pressure vessel

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Riveted pressure vessel with inspection opening and fittings
Compressed air storage (air tank ) of a small mine
Reactor pressure vessel at Shippingport nuclear power plant in 1956

Pressure vessels are closed vessels , the pressure of which is above the ambient pressure inside. In contrast to pressurized gas containers and transport containers according to traffic law , in which fluids are transported, the operation of pressure containers is assigned to a specific installation location.

Definition and demarcation

Historically, a distinction is made between steam boilers and pressure vessels. Steam boilers are defined as being used to generate steam (steam pressure above 0.5 bar) or hot water (temperature above 110 ° C) by means of a furnace , electrical heating or waste heat.

Pipelines for conveying fluids are not pressure vessels. Pressure vessels are defined as having a specific function assigned to them (storage, filtering , heat transfer ).

Types of pressure vessels

Condenser, subcooler, evaporator and filter dryer in one refrigeration system

In terms of function, pressure vessels can be classified as follows

The ideal design shape for pressure vessels is the ball and it is chosen as a design especially with very high internal pressures or very large volumes ( gas containers ). However, the cylindrical shape is most commonly used. The cylindrical shell is closed on both sides by flat or curved bottoms ( dished bottoms or basket arch bottoms ).

Quality requirements

According to European law on the free movement of goods (New Approach), a distinction is made between “simple pressure vessels” (Directive 2014/29 / EU ) and “ pressure equipment ” ( Pressure Equipment Directive 2014/68 / EU ).

Pressure Equipment Directive

Since 2002, pressure vessels with a maximum permissible operating pressure of over 0.5 bar ( overpressure ) have been subject to the Pressure Equipment Directive. Pressure vessels are in accordance with the Pressure Equipment Directive, in addition pipelines, boilers and pressure accessory parts (such, valves, filters) as a printing device referred and must be placed on the market by an EC declaration of conformity and a CE mark. Depending on the hazard potential (criteria: maximum operating pressure, volume, gaseous or liquid fluid, fluid group), pressure equipment is classified in categories I to IV. The manufacturer can choose from different conformity assessment procedures (prototype, quality assurance system, individual test) in order to meet the requirements of the Pressure Equipment Directive. The pressure vessel is given a manufacturer's plate on which the individual manufacturing number as well as the permissible operating data (operating pressure, test pressure, temperature, volume) and the CE mark are listed. If a notified body was involved in production monitoring, its identification no. to be affixed next to the CE mark. As part of the conformity assessment procedure, the manufacturer must prepare a risk analysis and operating instructions with information on intended use, assembly, commissioning, use and maintenance and, if necessary, improper use.

The construction and dimensioning of pressure vessels is described in various regulations. The Pressure Equipment Directive leaves the choice of the applicable regulations open. In Germany, the AD 2000 rules are mostly used. Harmonized European standards for pressure vessels (series of standards EN 13445) have been developed, which are still being applied hesitantly. Other regulations (ASME, CODAP) may also be used if they meet the basic safety requirements of Appendix I of the Pressure Equipment Directive. The implementation of the directive into German law was implemented by the 14th ProdSV ( Pressure Equipment Ordinance).

The operating regulations for pressure vessels are contained in the Industrial Safety Ordinance.

Simple Pressure Vessel Directive

Since January 1, 1993, simple pressure vessels were only allowed to be placed on the market if they met the basic requirements of the "Directive 87/404 / EEC for simple pressure vessels". The directive was initially superseded by Directive 2009/105 / EC and has since been replaced by the now valid Directive 2014/29 / EU.

Simple pressure vessels are mass-produced pressure vessels for certain limited applications and constructions:

  • only to absorb air or nitrogen
  • Containers must not be exposed to flames
  • only made of carbon steel or aluminum
  • consists of a cylindrical part that is closed by two outwardly curved bottoms or flat bottoms
  • Max. Operating pressure = 30 bar
  • Max. Pressure content product (pressure times volume; p · V) must not exceed 10,000 bar · liters
  • Max. Temperature of 300 ° C (steel) and max. 100 ° C (aluminum)

Typical areas of application for these tanks are industrial compressed air tanks or compressed air tanks in vehicle brake systems.

