Required Straight Pipe Lengths for Vortex Flowmeters

Contents

 

1. Introduction

Yokogawa launched the world’s first vortex flowmeter in 1969, and in 1979, we launched general-purpose vortex flowmeters. For over 50 years, Yokogawa’s vortex flowmeters have sold more than 500,000 units worldwide, improving our customers' productivity. In 2022, we launched the latest VY Series, continuing that tradition of improved productivity. One of the features of the sensing structure of the VY Series is that only a short straight pipe length on the upstream side is required.

In general, unbalanced velocity distribution and fluid turbulence in a pipe disturb Karman Vortices generated by the shedder bar and affect the accuracy of flow measurement. Therefore, each flowmeter manufacturer specifies the required straight pipe length to reduce unbalanced distribution and turbulence. This article shows the reasons why different straight pipe lengths are necessary, using Vortex Flowmeter VY Series as examples.

 

2. Straight Pipe Lengths

Straight pipe lengths are the upstream or downstream lengths of straight pipe between the vortex flowmeter and the point at which the structure of the piping causes the flow to change, such as by a reducer, expander, or bend pipe. The orientation of the straight pipe can be horizontal, vertical, or inclined.

Fig.1: Straight Pipe Lengths (Excerpt from General Specification)

 

2.1. Description and Actual Size of Straight Pipe Lengths

Straight pipe lengths are generally expressed as  5D, 10D, or 20D. DN stands for the inner diameter of the pipe; DN25 is about 25 mm and DN50 is about 50 mm. Inner diameters vary by schedule number, so please check the inner diameter standards for your particular pipes.
For example, 10D of DN25 is about 10*25 = 250 mm and 10D of DN50 is about 10*50 = 500 mm (Fig.2). Therefore, the lengths of the same types of straight pipe are different depending on the inner diameter (DN). 10D of DN200 is about 2000 mm, so straight pipes can be long and require more space. Long straight pipes can be a constraint when vortex flowmeters are installed in existing pipelines and narrow facilities.
Therefore, the short straight pipe requirement of the VY Series has the advantage of having fewer space constraints during installation. 

Fig.2: Even with the Same Straight Pipe Length, the Dimensions can Differ

 

2.2. Straight Pipe Lengths Reduce Turbulence

Fig.3 depicts flowlines in a simulated flow, with their velocities represented by colors. The flowlines from the inlet to the bend are straight without turbulence. The flowlines are disturbed by the pipe bend. The relationship between the number of vortices and flow velocity is based on stable flow conditions. Therefore, accurate measurement requires low turbulence, which requires straight pipe lengths.

Fig.3: Flow Turbulence before and after a Single-Bend Pipe

Fig.4 shows an animated flow simulation through a single-bend pipe. The flowlines reach the bend without turbulence but are disturbed as they pass through it. The turbulences in each straight pipe length are different, but in a 10D pipe the flow is roughly stable (Fig.5). In fact, vortex flowmeters can take accurate measurements in 10D pipes, and they match well with the simulation results.

Fig.4: Animation of Turbulence in a Single-Bend Pipe
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D, upstream and downstream)

 

Fig.5: Difference of turbulence in straight lengths (condition is the same as in Fig.3)

Moreover, Fig.6 compares the simulation results for DN50 and DN25 pipes. The results are nearly the same, and 10D reduces turbulence and provides stable flow. The physical phenomenon in which turbulence decreases in a straight pipe is based on the law of similarity.

 

Fig.6: Turbulence Follows the Law of Similarity Note: Figures adjusted to the same scale
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 and DN25 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

Also, the simulation results for water and air are similar, demonstrating that the type of fluid has only a very small affect (Fig.7). Therefore, straight pipe lengths can be based on inner diameter D, such as 5D and 10D.

 

Fig.7: Fluid Type Doesn’t Change Straight Pipe Lengths
(Fluid Type: Water and Air, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

3. Required Straight Pipe Lengths for Different Installation Conditions

Using the VY Series as an example, this section describes detailed straight pipe lengths under various conditions with combinations of different pipe elements. Straight pipe lengths are based on inner diameter D regardless of fluid types.
Straight pipe lengths don’t change by orientation because the VY Series has a unique integrated sensing structure of the vortex shedder bar and sensor to catch Karman vortices without the effects of position. Furthermore, the VY Series is resistant to unbalanced velocity distribution and turbulence, so it requires shorter upstream straight pipe lengths than other vortex flowmeters. On the other hand, downstream straight pipe lengths are decided by the vortex flowmeters’ structure. Downstream straight pipe lengths are over 5D without upstream piping because the VY Series have a unique integrated sensing structure of the vortex shedder bar and sensor.
Please note that there are various shapes and combinations of pipe, such as bent pipe. Additionally, there are changes due to fluid types, fluid viscosity, Reynolds number, and many other factors. Therefore, we recommend carefully assessing measurement conditions when using vortex flowmeters. In this case, the FSA130 Magnetic Flowmeter/Vortex Flowmeter Verification Tool provides a vortex waveform monitor function to check the integrity of flow measurement in the VY Series. Please consider using it as needed.

