High pressure seal design

Master's Thesis in Mechanical Engineering - Applied Design

Abstract

Today, in various industries such as aerospace, atomic energy, chemical industries, refineries, petrochemical industries, etc., there is a need to use high-pressure mechanisms to enable the completion of relevant processes. The increase in pressure is done inside the cylindrical chambers and as a result, such an area must be sealed against the penetration of the fluid inside the chamber to the outside. With the increase in pressure, many problems arise in the field of fluid settling, because with the progress of the process, the pressure difference outside and inside the system increases, and as a result, the tendency of the fluid to settle increases. In such a way that the existing tank becomes like a bomb that the smallest disturbance in the sealing system causes leakage and even explosion and irreparable damages. Therefore, to carry out such processes (high pressure processes), safe sealing is one of the most essential requirements. This research deals with the design of a high pressure seal based on Bridgman's design, in such a way that by using fluid pressure and increasing it by a set of seals, the sealing operation can be done reliably. To check the performance of the seal, simulation using Abaqus software has been used. Different sealing rims are checked and finally, a rim that can not only create enough pressure (pressure more than the fluid pressure), but also has a more even distribution of pressure (to prevent damage to the seal) is selected. In this research, PA6, UHMWPE-glass, UHMWPE-ceramic, NBR and silicone are used for high pressure seals. PA6, UHMWPE-glass, UHMWPE-ceramic have elasto-plastic behavior and time function. Hence, elastic and viscoelastic models are used to simulate them.  NBR and silicone have hyperelastic behavior and time function, and hyperelastic and viscoelastic models are used to simulate them.

Key words: sealing, high pressure, Bridgman, polymers, hyperelastic

Introduction

1-1 History

Lundberg[1] registered the first invention related to O-ring in Sweden in 1896. In 1937, Christensen[2] registered a patent in America in which he used rubber rims to improve sealing in a hydraulic brake system. A longitudinal section of this system is shown in Figure 1-1. In Figure 1-2, an enlargement of a part of Figure 1-1 including the rubber o-ring can be seen. This O-ring is made of high-density compressible rubber.

At first, O-ring, as a simple and strong seal in hydraulic systems, was used by the aviation industry during World War II (1939-1945), but since 1950, its use by automobile manufacturers and other industries has expanded [1], [2].

So far, many examples of rubber simulation have been done with the help of finite elements, some of which are mentioned below.

In 2002, Anis et al. modeled a type of Push-Button Diaphragm rubber seal [3].

In 2009, Javan et al. They optimized the sealing of a type of milk. This seal is made of NBR material [4].

In 2009, Lee et al. investigated the failure of a bush-like rubber used in cars [5].

In 2013, Tasura et al. simulated the deformation of an elastomer lip seal [6].

In 2014 Liu and his colleagues investigated the life of a silicone rubber sealing ring, which is used in a jet engine [7].

In 2014, Zooey and colleagues simulated the failure of a pump seal [8].

In all of these cases, the Mooney-Rivlin function was used to model the rubber, except for the fourth case, the parameters of this function are available from uniaxial tensile test data.In the fourth case, Tasura and his colleagues considered different parameters for the Mooney-Rivelin function and selected the best parameters using experimental data and the least squares method. Certainly, without them, many products could not find their way to the market.

Some of the O-ring features that have made it used for sealing since the beginning are:

It can be used to seal cylinders and static pistons up to 5000 psi pressure (the pressure can be constant or variable).

O-rings perform satisfactorily for cylinder and reciprocating pistons up to 5000 psi and may leak a small amount (a few drops per 100 strokes). They have the same function for rotary systems, but in all these cases, the speed of movement of the surfaces must be kept low. In heavy loading or other special cases, to increase the life of the seal set, the mechanism can be designed in such a way that pressure is applied to each O-ring in only one direction. Sometimes, several rows of O-rings are used instead of one O-ring for more certainty. In this case, before the damage, the first o-ring that is under pressure bears the entire applied force.

The effects of temperature change from +18°C to +121°C on the performance of o-rings, depends on the material used for them. Synthetic rubber can be permanently exposed to high temperature or low temperature and can be exposed to wide temperature changes for a short time. Seals can become brittle at very low temperatures, but when heated, they regain their normal flexibility without causing any problems. If the seals are exposed to extreme heat for a long time, their hardness increases permanently and they are destroyed. Usually, the coefficient of thermal expansion of synthetic rubber is small enough that temperature changes do not create a problem for design (these characteristics are not true for all elastomeric compounds).

O-ring seals are more useful than other seals due to their simplicity, high strength, cheap price, and easy installation.

Although O-rings are widely used, they are not a suitable solution for all sealing problems. For example, in cases where:

rotational speed is more than min/ft1500.

The environment is incompatible with elastomeric materials.

There is not enough space.

They cannot be used

Abstract

Nowadays, in various industries such as aerospace industry, gas and oil, chemical industry, refineries and petrochemical industries, etc., mechanisms at high high pressure are required to allow the process to be completed. Pressure rise of the fluid occurs inside the chamber which usually has a cylindrical shape. Therefore, it is vital to design a sealing system in order to prevent the leakage of the fluid. The leakage happens when the fluid pressure is increased. The reason for this is that as the process progresses, the pressure differences between the outside and inside of the system goes up and consequently the tendency of the fluid leakage increases in a way that the tank turns into a kind of uncontrollable vessel chamber that even the least disruption in sealing system causes fluid leakage and it's damage of which maybe irreparable. Therefore, for such processes (high pressure processes), sealing securely is an extremely important prerequisite.

This research aims at designing a high-pressure seal, based on the Bridgman plan, by means of which the fluid pressure is increased through a sealing system, as a result of which the sealing operation is carried out securely.