The implementation of the current Directive 2014/29 / EU in German law was carried out by the 6th ProdSV, a statutory ordinance on the Product Safety Act .

Requirements for the installation and testing of pressure vessels are specified in the national industrial safety ordinance.

business

Industrial Safety Ordinance

Pressure vessels and pressure vessel systems are subject to monitoring in the sense of the Ordinance on Industrial Safety and Health and must be subjected to a test prior to commissioning and periodic tests (external, internal test, strength test). This test can be carried out by a qualified person for containers with a lower risk potential (depending on the permissible operating pressure and the container volume) , and for containers with a higher risk potential by an approved monitoring body (ZÜS) .

For the pressure vessels that have to be checked by the ZÜS, the inspection intervals for the recurring inspections must be specified by the operator as part of the risk assessment. The maximum test periods for recurring tests are specified in the industrial safety regulations and are five years (internal test) and ten years (strength test, usually a pressure test). For the pressure vessels that can be checked by a qualified person, the inspection intervals must also be set by the operator. The maximum test period in these cases is 10 years (strength test, if necessary 15 years).

Equipment parts with safety function

Pressure vessels must be equipped with equipment parts with a safety function in order to protect the vessel from inadmissible operating conditions (exceeding the design pressure or temperature). Equipment parts with a safety function are:

  • Safety valve (for pressure relief in the environment or in a collection system),
  • Rupture disc (pressure relief mostly with large mass flows to be discharged or use with low response pressures),
  • Pressure limiter (switching off the pressure generator, compressor , heating),
  • Temperature limiter (switching off the heating source),
  • Liquid shortage protection (for fired pressure vessels as protection against overheating),
  • Overfill protection (securing a gas cushion above a liquid level to avoid overpressure due to thermal liquid expansion),
  • Vacuum breaker (protection against external overpressure),
  • Special applications in process engineering (reaction blockers in the event of violent exothermic reactions in a pressure vessel, running monitoring of a stirrer to ensure an even course of the reaction in a reactor),
  • Water sprinkler systems (external heat dissipation for cooling as in the event of fire).

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

Operator responsibility

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

  • external protection of the container (collision protection),
  • Access bans,
  • Warning signs ,
  • Mandatory signs (wearing personal protective equipment),
  • Training and instruction of employees,
  • Use only approved spare parts (seals, screws),
  • regular leak tests when using dangerous fluids,
  • regular drainage of compressed air tanks,
  • regular inspection of the container for damage,
  • Functional test of the safety equipment,
  • Protection against contact with hot surfaces,
  • Emergency exercises, if necessary with external bodies ( fire brigade ),
  • Regular maintenance .

The measures identified as necessary are included in operating instructions.

hazards

The hazard posed by pressure vessels comes from the energy stored in the pressurized fluid. The energy is the applied volume change work, which depends on the fluid properties, the volume of the container and the pressure. Because the compressibility (symbol: κ or χ ) of liquids is low, the energy stored in containers with liquids without gas cushions and the risk potential are also low. This influence is taken into account in the Pressure Equipment Directive and the Ordinance on Industrial Safety and Health. The requirements with regard to production and testing according to these regulations are lower for containers that are exposed to liquids than for containers that are exposed to gases. At very high pressures (p> 100 bar), however, the risk of leaks from liquid jets at very high speeds must be taken into account.

