FF_CRIT_PRESS_L

Overview

The FF_CRIT_PRESS_L function calculates FF, the liquid critical pressure ratio factor, which is an essential parameter in control valve sizing for liquid service according to the IEC 60534 standard. This factor accounts for the thermodynamic behavior of the liquid as it approaches critical conditions during choked (cavitating) flow through a valve.

This implementation uses the fluids library, specifically the FF_critical_pressure_ratio_l function from the control valve module. For detailed documentation, see the fluids control valve documentation.

The FF factor relates the vapor pressure at the vena contracta to the inlet saturation pressure and is calculated using the following correlation from IEC 60534-2-1:

FF = 0.96 - 0.28\sqrt{\frac{P_{sat}}{P_c}}

where P_{sat} is the saturation pressure of the liquid at inlet temperature (in Pa) and P_c is the critical pressure of the liquid (in Pa). The result is a dimensionless factor typically ranging between 0.85 and 0.96.

The FF factor is used in conjunction with other valve sizing parameters—such as the liquid pressure recovery factor (F_L)—to determine whether choked flow conditions exist and to calculate the allowable pressure differential before cavitation occurs. This is critical for proper valve selection and preventing damage from cavitation or flashing in process piping systems.

This example function is provided as-is without any representation of accuracy.

Excel Usage

=FF_CRIT_PRESS_L(psat, pc)
  • psat (float, required): Saturation pressure of the liquid at inlet temperature [Pa]
  • pc (float, required): Critical pressure of the liquid [Pa]

Returns (float): FF - Liquid critical pressure ratio factor (dimensionless), or an error message (str) if input is invalid.

Examples

Example 1: Water at 90°C

Inputs:

psat pc
70100 22120000

Excel formula:

=FF_CRIT_PRESS_L(70100, 22120000)

Expected output:

0.9442

Example 2: High pressure ratio

Inputs:

psat pc
1000000 20000000

Excel formula:

=FF_CRIT_PRESS_L(1000000, 20000000)

Expected output:

0.8883

Example 3: Low pressure ratio

Inputs:

psat pc
10000 25000000

Excel formula:

=FF_CRIT_PRESS_L(10000, 25000000)

Expected output:

0.9572

Example 4: Near critical conditions

Inputs:

psat pc
5000000 22120000

Excel formula:

=FF_CRIT_PRESS_L(5000000, 22120000)

Expected output:

0.8269

Python Code

import micropip
await micropip.install(["fluids"])
from math import isfinite
from fluids.control_valve import FF_critical_pressure_ratio_l as fluids_ff_critical_pressure_ratio_l

def ff_crit_press_l(psat, pc):
    """
    Calculates FF, the liquid critical pressure ratio factor, for use in IEC 60534 liquid valve sizing calculations using fluids.control_valve.FF_critical_pressure_ratio_l. See https://fluids.readthedocs.io/fluids.control_valve.html#fluids.control_valve.FF_critical_pressure_ratio_l for details.

    See: https://fluids.readthedocs.io/fluids.control_valve.html#fluids.control_valve.FF_critical_pressure_ratio_l

    This example function is provided as-is without any representation of accuracy.

    Args:
        psat (float): Saturation pressure of the liquid at inlet temperature [Pa]
        pc (float): Critical pressure of the liquid [Pa]

    Returns:
        float: FF - Liquid critical pressure ratio factor (dimensionless), or an error message (str) if input is invalid.
    """
    try:
        psat = float(psat)
        pc = float(pc)
    except (ValueError, TypeError):
        return "Invalid input: psat and pc must be numeric values."

    if not isfinite(psat) or not isfinite(pc):
        return "Invalid input: psat and pc must be finite numbers."

    if psat <= 0:
        return "Invalid input: psat must be positive."
    if pc <= 0:
        return "Invalid input: pc must be positive."

    try:
        result = fluids_ff_critical_pressure_ratio_l(psat, pc)
        if not isfinite(result):
            return "Error: Calculation resulted in a non-finite value."
        return result
    except Exception as e:
        return f"Error: Failed to compute FF: {str(e)}"

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