**Preface**
A pressure reducing valve works by controlling the opening and closing of a throttling element to lower the inlet pressure to a desired outlet pressure. It uses its own medium energy to maintain a relatively stable outlet pressure, even when the inlet pressure or flow fluctuates. However, as fluid flows through the valve, pressure loss occurs, which results in energy dissipation. This energy loss is the primary cause of vibration and noise generated by the valve. It is inevitable for a pressure reducing valve to produce some level of vibration and noise during operation.
If the operating pressure is too high, it not only poses health risks to operators but also threatens the safe functioning of the entire system. Such noise is recognized as a potential hazard. According to environmental regulations, if the noise level of the pressure reducing valve exceeds acceptable limits, noise control measures must be implemented. As a result, noise-reducing valves have gained increasing attention in piping systems.
This paper discusses how to determine the allowable noise levels for pressure reducing valves, analyzes the mechanisms that generate noise during their operation, and proposes effective noise reduction strategies. Practical applications have shown that these methods significantly reduce noise, yielding satisfactory results.
**1. Allowable Noise Level of the Valve**
In practice, determining the appropriate noise control range for a pressure reducing valve has always been a critical concern. By referencing environmental standards, labor safety regulations, and valve manufacturing guidelines, Table 1 provides the allowable noise levels for pressure reducing valves [1].
| Duration (h) | Allowable Noise (dB) |
|--------------|----------------------|
| 8 | 85 |
| 4 | 88 |
| 2 | 91 |
| 1 | 94 |
| *Max: ≤115 dB |
When determining the allowable noise level, several factors should be considered, such as the installation location, working environment, noise source power, and operator proximity. For instance, if the operator is not near the valve, the allowable noise level can be relaxed due to sound attenuation with distance. At 1 meter from the noise source, the maximum noise intensity should not exceed 85 dB. Before implementing noise control measures, it is essential to first define the allowable noise level and perform noise prediction calculations. If the calculated value exceeds the limit, noise reduction steps must be taken.
**2. Noise Sources and Mechanisms of Noise Reduction**
During the pressure reduction process, the fluid medium’s energy can be converted into heat, mechanical energy, and acoustic energy. To minimize noise, it is crucial to maximize the conversion of this energy into heat. The main noise sources in a pressure reducing valve can be categorized into three types:
- **Mechanical Vibration Noise**
- **Hydrodynamic Noise**
- **Aerodynamic Noise**
**2.1 Mechanical Vibration Noise**
When fluid flows through the valve, it can excite mechanical vibrations. These vibrations are typically classified into low-frequency (50–500 Hz) and high-frequency (1000–8000 Hz) types. Low-frequency vibrations are often caused by high flow velocity at the valve outlet, poor piping layout, or insufficient rigidity of moving parts. High-frequency vibrations occur when the natural frequency of the valve resonates with the flow-induced excitation frequency, leading to sudden noise increases. These vibrations are independent of flow speed and cannot be predicted easily. To reduce mechanical vibration noise, design improvements such as optimizing the valve cavity shape, adjusting the clearance between components, and selecting appropriate materials are recommended.
**2.2 Hydrodynamic Noise**
Hydrodynamic noise arises from turbulence and eddies formed after the fluid passes through the valve’s relief port. This includes turbulent noise and cavitation noise. Cavitation noise occurs when the fluid vaporizes due to pressure reduction, creating bubbles that collapse and generate shock waves. These shock waves can reach pressures up to 196 MPa, causing significant noise. To prevent cavitation, the pressure drop across the valve must be controlled below a critical threshold. Proper fluid flow direction and valve design are essential in minimizing this type of noise.
**2.3 Aerodynamic Noise**
Aerodynamic noise occurs when compressible fluids, such as steam, pass through the valve. This noise is generated as mechanical energy is converted into acoustic energy. It typically ranges from 1000 to 8000 Hz and is difficult to eliminate entirely due to unavoidable turbulence during decompression. Measures such as optimizing the flow path and using specialized valve structures can help reduce its impact.
**3. Methods to Reduce Pressure Relief Valve Noise**
During operation, the pressure reducing valve consumes a significant amount of energy, part of which is lost through friction, eddy currents, and mechanical vibrations, ultimately resulting in noise. To reduce noise, it is essential to convert as much of this lost energy into heat as possible.
Figure 1 shows the structure of a pneumatic valve, where the decompression hole plays a key role in noise reduction. The resistance of the hole is governed by equation (3):
$$ R = k_1 \times Q $$
Where $ R $ is the resistance, $ Q $ is the flow rate, and $ k_1 $ is the drag coefficient, which depends on the hole’s length, shape, and roughness. A longer or rougher hole increases the drag coefficient, thus enhancing noise reduction.
Through extensive research, threaded holes have been introduced to increase resistance and promote vortex formation, thereby converting more energy into heat. By adjusting the diameter and spacing of the threaded holes, optimal noise reduction can be achieved. The ideal hole size ranges from M4 to M8, with a pitch between 0.77 and 0.80 times the diameter. This design has proven effective in large-scale chemical plants, demonstrating improved noise control performance.
**4. Conclusion**
The issue of noise in pressure reducing valves can now be predicted and effectively mitigated through proper design and implementation. After defining the allowable noise level, the most suitable and cost-effective method can be selected. Practical experience has shown that using special decompression holes with enhanced sleeve designs, such as replacing light holes with threaded ones, is both feasible and highly effective in reducing noise. This approach ensures that noise remains within acceptable limits, making it an efficient solution for noise control in pressure reducing valves.
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