Pressure vessel

Pressure vessel are closed container whose inside pressure is above ambient pressure. Unlike pressurized gas tanks and transport containers according to traffic law, in which fluids are conveyed, the operation of pressure vessels is assigned to a particular site.

• 4.1 Operational Safety Ordinance
• 4.2 equipment with safety function
• 4.3 operator responsibility
• 4.4 hazards
• 4.5 damage to pressure vessels 4.5.1 External Corrosion
• 4.5.2 bursting

Definition and delimitation

Historically distinction is made between steam boilers and pressure vessels. Boilers are defined in that they are used for the generation of water vapor ( steam pressure above 0.5 bar) or hot water ( temperature 110 ° C) by means of a furnace, the electrical heating or waste heat.

Pipelines for the transport of fluids are not pressure vessels. Pressure vessels are defined as having a specific function assigned to them ( bearings, filters, heat transfer).

Pressure vessel types

Pressure vessel can be divided on the function as follows in terms

• Storage containers for gases ( LPG storage tanks, cryogenic liquefied gases stored )
• Compressed air tank,
• Silos with compressed air for pneumatic conveying of granular materials ( silo to truck )
• Tanks / containers which are discharged under pneumatic pressure
• Pressure reservoir ( hydraulic accumulators, diaphragm expansion tank, air tanks )
• Heater in a thermal oil system,
• Separators, collectors, filter ( water separator),
• Heat exchangers (condensers, a condenser ),
• Procedural containers and vessels ( agitator, columns ),
• Heated or cooled pressure vessel as part of a system ( steam-heated cylinders in paper making, plate freezers )
• Reactor pressure vessels of nuclear power plants

Quality requirements

According to European law of free movement of goods (New Approach), a distinction between the " simple pressure vessels " (Directive 2009/105/EC ) and the " pressure equipment " (Pressure Equipment Directive 97/23/EC).

Pressure Equipment Directive

Since 2002, pressure vessels subject to a maximum allowable operating pressures above 0.5 bar (gauge pressure ) of the EC Directive 97/23/EC ( Pressure Equipment Directive). Pressure vessels in accordance with the Pressure Equipment Directive, other pipelines, boilers and pressure- containing equipment items ( such, valves, filters) called a print device and must be placed on the market by an EC declaration of conformity and CE marking. Depending on the potential risk (criteria: maximum operating pressure, volume, gaseous or liquid fluid, fluid group ) Pressure equipment are classified in categories I to IV. The manufacturer can choose from different conformity assessment procedures (type, quality assurance system, unit verification), to meet the requirements of the Pressure Equipment Directive. The pressure vessel receives a Herstellschild on which the individual production number and the permitted operating data (operating pressure, test pressure, temperature, volume ) and the CE mark are listed. If a notified body was involved in the production control, is their code no. to install in addition to the CE mark. The manufacturer must produce a hazard analysis as part of the conformity assessment procedure and create an instruction manual containing information intended for use, on installation, commissioning, use and maintenance and possibly risk of misuse.

The design and sizing of pressure vessels is described in various regulations. The Pressure Equipment Directive leaves open the choice of the applicable rules. In Germany, AD 2000 Code is most often applied. There have been developed that are still used hesitant European harmonized standards for pressure vessels ( standard series EN 13445 ). Other rules ( ASME, CODAP ) may also be used, if so, the essential requirements of Annex I of the PED are satisfied. The implementation of the Directive into German law has been implemented by 14 ProdSV ( Pressure Equipment Directive ).

The operating rules of pressure vessels are included in the Operational Safety Ordinance.

Directive Simple Pressure Vessels

Since 1 January 1993, Simple Pressure Vessels may only be placed on the market if they are " simple pressure vessels Directive 87/404/EEC " meet the basic requirements of ( Superseded by 2009/105/EC ). Simple pressure vessels are serially produced pressure vessels for certain limited applications and constructions:

• Only to contain air or nitrogen
• Containers should not be exposed to flame
• Only made ​​of carbon steel or aluminum
• Consists of a cylindrical part which is closed by two outwardly curved plates or flat bottoms
• Max. Operating pressure = 30 bar
• Max. Pressure capacity ( pressure times volume, P · V) must be less than 10,000 bar.litre
• Max. Temperature of 300 ° C ( steel) and max. 100 ° C ( aluminum)

Typical applications of these containers are industrial compressed air tank or compressed air tank in vehicle braking systems. The implementation of the Directive into German law was implemented by the 6th ProdSV ( an ordinance of the Product Safety Act ).

