BIZOMAT MAKEOS · Hydraulics
Pump Suction Line Check (Friction Loss, NPSH Available, Lowest Liquid Level)
Will my pump cavitate with this suction line, how low can the liquid level fall before it does, and is the suction velocity reasonable?
Check of a pump suction line: velocity in the suction pipe against your limit, friction factor and head loss through the pipe, foot valve and fittings (as the pipe friction tool), NPSH available from the surface pressure, vapour pressure, liquid level and suction losses compared with the pump’s NPSH required plus your margin (as the NPSH available tool), and the lowest liquid level that still keeps the margin.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Flow | L/min | Flow the pump delivers at duty. |
| Suction pipe inside diameter | mm | Inside diameter of the suction pipe. |
| Suction pipe length | m | Length of the suction pipe. |
| Sum of fitting loss coefficients (foot valve, bends) | Sum of loss coefficients for the entry, foot valve, strainer and bends. | |
| Kinematic viscosity | cSt | Kinematic viscosity at the pumping temperature. |
| Liquid density | kg/m³ | Density of the liquid. |
| Pipe roughness | mm | Roughness of the pipe bore. |
| Pressures and levels | ||
| Pressure on the liquid surface (absolute) | kPa | Absolute pressure on the liquid surface (atmospheric for an open sump). |
| Vapour pressure at pumping temperature | kPa | Vapour pressure of the liquid at the pumping temperature. |
| Liquid level above pump centreline (negative = below) | m | Height of the liquid surface above the pump centreline; negative for a suction lift. |
| Pump NPSH required at duty | m | NPSH the pump needs at the duty flow, from its curve. |
| Required margin | m | Extra head you want above the NPSH required. |
| Largest suction velocity | m/s | Largest velocity you accept in the suction pipe. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Flow
- What it is
- The volume of liquid the pump delivers at its duty point, which is also the flow through the suction pipe.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 1000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Suction velocity rises in proportion; head loss rises roughly with the square of flow.
- Get it right
- Type litres per minute; check the duty point on the pump curve, not just the nameplate.
What this means: Suction pipe inside diameter
- What it is
- The bore of the suction pipe between the sump or tank and the pump inlet, from the pipe standard or a measurement.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Velocity falls with the square of the bore, so a larger pipe cuts suction loss strongly.
- Get it right
- Use the actual bore, not the nominal pipe size or the pump inlet flange size.
What this means: Suction pipe length
- What it is
- The length of the suction pipe from its inlet to the pump, measured along the pipe including any vertical drop.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 10000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pipe friction loss rises in proportion to length.
What this means: Sum of fitting loss coefficients (foot valve, b…
- What it is
- The total of the loss coefficients for the pipe entry, foot valve, strainer and every bend in the suction line, each from fittings data and added together.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fitting loss rises in proportion to this sum.
- Get it right
- Foot valves and strainers often have large coefficients; check the maker’s data and allow for fouling.
What this means: Kinematic viscosity
- What it is
- The kinematic viscosity of the liquid at the pumping temperature, used to find the Reynolds number and friction factor.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.000001 cSt to 10000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- One centistoke is one square millimetre per second; viscosity falls as the liquid warms.
What this means: Liquid density
- What it is
- The mass per unit volume of the liquid at the pumping temperature. It converts the pressures into head of liquid.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 2500 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 998 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Pipe roughness
- What it is
- The average height of the roughness on the inside wall of the suction pipe, from pipe data for its material and condition.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.045 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type millimetres; corroded or scaled pipe is rougher than new.
Pressures and levels
What this means: Pressure on the liquid surface (absolute)
- What it is
- The absolute pressure acting on the liquid surface in the sump or tank: local atmospheric pressure for an open sump, or the vessel pressure for a closed one.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.3 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- NPSH available rises one for one with this pressure expressed as head.
- Get it right
- Use absolute pressure, not gauge; a gauge reading of zero on an open tank is atmospheric pressure in absolute terms.
What this means: Vapour pressure at pumping temperature
- What it is
- The pressure at which the liquid boils at the pumping temperature, from vapour pressure tables or the fluid data sheet.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.34 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher vapour pressure reduces NPSH available directly.
- Get it right
- Use the value at the highest pumping temperature; vapour pressure rises quickly as the liquid warms.
What this means: Liquid level above pump centreline (negative =…
- What it is
- The vertical distance from the pump centreline up to the liquid surface. A surface below the pump means the pump has to lift the liquid.
- Units and format
- Enter a number in m: metres. The form accepts -100 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -2 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each metre higher adds a metre to NPSH available.
- Get it right
- Type a negative value when the liquid surface is below the pump centreline (a suction lift).
What this means: Pump NPSH required at duty
- What it is
- The net positive suction head the pump needs at the duty flow to avoid harmful cavitation, read from the pump maker’s curve.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read it at the duty flow; NPSH required rises with flow.
What this means: Required margin
- What it is
- The extra head you want NPSH available to exceed NPSH required by, as a safety allowance against cavitation.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 100 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger margin raises the lowest liquid level you can allow.
- Get it right
- Take it from the pump maker or your design standard.
What this means: Largest suction velocity
- What it is
- The highest velocity you accept in the suction pipe, to limit losses and air entrainment, from your design standard or the pump maker.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 20 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Velocity = Q ÷ A; head loss h_f = (f L ÷ d + ΣK) v² ÷ 2g with the Colebrook friction factor (as the pipe friction tool).
- NPSH_a = (p_s − p_v) ÷ ρg + z − h_f (as the NPSH available tool); margin = NPSH_a − NPSH_r.
- Lowest level = z − (margin − margin required).
- Example: the suction loses 0.23159 m, NPSH available is 7.8797 m against 3 m required, and the level may fall to -5.8797 m.
Limitations
- Steady flow at the duty point; NPSH required rises at higher flows.
- Entrained air and vortexing at a low submergence are not covered.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a pump installation before ordering
- Find how far a sump may be drawn down
- Size a suction pipe
Who it is for
- Plant engineers
- Maintenance engineers
- Pump installers
What you get
- Suction velocity and head loss
- NPSH available and margin
- Lowest liquid level for the margin
Interpreting the result
- Raising the liquid level or shortening the suction line helps more than any change to the pump.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Pipe flow, pressure drop and pump selection: getting fluid where it needs to go efficiently
In Fluids, HVAC and Building Services
What it covers: How head, velocity, friction and fittings set pressure drop, how system and pump curves fix the operating point, why efficiency and NPSH matter, and how pipe size affects energy cost.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Pump Suction Line Check (Friction Loss, NPSH Available, Lowest Liquid Level) calculator