When it comes to flanges, many people may not be p...MORE >>
When pumps and valves operate in high pressure Plastic […]
When pumps and valves operate in high pressure Plastic Valve ranges, you need to ensure that they are robust enough and don’t cause issues during operation. How to prove that a design will potential work within the specifications is unfortunately less obvious. Of course you can and most likely will use the as a guideline, but decoding, understanding and applying what ASME VIII says is unfortunately far from trivial.If a high pressure valve is at its operating pressure, or at is maximum pressure for testing purposes, the body of the valve will have small areas that already show stress results beyond the yield limit. So just taking the traditional approach to perform linear analysis and make sure that the components are kept out of the region of yield limits, simply doesn’t work.
These small regions that show stress values above yield limits generally don’t cause component failure, but how do you prove that?In the valve industry as well as for pressure vessels, it is quite common to perform Stress Linearization. Stress linearization is a technique which was developed to allow an assessment of the yielding in a structure, and judge whether this yielding leads to premature component failure. The issue with Stress Linearization is that it is essentially a judgement call based on codes and checks, and requires some degree of creativity in its application. It doesn’t model what actually happens in the failure process; which a proper elasto-plastic analysis would. In all fairness, stress linearization as an approach dates from a time when a decent linear model of a valve would have been quite an achievement.
Luckily it turns out that ASME does allow in its codes, the use of an elasto-plastic approach in the assessment of valve bodies, especially in extreme applications. And this has some real advantages; and not just in terms of compliance. By modelling the actual plasticity developing in the solid body you have a picture of what is really happening. And if the growth of the plastic zone is moderate, and doesn’t run away out of control at test pressures, you have a real assessment of the design’s response; which you can use to both show compliance, and understand where you are on the spectrum which ranges from test failure to commercial success.
We have done quite some projects for which, because of our current NDA’s, we are not allowed to talk about it. Therefore we have put together an example model which has all the features of a typical high pressure valve. We have modelled all the contact interfaces, applied 3D solid bolt representations, in order to be able to show how the yielded material is explicitly modelled, rather than having its influence inferred.As a Simulation Engineer, please feel tempted to try the Elasto-Plastic approach as an alternative to the Stress Linearization approach. We think that this is a good alternative and more accurate way of modelling what actually happens in the valve, for values that are close to, or beyond the yield limits.