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Useful information on pipe velocity

The careful design and selection of pipework in a system reduces frictional losses and improves the performance of pumps and other equipment.

At low velocities, fluids flow in a regular manner with a constant velocity and with no vertical mixing across the wave front. This is termed laminar flow. At high fluid velocities, eddies (flow currents) are formed which lead to random mixing throughout the flow cross-section. This is called turbulent flow. At intermediate flow rates, there is always a laminar flow region close to the pipe walls and this can vary in thickness depending on the roughness of the pipe material and the overall flow velocity. The point at which the flow ceases to be laminar and becomes turbulent is called the critical velocity.

Pumps, and especially centrifugal pumps, work most efficiently when the fluid is delivered in a surge-free, smooth, laminar flow. Any form of turbulence reduces efficiency, increases head loss and exacerbates wear on the pump’s bearings, seals and other components.

How is Pipe Velocity calculated?

Pipe velocity is an area averaged property which is independent of the pipe’s cross-sectional flow distribution and whether the flow is laminar or turbulent. For example, along the central axis, fluid may be travelling at twice the calculated pipe velocity.

What is head loss?

Within a pipe, frictional contact with the walls means that fluid flow is highest on the pipe axis and effectively zero at the pipe wall. The frictional contact causes a pressure and energy loss along the pipe and this is much greater with turbulent flow. Whereas with a laminar flow, pressure loss is proportional to pipe velocity, in turbulent flow it is proportional to its square.

What is the Reynolds Number?

The transition from laminar to turbulent flow can be assessed from a calculation of the Reynolds number. This is a dimensionless number determined from the pipe diameter, the density and viscosity of the flowing fluid and the flow velocity:

The Reynolds Number is effectively the ratio of the forces of mass flow and shear stress due to the fluid’s viscosity. Pipe flow can be considered to be laminar if the Reynolds number is less than 2000 and fully turbulent if it is greater than 4000. Flow characteristics are unpredictable if the value is between these two values.

What is a ‘good’ pipe velocity?

An installation engineer chooses pumps and sizes pipework to achieve a satisfactory pipe velocity. For water-like liquids with no entrained solids (for example: chemicals, paints, petrol, beverages), a pipe velocity of about 1 – 2 m/s is considered an acceptable value. If a system contains any narrow pipes or other constrictions, the pipe velocity will be a lot higher at these points.

If the liquid is shear-sensitive or can foam or change properties, a lower pipe velocity may be targeted with larger diameter pipework. On the other hand, if the fluid contains solids that could settle and form blockages at low flow rates, a higher pipe velocity (5-6 m/s) may be required.

The following table lists some typical pipe velocities for a range of common industrial feeds:

Fluid Typical Pipe Velocity (m/s)
Water 0.9 - 2.4
Carbon tetrachloride  1.8
Chlorine, liquid 1.5
Ethylene glycol 1.8
Hydrochloric acid 1.5
Oil lubricating 1.5
Sulfuric acid 1.2

 

Summary

Pumps, and especially centrifugal pumps, work most efficiently when the fluid is delivered in a surge-free, smooth, laminar flow. Any form of turbulence reduces efficiency, increases head loss and exacerbates wear on the pump’s bearings, seals and other components.

Pipe size and hence pipe velocity can have a significant effect on system performance on both the suction and discharge side of a pump. For water-like liquids with no entrained solids (for example: chemicals, petrol, beverages), a pipe velocity of about 1 – 2 m/s is considered suitable. However, with feeds containing entrained solids, pipe flow may need to be increased to eliminate the risks of sediment deposition. Fittings such as elbows and reducers should be selected to avoid restrictions that could encourage blockages. Conversely, with fluids containing dissolved gases, or sensitive to shear, turbulence may cause the liquids to degas and foam such that a lower flow and/or larger pipe size may be advisable.

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