Optimisation of Stem Sealing Elements in Armatures in View of Performance and Blow-out Safety by Surface Treatment

Project Number 317

The proper functioning of valves, gates and other armatures containing stuffing box sealing elements are of great importance for the proper performance of operational safety, availability and efficiency in plants of the power generating industry.

All these devices equipped with stuffing box sealing elements must comply with various demands. In addition to the external tightness, the closing and control function of the device must perform without failure. If the frictional resistance between stem and stuffing box packings is too high, the devices may fail. Abrasion and adhesive wear have a great influence on the leakage rates between stem and packing material.

The goal of this research project is to improve and increase the reliability of the performance by employing various coating technologies on the stem. The optimisation of the surface of the stem shall obtain high wear resistance and reduced friction in the stuffing box sealing within the application range up to 400 °C for packings. For this purpose the following coating technologies will be investigated:

  • Self-flowing alloys:
    • inductive melt-down in vacuum,
    • thermal post-treatment.
  • High-velocity flame spraying/High Velocity Oxygen Fuel (HVOF) (e.g. carbide coating).
  • Amorphous Diamond-Like Carbon Deposition (DLC).
  • Crystalline Chemical Vapour Deposition (CVD).
  • Nitration of the stems.
  • Al-Ti-Nitride Physical Vapour Deposition (PVD).

With mechanical-technological tests the quality of the stem surface and the stem sealing will be investigated accompanied by friction tests using enveloped leakage rate measurements. To assess these sealing properties a compilation of all results will be made about the obtained characteristics such as surface pressure, friction coefficient, leakage rate and metallographical and metallurgical properties of the surface coatings.

The coatings of the stem produced according to the above mentioned coating technologies will be subjected to mechanical-technological investigations in view of:

  • Surface hardness,
  • abrasion resistance,
  • impact strength,
  • thermal shock resistance,
  • resistance against thermal cycling
  • and resistance against corrosion due to water, steam and chloride.

General standard and qualification test methods will be carried out for these investigations.

An additional aim of this project will be the proof of blow-out safety of packings in the stuffing box sealing elements with coated and uncoated stems depending on the residual pre-stresses in the packings.

The research project is carried out from August 2008 to July 2010 at the MPA Stuttgart (person in charge: Dr. Hans Kockelmann) and supervised by a working group of the VGB-Working Panel Pipe Systems, Valves and Pressure Vessels . It is funded by the German Federal Ministry of Economics and Technology via the AiF (AiF project no. 15722 N).