Pressure control valve for maintaining preset pressure in fluid network
Thermal Liquid/Valves/Pressure Control Valves
The Pressure Relief Valve (TL) block represents a valve for maintaining a preset pressure in a fluid network. The valve remains closed until the pressure at port A reaches the valve set pressure. A pressure rise above the set pressure causes the valve to gradually open, allowing the fluid network to relieve excess pressure.
A smoothing function allows the valve opening area to change smoothly between the fully closed and fully open positions. The smoothing function does this by removing the abrupt opening area changes at the zero and maximum ball positions. The figure shows the effect of smoothing on the valve opening area curve.
Opening-Area Curve Smoothing
The mass conservation equation in the valve is
is the mass flow rate into the valve through port A.
is the mass flow rate into the valve through port B.
The momentum conservation equation in the valve is
pA and pB are the pressures at port A and port B.
is the mass flow rate.
is the critical mass flow rate.
ρAvg is the average liquid density.
Cd is the discharge coefficient.
SR is the valve opening area.
S is the valve inlet area.
PRLoss is the pressure ratio:
The valve opening area is computed as
SLeak is the valve leakage area.
SLinear is the linear valve opening area:
SMax is the maximum valve opening area.
pcontrol is the valve control pressure:
pset is the valve set pressure:
pMin is the minimum pressure.
pMax is the maximum pressure:
Δp is the portion of the pressure range to smooth.
λL and λR are the cubic polynomial smoothing functions
The critical mass flow rate is
The energy conservation equation in the valve is
ϕA is the energy flow rate into the valve through port A.
ϕB is the energy flow rate into the valve through port B.
Specification method for the valve set pressure parameter. Options
Pressure at port A and
Minimum gauge pressure at port A required to open the valve. A
pressure rise above the set pressure causes the valve to gradually open
until it reaches the fully open state. This parameter is active only
when the Pressure control specification parameter
is set to
Pressure at port A. The default
Minimum pressure differential between ports A and B required to open
the valve. A pressure differential rise above this value causes the
valve to gradually open until it reaches the fully open state. This
parameter is active only when the Pressure control
specification parameter is set to
differential. The default value is
Difference between the maximum and set pressures at port A. The valve
begins to open at the set pressure. It is fully open at the maximum
pressure. The default value is
Flow cross-sectional area in the fully open state. This state
corresponds to pressures lower than the set pressure. The default value
Aggregate area of all fluid leaks in the valve. The leakage
area helps to prevent numerical issues due to isolated fluid network
sections. For numerical robustness, set this parameter to a nonzero
value. The default value is
Fraction of the opening-area curve, expressed as a fraction from 0 to
1, to smooth. The block replaces the discontinuities in the opening area
curve with smooth transitions that span the specified fraction of the
curve. The default value is
A smoothing factor of 0 corresponds to a linear function that is discontinuous at the set and maximum-area pressures. A smoothing factor of 1 corresponds to a nonlinear function that changes continuously throughout the entire function domain.
A smoothing factor between 0 and 1 corresponds to a continuous piece-wise function with smooth nonlinear transitions at the set and maximum-area pressures and linear segments elsewhere.
Opening-Area Curve Smoothing
Flow area at the valve inlets. The inlets are assumed equal in size.
The default value is
Approximate length of the valve. This parameter provides a measure
of the longitudinal scale of the valve. The default value is
Semi-empirical parameter commonly used as a measure of valve performance. The discharge coefficient is defined as the ratio of the actual mass flow rate through the valve to its theoretical value.
The block uses this parameter to account for the effects of
valve geometry on mass flow rates. Textbooks and valve data sheets
are common sources of discharge coefficient values. By definition,
all values must be greater than 0 and smaller than 1. The default
Reynolds number corresponding to the transition between laminar
and turbulent flow regimes. The flow through the valve is assumed
laminar below this value and turbulent above it. The appropriate values
to use depend on the specific valve geometry. The default value is
Mass flow rate into the component through port A
at the start of simulation. The default value is
A — Thermal liquid conserving port representing valve inlet A
B — Thermal liquid conserving port representing valve inlet B