In contrast , a considerable amount of energy is stored in pressure vessels filled with gases or liquefied gases (liquids that are heated above atmospheric boiling point) . In the case of pressurized liquefied gases, for example hot water with T above 100 ° C, refrigerants, liquid gases, the re-evaporation of the superheated liquid must also be taken into account when a pressure vessel bursts. If a pressure vessel is significantly damaged in its strength and this leads to a crack extending through the wall during operation, then considerable stress peaks occur at these crack locations, which lead to the vessel tearing open further. This tearing open can cause the container to be thrown away like a rocket several tens to hundreds of meters by the escaping gas pulse. Furthermore, there are considerable hazards from debris being thrown away. When larger containers burst , pressure peaks occur when installed in rooms, which can lead to considerable damage to the building.

Other hazards that may have to be taken into account:

  • Throwing away unsecured quick-release fasteners that can be opened under pressure,
  • Release of toxic fluids,
  • Leakage of hot fluids,
  • Leakage of pressurized liquefied gases (cold burns),
  • Bursting due to external overpressure if this operating case was not taken into account in the design ( implosion ),
  • Formation of an explosive atmosphere,
  • Steam hammer during the expansion of overheated liquids (spontaneous evaporation) with the risk of rupturing pressure-bearing connections,
  • Rapid pressure increase in exothermic reactions,
  • Low temperature embrittlement at impermissibly low temperatures,
  • Crushing from moving parts
  • Pressure wave that tears the lungs like an air mine

Damage to pressure vessels

Damage to pressure vessels can occur due to inadequate manufacture and / or in connection with damaging operating conditions. Insofar as the damage to the wall occurs locally, this can lead to a leak in the event of rust perforation ("leak before break"). As long as the walls still have sufficient strength in addition to a locally limited damage area, a leak does not lead to sudden crack growth. If the damaged area is extended, however, if the critical threshold is not reached, the crack can suddenly grow further, with the result that the container tears open over a large area. This is known as bursting . If the container is exposed to compressible fluids, a considerable amount of energy is released in fractions of a second, which accelerates the container and broken fragments of the wall may form dangerous projectiles.

External corrosion

Corrosion on the outer wall can be triggered by an aggressive atmosphere, such as seawater, cleaning agents containing halogens or acid-forming gases. Containers whose walls are colder than the ambient temperature are at risk from external corrosion. Particularly in the case of black containers, sufficient corrosion protection and a flawless design of the vapor barrier must be ensured.

Burst

Bursting describes the sudden, sudden failure of a pressure vessel. In the case of steam boilers, one also speaks of a bang . Reasons for this can be:

  • Manufacturing defects (poor execution of welded joints ),
  • Material defects or the use of unsuitable or incorrect materials,
  • poor repairs,
  • Large area rusting through of the container with the consequence of the container wall being torn open under operating pressure . (The dimensioning of the wall thickness of a pressure vessel contains a wall thickness allowance, the so-called corrosion allowance )
  • special forms of corrosion that significantly influence the material properties (stress corrosion cracking, aging of the material at high temperatures, low temperature embrittlement),
  • external mechanical force that the pressure vessel cannot withstand,
    • Example: approaching a container by a vehicle
  • thermal influence; Inadmissible heating (fire, fire damage) of the container reduces the strength of the material and can lead to bursting,
  • Impermissible heating and pressure build-up in the container, especially in the case of fluids in the saturated vapor state [liquid gas, refrigerant] with a pressure increase for which the safety valve is not designed
  • Error in the pressure control and non-response of the safety valve , pressure relief valve ,
  • Missing or incorrect pressure protection (non-consideration of the thermal expansion of liquids),
  • Negative pressure damage (the container is not designed for negative pressure and there is no vacuum fuse).

Norms and standards

Austria:

Web links

Commons : pressure vessel  - album with pictures, videos and audio files

Individual evidence

  1. ↑ Directive 2014/29 / EU
  2. ↑ Sixth Ordinance on the Product Safety Act
  3. ↑ container. In: Machines, tools> Other work equipment. Federal Ministry of Economics and Labor, Labor Law and Labor Inspection Section, November 11, 2004, archived from the original on March 11, 2008 ; Retrieved June 14, 2008 .