 

3.1. Reducer Pipe: Over 5D Upstream and 5D Downstream

For the VY Series, when a reducer pipe is upstream, straight pipe lengths must be over 5D. The straight pipes are shorter than in other conditions because turbulence is less likely to occur in the reducer pipe (Fig.8 and Fig.9).
To ensure that flow velocity is measurable by the vortex flowmeter, a reducer pipe and expander pipe are used to return the pipe diameter. In such cases, VY Series reduced bore types, which integrate reducer and expander pipes, can simplify straight piping of the reducer and expander pipes.
Note that the required straight pipe lengths for the VY reduced bore type is the same as for the VY standard type.

Fig.8: Animated Flow Simulation with a Reducer Pipe
(Fluid Type: Water, Outlet Velocity: 5 m/s, Pipe Size: Upstream DN80 Sch40 and Downstream DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

Fig.9: Turbulence at a Reducer Pipe (Condition is the Same as in Fig.8)

 

3.2. Expander Pipe: Over 10D Upstream and 5D Downstream

For the VY Series, when a reducer pipe is upstream, straight pipe lengths must be over 10D. Compared to the reducer pipe, flowlines remain at near center immediately after the expander pipe and flow velocity is slow near the pipe walls (Fig.10). Straight pipe lengths are more than 10D to change balanced velocity distribution.

Fig.10: Animated Flow Simulation of an Expander Pipe
(Fluid Type: Water, Outlet Velocity: 5 m/s, Pipe Size: Upstream DN50 Sch40 and Downstream DN40 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

Fig.11: Turbulence by Expander Pipe (Condition is same as in Fig.10)

 

3.3. Single-Bend Pipe: Over 10D Upstream and 5D Downstream

For the VY Series, when a single-bend pipe is upstream, straight pipe lengths must be over 10D. Flow velocity changes in the pipe bend, and after the bend, flowlines are disturbed. The figure shows the reversal of flowlines in the vertical direction. Straight pipe lengths of 10D are needed to reduce turbulence and to stabilize flow velocity and direction (Fig.12, Fig.13).

Fig.12: Animated Flow Simulation of Single-Bend Pipe (Same as Fig.4)

 

Fig.13: Turbulence by Single-Bend Pipe (Same as Fig.5)

 

3.4. Double-Bend Pipe in Same Plane: Over 10D Upstream and 5D Downstream

For the VY Series, when a double-bend pipe in the same plane is upstream, straight pipe lengths must be over 10D. These straight pipe lengths are the same as the straight pipe lengths of single-bend pipe because turbulence by the double-bend pipe in the same plane is like that in the single-bend pipe (Fig.14, Fig.15).

Fig.14: Animated Flow Simulation of a Double-Bend Pipe in the Same Plane
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

Fig.15: Turbulence in a Double-Bend Pipe in the Same Plane (Condition is the Same as in Fig.14)

 

3.5.Double-Bend Pipe Not in the Same Plane: Over 10D Upstream and 5D Downstream

For the VY Series, when a double-bend pipe not in same plane is upstream, straight pipe lengths must be over 20D. Unlike in the previous pipe condition, a spiral flow can occur (Fig.16). This flow, called swirling flow, does not settle at 10D, and begins to reduce turbulence around 15D, so straight pipe lengths of more than 20D are required (Fig.17).

Fig.16: Animated Flow Simulation of a Double-Bend Pipe Not in the Same Plane
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and 20D in downstream)

 

Fig.17: Turbulence in a Double Bend Pipe Not in the Same Plane (Condition is the Same as in Fig.16)

 

3.6. Valve: Downstream over 5D

Valves should generally be installed downstream of vortex flowmeters. The structure of valves is complex, mechanical shapes vary by type (such as gate valves, globe valves, ball valves, and butterfly valves) and required straight pipe lengths differ for each valve. Valves should be installed downstream with straight pipe lengths of at least 5D because these may not reduce fluid disturbances depending on the valve structure and position. If you cannot avoid installing a valve upstream of the vortex flowmeter, use straight pipe lengths of at least 20D upstream.

 

Fig.18: Required Straight Pipe Lengths for Valves (Excerpt from General Specification)

 

4. Tips on Straight Pipe Length

Various piping combinations are possible other than the ones mentioned above. This section shows some examples for your reference.

 

4.1. Single Straight Pipe Best for Straight Pipe Lengths

If the straight pipes have any internal diameter steps or gasket protrusions at the flange connections, they can cause turbulence. Therefore, it is recommended to use a single continuous straight pipe for straight pipe lengths (Over 10D). Even a small step of about 3 mm can cause turbulence (Fig.19).

