Investigation and model development of pressure relief valve leak tightness

Student thesis: Doctoral Thesis

Abstract

Controlling and assessing the leak tightness of a Pressure Relief Valve (PRV) has been a challenge since the original design of the product. With more stringent demands from the nuclear power industry for leakproof PRV’s, closer to the set-pressure, there has been a drive by both industry and academia for a better design method for many known metal-to-metal contacting seal/surface problems. This thesis attempts to understand the surface metrology characteristics which facilitate leakage, from which a numerical modelling tool is developed.Drawn from industry experience, it is found that the main surface finish contributing factor to leakage is form, rather than roughness. Industry experience combined with metrology measurements allowed investigating the effects of poli-lapping and improving the surface form for a new disc. This led to reducing the leakage rate proving, this to be the key influencing parameter on leakage. A numerical modelling tool is created taking into consideration the surface form, waviness and roughness. The numerical approach requires efficient coupling of a non-linear structural Finite Element Analysis (FEA) with a Computational Fluid Dynamic (CFD) solver. This allows the examination of the relationship between deformation of the contacting surfaces, based on the applied spring force, and the resultingmicro-flow of gas through any available gaps and the overall leakage to be found.The API527 Seat Tightness methodology is followed to allow leakage rates to be measured and the computational model to be validated at a set-pressure of 0.5 MPa. It was found that the API527 standard falls short of being able to quantify leakage, rather it is understood to be an indicator of leakage. The FEA numerical model builds upon the current literature by using the sum surface technique, which is partially validated in this thesis. The CFD model is verified and validated using equations and experimental results found in the literature. There is confidence in the leakage results calculated via the CFD model up to 10MPa, after which no equations or experimental data exist in the literature which can be used for validation purposes against the CFD results. It is found by reducing the seat length to 0.5 mm and applying a force equivalent to 18.6 MPa, the contact between the metal surfaces reduced to 99.5% roughness contact. Another technique developed called the Surface Compliance Technique (SCT) showed that by applying 2.2x the spring force for a set-pressure of 18.6MPa, the gap spacing due to the form and waviness is plastically deformed, closing the gap spacing. A secondary factor known as valve swivel was also investigated finding that it has no effect on leakage. Also from the roughness numerical CFD model, the permeability coefficient is found. In addition, the numerical approach can potentially be applied to other metal-to-metal contacting surface components, such as flanges with metal gaskets, to help eliminate leakage.
Date of Award1 Jun 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsWeir Group plc (The) & University of Strathclyde
SupervisorWilliam Dempster (Supervisor) & Robert Hamilton (Supervisor)

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