Occupational Health & Safety Practitioner. Reading PRESSURE VESSELS - DESIGN

Transcription

1 Occupational Health & Safety Practitioner Reading PRESSURE VESSELS - DESIGN January 2007

2 Contents OVERVIEW...1 SECTION 1: INTRODUCTION...2 SECTION 2: DESIGN CONCEPTS...7 SECTION 3: DESIGN CONDITIONS...9 SECTION 4: MATERIAL SELECTION...10 SECTION 5: CORROSION...13 SECTION 6: PRINCIPLES INVOLVED IN SHELL DESIGN...14 SECTION 7: ENDS...17 SECTION 8: EXTERNAL PRESSURE...18 SECTION 9: OPENINGS...20 SECTION 10: VESSEL SUPPORTS...22 SECTION 11: TRANSPORTABLE VESSELS...23 SECTION 12: MOUNDED OR BURIED VESSELS...24 SECTION 13: EARTHQUAKES AND WIND LOADS...24 SECTION 14: DRAINAGE...25 SECTION 15: PROTECTIVE DEVICES...26 SECTION 16: MODIFICATIONS AND REPAIRS...27 REFERENCES AND FURTHER READING...28

3 Published by WorkSafe, Department of Consumer and Employment Protection, PO Box 294, WEST PERTH WA Tel: Toll Free The SafetyLine Institute material has been prepared and published as part of Western Australia s contribution to the National Occupational Health and Safety Skills Development Action Plan State of Western Australia. All rights reserved. Details of copyright conditions are provided at the SafetyLine Institute website. Before using this publication note should be taken of the Disclaimer,, which is published at the SafetyLine Institute website.

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5 OVERVIEW This reading provides a general outline of the basic concepts involved in the design of pressure vessels. Objectives After reading this information you should be able to outline some of the basic concepts involved in the design of pressure vessels. Author Prem Nagra B.E.(Mech) University of Madras, India Certificate of Competency - 1st Class Engineer (Motor) Institution of Engineers JANUARY 2007 SAFETYLINE INSTITUTE PAGE 1

6 Section 1: INTRODUCTION Glossary of terms When they are first used, glossary terms are indicated with an asterisk (*). Make sure that you are familiar with the Glossary of terms before going any further. Brittle Fracture Buried Vessel Creep Kilopascal (kpa) Spontaneous fracture of steel in a brittle non- - ductile manner with low toughness usually at low temperature when the applied stress generally is below yield. A storage vessel situated below ground level in a pit or trench that has been backfilled with sand or other suitable material. Long term exposure to high temperatures can severely reduce the strength of steel. For example, if mild steel grade 250 is exposed to 400 C for a period of 10,000 hours, its tensile strength is reduced to half when tested at room temperature Pascals. Megapascal (Mpa) 1,000,000 Pascals or 1000 kilopascals. Mounded Vessel Pascal (Pa) A storage vessel that may be sited above ground or partly buried and which is completely covered by a mound of earth or similar inert material. Force of one newton acting on an area of 1 square metre. Stress Corrosion Cracking produced under the combined effects of corrosion. Cracking and tensile stress starts where the stress is the highest and the material's metallurgical structure is the weakest. PAGE 2 SAFETYLINE INSTITUTE JANUARY 2007

7 Stress Force acting per unit area. The unit is Pascal. Strain Change in dimension per unit length. Impact Strength Tensile Strength Yield Strength Measure of the notch ductility of steel. A well-known test is the Charpy V-notch test, in which a special test piece is fractured in one blow by a falling pendulum. The energy absorbed by the test piece in fracturing is a measure of its impact strength at the test temperature. Maximum stress that a test specimen of a material can sustain before fracture. Unit Mpa. Lowest stress at which the strains are detected to grow without further increase of stress. Unit Mpa. 1.1 Types of pressure vessels A pressure vessel is defined as a vessel that is subject to either internal or external pressure. This definition generally includes air receivers, heat exchangers, evaporators, steam type sterilisers and autoclaves. Pigment Separator Image courtesy of Specialised Welding JANUARY 2007 SAFETYLINE INSTITUTE PAGE 3

