Kv COEFFICIENT (SI UNIT)

  Kv  =  Q × √(SG / ΔP)

  Variables :
    Kv   flow coefficient (m³/h at ΔP = 1 bar, water)
    Q    volumetric flow rate (m³/h)
    ΔP   pressure drop across the valve (bar)
    SG   specific gravity — water = 1, oil ≈ 0.85, brine > 1

  Rationale : Kv = 1 means that 1 m³/h of water (SG = 1) flows through
  the fully open valve with 1 bar of pressure drop. It is the
  "hydraulic ID card" of the valve, provided by the manufacturer.


Cv COEFFICIENT (US UNIT)

  Cv  =  1.156 × Kv

  Variables :
    Cv   flow coefficient (US gpm at ΔP = 1 psi, water)

  Rationale : US equivalent. Cv = US gal/min of water for ΔP = 1 psi.
  The 1.156 factor comes from the m³/h ↔ US gpm and bar ↔ psi
  conversions. Valid for incompressible, non-cavitating fluids ; for
  gases and steam, additional correction factors apply.


INVERSE FLOW FROM Kv

  Q  =  Kv × √(ΔP / SG)

  Rationale : inverse formula — useful to check that a given valve
  will pass the required flow under the available pressure drop. Also
  used to compute the actual pressure drop for a given flow.


DENSITY EFFECT

  SG  >  1  →  same Kv  →  less flow   (brine, glycol, acids)
  SG  =  1  →  water reference
  SG  <  1  →  same Kv  →  more flow   (gasoline, alcohols, light oils)

  Rationale : a denser fluid requires more energy to cross the valve
  — hence less flow at constant ΔP. The formula Q ∝ 1/√SG reflects
  this. Always provide the operational SG at process temperature, not
  at 15 °C — density varies with temperature.