1 Overview

Gas regulator station produces a lot of noise during operation, which, if not controlled, will cause serious interference to the staff inside the regulator station and the residents outside the regulator station.

With the enhancement of environmental protection consciousness of the whole society, the noise problem of the regulating station has also attracted great attention from all parties [1-2]. According to the requirements of GB 12348 - 2008 "Industrial Enterprise Factory Boundary Environmental Noise Emission Standards" and GB/T 15190 - 2014 "Technical Specification for the Delineation of Functional Areas of the Acoustic Environment", "industrial production, warehousing and logistics as the main function, and the need to prevent the industrial noise from having a serious impact on the surrounding environment of the area, the industrial enterprise factory boundary environmental noise daytime shall not be higher than 65 dB and 55 dB at night."

Therefore, reducing the noise of the regulator station is a problem we urgently need to solve.

2 Analysis of noise sources and noise reduction measures

Noise in the regulator station mainly comes from the high-speed flow of gas, a large gas pressure drop and turbulence generated by the sudden change in the direction of the gas flow, and it is necessary to take corresponding measures in the design of the regulator station to reduce the noise of the regulator station.

2.1 Noise generated by pipeline

Noise in the pipeline of the regulating station mainly comes from the friction and collision between the high-speed flowing gas in the pipeline and the inner wall of the pipeline. The faster the gas flow rate, the greater the noise generated in the pipeline. For the noise generated in the pipeline, we can take the following measures.

①Control gas flow rate

Under a given design flow rate, the gas flow rate in the pipeline of the gas regulating station depends on the caliber of the pipeline. Therefore, increasing the caliber of the pipeline is a good choice to reduce the gas flow rate.

However, when the pipeline caliber is increased, the valves on the pipeline and the receiver of the pressure vessel should be increased accordingly, which will greatly increase the construction cost of the regulating station. If the pipeline caliber is too small, it will not only cause excessive pipeline noise, but also cause serious erosion to the inner wall of the pipeline and equipment by the high-speed flow of gas, which will affect the service life of the regulator station and increase the pressure loss of the gas in the pipeline.

Therefore, it is necessary to consider the influence of the above factors to determine the caliber of the pipeline. According to domestic and international engineering experience, the pipeline flow rate in the regulator station is generally controlled within 25 m/s.

②Increase the wall thickness of pipeline

The wall thickness of the pipeline has an attenuation effect on the propagation of noise, and the thicker the wall thickness of the pipeline, the greater the noise attenuation. According to the calculation, every increase of a wall thickness series, the equivalent sound level of the noise of the gas in the pipeline can be reduced by about 2 dB.

③Increase the inner coating of piping

The open-air pipeline in the regulator station is usually only painted on the outside of the pipeline for corrosion prevention and aesthetic needs. Since the inner side of the pipeline is not in contact with the air, it is not generally considered to do the internal coating for corrosion protection.

By coating the pipeline, the roughness of the inner wall of the pipeline can be effectively reduced, and the friction between the gas and the inner wall of the pipeline can be reduced, thus reducing the noise.

④Choose valves that are consistent with the inner diameter of the pipeline.

There are a large number of valves connected to the pipeline in the regulating station, if the inner diameter of the two is not the same, the gas flow will produce strong turbulence at their connection.

Therefore, when choosing valves, try to choose valves with the same inner diameter of the pipeline, which can also reduce the noise generated in the pipeline.

2.2 Noise generated by the manifold

The manifold is also an important part of the regulator station, usually set up in the inlet and outlet of the regulator station, connected with the valve and pipeline. The manifold has the function of distributing airflow and multi-way convergence. The direction of the airflow in the manifold is very complex, and it will produce intense turbulence. Therefore, the manifold will produce large noise, which is one of the main noise sources in the pressure regulating station.

The influence of the manufacturing process of the manifold on the noise

The manifold is mainly composed of cylinder and branch pipe. The cylinder is usually made of seamless steel pipe or plate rolled and processed. The manufacturing of the branch pipe usually has two manufacturing processes. One is to cut holes directly in the barrel by gas cutting, and then weld a short tube to manufacture. The other is molded and drawn holes in the cylinder, and then welded from the short tube manufacturing. Molded and drawn holes between the branch pipe and the cylinder to form a circular arc transition, while gas cutting holes between the branch pipe and the cylinder without the transition surface but perpendicular to each other in the form. Therefore, the airflow of the former is more gentle than the latter, resulting in less noise. In addition, since the hole wall will be stretched and thinned when drawing the open hole, in order to ensure the design strength, a thicker wall thickness of the barrel will be selected. Therefore, under the same design pressure, the barrel of the drawn manifold is relatively thicker, and the noise generated is also lower.

② The effect of the diameter of the tube of the manifold on the noise

Gas from the branch pipe into the cylinder is the process of reducing the flow rate, gas from the cylinder into the branch pipe is the process of increasing the flow rate. If the noise is to be reduced, two points are usually ensured.

First, the sum of the cross-sectional areas of the inlet branch of the manifold and the sum of the cross-sectional areas of the outlet branch are substantially equal.

Secondly, the cross-sectional area of the pipe barrel is at least 1.5 times the sum of the cross-sectional area of the inlet branch or the sum of the cross-sectional area of the outlet branch.

③The influence of the arrangement of the manifold branch pipe on the noise

According to the needs of the process, usually the manifold branch pipe has four kinds of arrangement, see 1.

