Calculation of Npsh

NPSH Calculator (Net Positive Suction Head)

Absolute pressure head at the source (Atmospheric pressure head is ~10.33m at sea level).
Positive if liquid level is above pump; negative if liquid level is below pump.
Losses due to pipes, valves, and fittings in the suction line.
The pressure head at which the liquid starts to boil at its current temperature.

NPSH Available (NPSHa): 0.00 m

Understanding NPSH Calculation

Net Positive Suction Head (NPSH) is a critical factor in centrifugal pump design and operation. It defines the margin of pressure available to prevent the liquid from vaporizing (boiling) at the point of lowest pressure within the pump, usually the impeller eye.

The Formula for NPSHa

This calculator determines NPSH Available (NPSHa), which is the actual head at the pump suction flange. The formula used is:

NPSHa = Ha ± Hs – Hf – Hvp

  • Ha (Absolute Pressure Head): The pressure on the surface of the liquid in the supply tank.
  • Hs (Static Suction Head): The vertical distance from the liquid surface to the pump centerline. This is positive if the pump is below the liquid (flooded suction) and negative if the pump is above the liquid (suction lift).
  • Hf (Friction Head): The energy lost due to friction as the fluid moves through suction piping, elbows, and valves.
  • Hvp (Vapor Pressure Head): The pressure head exerted by the liquid's vapor at the operating temperature.

NPSHa vs. NPSHr

For a pump to operate correctly and avoid cavitation, the NPSH Available (NPSHa) must always be greater than the NPSH Required (NPSHr). NPSHr is a value provided by the pump manufacturer based on laboratory testing.

Why NPSH Matters

When NPSHa drops below NPSHr, the liquid pressure falls below its vapor pressure. Bubbles of vapor form and then collapse violently as they enter high-pressure areas of the impeller. This phenomenon, known as cavitation, causes:

  • Severe pitting and erosion of the impeller and pump casing.
  • Loss of pump efficiency and head.
  • Excessive noise and vibration.
  • Premature bearing and seal failure.

Practical Example

Imagine pumping water at 20°C from an open tank located at sea level. The pump is 2 meters above the water level. Friction losses are 0.5 meters.

  • Ha: 10.33m (Atmospheric pressure)
  • Hs: -2.0m (Suction lift)
  • Hf: 0.5m
  • Hvp: 0.24m (Vapor pressure of water at 20°C)
  • Calculation: 10.33 – 2.0 – 0.5 – 0.24 = 7.59m NPSHa
function calculateNPSH() { var ha = parseFloat(document.getElementById('absPressure').value); var hs = parseFloat(document.getElementById('staticHead').value); var hf = parseFloat(document.getElementById('frictionLoss').value); var hvp = parseFloat(document.getElementById('vaporPressure').value); var resultDiv = document.getElementById('npshResult'); var resultSpan = document.getElementById('npshValue'); var warningP = document.getElementById('cavitationWarning'); if (isNaN(ha) || isNaN(hs) || isNaN(hf) || isNaN(hvp)) { alert("Please enter valid numeric values for all fields."); return; } // Formula: NPSHa = Ha + Hs – Hf – Hvp // Note: User enters negative Hs for suction lift, so we add the signed value. var npsha = ha + hs – hf – hvp; resultSpan.innerText = npsha.toFixed(2); resultDiv.style.display = "block"; if (npsha < 0) { warningP.innerText = "CRITICAL: Negative NPSHa detected! Immediate cavitation and pump damage will occur."; warningP.style.color = "#d93025"; } else if (npsha < 2) { warningP.innerText = "WARNING: Low NPSHa. Ensure this exceeds your pump's NPSHr by a safety margin (usually 0.5m – 1.0m)."; warningP.style.color = "#e67e22"; } else { warningP.innerText = "NPSHa is positive. Compare this value to your pump's NPSH Required (NPSHr) curve."; warningP.style.color = "#188038"; } }

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