphur and phosphorus less than 0.05%.
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Have the following mechanical property: maximum tensile strength must be l ess than 580 n m2.
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Non-alloy aluminium must have an aluminium content of at least 99.5% and non -age hardening aluminium alloys must display adequate resistance to inter-crystalline corrosion at the maximum working temperature.
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Be supplied in an annealed (hardened) state.
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Have a maximum tensile strength no more than 350 n m2.
A written scheme of examination is a document containing information about selected items of plant or equipment that form a pressure system, covers compressed or liquefied gas, including air, at a pressure greater than 0.5 bar above atmospheric pressure; pressurised hot water above 110 °C; and steam at any pressure.
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Identification of the items of plant or equipment within the system.
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Those parts of the system which are to be examined.
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Nature of the examination required, including the inspection and testing to be carried out on any protective devices.
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Preparatory work needed for the item to be examined safely.
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Where appropriate, the nature of any examination needed before the system is first used.
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Maximum interval between examinations.
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Critical parts of the system which, if modified or repaired, should be examined by a competent person before the system is used again.
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Name of the competent person certifying the written scheme of examination.
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Date of certification.
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A compressed air receiver and the associated pipework, where th e product of the pressure in bars multiplied by the internal capacity in litres of the r eceiver is equal to or greater than 250 bar litres.
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A steam sterilising autoclave and associated pipework and protective devices.
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A steam boiler and associated pipework and protective devices.
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A pressure cooker.
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A gas loaded hydraulic accumulator.
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A vapour compression refrigeration system where the installed power exceeds 25 kw.
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Narrow gauge steam locomotive.
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Components of self-contained breathing apparatus sets (excluding the gas container).
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A fixed LPG storage system, supplying fuel for heating in a workplace.
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An office hot water urn (for making tea).
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A machine tool hydraulic system.
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A pneumatic cylinder in a compressed air system.
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A hand-held tool.
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A combustion engine cooling system.
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A portable compressed air receiver and the associated pipework, where th e product of the pressure in bars multiplied by the internal capacity in litres of the receiver is less than 250 bar litres.
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Any pipeline and its protective devices in w hich the pressure does not exceed 2 bar above
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atmospheric pressure. o
A portable fire extinguisher with a working pressure below 25 bar at 60 °C and having a t otal mass not exceeding 23 kilograms.
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A portable LPG cylinder.
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A tyre used on a vehicle.
The frequency of inspections should be based on how quickly the work equipment or parts of it are likely to deteriorate and therefore give rise to a significant risk. This should take into account the type of equipment, how iCt is used and the conditions to which it is exposed.
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Safety record and previous history of the system.
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Any generic information available about the particular type of system.
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Current condition, e.g. due to corrosion / erosion, etc.
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Expected operating conditions.
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Quality of fluids used in the system.
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Standard of technical supervision, operation, maintenance and inspection in the user's / owner's organisation.
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Applicability of any on-stream monitoring.
Equipment may need to be checked frequently to ensure that safety-related features are funct ioning correctly. A fault that affects production is normally apparent within a short time; however, a fault in a safety-critical system could remain undetected unless appropriate safety checks are included in maintenance activities.
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Frequency and maximum working limits. For example marine, outdoors. Is the equipment performing the same task all the time or does this change? Risk to health and safety from malfunction or failure.
Under the Pressure Systems Safety Regulations 2000 the requirements for examination have become much less prescriptive in that statutory reporting forms no longer need be used and the phasing of the examinations can be related more closely to operating circumstances.
Which of the following pressurised systems are not likely to require a written scheme of examination ?
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Lifting Operations and Lifting Equipment
Lifting equipment.
6-12 or as scheme of examination.
Relevant pressure plant.
As scheme of examination.
Control of Substances Hazardous to
Local exhaust ventilation plant and dust /
1-14.
Health Regulations 2002.
fume extraction plant.
