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Variable Head Tank

Tank with constant pressurization and volume-dependent fluid level

Library

Low-Pressure Blocks

Description

The Variable Head Tank block represents a pressurized hydraulic reservoir, in which fluid is stored under a specified pressure. The pressurization remains constant regardless of volume change. The block accounts for the fluid level change caused by the volume variation, as well as for pressure loss in the connecting pipe that can be caused by a filter, fittings, or some other local resistance. The loss is specified with the pressure loss coefficient. The block computes the volume of fluid in the tank and exports it outside through the physical signal port V.

For reasons of computational robustness, the pressure loss in the connecting pipe is computed with the equations similar to that used in the Fixed Orifice block:

The Critical Reynolds number is set to 15.

The pressure at the tank inlet is computed with the following equations:

where

pPressure at the tank inlet
pelevPressure due to fluid level
plossPressure loss in the connecting pipe
pprPressurization
ρFluid density
gAcceleration of gravity
HFluid level with respect to the bottom of the tank
KPressure loss coefficient
ApConnecting pipe area
dConnecting pipe diameter
qFlow rate
pcrMinimum pressure for turbulent flow
VInstantaneous fluid volume
V0Initial fluid volume
ATank cross-sectional area
tSimulation time

For a tank with a variable cross-sectional area, the relationship between fluid level and volume is specified with the table lookup

You have a choice of three interpolation methods and two extrapolation methods.

Connection T is a hydraulic conserving port associated with the tank inlet. Connection V is a physical signal port. The flow rate is considered positive if fluid flows into the tank.

    Warning   If fluid level becomes so low that the tank inlet gets exposed, no warnings will be issued. The simulation will continue and pressure at the inlet will be set to the pressurization pressure. If this is not acceptable, MathWorks recommends that you employ the necessary control measures to guard against this situation in your models.

Dialog Box and Parameters

Initial fluid volume

The initial volume of fluid in the tank. This parameter must be greater than zero. The default value is 20 l.

Pressurization

Gage pressure acting on the surface of the fluid in the tank. It can be created by a gas cushion, membrane, bladder, or piston, as in bootstrap reservoirs. This parameter must be greater than or equal to zero. The default value is 0, which corresponds to a tank connected to atmosphere.

Level/Volume relationship

Select one of the following block parameterization options:

  • Linear — Provide a value for the tank cross-sectional area. The level is assumed to be linearly dependent on the fluid volume. This is the default method.

  • Table-specified — Provide tabulated data of fluid volumes and fluid levels. The level is determined by one-dimensional table lookup. You have a choice of three interpolation methods and two extrapolation methods.

Tank cross-section area

The cross-sectional area of the tank. This parameter must be greater than zero. The default value is 0.8 m^2. This parameter is used if Level/Volume relationship is set to Linear.

Tabulated fluid volumes

Specify the vector of input values for fluid volume as a one-dimensional array. The input values vector must be strictly increasing. The values can be nonuniformly spaced. The minimum number of values depends on the interpolation method: you must provide at least two values for linear interpolation, at least three values for cubic or spline interpolation. The default values, in m^3, are [0 0.0028 0.0065 0.0114 0.0176 0.0252 0.0344 0.0436 0.0512 0.0574 0.0623 0.066 0.0688 0.0707 0.072 0.0727]. This parameter is used if Level/Volume relationship is set to Table-specified.

Tabulated fluid levels

Specify the vector of fluid levels as a one-dimensional array. The fluid levels vector must be of the same size as the fluid volumes vector. The default values, in meters, are [0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3]. This parameter is used if Level/Volume relationship is set to Table-specified.

Inlet pipeline diameter

The diameter of the connecting pipe. This parameter must be greater than zero. The default value is 0.02 m.

Pipeline pressure loss coefficient

The value of the pressure loss coefficient, to account for pressure loss in the connecting pipe. This parameter must be greater than zero. The default value is 1.2.

Interpolation method

Select one of the following interpolation methods for approximating the output value when the input value is between two consecutive grid points:

  • Linear — Uses a linear interpolation function.

  • Cubic — Uses the Piecewise Cubic Hermite Interpolation Polinomial (PCHIP).

  • Spline — Uses the cubic spline interpolation algorithm.

For more information on interpolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Level/Volume relationship is set to Table-specified.

Extrapolation method

Select one of the following extrapolation methods for determining the output value when the input value is outside the range specified in the argument list:

  • From last 2 points — Extrapolates using the linear method (regardless of the interpolation method specified), based on the last two output values at the appropriate end of the range. That is, the block uses the first and second specified output values if the input value is below the specified range, and the two last specified output values if the input value is above the specified range.

  • From last point — Uses the last specified output value at the appropriate end of the range. That is, the block uses the last specified output value for all input values greater than the last specified input argument, and the first specified output value for all input values less than the first specified input argument.

For more information on extrapolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Level/Volume relationship is set to Table-specified.

Ports

The block has the following ports:

T

Hydraulic conserving port associated with the tank inlet.

V

Physical signal port that outputs the volume of fluid in the tank.

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