FF_CRIT_PRESS_L

Computes the IEC 60534 liquid critical pressure ratio factor F_F from saturation pressure and critical pressure.

The factor is calculated as:

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

This dimensionless ratio is used in downstream liquid control-valve choking and sizing calculations.

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.

Example 1: Water at 90°C

Inputs:

psat	pc
70100	22120000

Excel formula:

=FF_CRIT_PRESS_L(70100, 22120000)

Expected output:

0.944238

Example 2: High pressure ratio

Inputs:

psat	pc
1000000	20000000

Excel formula:

=FF_CRIT_PRESS_L(1000000, 20000000)

Expected output:

0.89739

Example 3: Low pressure ratio

Inputs:

psat	pc
10000	25000000

Excel formula:

=FF_CRIT_PRESS_L(10000, 25000000)

Expected output:

0.9544

Example 4: Near critical conditions

Inputs:

psat	pc
5000000	22120000

Excel formula:

=FF_CRIT_PRESS_L(5000000, 22120000)

Expected output:

0.826878

Python Code

Show Code

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 the liquid critical pressure ratio factor FF for IEC 60534 liquid valve sizing.

    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)

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

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

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

Online Calculator

psat *

Saturation pressure of the liquid at inlet temperature [Pa]

pc *

Critical pressure of the liquid [Pa]