How It Works

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Total Dynamic Head (TDH) Calculation

Total Dynamic Head represents the total equivalent height that a pump must overcome. It combines static head, friction losses, and pressure requirements:

TDH = H_static + H_friction + H_pressure

Static Head: Vertical distance from suction to discharge level (H_discharge - H_suction)
Friction Head: Energy lost due to pipe friction, fittings, and valves
Pressure Head: Additional head to achieve required discharge pressure

NPSH Available vs Required

Net Positive Suction Head (NPSH) determines if the pump will cavitate. NPSHa must exceed NPSHr:

NPSH_a = (P_atm - P_vapor)/(rho*g) - H_suction - H_friction_suction

NPSHa (Available): Calculated from system conditions - what the system provides
NPSHr (Required): From pump manufacturer curve - what the pump needs
Margin: NPSHa should exceed NPSHr by at least 0.5m (1.5ft) or 15%

If NPSHa < NPSHr, cavitation occurs causing noise, vibration, impeller damage, and reduced performance.

Pump Affinity Laws

When pump speed changes, flow, head, and power follow these relationships:

Q2/Q1 = N2/N1 - Flow varies linearly with speed
H2/H1 = (N2/N1)^2 - Head varies with speed squared
P2/P1 = (N2/N1)^3 - Power varies with speed cubed

These laws also apply to impeller diameter changes (approximately).

Specific Speed and Pump Selection

Specific speed (Ns) helps select the optimal pump type for an application:

Ns = N * sqrt(Q) / H^0.75

Where N = RPM, Q = flow (GPM or m3/s), H = head per stage (ft or m).

500-1500: Radial flow - high head, low flow
1500-4000: Francis/mixed flow - medium head and flow
4000-10000: Mixed flow - low head, higher flow
10000+: Axial flow - very low head, high flow

Brake Horsepower (BHP) Calculation

BHP is the actual power required at the pump shaft:

BHP = (Q * H * rho * g) / (eta * 746)

Where eta = pump efficiency (decimal). Motor size should be 1.15-1.25x BHP for service factor.

NPSH Concept Visualization

Pump Sizing Calculator

Calculate total dynamic head, hydraulic power, shaft power requirements, and verify NPSH available for centrifugal pump applications.

Quick Select Presets

Unit System

Fluid Selection

Fluid Type

Flow Rate (m3/h)

Fluid Density (kg/m3)

Vapor Pressure (kPa abs)

Head Components

Static Suction Head (m) Positive if pump above liquid

Static Discharge Head (m)

Suction Friction Loss (m)

Discharge Friction Loss (m)

Discharge Pressure (kPa gauge)

Pump Efficiency (%)

NPSH Calculation

Atmospheric Pressure (kPa abs)

NPSH Required (m) From pump curve

Pump Speed (Optional)

Pump Speed (RPM)

Number of Stages

✔

Calculating...

Margin: --

NPSH Margin Indicator

NPSHr

-- m

0 NPSHr NPSHa

Results

Total Dynamic Head (TDH) --

Hydraulic Power --

Shaft Power (BHP) --

Motor Size (Recommended) --

NPSH Available --

NPSH Margin --

Specific Speed (Ns) --

Pump Type Suggestion --

Pump Curve Concept

Flow Rate (Q) Head (H) 0 0

Blue = Pump H-Q curve, Orange = System curve. Operating point (blue dot) is where curves intersect.

Reference Formulas

Total Dynamic Head:

TDH = Hd + Hs + hf_s + hf_d + Hp

Hydraulic Power:

P_hyd = rho * g * Q * H

Shaft Power:

P_shaft = P_hyd / eta

NPSH Available:

NPSHa = (P_atm - P_v)/(rho*g) - Hs - hf_s

Specific Speed (US):

Ns = N * sqrt(Q_gpm) / H_ft^0.75

Typical Pump Efficiencies

Pump Type	Flow Range	Efficiency
End Suction Centrifugal	Small	50-70%
End Suction Centrifugal	Medium	70-85%
Multistage Centrifugal	All	65-80%
Positive Displacement	All	80-95%
Submersible	Small	40-60%

Application Guidelines

Parameter	Guideline	Notes
NPSH Margin	> 0.5m or 15%	Minimum safety factor
Motor Sizing	1.15x - 1.25x BHP	Standard service factor
Suction Velocity	< 2.5 m/s	Prevent cavitation
Discharge Velocity	2 - 4 m/s	Typical design range
Specific Speed	500 - 10000	Centrifugal pump range