8 Oxydrolysis Reactor Image courtesy of Specialised Welding Digester Tank Image courtesy of Specialised Welding Heat Exchanger Image courtesy of Heat Exchangers International PAGE 4 SAFETYLINE INSTITUTE JANUARY 2007

9 1.2 Hazardous circumstances Pressure vessels are potentially hazardous because of the pressure energy stored in them and their contents. The associated hazard, which could contribute to leakage and rupture of a pressure vessel, increases in the following circumstances: Increased pressure (p) or volume (V) or both. A pressure vessel containing a gas has greater expansive energy as compared to an identical vessel containing a liquid at the same pressure. The more adverse effect of contents on humans and the environment. Location of the vessel, for example, transportable or in refineries. KEY POINT Pressure vessels are hazardous because of the pressure energy and contents stored in them. 1.3 Example For example, anhydrous ammonia (a hazardous chemical) is a gas at ordinary temperature and pressure. It can be stored as a liquid under pressure. Ammonia stored at pressure has considerable pressure energy and should a rupture of the pressure vessel occur, liquid would flash into a vapour spontaneously as the thermodynamic state of the ammonia adjusts itself to the diminished pressure. Since this flash evaporation takes place almost instantaneously throughout the bulk of the liquid, most of the contents of a ruptured pressure vessel will enter the atmosphere either as a vapour or as a fine mist. At low concentrations in air, ammonia vapour irritates the eyes, nose and throat. Inhalation of high concentrations produces a sensation of suffocation, quickly causes burning of the respiratory tract and may be fatal. For this reason, the design of the pressure vessel should be of a standard that virtually eliminates the possibility of a major failure. Due to the potential hazard of pressure vessels, they are subject to occupational safety and health legislation covering their design and construction. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 5

10 1.4 Pressure vessel failures Pressure vessel failures generally fall into the following categories: Improper vessel design. Operational or process failures, for example, overheating and explosion. Failure because of the service conditions of the vessel, for example, corrosion, stress corrosion* and cracking*. Failure due to unsuitable materials. Failure due to fabrication defects. Damaged Oil Tank The above oil storage tank was subjected to air pressure to expedite the removal of oil. The storage tank was not designed as a pressure vessel, and it exploded on application of pressure. The bottom of the tank was blown out and the inlet nozzle sheared off. A worker who was tending to the tank was lucky to suffer only minor injuries. PAGE 6 SAFETYLINE INSTITUTE JANUARY 2007

11 1.5 Hazard levels Australian Standard AS Pressure Equipment Hazard Levels- allocates hazard levels according to pv values (multiple of pressure and volume) and the contents of the vessel as follows: Hazard Level A (high) applies to high hazards such as large vessels containing lethal substances, ie. a very toxic substance or highly radioactive substance. Hazard Level B (average) applies to most pressure vessels and boilers. Hazard Levels C and D (low and extra low, respectively) applies to small equipment or equipment with low hazard contents. Hazard Level E (negligible) applies to all very low hazard vessels, which are generally exempt from regulatory controls. Section 2: DESIGN CONCEPTS In designing pressure vessels, the following risk management process has to be considered. 2.1 Hazard identification Assess all hazards that may arise with the vessel during its entire life. The possible sources of harm are as follows. KEY POINT Pressure energy, which contributes to rupture and leakage. Escape of toxic, flammable or harmful fluid from the vessel and its components. Legislation requires pressure vessel designers to manage risk by identifying hazards, assessing risks and controlling these risks. Rapid release of high-pressure fluid causing projection of vessel components or fragments or high pressure blast. Collapse during lifting. Collapse/vacuum implosion or detachment of vessel from equipment or vehicle. A domino effect - with subsequent damage to adjacent plant or property. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 7

12 2.2 Risk assessment Assess the above risks - their likelihood of occurring and their consequences. 2.3 Risk control The controls that are put into place to ensure that overall risk is acceptable. The proper design of pressure controls the above risks to a very low level. There are two ways in which vessels are designed: Design by Rule - This is based on the use of formulae and rules to calculate basic shell thickness and keeping stresses below allowable values. All design standards are based on the concept of design by rule and compliance with the standards is mandatory. Table 2.1 of AS1200 outlines design standards that are acceptable. Design by Analysis - The design by rule is not suitable when complex loads and shapes are encountered which are not covered by the standards. In such cases, rigorous mathematical analysis or finite element stress* analysis may be used. These means are quite complex as stresses in different directions are considered. The finite element analysis is done using special software that requires experienced and competent analysts to evaluate the results. Generally pressure vessels are designed to design by rule with the design by analysis being used for vessel details which are not covered by a standard. PAGE 8 SAFETYLINE INSTITUTE JANUARY 2007

13 Section 3: DESIGN CONDITIONS 3.1 Design pressure A pressure vessel has to be designed to sustain the stresses imposed on it due to the maximum internal or external pressure it is subjected to. This pressure is called the design pressure. The design pressure generally is determined from the maximum operating pressure of the vessel, which is increased by a margin to take into account probable surges of pressure during operation. KEY POINT Pressure vessels have to be designed to a design pressure and design temperature. The design pressures for pressure vessels, which contain liquefiable gases (eg, LPG, LNG and Chlorine), are determined by the most severe operating conditions or the vapour pressure of the liquid content at the maximum service temperature. 3.2 Design temperature The coincident metal temperature at the design pressure is called the design temperature. When steel is subjected to temperatures above 420 deg C, there is a marked change in its properties. At high temperatures, the design allowable stress has to be sufficiently high to take into account the risk of deformity of the vessel or rupturing. Also, under constant load, there is a continuous increase in permanent deformation known as creep*. The design allowable stress is based on keeping deformation due to creep within permissible limits during the service lifetime. At low temperatures of -250 C to -100 C (known as cryogenic temperatures), most carbon and low alloy steels are brittle* and unsuitable for use. Materials that are used at these low temperatures must have proven high impact* strength*. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 9

14 Section 4: MATERIAL SELECTION 4.1 Maintain stresses in elastic range The factors that have to be considered when selecting the most suitable materials are for constructing pressure vessels are: corrosion resistance; strength requirements for design pressure and temperature; cost; availability; and fabricability. The most commonly used factor in the design of pressure vessels is that of maintaining the induced stresses within the elastic region of the material of construction. This is done in order to avoid plastic deformation of the material when the yield* point is exceeded. The functional service of a member may be lost if the induced stresses exceed the yield point causing deformation. For example, a flange used as a closure for a vessel may no longer produce a pressure tight seal if the face of the flange is permanently deformed. PAGE 10 SAFETYLINE INSTITUTE JANUARY 2007

15 4.2 Factor of safety In the above diagram, when the loading of a pressure vessel part is within the region OA it is within the elastic region and on removal of the load the stress and strain* will both return to zero. This stress is considered to be satisfactory for designing the part. To ensure this, pressure vessels are designed with a margin that is called the factor of safety. Prior to 1940, vessels were manufactured with a factor of safety of 5, but during the Second World War (due to a shortage of materials) this factor was reduced to 4.0. Currently, there is a proposal to reduce this factor of safety to 3.5. The factor of safety is defined as the ratio between the design stress and the ultimate tensile* stress of the material. For example, the allowable design stress of a material having a tensile strength of 430 Mpa is: JANUARY 2007 SAFETYLINE INSTITUTE PAGE 11

16 = MPa All standards provide a table of allowable design stresses for various materials. Effect of Temperature on Safety Factor PAGE 12 SAFETYLINE INSTITUTE JANUARY 2007

17 Section 5: CORROSION 5.1 Vessels severely corrode Pressure vessels are affected by corrosive conditions internally and externally. Air receivers must have a minimum corrosion allowance of 0.75 mm. Process vessels are exposed to severe corrosion and may have a corrosion rate of mm per year of design life added to the calculated required thickness. KEY POINT The effects of corrosion have to be considered when designing pressure vessels. 5.2 Measures to reduce corrosion Corrosion can also be eliminated or reduced by: Using glass, enamel, teflon and lead as protective lining materials. Common and commercially available corrosion resistant materials are stainless steels, which are economical for vessels having shell thickness of below 9.0 mm. An integrally clad corrosion resistant plate which is attached to the shell material at the steel mill. The attachment of corrosion-resistant strip or sheet lining to the vessel shell after the vessel has been fabricated. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 13