Electricity at Work Regulations 1989.
Electrical installations.
3-60 depending on the application.
Provision and Use of Work Equipment
Power press.
6-12 depending on guard type.
Regulations 1998.
Pressure Systems Safety Regulations 2000.
Regulations 1998.
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Observe operating pressure.
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Observe water level.
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Observe general conditions.
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Determine cause of unusual noises or conditions and correct.
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Test water column or gauge glass.
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Test water condition and perform corrections as necessary.
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Record in log.
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Test low-water fuel cut-off.
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Blow down boiler.
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Observe condition of flame.
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Check oil supply.
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Observe operation of condensate or vacuum pump.
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Check safety valve-lift lever.
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Test flame-detection devices.
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Test limit controls.
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Test operating controls.
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Sludge blow down (if required).
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Check condition of heating surfaces for oil pre-heaters.
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Check combustion in air supply to boiler room.
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Inspect internally and externally after cleaning.
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Open and inspect low-water fuel cut-off.
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Routine burner maintenance.
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Routine maintenance of condensate or vacuum return equipment.
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Routine maintenance of all combustion control equipment.
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Perform combustion and draft tests.
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Perform safety valve pop test.
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Perform evaporation test of low-water fuel cut-off.
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Inspect gas or oil piping for proper support and tightness.
. IC11.3.1 Relevant fluids.
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A system comprising one or more pressure vessels of rigid construction, any associated pipework and protective devices.
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Pipework with its protective devices to which a transportable pressure receptacle is, or is intended to be, connected.
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Pipeline and its protective devices that contain, or are liable to contain, a relevant fluid, but do not include a transportable pressure receptacle.
Steam, or Any fluid or mixture of fluids which is at a pressure greater than 0.5 bar above atmospheric pressure, and which fluid or mixture of fluids is: A gas. A liquid which would have a vapour pressure greater than 0.5 bar above atmospheric pressure when in equilibrium with its vapour at either the actual temperature of the liquid or 17.5 °C, or A gas dissolved under pressure in a solvent contained in a porous substance at ambient temperature and which could be released from the solvent without the application of heat.
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A "pressure system" is a system comprising one or more pressure vessels of rigid construction, any associated pipework and protective devices; the pipework 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.
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Autoclaves or equipment sterilisers used in cleaning laboratory glass ware.
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Boiler or steam heating systems providing general heating in workplaces.
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Fixed or portable compressed air systems as used for painting or tyre inflation.
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Heat exchangers providing heating or cooling in industry.
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Pressure cookers normally present in industrial kitchens.
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Pressurised process plant and pipe work connected with chemical processes.
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Refrigeration plant found in a food storage building. Components and safety features of a pressure system.
Burning Discs.
A bursting disc is a type of sacrificial part because it has a one-timeuse membrane that fails at a predetermined differential pressure, either positive or vacuum.
Check Valves.
A check valve is essentially a non-return valve to prevent water escaping from the boiler should the pressure in the feed pipe be less than that in the boiler.
Fuel cut off / shut off.
Fuel shut off valves close a line and stops the flow of fuel when a preset condition occurs. For example: excess flow, pre ssure pulses from a broken line or a temperature change from an idle burner.
Fusible metal plugs.
A fusible metal plug that prevents over-heating and in turn over pressurisation if external heat sources are applied by melting and thus again relieves the pressure.
Level indicators /
Level sensors will indicate the liquid contents level and raise alarm if
sensors.
the level falls or rises outside of the operating parameters.
Pressure cut-out or
Cuts off the compressor when the working pressure is reached.
unloading device.
Pressure relief valves.
Relieve excess pressure when the maximum safe working pressure of
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the receiver is attained.
Safety valves which would r elieve excess pressure when the
Safety Valve.
maximum safe working pressure of the receiver is attained, or pressure cut-out or unloading device which cuts off the compressor when the working pressure is reached.