1 The arrangement of the manifold branch pipe

The first arrangement, the inlet branch pipe and the outlet branch pipe in the same axis.

The second arrangement, the inlet branch pipe and the outlet branch pipe is not in the same axis, staggered a certain distance from each other.

In the third arrangement, the inlet branch and the outlet branch are set on two manifolds, and the manifolds are connected by a pipe.

In the fourth arrangement, the inlet branch and outlet branch are centrally arranged on the same side or different side of the manifold.

Analyzed from the gas flow pattern, in the third and fourth arrangements, the gas enters into the manifold cylinder through different branch pipes and then flows out through the outlet branch pipes respectively. In the first and second arrangement, the gas enters the cylinder and flows out from the outlet branch directly without summarizing, and the gas flow pattern is relatively disordered. Therefore, the third and fourth arrangement of the noise is smaller.

2.3 Noise generated by the regulator

The regulator is the core component of the regulator station. Since the regulator relies on throttling to realize the purpose of reducing pressure, the noise it produces is very large and is the main source of noise in the regulator station.

2.3.1 Classification of regulator noise

The noise generated by the regulator is mainly of three types.

① Mechanical vibration noise

Mechanical vibration noise refers to mechanical vibration, intrinsic frequency vibration and the noise generated by the spool oscillatory displacement caused by the pressure fluctuations of the fluid. The reason for this type of noise and regulator design, component materials, processing technology, assembly quality.

② Fluid dynamics noise

Fluid dynamics noise is generated by the turbulence and eddy current after the fluid passes through the regulator's valve port, that is, the noise generated by the turbulent fluid interacting with the regulator or the inner surface of the pipeline.

③Aerodynamic noise

When the natural gas through the regulator decompression parts and regulator outlet expansion parts, the mechanical energy of the fluid is converted to acoustic energy and the noise generated is called aerodynamic noise. This noise in the regulator noise in a larger proportion. The frequency of the noise is about 1 000 ~ 8 000 Hz, generally not particularly steep peak frequency. The reason for this noise is mainly fluid turbulence and due to the fluid to reach the critical flow rate caused by the surge. 

Aerodynamic noise cannot be completely eliminated, but certain technical measures can be taken to reduce it.

2.3.2 Noise reduction measures for regulators

Currently the most widely used two types of regulators are cut-off valve regulator and axial flow regulator.

For the cut-off valve regulator, the noise source is mainly in the regulator valve mouth, so it can be installed in the valve mouth muffler to eliminate some of the noise. This muffler is generally multi-channel multi-flow structure [3], see 2.

2 Cut-off valve regulator muffler

The principle of the muffler is that when the gas passes through a small gap in the cage-like cylinder, the flow rate will increase rapidly to the speed of sound, thus generating a frequency greater than 8 000 Hz sound waves. This sound wave is easily absorbed by the surrounding sound-absorbing material, which prevents the noise from being emitted and transmitted downstream of the regulator. This muffler reduces noise by 10 to 20 dB.

For axial flow regulator, the noise source is mainly at the regulator outlet. In order to eliminate the noise generated by high-speed air jet, the axial regulator uses a flare structure [3], see 3.

3 axial regulator in kind

This structure will be the regulator outlet gas flow rate reduction, through the internal hole pipe, hole plate part of its external cavity to form a multi-stage small hole injection muffler, the human ear can feel the low-frequency noise into the human ear is difficult to detect the high-frequency noise.

At the same time, in the regulator valve seat installed on the cage muffler [3], see 4. The cage muffler set in the valve seat outside, the structure of the 5, by the upper and lower fixed ring and metal sintered wire mesh composition. The small holes on the metal sintered wire mesh can make the gas channel dispersion, increase the friction resistance, so that the acoustic energy is converted to thermal energy, and at the same time to make the gas velocity field distribution more uniform, thus reducing the noise. This approach reduces noise by 8 to 15 dB.

4 Installation position of axial regulator cage muffler

5 Cage muffler structure set on the outside of the valve seat

In addition, we can also set the pipe muffler behind the regulator to reduce the noise generated by the regulator. 

This kind of muffler adopts the resonance sound absorption principle of micro perforated plate, there is a circle of double-layer perforated plate in the inner wall of the pipe shell, there is a sound-absorbing packing made of stainless steel wire sandwiched between the two layers of perforated plate, and there is a certain distance between the perforated plate and the shell, which forms a resonance cavity. When the frequency of the noise is the same as the resonance frequency of the structure, the noise is absorbed and converted into thermal energy.

This kind of muffler can reduce the noise by about 10 dB.

2.4 Other noise reduction methods

①Siting of regulator station

Noise spreads in the air and can spread farther when it spreads downwind.

Therefore, the regulating station should be arranged on the downwind side of the monsoon wind in towns and residential areas as much as possible to reduce the impact of noise on towns and residential areas.

②Buried and closed

Buried, wrapped with sound-absorbing materials, closed building or underground regulator box is also an effective measure to control noise.

The use of sink pipe buried can reduce the noise by about 20 dB, the use of underground regulator box can reduce the noise by about 40 dB, the use of pipeline outsourcing sound-absorbing materials can reduce the noise by about 15 dB, the use of closed building can reduce the noise by about 20 dB, and the use of regulator closed box can reduce the noise by about 15 dB.