Centrifugal Pump Sizing: TDH, BHP & NPSH Calculator
Calculate Total Dynamic Head (TDH), Water Horsepower (WHP), Brake Horsepower (BHP), electrical motor kW, annual operating costs, Net Positive Suction Head available (NPSHa vs NPSHr cavitation check), and Affinity Laws.
Flow & Head Parameters
Pump Efficiencies & Suction
Hydraulic & Power Output
Pump Characteristic vs System Friction Curve (Operating Point)
Vector H-Q diagram showing the parabolic system head curve ($H = H_{\text{static}} + k Q^{1.852}$) intersecting the centrifugal pump head curve at the Best Efficiency Point (BEP).
Hydraulic Engineering Principles: TDH, Power & Cavitation Limits
Pumping fluid requires transferring mechanical shaft energy into fluid pressure and velocity head. Proper pump selection requires evaluating system friction and suction pressure to prevent cavitation.
\text{TDH} = H_{\text{static}} + \frac{P_{\text{discharge}} - P_{\text{suction}}}{\gamma} \times 2.31 + H_{\text{friction}} + \frac{v^2}{2g}
2. Hazen-Williams Friction Head Loss:
H_f = 10.44 \times \left(\frac{Q}{C}\right)^{1.852} \times \frac{L}{D^{4.8655}} \quad (C = 150 \text{ for PVC})
3. Water Horsepower (Hydraulic Power):
\text{WHP} = \frac{Q (\text{GPM}) \times \text{TDH} (\text{ft}) \times \text{SG}}{3960}
4. Brake Horsepower (Shaft Mechanical Power):
\text{BHP} = \frac{\text{WHP}}{\eta_{\text{pump}}} \quad \Big(\text{Motor kW} = \frac{\text{BHP} \times 0.7457}{\eta_{\text{motor}}}\Big)
5. Net Positive Suction Head Available (NPSHa):
\text{NPSHa} = H_{\text{barometric}} \pm H_{\text{static suction}} - H_{\text{friction suction}} - H_{\text{vapor pressure}}
\text{Cavitation Margin} = \text{NPSHa} - \text{NPSHr} \ge 3.0\text{ to }5.0\text{ ft}
5 Critical Centrifugal Pump & Hydraulic Traps
1. The Cavitation Implosion Catastrophe
When NPSHa falls below NPSHr, local fluid pressure drops below liquid vapor pressure, creating vapor bubbles that violently collapse against the impeller at $100,000\text{ PSI}$. It sounds like pumping marbles and pit-corrodes stainless steel impellers within weeks.
2. Throttling Far Left of the BEP
Choking down a discharge valve to reduce flow pushes the pump far left of its Best Efficiency Point (BEP). Asymmetric pressure builds inside the volute, generating violent radial thrust that flexes the shaft, destroys mechanical seals, and causes catastrophic bearing seizure.
3. Deadheading & Superheated Boiling
Running a centrifugal pump with the discharge valve completely closed converts 100% of motor brake horsepower directly into heat. Trapped water quickly flashes into superheated steam, melting elastomer seals, shattering silicon carbide faces, or detonating the casing.
4. System Runout Motor Overload
Oversizing a pump or operating without sufficient system backpressure shifts the duty point far to the right into "runout." High flow rates spike horsepower demand dramatically ($BHP \propto Q$), pulling continuous locked-rotor amperes and tripping motor thermal overloads.
5. Ignoring Affinity Law Cube Savings
Throttling flow mechanically wastes massive electrical energy across the valve. By the Affinity Laws ($P \propto N^3$), installing a Variable Frequency Drive (VFD) and reducing pump speed by just 20% slashes electrical power consumption by nearly 50% ($0.8^3 = 0.512$).