Fig.19: Turbulence by 3 mm Steps
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

4.2. Over 2 Size Reducer or Expander Pipe

It is recommended to multiply the number of reducer or expander sizes by required straight pipe length for one size. For a two-size expander, the straight pipe lengths should be 10D × 2 = 20D or more, and for a three-size reducer, they should be 5D × 3 = 15D or more. As a reference, the simulation results for a three-size reducer are shown (Fig.20). In this simulation, flowlines stabilize at 10D. However, it is expected that the structures of multi-step reducers and expanders will vary significantly from plant to plant. Therefore, please determine your straight pipe lengths based on flow measurement values and vortex waveforms.

Fig.20: Animated Flow Simulation of Three-Size Reducer Pipe
(Fluid Type: Water, Outlet Velocity: 5 m/s, Pipe Size: Upstream DN150 Sch40 and Downstream DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

4.3. Combination of Reducer or Expander Pipe and Bent Pipe

It is recommended to add the D values of the straight pipe lengths. For example, an expander (10D or more) + a single bend pipe (10D or more) requires 20D or more. Additionally, it is recommended to have 10D or more between an expander pipe and a bent pipe.
As a reference, the simulation results without the 10D between the expander pipe and the double-bend pipe in the same plane are shown (Fig.21). Swirling flow occurs, and the flow disturbance is greater than in the case of a double-bend pipe in the same plane (Fig.14). In such cases, please determine straight pipe lengths based on the flow measurement values and vortex waveforms.

Fig.21: Animated Flow Simulation of the Combination of an Expander Pipe and Double-Bend Pipe in the Same Plane
(Fluid Type: Water, Outlet Velocity: 5 m/s, Pipe Size: Upstream DN50 Sch40 and Downstream DN40 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

4.4. Length between Bends

The longer the lengths between bends in a double-bend pipe, the more stable the flowlines become. When the distance is 10D or more, the impact of swirling flow is reduced (Fig. 22).

Fig.22: Animated Flow Simulation of a Double-Bend Pipe Not in the Same Plane and 10D between Bends
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and downstream)

 

4.5. Multiple Flow Meters

It is not recommended to install another flowmeter, such as a throttle or orifice structure, nearby and upstream of the vortex flowmeter. The simulation results of a constricted structure like an orifice flowmeter are shown in Fig.23. Turbulence occurs at 10D. Even with a straight pipe length of 20D, the turbulence is not completely reduced (Fig.24).
If you cannot avoid installing another flowmeter, it should generally be installed downstream of the VY Series. In this case, straight pipe lengths are at least 20D from the VY Series.

Fig.23: Animated Flow Simulation with an Orifice Structure
(Fluid Type: Water, Inlet Velocity: 5 m/s, Pipe Size: DN50 Sch40, Straight Pipe Lengths: 10D upstream and 20D in downstream, Beta Ratio: 0.4)

 

Fig.24: Flow Turbulence with an Orifice Structure (Condition is the Same as in Fig.23)

 

4.6. Are Longer Straight Pipe Lengths Better?

The longer the straight pipe length, the more stable the flow. On the other hand, the accuracy of the vortex flowmeter does not change if the required straight pipe length is met. Therefore, please determine the straight pipe lengths beyond the required lengths based on the installation location. However, if there are factors that could cause turbulence, it is recommended to have longer straight pipe lengths from pipe elements.

 

5. Conclusion

This article described the reasons that vortex flowmeters need straight pipe lengths, the required straight pipe lengths for different installation conditions of the VY Series, and tips on determining straight pipe lengths. The latest Vortex Flowmeter VY Series represent an evolution in the original sensing structure, are resistant to unbalanced velocity distribution and turbulence, and require shorter upstream straight pipe lengths.
The piping conditions vary by facility, so you may need to check measurement health. In that case, the FSA130 Magnetic Flowmeter/Vortex Flowmeter Verification Tool provides a vortex waveform monitoring function to check the integrity of flow measurements in the VY Series. Please visit the pages for the VY Series and the FSA130 through the related product links.

 

Table1: Required Straight Pipe Lengths of the VY Series

Upstream Element	Reducer Pipe	Expander Pipe	Single Bend Pipe	Double Bend Pipe in Same Plane	Double Bend Pipe not in Same Plane	Over 2 Size Reducer Pipe	Over 2 Size Expander Pipe	Combination	Valve or Another Flowmeters
Required Upstream Straight Pipe Lengths from VY	5D	10D	10D	10D	20D	5D * Size	10D * Size	Reference: Sum of Required Straight Pipe Lengths	Installed Downstream, not Upstream
Required Downstream Straight Pipe Lengths from VY	5D because the VY Series Structure Decide Downstream Straight Pipe Lengths

We hope this article helps you understand vortex flowmeters and piping to improve your digitalization and energy management.

 

Looking for more information on our people, technology and solutions?

Contact Us