Safety Engineering -Pressure Systems-

Transcription

1 Safety Engineering -Pressure Systems- M. Jahoda

2 Introduction Pressure Systems means a system comprising one or more pressure vessels of rigid construction, any associated piping and protective devices and pressure accessories which contains or is liable to contain a relevant fluid 2 Examples of pressure systems include: steam boilers and steam heating systems pressurised process plant and piping compressed air systems refrigeration systems Source:

3 Pressure Systems Compressed Air Systems 3 Source:

4 Pressure Systems Causes of Faliur 4 Faulty Design Operator error or poor maintenance Operation above max allowable working pressures Change of service condition Over temperature Safety valve Improper installation Corrosion Cracking Welding problems Erosion Stress Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

5 Pressure Systems Causes of Faliure 5 Improper selection of materials or defects Low water condition Improper repair of leakage Burner failure Improper installation Fabrication error Over pressurisation Failure to inspect frequently enough Erosion Unsafe modifications or alteration Unknown or under investigation Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

6 Pressure Systems Hazard Awareness 6 Blast Effects Fragmentation damage Suffocation Poisoning Fire Explosion - leakage of fuel or gas BLEVE Chemical burn Thermal burns Loss of lives and property Permanent injuries or disabilities on affected people Loss of earning/income Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

7 Pressure Systems Hazard Awareness - BLEVE Boiling Liquid Expanding Vapor Explosion 7 This is a type of explosion that can occur when a vessel containing a pressurized liquid is ruptured. Such explosions can be extremely hazardous. A BLEVE results from the rupture of a vessel containing a liquid substantially above its atmospheric boiling point. The substance is stored partly in liquid form, with a gaseous vapor above the liquid filling the remainder of the container.

8 Pressure Systems Safety Approach 8 There are standards and codes laid down by approved regulatory bodies for the design, construction, welding, testing, marking, operation, inspection, and repair of any pressure systems, which provides fundamental safeguards and good safety practices. The standards and codes vary from country to country. The commonly used codes are: ASME Boilers and Pressure Vessel Codes (codes and standards, since 1914) API Standards (The American Petroleum Institute, since 1924) PD5500 (Specification for unfired, fusion welded pressure vessels), UK British Standards European Codes and Standards Other International Codes Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

9 Pressure Systems Regulations for design, construction and safe operating limits 9 The pressure system should be manufactured from materials suitable for the substances it will contain. The supplier of the equipment should be able to demonstrate that it complies with the relevant European Directives and standards. The adequate design, installation and maintenance of pressure systems includes measures to protect against vacuum in situations where vacuum can occur. It is good practice to have a preventive maintenance programme and maintenance file for the system as a whole and for some systems where there is the potential for serious injury or damage, they are a necessity.

10 Pressure Systems Regulations for design, construction and safe operating limits 10 It is not operated or allowed to be operated beyond its safe operating limits except for testing purposes as specified by, and under the direction of, a competent person. The only reason a system should be operated above the safe operating limits is for the purposes of testing under specialist supervision. It is provided with adequate and appropriate protective devices, and any such device designed to release contents shall do so safely. A person who installs pressure equipment or a pressure system at a place of work shall ensure that it is installed so that it may be used safely, without risk to health or impairing the operation of any protective device or inspection facility.

11 Pressure Systems Boiler and High-temperature Water Heaters Hazards Explosion hazards Fire hazards leakage of fuel or gas Where Pressure part ruptures Furnace explosions 11 Source:

12 Pressure Systems 12 Prevention for Boiler Fires Fully enclosed boiler rooms Large door openings on boiler rooms Noncombustible ceiling and automatic sprinklers Proper clearance around exterior walls Clean boilers promptly. Follow correct guidelines. Schedule maintenance guidelines. Follow general precautions for entrance. Wear proper attire when entering.

13 Pressure Equipment Types vessels pressure cookers, autoclaves and retorts heat exchangers valves, steam traps and filters piping and hoses protective devices 13 Autoclave MEGA

14 Pressure Vessels Types 14 Pressure vessels are air-tight containers used mostly in process industry, refinery and petrochemical plant to carry or hold liquid, gases or process fluids. They are typically subjected to pressure loading and internal or external operating pressure different from ambient pressure. Cylindrical Pressure Vessel Spherical Pressure Vessel

15 Pressure Vessels Description A typical pressure vessel consists of shell (body of the vessel), closure heads, openings for inspection and instrumentations, attachments and a combination of nozzles for pressure relief or other purpose, and supports. 15 Pressure Relief Valve Crane Transportation Eye Pressure Head Temperature Sensor Man way Pressure Adjusting Apparatus Sample Valve Pressure Cylindrical Jacket Sanitary Input Fill Level Indicator Inlet/Outlet Pressure Head Source The tank for beer maturation Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

16 Pressure Vessels Shape 16 What shape of pressure vessel uses the least amount of metal to contain a given volume, pressure? A sphere! Why is this shape not more widely used? Usually need to have an extended section of constant cross-section to provide support for vessel internals, trays, distributors, etc. It is much easier to obtain and maintain uniform flow in a cylindrical bed of catalyst or packing than it is in a non-uniform cross-section. A cylinder takes up a lot less plot space for the same volume. A sphere is more expensive to fabricate.