18 Section 6: PRINCIPLES INVOLVED IN SHELL DESIGN 6.1 Circumferential (hoop) and longitudinal stress The shell thickness is calculated based on simple methods of stress analysis. The basic equations for stresses in the shell are: Hoop Stress: Longitudinal Stress: Where D = inside diameter of shell in millimetres P = design pressure in megapascals* f = allowable design tensile stress in megapascals t = minimum calculated thickness in millimetres Longitudinal Forces acting on thin Cylinder under Internal Pressure D = inside diameter of shell in millimetres t = thickness of shell in millimetres p = design pressure in megapascals PAGE 14 SAFETYLINE INSTITUTE JANUARY 2007

19 Circumferential (Hoop) Forces acting on thin Cylinder under Internal Pressure The above equations are modified in AS1210 as follows to take into account the stress in the middle of the plate thickness and weld efficiency: Based on circumferential stress the thickness required is: Based on longitudinal stress the thickness required is: Where η = welded joint efficiency. The shell thickness for the vessel is the higher of the above two thicknesses. The actual plate thickness selected has to take into account any other allowances and mill tolerances. 6.2 Vessel weld classification Pressure vessels that are of welded construction are classed according to their construction. This classification is based on the welded joint efficiency utilised in calculations. This in turn determines the extent of non-destructive examination (NDE) of the welds required. The classification of a vessel is determined by: contents; material specification and thickness; and JANUARY 2007 SAFETYLINE INSTITUTE PAGE 15

20 transportable or stationary. According to Australian Standard AS , welds are classified as: Class 1 - A weld joint efficiency of 1 is used. Pressure vessels that contain lethal substances such as hydrogen cyanide, carbonyl chloride and highly radioactive substances are required to be of forged, seamless or Class 1 welded construction. These vessels require 100 % non-destructive examination of welds. Class 2 - This is further subdivided into Class 2A and Class 2B. These allow higher weld efficiencies of 0.65 to 0.85 with spot examination of welds. Class 3 - Low weld joint efficiencies of 0.45 to These vessels do not require any non-destructive examination of welds. 6.3 Worked example Calculate the shell thickness of a pressure vessel having an internal diameter of 900 mm, design pressure 1200 kpa, constructed to AS1210-2A with material design strength of 115 Mpa at the design temperature of 100 C, corrosion allowance of 0.75 mm and weld efficiency of Convert design pressure to megapascals = 1.2 Mpa. Using the formula for circumferential (hoop) stresses: = 5.56 mm Minimum shell thickness required = Corrosion allowance = = 6.31 mm PAGE 16 SAFETYLINE INSTITUTE JANUARY 2007

21 Section 7: ENDS 7.1 Dished ends Dished ends of torispherical, semi - ellipsoidal or spherical shapes normally close the ends of pressurised cylinders. These shapes are considerably stronger than flat plate closures. Also, the use of a flat plate as a closure results in the plate being considerably thicker than the cylinder to which it is attached. The choice of the type of head is dependent on economy and the pressure to which they are subjected. Typical ends are: Torispherical End Ellipsoidal End Spherical End Conical End JANUARY 2007 SAFETYLINE INSTITUTE PAGE 17

22 Toriconical End Torispherical ends are the least expensive to fabricate. They are frequently used for low design pressures below 1000 kpa. The semi - ellipsoidal ends are the most popular for design pressures above 1000 kpa. Spherical ends are more difficult to fabricate and consequently most expensive, but require half the shell thickness of a cylindrical shell for the same pressure. Toriconical and conical heads are only used for special applications. Section 8: EXTERNAL PRESSURE 8.1 Internal or external stiffening used to prevent collapse Pressure vessels which are subject to vacuum conditions have to be designed for external pressure, otherwise they would collapse. The length of a pressure vessel affects the magnitude of the external stresses: the longer the unsupported length of the shell, the greater is the stress due to external pressure. The normal procedure is to provide external or internal stiffening at regular intervals to reduce the vessel length, as shown below. Internal or external fittings contribute to the stiffening of the vessel. PAGE 18 SAFETYLINE INSTITUTE JANUARY 2007

