Hans Kockelmann

Gas Leakage Correlation

A project has been carried out on the correlation of leakage measurements of different gases in specific conditions in bolted flanged assembly. The objectives were: • to check if the relationship between the leakage measurements of Helium, CO2 , refrigerant R134a and CH4 would be similar to results of calculations that can be found when using correlation formulas, • to compare emissions of a typical laboratory fluid (Helium) with process fluids like methane, used in petrochemical plants, as well as CO2 and R134a, used in the refrigeration industry. Referring to TA Luft and VDI 2440, which specify a unique test for the certification of gaskets, the configurations of the testing installation used consist of flanges assembled with either PTFE based, graphite or fibre based gaskets. Different gasket loads and internal pressure were applied in order to simulate molecular, intermediate or laminar leakage flow rates. This article describes the test configurations and shows the leakage measurement results. It also compares the ratios between the leakage values and the results that would be obtained by predicting the leakage of another gas and/or in other pressure conditions, using Poiseuille or Knudsen laws.Copyright

Application and evaluation of fracture mechanics failure concepts to precracked vessels

Abstract The elastic-plastic fracture mechanics concepts R -curve method, Two Criteria Approach, COD concept and Battelle formula were applied on three tested vessels made of low and high tough and fine grained structural steel and X 8 Ni 9 with an axial notch on the outer surface. Apart from the R -curve method for the vessel made of low tough material, failure loads were conservatively calculated. The COD concept could not be applied to these vessels and failure geometries using the well known design curve. A modification of the COD concept by means of FE-calculations was made so that a correlation between the displacement at the crack tip and at the vessel surface could be determined. With this procedure, the calculated load at fracture was only 8% below the experimental result.

Fracture mechanics evaluation of small diameter piping considering the latest experimental results

Abstract The results obtained from investigations carried out on austenitic piping of small nominal diameter (DN80 and DN50) are introduced and discussed together with their assessment using fracture mechanics methods. Essential results are summarised as following. The pipes with flaws (fatigue crack) down to a depth to a max / t =0.51 (DN80) as well as a max / t =0.62 (DN50) and a circumferential extension of results 2 α =120° reached bending angles up to 26°. The ASME collapse load (test collapse load) was exceeded considerably and the experimental maximum load could not be reached. Failure due to a leakage or rupture did not occur in any test. The maximum crack extension was 0.69 mm (DN80, a max / t =0.51) resp. 0.3 mm (DN50, a max / t =0.62). The experimental maximum load can approximately be assessed by the limit analysis. The fracture mechanics approximation methods GE/EPRI and LBB/NRC calculated a / t =0.4 and 2 α =120° initiation loads above the experimental maximum load for pipes containing flaws. These results confirmed the procedures for the proof of integrity of small diameter piping by updating information on load, deformation and failure behaviour of austenitic piping damaged with circumferential flaws. Using these results may formulate a final safety concept for the proof of integrity of small diameter piping by completing the current concepts.

The basic concept to ensure the integrity of components during operation

Abstract The integrity of components which are relevant to safety is one of the most important prerequisites for the safe operation of a power plant. A systematic way of proceeding can be achieved by the basic safety concept. In doing so attention has to be paid to the balanced application of the so-called ‘redundant measures’. Using the pressuriser spray system of a nuclear power plant as an example, the relevant factors of influence and the way of proceeding are demonstrated for piping system components which are important to safety.

Operational monitoring in German nuclear power plants

The Atomic Energy Act requires that measures made feasible by state of the art technology be adopted to avoid damage that could be caused as the result of the construction and operation of a nuclear plant. This stipulation constitutes the basis for deriving requirements for planning, design, construction, operation and decommissioning. Ensuring the function and integrity of those components and systems that are relevant to plant safety is of major significance with regard to operation of a nuclear power plant. The basis for ensuring these features is laid in planning, design and construction. Important as these foundations may be, it is absolutely essential to monitor the quality originally planned and achieved in an object as undeniably complex as a nuclear power plant. The RSK-Leitlinien fur Druckwasserreaktoren (Reactor Safety Commission Guidelines for Pressurized Water Reactors) incorporate fundamental requirements for design, mechanical design, materials, manufacturing, testing and examination, and operation. Meeting these requirements makes it possible to exclude a catastrophic rupture of the components in the reactor cooling system pressure boundary (primary system), as has been demonstrated in detailed research and development work. The term basic safety was defined for this concept. Basic safety coupled with multiple redundancy suffices to exclude the possibility of large ruptures (rupture preclusion). The principle of plant monitoring and documentation (operational monitoring) implements redundancy in a significant manner within this concept. The monitoring techniques used in Germany have reached an advanced state of development and are still being optimized. Thus, operational monitoring is a major contributory factor in the safety and high availability of nuclear power plants. It also provides a means of expanding our knowledge of life time expectation.

