Análise e pesquisa sobre a influência da estrutura do canal de fluxo da válvula nas características do fluido

Análise e pesquisa sobre a influência da estrutura do canal de fluxo da válvula nas características do fluido

Analyzed the influence of valve channel structure on the fluid flow state and flow resistance coefficient of the system, discussed the optimization design, experimental detection, and practical application of valve channel and throttling internal components, and introduced the calculation of flow resistance coefficient and pressure loss of typical valves.

1. Overview

Válvulas are the main pressure pipeline components that transport fluids. In the process of fluid transportation, the structure of valve flow channels is closely related to the medium, flow rate, flow velocity, flow resistance, and pressure loss of the operating system, such as the structure of valve flow channels and throttling components, as well as pipe turning and nozzle scaling. This article discusses the typical structures of valves and pipelines.

2. Structural characteristics

Typical valve structures include truncated shapes (Figure 1), adjustable shapes (Figure 2), check shapes, and split shapes. Typical pipeline bending connection structures (Figure 3) include circular and oblique connections, and typical nozzle types include scaled pipes (Figure 4) and straight pipes. Technical data related to fluid characteristics, such as flow resistance, flow rate, flow rate, the cross-sectional area of the body cavity channel, and the throttle surface and pressure difference controlling the fluid, are often encountered in the fluid design and calculation of valves and pipelines.

3. Flow resistance calculation

3.1 Flow resistance characteristics

There are two states of medium flow in the channel, laminar flow and turbulent flow. The fluid particles in the laminar flow are very neat, do not interfere with each other, and push in parallel. The energy loss through the channel is linear with the velocity, and the Reynolds number of the fluid is less than 2000. Turbulence is a high-free state. The media interfere with each other. The velocity and direction at any point constantly change, but there is an average velocity in a specific direction. The energy loss through the channel is proportional to the square of the velocity. The Reynolds number of the fluid is higher than 3000.

Figure.1 Inner Chamber Flow Path of Truncated Valves
(a) Direct flow channel; (b) Right angle channel

Combined with the fluid test of the flow channel, taking the water medium as an example, the Reynolds number Re is:

In the formula:

• V – Average flow velocity of the fluid in the flow channel, m/s
• D – Inner diameter of the flow channel, m
• U – Fluid motion viscosity in the flow channel, m²/s

During the test, to ensure that the fluid is in a turbulent state, the Reynolds number should be greater than 40000, and the average flow velocity v is:

In the formula:

• Q – Fluid flow rate in the flow channel, m³/s

Figure.2 Throttling Forms of Regulating Valves
(a) Plunger type (b) sleeve type

The enormous kinetic energy of fluid transportation flow is the pressure difference, and the flow channel will generate local resistance to the fluid flow and consume kinetic energy. Therefore, pressure differences should be well utilized when needed and effectively overcome when unfavorable.

Figure.3 Bend

(a) Arc; (b) Miter joint

Figure.4 Concentric swage nipple
(a) Gradually shrinking; (b) Gradual release

3.2 Pipeline flow resistance

When the medium passes through the flow channel, local resistance will be generated, and overcoming the resistance will consume energy, reflected in the pressure difference and flow resistance coefficient.

The pressure difference refers to the pressure loss value △ p₁ between the inlet p₁ and outlet p₂ of the flow channel, which is:

The total pressure difference △ p of the system is equal to the sum of the pressure differences of each unit flow channel, namely:

So, the net pressure difference △ pi of a certain unit flow channel is subtracted from the measured total system pressure difference △ p by the pressure differences of other flow channels, namely:

Flow resistance coefficient of flow channel ξ. The flow resistance coefficient obtained from the experiment depends on the pressure difference in the flow channel ξ for:

In the formula:

• △ p – Net pressure difference of the flow channel, kPa
• ρ —  Medium density, kg/m³

The local flow resistance coefficient of a 90 ° curved pipe is determined through experimental and theoretical derivation ξ_{90 °} is:

If the corner α When not equal to 90 °, then ξα for:

The Conduit α Local flow resistance coefficient of angled mitered bends ξα for:

The experiment shows that the pipeline corner α, the larger the ratio of pipeline diameter D to pipeline bending radius R, the higher the flow resistance coefficient. So, at the corner α, the flow resistance coefficient in the straight pipe section at 0 ° can be ignored. Similarly, in 90 ° arc bends and mitered bends with the same diameter D, when D/R is greater than 1.6, the local flow resistance of the arc bend is equivalent to that of the mitered bend (Figure 5, Table 1). Therefore, a continuous 90 ° diagonal bend can be used to increase fluid resistance and consume energy. To reduce energy loss, straight pipes or large radius circular flow channels can be used (considering smooth flow channels).

Figure.5 Bend Structure
(a) Arc curved pipe; (b) Mitered bend

Table.1 Local Flow Resistance Coefficients of Arc Bend and Miter Bend ξ

Corner	Arc bend D/R				Mitered bend
	0.5	0. 67	1	1. 6
90°	ξ = 0.145	ξ = 0.17	ξ = 0.29	ξ = 0.96	ξ  = 1
60°	ξ = 0.10	ξ = 0.11	ξ = 0.19	ξ = 0.6	ξ = 0.375
30°	ξ = 0.05	ξ = 0.07	ξ = 0.1	ξ = 0.3	ξ =  0.076
0°	0	0	0	0	ξ = 0

Gradually reducing pipes are common in fluid conveying systems, especially in nozzles where necking increases flow velocity. The experiment shows that the local flow resistance coefficient of the concentric swage nipple ξ_S depends on the necking section and angle.