23 Three cross-sectional views of the stiffener: These stiffeners may extend partly or all around the vessel as shown below: JANUARY 2007 SAFETYLINE INSTITUTE PAGE 19

24 Section 9: OPENINGS 9.1 Inspection openings to look at and clean surfaces Inspection openings are required to permit visual examination and cleaning of internal surfaces and for attaching a pipe or nozzle. Openings are preferably circular in shape but they may be elliptical or obround (ie. formed by two parallel sides with semi-circular ends). Openings may be of other shapes, provided that all corners, which have maximum stress concentration, have a suitable radius. Openings give rise to increased stresses around the edge of the hole. For this reason, openings in a vessel should be kept to the minimum. The strength of an opening has to be calculated for the safety of the vessel. The integrity of the vessel can be increased, by adding a reinforcement plate around the hole to reinforce the opening. 9.2 Entry holes for entry and rescue Entry-holes have to be positioned to allow easy ingress for inspection and safe recovery of an incapacitated person. Consideration has to be given to provide a safe landing place either inside the vessel or by locating the top rung of a ladder close to the opening. Vessels which contain an unsafe or oxygen deficient atmosphere may require two entry-holes to accommodate power lines, hoses, ventilation ducts or similar. Reinforcement of an Opening PAGE 20 SAFETYLINE INSTITUTE JANUARY 2007

25 TellTale Holes, above, are provided for testing of welding sealed off by the reinforcing plates, and as a vent during heat treatment. Reinforced Access Hole and other Pipe Openings Courtesy of Specialised Welding Reinforced Opening Courtesy of Heat Exchangers International JANUARY 2007 SAFETYLINE INSTITUTE PAGE 21

26 Section 10: VESSEL SUPPORTS 10.1 Must meet design recommendations Vessels are supported in many ways. In all cases care has to be taken that the supports are able to withstand all loadings without causing deformation of the vessel wall or increasing local stresses excessively and causing vessel instability. The design of supports should follow the recommendations of the code to which the vessel is designed. Supports should permit movement of the vessel due to changes in temperature and should be designed to prevent or drain any accumulation of water. The vessel and supports should be installed on firm foundations. Types of supports: PAGE 22 SAFETYLINE INSTITUTE JANUARY 2007

27 Section 11: TRANSPORTABLE VESSELS 11.1 Types of transportable vessels The following vessels are designed for the transport of fluids under pressure: Road Tanker Vessels - Vessels permanently mounted on, or forming an integral part of, a road vehicle. Rail Tanker Vessels - Vessels that form part of a rail tank wagon. Demountable or Skid Tanks - Portable tanks which can be moved by road or rail to various locations. Tank Shipping Containers - Vessels contained in a standard frame for sea, rail or road transport Design issues for transportable vessels The design of transportable vessels should take the following into account: The inertia forces on the vessel and its supports due to sudden impact. These forces are substantial. For example, in the case of skid tank design they are assumed to be four times the weight of the vessel and contents. Providing internal baffles, which break up the vessel into smaller sections, can reduce these forces. The fittings, eg, for filling, emptying, inspection and pressure relief, which have to be protected, where there is likelihood of them being damaged if the vehicle overturns, by enclosing them with protective guards. The vehicle requiring additional protection in the form of bumpers or barriers in the event of rear impact. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 23

28 Section 12: MOUNDED OR BURIED VESSELS 12.1 Design recommendations Pressure vessels for LPG (liquefied petroleum gas) bulk storage may be buried* or mounded* for fire protection. The design of the vessel has to take the following additional loads into consideration: The dead weight of the vessel, contents and earth mound assumed to be supported by it. Live loads by machinery and equipment. Soil pressure and friction loads during expansion and contraction of the vessel. Buoyancy forces due to buoyancy of the vessel in fully flooded site conditions (special care has to be given to the holding down arrangement). Section 13: EARTHQUAKES AND WIND LOADS 13.1 Must meet design standards Tall vertical vessels operate under severe conditions. The contents, in the case of process vessels, could be hazardous and their structural failure is a serious matter. Wind loads have to be taken into account. The wind can also induce vibration in the vessel that could cause failure of the vessel. Pressure vessels that are located in seismic activity zones have to be designed to withstand the lateral seismic forces during an earthquake. The appropriate wind velocity and earthquake activity in the area where the vessel is to be located is determined from the standards such as AS Wind loads and AS Earthquake loads. KEY POINT Pressure vessels that are located in seismic activity zones have to be designed to withstand the seismic forces. PAGE 24 SAFETYLINE INSTITUTE JANUARY 2007