Fatigue assessment of power plant components and monitoring during operation

Abstract Practical experience has proved that not all transient loadings have been taken into consideration during design of components against fatigue. Specific monitoring of all components susceptible to fatigue during operation guarantees recording of all transient loadings (including unspecified), realistic fatigue analyses, adjustment of the mode of operation and improved assessment of the remaining service life. The experience of in-service monitoring at nuclear power plant GKN over some years serves as a basis for the above.

Optimization of Stuffing Box Stem Sealings in Valves by Means of Surface Treatment and Coating

The optimization of the frictional behavior of valve stems in stuffing box sealing systems by means of stem surface treatment and coating is subject of a research project carried out at MPA Stuttgart. Low friction should lead to a high compression and densification of the packing already during assembly resulting in a reduced drop of packing stress with service time and improved tightness. Several surface treatment and coating technologies (Nitrogen and Boron Hardening, inductive coat, Si- and Me-DLC, AlTiN, Chromium-Nitride Multilayer, Tungsten-Carbide and Chromium-Carbide) were examined. Friction tests were carried out at 400 °C followed by leakage tests (160 bar, test fluid Nitrogen) on simulated stuffing box sealings with usual graphite packings and coated or surface treated stems in comparison to a “standard” stem without coating and surface treatment. These combined friction and leakage tests were accompanied by mechanical, technological and metallographical investigations. Visual inspection of the stems after the friction tests suggests a classification in 3 categories depending on the graphite adhesion to the stem surface. Some coatings caused a deterioration (increase of leakage rate) compared to the standard stem (without any coating or surface treatment). In the other cases the leakage rates were comparable to that of the standard stem. Most favorable behavior was observed for the stem with inductive coat. This technology can be seen as a repair technology for damaged stems.Copyright

Long Term Behaviour of Stuffing Box Packings Under the Influence of Fluids at High Temperature

Testing techniques were developed to investigate the long-term behaviour of stuffing box packings under real service conditions. This equipment meets all requirements for the simulation of high temperature applications with gaseous fluids, hot water and steam: • controlled pre-tightening (gland load); • variation of the stiffness of the assembly (e.g. Bellville springs); • determination and monitoring of packing stress - respective gland load; • control of stroke and stem force; • heating-up, pressurization and temperature control; • conditioning of water and steam chemistry. All relevant influencing parameters on the long-term behaviour of varied stuffing box packings based on flexible Graphite, woven PTFE fibres and special packing designs were taken into consideration: • assembly procedure (packing stress); • stroke and number of cycles; • effect of Bellville springs; • medium and temperature. In detail, friction, relaxation and leakage tests were carried out. On the basis of these results an evaluation of the long-term performance of the different packing types based on Graphite and PTFE is possible.Copyright

Blow-Out Safety of Gaskets for Bolted Flange Connections

Safety-relevant bolted flange connections used in industrial plants require in addition to the strength and tightness proof a proof of blow-out safety of the gaskets within the scope of design. In order to assess the behaviour of a gasket concerning its blow-out safety it is essential to know the gasket characteristics which describe the sealing and creep-relaxation behaviour. To prove the blow-out safety of bolted flange connections a reproducible experimental test method was developed. This method can be used to complement the design of bolted flange connections with the aspect blow-out safety. The investigated soft gaskets show four different types of behaviour with regard to the blow-out safety. They characterize the hazardous potential in form of blow-out and cause the classification of different gasket types and materials.© 2008 ASME

Design of Floating Type Bolted Flange Connections With GRP Flanges

Essential results of a research project “design of floating type bolted flange connections with glass-fibre reinforced plastic flanges for the chemical industry” are summarized in the following: • Various gaskets on the basis of rubber (EPDM) and PTFE were investigated in view of the boundary conditions in bolted flange connections with glass-fibre reinforced plastic (specified in the following as GRP) flanges in compliance with DIN EN 13555. The rubber gasket, the rubber-steel gasket with PTFE envelope and the PTFE based gasket with diffusion barrier meet the leakage rate criterion of TA Luft under the conditions of bolted flange connections with GRP flanges. • The mechanical behaviour of bolted flange connections diameter 50, DN50, with GRP flanges was investigated and is taken under consideration in a new calculation procedure. This draft procedure in the context with experimental pre-tests (already demanded in DIN 16966) is now available to design bolted flange connections with GRP flanges. Thus, the high materials reduction factors defined in AD-Merkblatt N1 can be ignored.Copyright