In the formula:

• γ —  Shrinkage reduction coefficient (Table 2)

Table.2 Shrinkage reduction coefficient γ

L/d	Reduced angle a
	5°	10°	20°	30°	40°	50°	60°	70°	80°
0.25	0.8	0.67	0.45	0.3	0.22	0.18	0.16	0.17	0.19
0.6	0.7	0.45	0.27	0.19	0.14	0.12	0.12	0.14	0.16
1	0.6	0.31	0.19	0.12	0.1	0.1	0.1	0.1	0.12

With angular reduction α, When it increases to 180 °, it becomes a sudden contraction tube, which results in significant pressure loss and energy consumption in the pipeline system. Its shrinkage reduction coefficient γ Increases to 5.0.

From the local flow resistance coefficient of the concentric swage nipple (Figure 6, Table 3), it can be seen that the reducing tube is at a reduced angle α in structures less than 10 °, their local flow resistance coefficient ξ as small as 0.07-0.005. The pressure difference and energy loss are both small. So, the concentric swage nipple with a small reduction angle is widely used and promoted in pipeline systems.

Figure.6 Concentric swage nipple with different L/d
(a)L/d=0.25; (b)L/d=0.6; (c)L/d=1.0

Table.3 Local flow resistance coefficient of concentric swage nipple ξ_s

L/d	Reduced angle a
	5°	10°	20°	30°	40°	50°	60°	70°	80°
0.25	0.031	0.05	0.072	0.063	0.061	0.061	0.064	0.076	0.095
0.6	0.063	0.076	0.081	0.08	0.073	0.071	0.078	0.098	0.12
1	0.096	0.084	0.085	0.07	0.066	0.073	0.078	0.082	0.1

The pressure loss of the concentric swage nipple in the fluid transportation pipeline system is relatively large, especially in the sudden release pipe, which will result in greater pressure loss and the flow rate of the fluid consumed. Experiments have shown that the local flow resistance coefficient of the gradual release pipe ξ_F depends on the ratio of the taper angle to the flared section.

In the formula:

• β —  Gradual angle（β≤ 12°),(°)
• F_D/f_d – the ratio of inlet area to flare area

When the ratio of the expansion and inlet section of the concentric swage nipple is more than 3 times, the fluid generates significant resistance, so technical data related to fluid characteristics should be avoided as much as possible in the pipeline system. The use of small expansion angles and long tapers in the structure is beneficial for minimizing the pressure loss of the fluid (Figure 7, Table 4).

Figure.7 F_D/F_d Ratio of Gradients

Table.4 Local Flow Resistance Coefficient of Concentric Swage Nipple ξ_f

FD/fd	0.9	0.8	0.7	0.6	0.5	0.4	0.3
β	5°	10°	12°	12°	12°	12°	12°
ξ	0	0.007	0.01	0.03	0.05	0.07	0.1

3.3 Valve flow resistance

In the pipeline system, valves account for a large proportion, and the internal flow channels and movable parts of valves are more complex than those of pipe fittings. So, valves’ fluid throttling and cut-off processes are closely related to the study of technical characteristics such as flow rate, flow rate, pressure difference, and flow resistance.
The flow resistance of cut-off valves at full open position depends on the area, shape, and roughness of the inner wall of the casting. The local flow resistance coefficient of the straight structure is 4-7, and the local flow resistance coefficient of the angle structure is 3-4. The theoretical calculation method is shown in equation (6).

The characteristic of regulating valves is to throttle the fluid at different stroke opening positions through the set movable parts inside the valve, fully utilizing the controllable changes in flow resistance to achieve flow and pressure characteristics adjustment and control. The experiment shows that the plunger type control valve’s flow resistance coefficient can refer to the reducing tube’s values throughout the adjustment process. In contrast, the flow resistance coefficient of the sleeve type control valve can refer to the values of the reducing tube and the elbow throughout the entire adjustment process. Through the analysis of flow resistance research on typical structures, there is inevitably flow resistance pressure loss in the flow channel, and the pressure loss △ p of different media is:

In the formula:

• δ —  Medium expansion coefficient

4. Calculation examples

The flow resistance and pressure loss of the flow channel are often encountered in practical applications. For example, in the pipeline conveying air medium, a block type straight through stop valve has a nominal size of DN50, a flow of 1000kg/h, a pressure of 0.5MPa, and a temperature of 20 ℃. According to the pressure and temperature of the air medium, the density of air can be found ρ= 0.0058g/cm³, and the flow rate v of the medium passing through the valve is:

According to the flow resistance coefficient of the cut-off type direct shut-off valve, ξ It is 4-7.

ξ= At 4 o’clock, the pressure loss Δ P is:

ξ= At 7 o’clock, the pressure loss Δ P is:

5. Conclusion

Through design and calculation examples, it can be seen that the pressure loss of the through globe valve is 1.4% -2.4%, and the pressure loss generated by the fluid in the pipeline system increases with the increase of the flow resistance coefficient of the flow channel. When designing flow channels for cut-off valves, the smaller the flow resistance, the better. The flow channel of regulating valves has the functions of throttling and reducing pressure. A typical example is a high-pressure differential regulating valve designed with a 90 ° angled elbow and a labyrinth valve core.

Author: Zhang Ming

Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)