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Segmented Pipe LP

Hydraulic pipeline with resistive, fluid inertia, fluid compressibility, and elevation properties

Library

Low-Pressure Blocks

Description

The Segmented Pipe LP block models hydraulic pipelines with circular cross sections. Hydraulic pipelines, which are inherently distributed parameter elements, are represented with sets of identical, connected in series, lumped parameter segments. It is assumed that the larger the number of segments, the closer the lumped parameter model becomes to its distributed parameter counterpart. The equivalent circuit of a pipeline adopted in the block is shown below, along with the segment configuration.

Pipeline Equivalent Circuit

Segment Configuration

The model contains as many Constant Volume Hydraulic Chamber blocks as there are segments. The chamber lumps fluid volume equal to

where

VFluid volume
dPipe diameter
LPipe length
NNumber of segments

The Constant Volume Hydraulic Chamber block is placed between two branches, each consisting of a Hydraulic Resistive Tube block and a Fluid Inertia block. Every Hydraulic Resistive Tube block lumps (L+L_ad)/(N+1)-th portion of the pipe length, while Fluid Inertia block has L/(N+1) length (L_ad denotes additional pipe length equal to aggregate equivalent length of pipe local resistances, such as fitting, elbows, bends, and so on).

The nodes to which Constant Volume Hydraulic Chamber blocks are connected are assigned names N_1, N_2, …, N_n (n is the number of segments). Pressures at these nodes are assumed to be equal to average pressure of the segment. Intermediate nodes between Hydraulic Resistive Tube and Fluid Inertia blocks are assigned names nn_0, nn_1, nn_2, …, nn_n. The Constant Volume Hydraulic Chamber blocks are named ch_1, ch_2, …, ch_n, Hydraulic Resistive Tube blocks are named tb_0, tb_1, tb_2, …, tb_n, and Fluid Inertia blocks are named fl_in_0, fl_in_1, fl_in_2, …, fl_in_n.

The number of segments is the block parameter. In determining the number of segments needed, you have to find a compromise between the accuracy and computational burden for a particular application. It is practically impossible to determine analytically how many elements are necessary to get the results with a specified accuracy. The golden rule is to use as many elements as possible based on computational considerations, and an experimental assessment is perhaps the only reliable way to make any conclusions. As an approximate estimate, you can use the following formula:

where

NNumber of segments
LPipe length
cSpeed of sound in the fluid
ωMaximum frequency to be observed in the pipe response

The table below contains an example of simulation of a pipeline where the first four true eigenfrequencies are 89.1 Hz, 267 Hz, 446 Hz, and 624 Hz.

Number of Segments1st Mode2nd Mode3rd Mode4th Mode
1112.3
2107.2271.8
497.7284.4432.9689
893.2271.9435.5628

As you can see, the error is less than 5% if an eight-segmented version is used.

The difference in elevation between ports A and B is distributed evenly between pipe segments.

The block positive direction is from port A to port B. This means that the flow rate is positive if it flows from A to B, and the pressure loss is determined as .

Basic Assumptions and Limitations

Flow is assumed to be fully developed along the pipe length.

Dialog Box and Parameters

The block dialog box contains three tabs:

Basic Parameters

Pipe internal diameter

Internal diameter of the pipe. The default value is 0.01 m.

Pipe length

Pipe geometrical length. The default value is 5 m.

Number of segments

Number of lumped parameter segments in the pipeline model. The default value is 1.

Aggregate equivalent length of local resistances

This parameter represents total equivalent length of all local resistances associated with the pipe. You can account for the pressure loss caused by local resistances, such as bends, fittings, armature, inlet/outlet losses, and so on, by adding to the pipe geometrical length an aggregate equivalent length of all the local resistances. This length is added to the geometrical pipe length only for hydraulic resistance computation. Both the fluid volume and fluid inertia are determined based on pipe geometrical length only. The default value is 1 m.

Internal surface roughness height

Roughness height on the pipe internal surface. The parameter is typically provided in data sheets or manufacturer's catalogs. The default value is 1.5e-5 m, which corresponds to drawn tubing.

Laminar flow upper margin

Specifies the Reynolds number at which the laminar flow regime is assumed to start converting into turbulent. Mathematically, this is the maximum Reynolds number at fully developed laminar flow. The default value is 2000.

Turbulent flow lower margin

Specifies the Reynolds number at which the turbulent flow regime is assumed to be fully developed. Mathematically, this is the minimum Reynolds number at turbulent flow. The default value is 4000.

Wall Compliance

Pipe wall type

The parameter can have one of two values: Rigid or Flexible. If the parameter is set to Rigid, wall compliance is not taken into account, which can improve computational efficiency. The value Flexible is recommended for hoses and metal pipes where wall compliance can affect the system behavior. The default value is Rigid.

Static pressure-diameter coefficient

Coefficient that establishes relationship between the pressure and the internal diameter at steady-state conditions. This coefficient can be determined analytically for cylindrical metal pipes or experimentally for hoses. The parameter is used if the Pipe wall type parameter is set to Flexible. The default value is 2e-12 m/Pa.

Viscoelastic process time constant

Time constant in the transfer function that relates pipe internal diameter to pressure variations. By using this parameter, the simulated elastic or viscoelastic process is approximated with the first-order lag. The value is determined experimentally or provided by the manufacturer. The parameter is used if the Pipe wall type parameter is set to Flexible. The default value is 0.01 s.

Specific heat ratio

Gas-specific heat ratio for the Constant Volume Hydraulic Chamber block. The default value is 1.4.

Vertical Position

Port A elevation wrt reference plane

The parameter specifies vertical position of the pipe port A with respect to the reference plane. The default value is 0.

Port B elevation wrt reference plane

The parameter specifies vertical position of the pipe port B with respect to the reference plane. The default value is 0.

 Restricted Parameters

Global Parameters

Parameters determined by the type of working fluid:

  • Fluid density

  • Fluid kinematic viscosity

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.

Ports

The block has the following ports:

A

Hydraulic conserving port associated with the pipe inlet.

B

Hydraulic conserving port associated with the pipe outlet.

References

[1] White, F.M., Viscous Fluid Flow, McGraw-Hill, 1991

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