Requirements for the establishment and testing of pressure vessels are specified in the national industrial safety regulations.

Operation

Ordinance on Industrial Safety

Pressure vessels and pressure vessels are a installation subject to monitoring in accordance with the Ordinance on Industrial Safety and must be subjected to an inspection prior to commissioning and in-service inspections ( external, internal testing, strength testing ). This test can ( depending on the permissible operating pressure and the container volume) by a competent person on a vessel with lower risk potential; in containers with a higher hazard potential of an approved body ( ZÜS ), these include the TÜV, DEKRA, GTÜ and the approved testing laboratories are carried out by large companies.

When the pressure vessels that need to be examined by the relevant inspection, the inspection intervals for periodic inspections by the operator as part of a safety evaluation, but considering the maximum inspection intervals to define. The maximum inspection intervals for periodic inspections are specified in the Ordinance on Industrial Safety and five years (internal audit ) and ten years ( strength test, usually a pressure test ). When the pressure vessels that can be tested by a competent person, the inspection intervals are also set by the operator. For this purpose, as specified in the Ordinance on Industrial Safety maximum inspection intervals shall not apply.

Equipment with safety function

Pressure vessel must be equipped with safety accessories function to protect the tank against unacceptable operating conditions (exceeding the design pressure or temperature). Equipment with safety function are:

• Safety valve ( for pressure relief in the environment or in a fall arrest system),
• Rupture disc ( pressure relief usually with large mass flows payable or use at low set pressures ),
• Pressure limiter ( off the pressure generator, compressor, heating),
• Temperature limiter ( switching off the heating source )
• Liquid indicator ( when fired pressure vessels as protection against overheating )
• Overfill protection (ensuring a gas cushion above a liquid level to prevent an excess pressure due to thermal expansion of liquid )
• Vacuum breakers (protection against external pressure ),
• Special applications in process engineering ( reaction blockers in violent exothermic reactions in a pressure vessel, run monitoring of an agitator to ensure smooth progress of the reaction in a reactor ).
• Water sprinkling devices ( external supply of heat for cooling as in the case of fire ).

When the container with the safety equipment is brought by a manufacturer as a functional unit on the market, this is referred to as an assembly. He must create an EC declaration of conformity for this module and describe the limits in the operating instructions.

Operator responsibility

The operator of a pressure tank system must ensure safe operation. He must produce a risk assessment and provide measures to avoid or hazards. These can be:

• External protection of the vessel ( collision ),
• Denial of entry,
• Warning signs,
• Mandatory Signs ( wearing of personal protective equipment ),
• Training and instruction of employees,
• Use only approved spare parts (gaskets, bolts),
• Regular leak tests on use of hazardous fluids,
• Regular drainage of compressed air receivers,
• Regular monitoring of the container for damage,
• Function test of safety equipment,
• Protection against contact with hot surfaces,
• Emergency exercises possibly with external agencies ( fire department),
• Regular maintenance.

The measures identified as necessary to be included in an operating statement.

Hazards

Exposure to the pressure vessel is based on the energy stored in the pressurized fluid. The energy, the applied volume change operation, which depends on the fluid properties, the volume of the container and the pressure. Since the compressibility (symbol: κ or χ ) is of fluids small, the energy stored in containers with liquids without gas cushion and also the potential risk is low. In the Pressure Equipment Directive and industrial safety regulations, this influence is taken into account. The requirements regarding manufacture and testing of these prescriptions are lower than for flüssigkeitsbeaufschlagte container for containers that are exposed to gases. At very high pressures (P> 100 bar), but the hazard leaks caused by jets of liquid must be considered at very high speed.

In contrast, in the pressure vessel, the pressure-liquefied gases or gases ( liquids which are heated above the atmospheric boiling point ), stored in considerable energy. In pressure-liquefied gases, such as hot water with T above 100 ° C, refrigerants, liquid is at the bursting of a pressure vessel nor the re-evaporation of superheated liquid to be considered. If a pressure vessel is damaged considerably in strength and this leads, in operation to a wall by cross plan, then replace these crack sites on significant voltage spikes which lead to a further rupture of the container. This tearing can cause the container is like a rocket thrown by the exiting gas pulse several tens to hundreds of meters. Furthermore, significantly hazards occur on by flying debris. When rupture occurs at a larger tank placed in rooms to pressure spikes that can lead to a substantial building damage.

