Spring-Loaded Accumulator

Hydraulic accumulator with spring used for energy storage

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Accumulators

Description

This block models a spring-loaded hydraulic fluid accumulator. The accumulator consists of a preloaded spring and a fluid chamber. The fluid chamber is connected to a hydraulic system.

As the fluid pressure at the accumulator inlet becomes greater than the preload pressure, fluid enters the accumulator and compresses the spring, storing hydraulic energy. A decrease in the fluid pressure causes the spring to decompress and discharge stored fluid into the system.

During typical operations, the spring pressure is equal to the pressure in the fluid chamber. However, if the pressure at the accumulator inlet drops below the preload pressure, the spring becomes isolated from the system. In this situation, the fluid chamber is empty and the spring pressure remains constant and equal to the preload pressure while the pressure at the accumulator inlet depends on the hydraulic system to which the accumulator is connected. If the pressure at the accumulator inlet builds up to the preload pressure or higher, fluid enters the accumulator again.

The motion of the spring is restricted by two hard stops that limit the expansion and contraction of the fluid volume. The fluid volume is limited when the fluid chamber is at capacity and when the fluid chamber is empty. The hard stops are modeled with finite stiffness and damping. This means that it is possible for the fluid volume to become negative or greater than the fluid chamber capacity, depending on the values of the hard-stop stiffness coefficient and the accumulator inlet pressure.

The diagram represents a spring-loaded accumulator. The fluid chamber is on the left and the spring is on the right. The distance between the left side and the spring defines the fluid volume (V_F).

The hard stop contact pressure is modeled with a stiffness term and a damping term. The accumulator spring is assumed to have a linear relationship between the spring pressure and the fluid volume, with pressure balanced at the end of the spring:

$p_{s p r} - p_{p r} = K_{s p r} V_{F}$

$p_{F} = p_{s p r} + p_{H S}$

$K_{s p r} = \frac{p_{\max} - p_{p r}}{V_{C}}$

$p_{H S} = {\begin{array}{l} K_{S} (V_{F} - V_{C}) + K_{d} q_{F}^{+} (V_{F} - V_{C}) & if V_{F} \geq V_{C} \\ K_{S} V_{F} - K_{d} q_{F}^{-} V_{F} & if V_{F} \leq 0 \\ 0 & otherwise \end{array}$

$q_{F}^{+} = {\begin{array}{l} q_{F} & if q_{F} \geq 0 \\ 0 & otherwise \end{array}$

$q_{F}^{-} = {\begin{array}{l} q_{F} & if q_{F} \leq 0 \\ 0 & otherwise \end{array}$

where

V_F	Volume of fluid in the accumulator
V_init	Initial volume of fluid in the accumulator
V_C	Fluid chamber capacity
p_F	Pressure at the accumulator inlet (gauge)
p_pr	Preload pressure (gauge)
K_spr	Spring gain coefficient
p_max	Pressure needed to fully fill the accumulator
p_spr	Pressure developed by the spring
p_HS	Hard-stop contact pressure
K_s	Hard-stop stiffness coefficient
K_d	Hard-stop damping coefficient
q_F	Fluid flow rate into the accumulator, which is positive if fluid flows into the accumulator

The flow rate into the accumulator is the rate of change of the fluid volume:

$q_{F} = \frac{d V_{F}}{d t}$

At t = 0, the initial condition is V_F = V_init, where V_init is the value you assign to the Initial fluid volume parameter.

The Spring-Loaded Accumulator block does not consider loading on the separator. To model additional effects, such as the separator inertia and friction, you can construct a spring-loaded accumulator as a subsystem or a composite component, similar to the block diagram below.

Basic Assumptions and Limitations

The accumulator spring is assumed to be behave linearly.
Loading on the separator, such as inertia or friction, is not considered.
Inlet hydraulic resistance is not considered.
Fluid compressibility is not considered.

Parameters Tab

Fluid chamber capacity: Amount of fluid that the accumulator can hold. The default value is 8e-3 m^3.
Preload pressure (gauge): Spring pressure (gauge) when the fluid chamber is empty. The default value is 10e5 Pa.
Pressure at full capacity (gauge): Spring pressure (gauge) when the fluid chamber is at capacity. The default value is 30e5 Pa.
Hard-stop stiffness coefficient: Proportionality constant of the hard-stop contact pressure with respect to the fluid volume penetrated into the hard stop. The hard stops are used to restrict the fluid volume between zero and fluid chamber capacity. The default value is 1e10 Pa/m^3.
Hard-stop damping coefficient: Proportionality constant of the hard-stop contact pressure with respect to the flow rate and the fluid volume penetrated into the hard stop. The hard stops are used to restrict the fluid volume between zero and fluid chamber capacity. The default value is 1e10 Pa*s/m^6.

Variables Tab

Accumulator flow rate: Volumetric flow rate through the accumulator port at time zero. Simscape™ software uses this parameter to guide the initial configuration of the component and model. Initial variables that conflict with each other or are incompatible with the model may be ignored. Set the Priority column to High to prioritize this variable over other, low-priority, variables.
Volume of fluid: Volume of fluid in the accumulator at time zero. Simscape software uses this parameter to guide the initial configuration of the component and model. Initial variables that conflict with each other or are incompatible with the model may be ignored. Set the Priority column to High to prioritize this variable over other, low-priority, variables.
Pressure of liquid volume: Gauge pressure in the accumulator at time zero. Simscape software uses this parameter to guide the initial configuration of the component and model. Initial variables that conflict with each other or are incompatible with the model may be ignored. Set the Priority column to High to prioritize this variable over other, low-priority, variables.