Reducing the range of vibrations of pipes of gas pressure reduction stations

Mechanical engineering-applied design orientation

Abstract:

Vibration is one of the important and basic concepts in engineering sciences. In simple words, any system that has mass and spring property has the ability to vibrate and can vibrate at a natural frequency. It can be said with certainty that 95% of vibrations in industries are harmful to systems. If the system is in any case intentionally or unintentionally stimulated with a force so that the stimulation frequency coincides with the natural frequency, the oscillation amplitude will increase so much that it will cause the collapse of the system. In this case, one of the limiters of vibrations is damping in the system. In station installations, the main cause of vibration is the appointed regulators, which is confirmed by using experience and tests. At first, the best way to prevent vibration is to eliminate vibration at the source of production. But this work is not always practical, so in most cases it will be impractical due to high costs and they are looking for ways to balance the vibrations. This process is done in the pressure reduction stations by changing the distribution of mass and stiffness by installing the harness and base, in which case the natural frequency changes, so that the excitation frequency no longer matches the natural frequencies. The frequency range of tubes is classified into two categories. The first category of frequencies below 300 Hz, which happens mostly in oil and gas and petrochemical refineries. In this case, there is an exact solution for some situations and the rest have experimental solutions (based on operating conditions, work and type of fluid). But at frequencies above 300 Hz, which often happens in nuclear power plants, there is no exact solution and more information is based on numerical solutions, test results and experience. In this thesis, taking into account that the vibrations are in the first type, the supports and constraints have been investigated, and due to the modeling and analysis with the finite element program, the amplitude of the vibrations has been reduced.

Key words:

oscillation, vibration, natural frequency, mechanical resonance, pressure pulses, main pipeline, branches, supports and limitations, limited components and reduction of vibrations

Introduction:

In this chapter, the impact of vibrations and stresses on various structures and their losses and damages even on the human body will be discussed.

1.1 Vibrations:

Vibrations are related to the free and oscillating movement of the entire structure or a part of the structure. Here the golden and key word is "oscillation" and it is very obvious that vibrations are one of the rules governing the world. Examples of the impact of vibrations are given in Table 1.1.

Natural phenomena and vibrations

Perhaps before engineers can understand the aspects of the vibration phenomenon, it is necessary to investigate the impact of vibrations on our own body. As an introduction to ship passengers who are on long journeys at sea, as the ship goes up and down due to the occurrence of waves, their bodies will also be in an oscillating state. In limited ranges of vibrations, the human body will be within its comfort zone. But with the increase in the amplitude of oscillations and the prolongation of their duration, the body no longer feels comfortable and will face various reactions.

Physiological effect of vibrations:

The effect of vibrations on the human body will cause a person's illness or disharmony of body parts. This case will vary from the discomfort of the body in a ship or plane to the breaking of bones.

When the human body is in contact with moving tools, it will be stimulated by vibrations. Low-frequency and high-amplitude vibrations will bring water to the lungs and nervous states in the brain and stimulate the mucous fluid in the spine. Accordingly, sudden shocks will have a direct effect on the eyes, stomach, sweat gland secretion, and therefore may cause abnormal reactions in the body and the feeling of illness.. In some cases, vibrations caused by fluctuating forces and blows may cause organ failure or disability.

Tests show that a sudden decrease in the speed of an airplane or car (with a change in acceleration of 4.5g) may cause a fracture of the skull, and while the change in acceleration is 5.75g, it will cause a fracture in the leg bone.

Experience has shown that the effect of vibrations even It is harmful to a sleeping person. Below 3 Hz will cause stomach ache, above 8 Hz will affect the head, 8 to 20 Hz will cause dizziness and between 20 to 100 Hz will reduce vision and not see objects clearly. A large range of oscillation causes the muscles to relax so that a person will not be able to work.

Tensions caused by vibrations:

If a rod or pipe is subjected to lateral loads, then the form of deformation is bending or buckling. This state is related to shear stress regardless of the cross section. All components in our real life are exchanging forces and as a result exerting tension and bending. All components have a corresponding frequency and mode shape due to the distribution of mass and stiffness.

When a part of the structure is bent due to vibrations, stress is applied to the structure. According to the condition of the object, this tension may be tensile in some points and compressive or even bending or twisting in some points. These processes may increase or decrease according to the range of vibrations.

Vibrations and fatigue:

In the vibrations occurring to the structures, the stress may alternately be tensile-compressive or compressive-tensile and this cycle changes and continues. If the amplitude of the occurred vibrations becomes so large that the resulting stress exceeds the bearing limit stress of the object, then the structure will break in one cycle. Therefore, even a stress less than the final stress may cause failure due to fatigue.

Figure 5.1 (which is related to the number of reversals and the final stress on the object) shows that when the stress due to the increase in the amplitude of vibrations increases, the number of reversals decreases. It is also clear that if the amplitude of vibrations is large enough, the structure will fail in a certain number of vibrations. So, a specific life will be defined for parts and structures. Figure 5.1 also shows that if the stress is less than a certain value, then the number of cycles will be infinite. Also, if the range is so small that the stress is less than a fixed limit and controlled, the probability of failure will be very low.

Range of vibration range

Permissible stress related to fatigue and therefore the range of vibrations depends on factors such as the nature and type of materials, stress concentration, corrosion, temperature and resonance.

phenomenon Rosenance is one of the reasons for the failure of blades and vanes in tools. If the frequency of impact of the fluid jet coincides with the natural frequency of its container, eventually the container will break due to excessive stimulation.

       Vibrations in pipes

Vibrations in pipes are very important. Because in the case of good control, it will cause damping of production waves (these waves can cause damage to the pipe itself and the equipment by moving in the accessories assigned to the pipe). When a tube vibrates at a natural frequency, it occurs in the cycle of stress absorption and in this case resonance occurs according to the amplitude of the oscillation. The force of vibrations will be significant in resonance and in these cases it will cause damage to the system. Resonance usually causes damage and erosion of vanes, seat[1] and orifice in regulators and compressors.

In many oil and gas refining units, a natural frequency of about 4 Cps is enough to avoid tube resonance without flow pulse. Therefore, the natural frequency depends on the maximum bending. For example, the natural frequency of a rod with a length of 1 in is approximately equal to 3.12 Cps. One of the ways to limit bending in pipes is to harden the pipe sufficiently or install a suitable support. so as to prevent the occurrence of large fluctuations.