Other hazards which are to be observed:

• Sliding away from unsecured quick conclusions that can be opened under pressure
• Release of toxic fluids,
• Leakage of hot fluids,
• Leakage pressure-liquefied gases ( cold burns),
• Bursting through external pressure, if this operating case was not considered in the design ( implosion )
• Formation of an explosive atmosphere,
• Steam blows in the relaxation of superheated liquids ( spontaneous evaporation ) with the risk of tearing of pressure-bearing compounds,
• Fast pressure increase in exothermic reactions,
• Low temperature embrittlement at low temperatures inadmissible,
• Crushing by moving parts
• Pressure wave that leads to type an Aerial Mine for lung crack

Damage to pressure vessels

Damage to the pressure vessel can occur due to faulty manufacture and / or in connection with adverse operating conditions. As far as the damage to the wall occurs locally, this may result in a rust to leakage. As long as the walls next to a localized damage area still have sufficient strength, performs a leakage not to abrupt crack growth. If the damaged area stretched but then falls below the critical threshold, the crack suddenly continue to grow, with the result that the container ruptures over a large area. This is called a burst. When the container is pressurized with compressible fluids is in a split second free a considerable energy that accelerates the container and possibly form torn fragments of the wall dangerous projectiles.

External corrosion

The corrosion of the outer wall can be triggered by an aggressive atmosphere, so by seawater, halogen- containing cleaning agents or by acid-forming gases. Container whose walls are colder than the ambient temperature are at risk from external corrosion. Especially with black containers must be paid to adequate corrosion protection and flawless execution of the vapor barrier.

Burst

With the sudden rupture, sudden failure of a pressure vessel is called. With steam boilers, one also speaks of a to bursting. Reasons for this may include:

• Workmanship ( poor workmanship of welds )
• Material defects or improper use or improper materials,
• Faulty repairs,
• Large rusting of the container with the result of splitting the container under operating pressure. ( The dimensions of the wall thickness of a pressure vessel containing a wall thickness surcharge, the so-called corrosion allowance )
• Special forms of corrosion, which greatly affect the material properties ( stress corrosion cracking, aging of the material at high temperatures, low temperature embrittlement )
• External mechanical force, which the pressure vessel can not withstand, Example: Approaching a container by a vehicle

Developments

When nuclear power plant THTR -300 by Rudolf Schulten, which went into operation in 1985, a reactor pressure vessel made of prestressed concrete was used in which rupture is impossible. This type of container was chosen because classic steel containers can not be built in the required size. His performance was, however, partly because of thermal decomposition concrete, are not convincing. For nuclear power plants with high- temperature reactors is therefore also thought about pre-stressed container made ​​of cast steel or ductile iron, but are still in the early stages of development.

The constructive ideal shape for pressure vessels is the ball and it is often chosen in particular at very high internal pressures or very large volume (gas tank) as design. Most, however, the cylindrical shape is used. The jacket is closed by flat or dished ends ( dish bottoms or ellipsoidal heads ) on both sides.

Norms and Standards

Austria:

• Provisions of the boiler right: Simple Pressure Vessels Ordinance Template: § § / Maintenance / old URL, Federal Law Gazette No. 388/1994
• Shipping Container Regulations 2002 (VBV 2002) Template: § § / Maintenance / old URL, Federal Law Gazette II No. 202/2002
• Pressure Equipment Directive - DGVOVorlage: § § / Maintenance / old URL, Federal Law Gazette II No. 426/1999
• Regulation on the establishment and operation of steam boilers - ABVVorlage: § § / Maintenance / old URL, Federal Law Gazette No. 353/1995
• Pressure vessel Aufstellungs Regulation - DBA VOVorlage: § § / Maintenance / old URL, Federal Law Gazette II No. 361/1998
• Transportable Pressure Equipment Regulation ( ODGVO ) Template: § § / Maintenance / old URL, Federal Law Gazette II No. 291/2001
• Pressure Equipment Monitoring Regulation ( DGÜW -V)

• § § 59, 60Vorlage: § § / Maintenance / old URL General Workers Protection Ordinance, Federal Law Gazette No. 218/1983