17 Pressure Vessels Shape A common design is a cylinder with end caps called heads. 17 Distillation columns are obviously an exception: diameter is set by flooding correlations and height by number of trays.

18 Pressure Vessels 18 Orientation Usually vertical Easier to distribute fluids across a smaller cross section Smaller plot space Reasons for using horizontal vessels To promote phase separation Increased cross section = lower vertical velocity = less entrainment Decanters, settling tanks, separators, flash vessels To allow internals to be pulled for cleaning Heat exchangers

19 Pressure Vessels Head (Closure) Designs Head shapes are frequently either hemispherical or dished (torispherical). 19 Hemispherical Ellipsoidal Torispherical Spherical Flat Conical Hemispherical (a) Good for high pressures Higher internal volume Most expensive to form & join to shell Half the thickness of the shell Ellipsoidal (b) Cheaper than hemispherical and less internal volume Depth is half diameter Same thickness as shell Most common type > 15 bar Torispherical (c) Part torus, part sphere Similar to elliptical, but cheaper to fabricate Cheapest for pressures less than 15 bar

20 Pressure Vessels Tangent and Weld Lines 20 Tangent line is where curvature begins Weld line is where weld is located Usually they are not the same, as the head is fabricated to allow a weld away from the geometrical joint

21 Pressure Vessels Vessel Specifications Set By the Process Engineer The process engineer will usually specify the following parameters based on process requirements: Vessel size and shape (volume, L and D) Vessel orientation and elevation Maximum and minimum design pressure Maximum and minimum design temperature Number of nozzles needed (& location) Vessel internals Distillation trays Packing supports Distribution grids Heating or cooling coils And often also: Material of construction Corrosion allowance There is often a lot of dialogue with the mechanical engineer to set the final specifications 21

22 Pressure Vessels Design Pressure 22 Normal operating pressure The pressure at which you expect the process to usually be operated Maximum operating pressure The highest pressure expected including upset conditions such as startup, shutdown, emergency shutdown Design pressure Maximum operating pressure plus a safety margin Margin is typically 10% of maximum operating pressure, whichever is greater Usually specify pressure at top of vessel, where relief valve is located

23 Pressure Vessels Design Vacuum 23 Vacuum can be caused by: Intentional process operation under vacuum (including start-up and shutdown) Cooling down a vessel that contains a condensable vapor Pumping out or draining contents without allowing enough vapor to enter Operator error Vacuum puts vessel walls into compressive stress.

24 Pressure Vessels Pressure testing incidents What happens if vessel is not designed for vacuum conditions? 24 A railroad tank car demonstration imploding after placing a vacuum on the tank with the vacuum safety valves disabled or removed Source:

25 Pressure Vessels Commonly Used Materials 25 Steels Carbon steel, Killed carbon steel (deoxidized with a strong deoxidizing agentsuch as silicon or aluminum) cheap, widely available Low chrome alloys (<9% Cr) better corrosion resistance than CS, KCS Stainless steels: 304 cheapest austenitic stainless steel 316 better corrosion resistance than 304, more expensive 410 Nickel Alloys Inconel, Incolloy high temperature oxidizing environments Monel, Hastelloy expensive, but high corrosion resistance, used for strong acids Other metals such as aluminum and titanium are used for special applications. Fiber reinforced plastics are used for some low temperature & pressure applications.