Temperature and
Gauges should indicate the temperature and pressure levels and
pressure gauges.
should also be marked with the maximum safe temperature and pressure limit. High integrity temperature / pr essure detection systems should be alarmed, to indicate a change in normal operating procedures.
Water Treatment.
Filtering and treatment of water is used to prevent metal corrosion and reduce heat transfer rates, leading to overheating and loss of mechanical strength.
Table 1. Components and Safety Features of a Pressure System.
. IC11.4.1 Hazards of over pressure (under pressure) and over temperature.
o
Every employer shall ensure that work equipment, parts of w ork equipment and any article or substance produced, used or stored in work equipment which, in each case, is at a high or very low temperature shall have protection where appropriate so as to prevent injury to any person by burn, scald or sear.
Pressure systems can be a source of many serious injuries and property destruction due to poorly understood engineering principles. Thin and brittle metal shells can ru pture, while poorly welded or riveted seams could open up, leading to a violent eruption. Collapsed or dislodged pipework / vessels could also spray scalding-hot steam and smoke. Overpressure can be caused by a breakthrough from a high pressure system t o a low pressure system at the HP / LP interface. This can be caused by a failure of a level control valve where there is a liquid level control scheme (in a separator, for example). This condition is sometimes described as gas blowby. Blocking in a liquid level can also lead to overpressure of equipment, if a hydrocarbon liquid is blocked in with no means of venting, an increase in ambient temperature will cause vaporisation with a subsequent SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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pressure increase. A rising temperature caused by a loss of cooling, for any reason, can lead to vaporisation with a subsequent pressure increase. Exposure to excessive heat can cause a significant pressure increase. Such heat can be generated by ambient conditions where equipment is insufficiently insulated against heat ingress or in some cases by chemical reactions within the process. At its most extreme, pressure increase caused by rising temperature is manifested in fire conditions where equipment has been known to cause aBoiling Liquid Expanding Vapour Explosion (BLEVE). = Boiling. = Liquid. = Expanding. = Vapour. = Explosion. Overpressure of equipment might be caused by an internal explosion initiated by gas mixtures, mists or dusts finding a source of ignition in the presence of air. In enclosed spaces, a likely source of ignition is static electrical discharge. Typical overpressure failures caused by mechanical forces are those due to water hammer (see below t able for definition) in steam or condensate lines. Damage to vessels that are designed to withstand high pressure is easily caused by even a small amount of under pressure. Such under pressure may be the result of excessive cooling of liquids or vapours, or the movement or transfer of liquids where no provision has been made to relie ve under pressure conditions. In tankage, provision is often made to avoid vacuum conditions by fittin g pressure / vacuum relief valves. These valves will open to vent high pressure to a flare system or atmosphere on a rising pressure. On a falling pressure the valves will open to admit hydrocarbon gas, nitrogen or air to protect against under pressure or vacuum conditions. The metals of which vessels or pipelines are constructed can easily fail if su bject to high temperatures due to fire, local overheating or inadequate fluid flow through a pipeline. The latter is an important consideration in direct fired heaters where process fluids are being heated in a coil. In such cases a low flow through the coil will activate a trip of the fired heater.
Jet fire.
A jet or spray fire is a turbulent diffusion flame resulting from the combustion of a fuel continuously released with some significant momentum in a particular direction or directions. Jet fires can arise from releases of gaseous, flashing liquid (two phase) and pure liquid inventories.
Boiling Liquid
This is a phenomenon that occurs when a vessel containing a pressurised liquid substantially above
Expanding Vapour
its boiling point is ruptured, releasing the contents explosively.
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Explosion (BLEVE).
The process of a BLEVE is:
1. External heat source heats up the liquid which boils. 2. This creates extra vapour pressure inside the vessel. 3. Vapour is released via a safety vent. 4. External heat source, continues to heat up the liquid which boils. 5. With the vapour liquid now released the casing of the vessel heats up. 6. Vessel wall will eventually rupture - caused by thermal shock. 7. Pressure vessel will explode. 8. Released vapour cloud will come into contact with the external heat source and explode, once it is within its flammable range.
