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Valve Design

The design of internal parts for every type of valve at REMOSA is carried out following three distinct approaches adapted to the requirements of two prominent process companies and one large oil company.

The design approaches take into account a temperature and a pressure upset. Dimensioning of internal parts is carried out by considering a long term design pressure at which the valve must be able to safely operate for its expected life - 20 years - and also a pressure upset assumed to last for 1,000 hours.

Due to the high temperatures at which the valve must operate, the most critical failure mode for internal parts is creep. Therefore the allowable stress for a given material is influenced by the temperature and the length of time of load application.

Procedure for Establishing the Allowable Stress

• a.   The expected operating times ( at design temperature and at upset temperature) are established;
• b.   A rupture stress value is assumed;
• c.   From the ASME Nuclear Code Case N-47-29 Tables I-14, the expected minimum times to rupture are extrapolated as a function of the temperature (design and upset ), the assumed rupture stress and material type. Two different minimum times to rupture will be determined, for design and for upset temperature;
• d.   The and times found in c. are compared with the respective expected operating times and ;
• e.   The condition

  

  is verified and the is iteratively changed until the condition is met under ;
• f.   Once the condition is met, the allowable stress is determined using the following equation:

  

Data about rupture strength for the materials used are taken from the ASME Nuclear Components Code Case, where the rupture strength is given as function of time and temperature. Since data are only available for a limited time and temperature range, it is necessary to get data by interpolation making use of the Larson-Miller parameter , which allows to determine the rupture strength as function of just one parameter.

to part 2 - Finite Elements Analysis to part 3 - Thermal Analysis

Copyright © REMOSA spa 1998-2001 Last updated on May 6, 1998