26 Pressure Vessels Inspection Internal inspection Periodic internal inspections to evaluate integrity of equipment. Inspections should be carried out by licensed inspectors to ensure compliance with jurisdictional or insurance requirements. Preventive maintenance Inspection or maintenance department should keep file or log of original design documents, records of inspections, and vessel history documents. manufacturer s data reports and instructions design data, including location of dimensional checkpoints installation information records of process changes vessel s historical profile, including records of all repairs and conditions found during inspections 26

27 Pressure Vessels Inspection 27 Potential hazards of confined spaces: Toxic materials including inert gases Flammable vapors which may be ignited Insufficient oxygen Heat or smoke from fire inside vessel Introduction of hot gases or fluids from external sources Start up of agitators or putting confined space in motion Prior to entry into pressure vessel: Ensure it is properly drained, ventilated, and cleaned Test atmosphere for oxygen content, explosiveness, and toxic levels Disconnect and blank all connecting pipelines, or close, lock out, and tag valves on the line All power-driven devices disconnected, locked out, and tagged Confined-space permit issued Provide access for safe entry and exit from vessel Use safety harnesses and lifelines

28 Pressure Vessels Testing Hydrostatic tests Can be used for vessels constructed without access for an internal inspection to ensure the vessel is fit for continued service. Targeted for the maximum allowable working pressure, the set pressure of the lowest installed safety valve, or the normal operating pressure of the boiler or pressure vessel at discretion of the authorized inspector. Testing should follow Codes and Standards. Minimize hazards to personnel during testing, isolate test area from operations, and provide suitable barricades. Detecting cracks/measuring thickness Non-destructive methods radiography ultrasonic examination 28

29 Pressure Vessels Pressure testing incidents 29 Filling of a vertical tank The filling of the tank was made with water from a fire hydrant. The top the tank blew off because the relief valve could not displace the air fast enough for the volume of water that was being pumped in. Fortunately no injury happened, but an operator was on top of the tank a few seconds before. Source:

30 Pressure Vessels Pressure Testing Incidents Emptying of a vertical tank 30 The collapse of this tank happened while it was being emptied. A plastic sheet protecting the roof was trapped in the vent; hence a vacuum was caused. There was no injury. It should be noted that this type of incident is not that unusual. Source:

31 Pressure Equipment Protective Devices means devices designed to protect the pressure equipment against the safe operating limits being exceeded 31 Devices for direct pressure limitation safety valves bursting disc safety devices (membranes) controlled safety pressure relief systems Limiting devices, which either activate the means for correction or provide for shutdown or shutdown and lockout pressure switches, temperature switches, fluid level switches safety related measurement control regulation devices Devices designed to give warning that the safe operating limits are being exceeded

32 Protective Devices Relief Design Methodology 32 To ensure that limits are not exceeded, these protective devices should be correctly specified and, where applicable, adjusted to the correct settings. LOCATE RELIEFS CHOOSE TYPE DEVELOP SCENARIOS SIZE RELIEFS Where? Solve Maximum Outflow Spring-Operated Valves Rupture Devices (membranes) CHOOSE WORST CASE DESIGN RELIEF SYSTEM

33 Protective Devices Relief Spring-Operated Valves 33 Source: plastomatic.com Source:

34 Protective Devices Relief Valves - Anything wrong here? 34 Reduced Inlet Piping Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil), Pressure Relief

35 Protective Devices Relief Valves - Anything wrong here? 35 Discharges Pointing Down Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil), Pressure Relief

36 Protective Devices Relief Valves - Anything wrong here? 36 Long Moment Arm Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil), Pressure Relief

37 Protective Devices Relief Valves - Anything wrong here? 37 Will these bolts hold in a relief event? Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil), Pressure Relief

38 Protective Devices Relief Valves - Anything wrong here? 38 Major Contributing Cause: Missing Safety Valve Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil), Pressure Relief

39 Pressure Equipment Preventions of Accidents The catastrophic disasters caused by pressure vessel accidents could be totally prevented by: 39 Designing, fabricating and constructing pressure vessels to comply with applicable codes and standards, and where no pressure vessel law exists, to internationally recognised pressure vessel safety codes. Operating the vessel at pressure below the maximum allowable working pressure with proper pressure setting of relief devices, to handle design pressures and temperatures. Periodically testing and inspecting the vessel as well as the relief devices in order to detect corrosion or erosion of the vessel that can cause holes, leaks, cracks, general thinning of the vessel walls or any other defects. Safety relief valves must be taken off during safety inspection to verify whether their settings are correct. Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010

40 Pressure Equipment Preventions of Accidents The catastrophic disasters caused by pressure vessel accidents could be totally prevented by: Keeping records of inspection reports and monitoring potential problem, so that the vessel may be taken out of service before it becomes dangerous. Also, having all information displayed prominently. Ensuring that alterations or repairs of vessels are only done by competent and authorised persons and the repair must meet the accepted industry quality standards for pressure vessel repair. Providing safety training for employees on job hazard and anticipated conditions that could jeopardize their safety or the safety of others. Periodically provide training for operators on vessels operating procedures to avoid over pressurizing, as well as providing them with adequate and suitable instructions for vessel safe operations. 40 Ladokun T et al., Accidents in Pressure Vessels: Hazard Awareness, Proc. of the World Congress on Engineering 2010