Please Select the Correct Answer. This is a phenomenon that occurs when a vessel containing a pressurised liquid substantially above its boiling point is ruptured, releasing the contents explosively.
At approximately 05:35 hours on 19th November 1984, a major fire and a series of catastrophic explosions occurred at the government-owned and operated PEMEX LPG Terminal at San Juan Ixhuatepec, Mexico City. As a consequence of these events, some 500 indivi duals were killed and the terminal destroyed. Three refineries supplied the facility with LPG on a daily basis. The plant was being filled from a refinery 400 km away, as on the previous day it had become almost empty. Two large spheres and 48 cylindrical vessels were filled to 90%, and 4 smaller spheres to 50% full. A drop in pressure was noticed in the control room and also at a pipeline pumping station. An 8-inch pipe between a sphere and a series of cylinders had ruptured. Unfortunately, th e operators could not identify the cause of the pressure drop. The release of LP G had been going on for about 5-10 minutes when the gas cloud, estimated at 200 m x 150 m x 2 m high, drifted to a flare stack. It ignited, causing violent ground shock. A number of ground fires occurred. Workers on the plant now tried to deal with the escape, taking various actions. At a late stage, somebody pressed the emergency shut-down button. About fifteen minutes after the initial r elease, the first BLEVE occurred. For the next hour and a half, there followed a series of BLEVEs as the LPG vessels violently exploded. LPG was said to rain down and surfaces covered in the liquid were set alight . The explosions were recorded on a seismograph at the University of SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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Mexico. : o
The total destruction of the terminal occurred because there wa s a failure of the overall basis of safety, which included the layout of the plant and emergency isolation features. Positioning of the vessels.
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Emergency isolation means. The terminal's fire water system was disabled in t he initial blast. Also, the water spray systems were inadequate.
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Survivability of critical systems, insulation thickness, water deluge. The installation of a more effective gas detection and e mergency isolation system could have averted the incident. The plant had no gas detection system and therefore, when the e mergency isolation was initiated, it was probably too late.
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Gas detection. Hindering the arrival of the emergency services was the traffic chaos, which built up as local residents sought to escape the area.
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Site emergency plan, access of emergency vehicles.
Figure 1. PEMEX Tank Area Before BLEVE.
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Figure 2. PEMEX Tank Area After BLEVE.
11th July 1978 216 people were killed in a ca mp site in Spain when a truck carrying propylene exploded as it passed the site. It has skidded around a bend and slammed into a wall. The cargo was released, caught fire and BLEVEd, sending 100-ft high flames into a campsite where 780 tourists were holidaying. The fireball injured 200 people.
Figure 1. Front Page of 'Irish Times' Reporting the 1978 San Carlos de la Rapita Explosion.
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Figure 2. Remains of Rear Axle of Truck.
Mechanisms of mechanical failure.
'Abnormal external loading' comes about when external fo rces are applied to the system, for example, a ladder being rested against pipework.
Brittle fracture.
Brittle fracture is caused by cold changing the characteristics of the material from which the system is made. For example, polymer seals need to be selected with care as their use in 'cold' systems makes the material brittle and liable to failure.
Caustic
In processes using certain chemicals, stresses have been shown to make the parts prone to
Embrittlement.
corrosive attack that can reduce their strength. Similarly failures have occurred where a chemical in contact with the part has affected its ability to carry stresses through causing embrittlement such as zinc embrittlement of stainless steel.
Corrosion with
Substances in the relevant fluid attacking the material from which the system is made cause
Internal Fluids.
corrosion. This is usually because of i mpurities within the fluid, a s the system has to be designed to take the fluid. Boilers or other systems using water are particularly susceptible, particularly where the system supply has to be regularly replenished. Corrosive failure will often occur inside the system, making detection difficult, so the examination process must set up a means of detection.