29 Section 14: DRAINAGE 14.1 To prevent moisture build up, and drain prior to inspection Pressure vessels containing material that is corrosive to the vessel or toxic or inflammable should have adequate drainage provided. The drainage is to prevent build up of moisture in air receivers and for completely evacuating vessel contents before inspection and repairs. This drain should be located at the lowest part of the vessel. Completed Pressure Vessel showing access hole, liquid level indicators and drain valve. Image courtesy of Heat Exchangers International JANUARY 2007 SAFETYLINE INSTITUTE PAGE 25

30 Section 15: PROTECTIVE DEVICES 15.1 Common types of pressure relief devices All pressure vessels have to be provided with pressure-relief devices that will prevent the pressure from rising more than 110 percent of the design pressure. In case of fire, the pressure relief device should not allow the pressure to exceed by 121 percent of design pressure. The common types of pressure-relief devices are as follows: Safety valve - Valve that automatically discharges fluid to the atmosphere above a predetermined pressure and closes after normal condition has been achieved. These are used for Safety Relief Valve compressible fluids which require quick over-pressure relief. Relief valve - Valve that automatically discharges fluid to the atmosphere as above but is used for liquids. Bursting discs - Used for conditions where the pressure rise is very rapid and where even minute leakage of the fluid is not permissible during normal conditions. Fusible plugs - Made of low melting point materials and may be provided in addition to safety/relief valves or bursting discs. Their purpose is to activate if the vessel is exposed to fire and subsequent rapid increase of pressure. KEY POINT Pressure vessels have to be provided with pressurerelief devices. PAGE 26 SAFETYLINE INSTITUTE JANUARY 2007

31 Section 16: MODIFICATIONS AND REPAIRS 16.1 Repairs and modifications must meet accepted standards One of the commonest threats to a pressure vessel's inherent safety and reliability are improperly instituted and completed modifications and repairs. Legislation requires modifications to be carried out and tested according to an accepted standard Example of an incompetent modification In this example, a 5000 litre bulk transfer vessel was modified by cutting a 1350mm x 1000mm rectangular opening in the end. This opening was fitted with a cover plate and secured to the vessel by 24 steel bolts. The design alteration was not checked to any pressure vessel standard nor was the vessel subjected to a hydrostatic pressure test before use. As the vessel was being filled with compressed air for the first time after the modification (to discharge its contents), there was a loud explosion. The pressure inside the vessel at the time of the explosion was 150 kpa, which was well below the design pressure. The cover plate weighing 85 kg was propelled by the pressure energy in the tank, landing 50 metres away in a workshop after penetrating the roof. Fortunately, no injuries were incurred. Investigations revealed a competent person had not designed the modifications: The 10mm diameter securing bolts were insufficient to withstand the pressure inside the vessel and failed. Unusually, rectangular openings were used, rather than the more usual circular or elliptical which are generally used for pressure vessels. WorkSafe WA prosecuted the employer for not providing safe plant, namely the bulk transfer vessel, at its workplace such that its employees were not exposed to hazards. As a result of the successful prosecution, the company was fined. JANUARY 2007 SAFETYLINE INSTITUTE PAGE 27

32 Your feedback WorkSafe is committed to continuous improvement. If you take the time to complete the online Feedback Form at the SafetyLine Institute website you will assist us to maintain and improve our high standards. REFERENCES AND FURTHER READING Australian Standard AS Pressure vessels. Spence J. and Tooth A. S., Pressure Vessel Design - Concepts and Principles. E & FN Spon, Bednar H. H., Pressure Vessel Design Handbook. Krieger, 2nd. Edn PAGE 28 SAFETYLINE INSTITUTE JANUARY 2007