l’Estrazione dei fasci tubieri è stata possibile grazie all'impiego di un estrattore progettato e costruito dalla VED di Priolo Gargallo Su due scambiatori di calore ad asse verticale I fasci...
Great participation this year at SAFAP!
More than 400 people present and high commitment in presentations.
From 2004 until today it is considered the most important national appointment of technical and scientific skills in the sector; the presentation of the results of research and new studies, on the safety issues of pressure equipment, and related experiences gained in the practice of prevention.
Among the topics covered:
Structural Integrity – Materials and Mechanisms of Damage – Design and Prefabrication – Natech Risks – Diagnostics – Risk Analysis – Seveso Directive – Poster Room
And what about VED? What have we been involved in this 2018 edition? Did we bring any news? or revisions on the contents of previous editions?
Yes, this year, three VED departments have given their availability to work with the technical / engineering sector of INAL; specifically, we participated in two public presentations and a poster review.
At SAFAP, every two years, dozens of memories are published on studies and research for solutions to problems related to the world of pressure equipment. VED for over 40 years in the field of PROBLEM SOLVING has exposed the following issues:
- How to prevent damage during maintenance in the bolts of flanged joints
- Improved the characteristics of the STP: the threaded cap to exclude tubes of shell and tube exchangers
- Developments on the use of composite materials for repairs of damaged lines due to corrosion effects
Here we will focus on the first theme, which has already published the entire memory and the poster (Read the entire memory presented to the SAFAP on LinkedIn or download the pdf) and we will also answer the questions:
- What are the elements that make up a bolted flanged joint with an interposed seal?
- What kind of damages have been found during the maintenance of these types of joints?
- What considerations have been reached in the face of tests carried out to find solutions?
- What kind of solutions are proposed?
It seems a contradiction to assert that damage may arise during maintenance, to restore the functionality of the features to be maintained, but it is a fact that in some conditions the damage is caused by maintenance activities. VED and INAIL present the solutions, to be applied in the assembly phases of the equipment, to avoid further damage during subsequent maintenance.
What are the elements that make up a bolted flanged joint with interposed gasket?
For those who have never seen and heard of flanged joints, and want to continue to deepen their knowledge, we explain immediately that to connect more elements between them, within which pass a fluid (liquid / gas), will be used bolts and tie rods. These approach the two elements to the point where the seals are placed between the sealed faces. So the elements needed to tighten a joint will be: two flanges, tie rods / bolts and the gasket.
Is it possible to use other? Yes, washers; we’ll see more later in this article.
What kind of damages have been found during the maintenance of these types of joints?
During the disassembly of the threaded rods (tie rods), sometimes problems of seizure of the nuts on the rods are encountered, above all where the operating temperatures are high. This happens because the thread has been compromised. The load that the bolt or tie-rod must withstand and the addition of a high temperature triggers plastic deformations on the threads, creating a single screw-nut block. It will therefore be necessary to destroy the rod by oxyacetylene cutting.
But there are other conditions that connected to each other trigger a series of further damage. The following image, extrapolated from the Poster, shows them to us:
- damage caused by poor manufacturing of nuts
- deformation phenomena of the flange due to the friction of the nuts on it
- damage to the first threads of the nut
What considerations have been reached in the face of the tests performed for the search for solutions?
The same operating conditions were replicated in the laboratory so as to be able to examine the behavior and, through the use of measuring instruments, to detect the data to be studied. For the entire explanatory text we refer to the “integral memory”, but we highlight some relevant technical details:
- High temperate plays a significant role in triggering seizure phenomena
- Using the washers, the friction coefficient between nut and flange is reduced
- The tensile load can be distributed to a larger number of threads by the use of concave whasher in contact with the nut
- It is possible to redistribute the load on several threads of the nut, removing the first 3, thus avoiding the seizure
What kind of solutions are proposed?
The solution includes an effective preventive action, which proposes to use nuts and tie rods together with washers; made with a geometry such as to cancel any seizure phenomena. In this way, the tightening loads are redistributed and additional damage to the flange and tie rods is avoided.
The following image summarizes in detail how the modifications applied to nuts and washers are of great advantage in avoiding the deterioration of the nuts and bolts making a more reliable and safe coupling: