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Fixed-Displacement Pump - Simulate fixed-displacement hydraulic pump

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Pumps and Motors

Description

The Fixed-Displacement Pump block represents a positive, fixed-displacement pump of any type as a data-sheet-based model. The key parameters required for this block are pump displacement, volumetric and total efficiencies, nominal pressure, and angular velocity. All these parameters are generally provided in the data sheets or catalogs. The fixed-displacement pump is represented with the following equations:

where

`q`	Pump delivery
`p`	Pressure differential across the pump
`p_P,p_T`	Gauge pressures at the block terminals
`T`	Torque at the pump driving shaft
`D`	Pump displacement
ω	Pump angular velocity
`k_leak`	Leakage coefficient
`k_HP`	Hagen-Poiseuille coefficient
η_V	Pump volumetric efficiency
η_mech	Pump mechanical efficiency
ν	Fluid kinematic viscosity
ρ	Fluid density
`p_nom`	Pump nominal pressure
ω_nom	Pump nominal angular velocity
ν_nom	Nominal fluid kinematic viscosity

The leakage flow is determined based on the assumption that it is linearly proportional to the pressure differential across the pump and can be computed by using the Hagen-Poiseuille formula

where

`q_leak`	Leakage flow
`d, l`	Geometric parameters of the leakage path
μ	Fluid dynamic viscosity, μ = ν^.ρ

The leakage flow at p = p_nom and ν = ν_nom can be determined from the catalog data

which provides the formula to determine the Hagen-Poiseuille coefficient

The pump mechanical efficiency is not usually available in data sheets, therefore it is determined from the total and volumetric efficiencies by assuming that the hydraulic efficiency is negligibly small

The block positive direction is from port T to port P. This means that the pump transfers fluid from T to P provided that the shaft S rotates in the positive direction. The pressure differential across the pump is determined as .

Basic Assumptions and Limitations

The model is based on the following assumptions:

Fluid compressibility is neglected.
No loading on the pump shaft, such as inertia, friction, spring, and so on, is considered.
Leakage inside the pump is assumed to be linearly proportional to its pressure differential.

Dialog Box and Parameters

Pump displacement: Pump displacement. The default value is 5e-6 m^3/rad.
Volumetric efficiency: Pump volumetric efficiency specified at nominal pressure, angular velocity, and fluid viscosity. The default value is 0.92.
Total efficiency: Pump total efficiency, which is determined as a ratio between the hydraulic power at the pump outlet and mechanical power at the driving shaft at nominal pressure, angular velocity, and fluid viscosity. The default value is 0.8.
Nominal pressure: Pressure differential across the pump, at which both the volumetric and total efficiencies are specified. The default value is 1e7 Pa.
Nominal angular velocity: Angular velocity of the driving shaft, at which both the volumetric and total efficiencies are specified. The default value is 188 rad/s.
Nominal kinematic viscosity: Working fluid kinematic viscosity, at which both the volumetric and total efficiencies are specified. The default value is 18 cSt.

Global Parameters

Fluid kinematic viscosity: The parameter is determined by the type of working fluid selected for the system under design. Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.

Ports

The block has the following ports:

T: Hydraulic conserving port associated with the pump suction, or inlet.
P: Hydraulic conserving port associated with the pump outlet.
S: Mechanical rotational conserving port associated with the pump driving shaft.

Examples

The Power Unit with Fixed-Displacement Pump demo (sh_power_unit_fxd_dspl_pump) contains a fixed-displacement pump, which is driven by a motor through a compliant transmission, a pressure-relief valve, and a variable orifice, which simulates system fluid consumption. The motor model is represented as an Ideal Angular Velocity Source block, which rotates the shaft at 188 rad/s at zero torque. The load on the shaft decreases the velocity with a slip coefficient of 1.2 (rad/s)/Nm. The load on the driving shaft is measured with the torque sensor. The shaft between the motor and the pump is assumed to be compliant and simulated with rotational spring and damper.

The simulation starts with the variable orifice open, which results in a low system pressure and the maximum flow rate going to the system. The orifice starts closing at 0.5 s, and is closed completely at 3 s. The output pressure builds up until it reaches the pressure setting of the relief valve (75e5 Pa), and is maintained at this level by the valve. At 3 s, the variable orifice starts opening, thus returning the system to its initial state.