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Creep.
Within the elastic limits of a material, stress is proportional to strain. When, however, a material is put under stress near to its elastic limit, it undergoes a process of plastic deformation, referred to as 'creep'. The extent to which creep a cts is dependent upon two main factors: time and temperature: 'time' as creep is a slow process, and 'temperature' as creep can be a ccelerated by increasing the temperatures. Creep has been known to lead to the rupture of pressure systems, for example, through fractured steam pipes. It is primarily controlled by design, in the shape of the components and the choice of materials, for example, chrome-molybdenum steels have low creep characteristics.
Excessive external
When a system is installed and pressured up for the first time, the various parts will move slightly
stress.
and 'settle in'. If this movement is prevented by , for instance, pipework being tightly clamped in place, the system will become stressed and this could lead to failure. Residual stresses from manufacturing produce the same effect, and it is difficult when investigating failure to determine which of the two has caused the failure.
Hydrogen
Hydrogen attack' is a particular problem within steam boilers. The heat and temperature lead to
Embrittlement at
some of the water molecules breaking down into hydrogen and oxygen. The hydrogen atoms, being
Welding Repairs.
positively charged and small, move out of the solution and into the material of the pressure system. This affects the properties of the material, generally by weakening it, so that failure becomes more possible.
Mechanical Fatigue.
This is caused by the physical movement of parts of the system, so setting up fatigue failure. An example of this is the bellows failure at Flixborough, in that the bellows eventually could not withstand the movement. The effects of the movement of t he fluid may cause such movement, particularly if it flows in 'pulses', for example, as a result of a water hammer.
Overheating.
Overheating occurs when the system runs faster t han designed and the pressure relief system fails to work, for example, an oil-burning boiler runs at full pressure due to failure of the thermostatic controls. Explosion would occur if the safety valve fails to function. Boilers may suffer this if they have low water levels.
Thermal Fatigue.
This is caused by the constant changes in temperature from hot to col d, and vice versa. These changes have the effect of making the material expand and contract, so setting up cyclic stress reversal leading to fatigue failure.
Water Hammer.
Water hammer is a pressure surge or wave resulting when a liquid / fluid / steam in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system and a pressure wave propagates in the pipe e.g. turning a water tap off suddenly.
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. Materials used in construction should be suitable for the intended use. Designers and manufacturers should, therefore, consider at the manufacturing stage both the purpose of th e plant and the means of ensuring compliance with Pressure Systems Safety Regulations 2000.
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All extreme operating conditions including start-up, shutdown and reasonably foreseeable fault or emergency conditions.
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Conditions for standby operation.
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Expected working life (the design life) of the system.
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External forces expected to be exerted on the system including thermal loads and wind loading.
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Need for system examination to ensure continued integrity throughout its design life any foreseeable changes to the design conditions.
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Properties of the contained fluid.
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Protection against system failure, using suitable measuring, control and protective devices as appropriate.
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Safe access for operation, maintenance and examination, including the fitting of access (e.g. door), safety devices or suitable guards, as appropriate.
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Suitable materials for each component part.
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Need to issue a permit to work which would specify the control measures.
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Allowing hot machinery to cool to at least 50°C.
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Isolation and locking off of the electrical power to the pressure vessel / pump.
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Isolation of pipelines by locking valves or inserting blanks.
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Provision of a good standard of lighting and ventilation.
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Ensuring coordination with the person conducting the examination of the pressure vessel / pump / boiler.
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Releasing stored energy, de-pressurising, draining and decontaminating the pressure vessel / pump / boiler.
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Segregating the work by the use of barriers and signs and providing safe means of access for employees who are to carry out the repair work.
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Use of skilled and competent personnel to carry out the work.
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Provision and use of personal protective equipment such as head protection, eye protection and gloves.
