Because valves are rarely installed in perfect, straight-pipe conditions.
Enter the Fp factor — the Pressure Drop Correction Factor.

🔹 What is Fp?
Fp represents how much the actual installation (piping layout, fittings, coil proximity) affects the valve’s flow performance.

Formula:
Cv_actual = Cv_catalog × Fp

🔹 Typical Scenarios
Fp = 1.0 → Perfect straight pipe (ideal condition)
Fp = 0.9 → Minor fittings or 5D/10D straight length
Fp = 0.7 → Elbows or coil connection near valve
Fp = 0.5 → Tight installation, high turbulence

So if a valve has a catalog Cv of 10 but an Fp of 0.8 —
👉 the actual effective Cv is only 8 in your installation!

🔹 Why It Matters
Ignoring Fp leads to:
* Under-performing coils
* Poor control authority
* Unstable temperature regulation

Always check manufacturer data (like Belimo or ASHRAE tables) to apply the right Fp correction — especially for coil-mounted valves.

🔹 Key Takeaway
Fp < 1 → Actual capacity is lower than catalog Cv.

Smart engineers always consider it during valve selection.