The designer or supplier of a pressure system or component part covered Pressure Systems Safety Regulations 2000. should consider the most effective way of providing the appropriate information to those who need it. SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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Corrosion allowances.
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Creep life.
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Design pressures (maximum and minimum).
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Design standards used and evidence of compliance with national / European / international standards or documentation showing conformity.
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Design temperatures (maximum and minimum).
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Fatigue life.
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Flow rates and discharge capacities.
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Details of intended contents, especially where the design has been carried.
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Materials of construction.
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Volume capacities, especially for storage vessels.
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Wall thickness. .
Where the system consists of a standard production item, the designer / manufacturer should assess the safe operating limits and pass the relevant information to the user / owner. In these circumstances, the user / owner will not always need to carry out the detailed work required to establish the safe operating limits of the system. In cases where the user / owner has specified the design, the responsibility for establishing the safe operating limits rests with the user / owner. If the user / owner does not have sufficient technical expertise to establish the safe operating limits, an organisation that is competent to carry out the task should be used. The exact nature and type of safe operating limits that need to be specified will depend on the complexity and operating conditions of the particular system. 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, e.g. maximum and minimum temperatures and pressures, nature, volumes and flow rates of contents, operating times, heat input or coolant flow. In all cases, the safe operating limits should incorporate a suitable margin of safety. . A typical written scheme of examination for pressure systems would cover all protective devices an d would include all pressure vessels and pipe work that could give rise to danger if they failed. The type of examination would be specified, for example, as a result of a statutory requirement, a s would also the frequency with which the examination should be carried out. The scheme would stipulate the special measures needed in preparing the plant for examination and in the case of a fired or heated pressure systems, the need for them to be examined when cold and also when in operation with a final requirement that the examination should be carried out by a competent person.
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Identification of the items of plant or equipment within the system.
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Parts of the system which are to be examined.
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Nature of the examination required including the inspection and testing to be carried out on
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protective devices. o
Preparatory work needed for the item to be examined safely.
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Maximum interval between examinations.
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Imminent danger report is required to be given t o the enforcing authority and / or the employer.
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Critical parts of the system - if modified or repaired - which must be examined by a competent person before the system is used again.
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Name of the competent person certifying the written scheme.
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Date of certification. .
Maintenance is an important control measure and a programme should be drawn up for the whole system, monitoring it for defects and for signs of corrosion and wear and ensuring that both the examination and any necessary remedial work is carried out using safe systems of work and under competent supervision. The type and frequency of maintenance for the system should be assessed and a suitable maintenance programme planned.
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Age of the system.
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Any previous maintenance history.
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Manufacturer's / supplier's instructions.
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Operating / process conditions.
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Repairs or modifications to the system.
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Reports of examinations carried out under the written scheme of examination by the competent person.
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Results of other relevant inspections.
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Risks to health and safety from failure or deterioration.
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Working environment.
. The United Kingdom Accreditation Service (UKAS) is recognised by UK Government as the UK national accreditation body responsible for assessing and accrediting the competence of organisations in the fields of inspection, measurement, testing and the certification of systems, products and personnel.
Accreditation for In-Service Inspection of Pressure Systems / Equipment, details a series of five qualification categories from Category 1, which is a Chartered Engineer wit h at least 3 years relevant experience, through to Category 5, which is a person with a tradesman's apprenticeship and a minimum of 5 years relevant experience. Training should to be provided for all operators, maintenance personnel, and those responsible for carrying out inspections of the system to ensure they are in possession of the necessary knowledge and SHEilds Ltd www.sheilds.org eLearning: www.sheilds-elearning.com NEBOSH International Diploma v 2.0 (21/08/2014)
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skills to perform their duties
Now that you have convered the physical hazards section of this course it would be now worth looking at your company and indentifying the physical hazards present there, ready for your UNIT D assignment. You should start collecting your information for your assignment as you go through your studies.
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