How Pipe Size Affects Flow, Pressure Drop and Pump Performance

Pipe size can have a significant impact on the performance of a hygienic processing system. For a given flow rate, pipe diameter determines fluid velocity, which influences friction loss, pressure requirements and the amount of energy required to move product through the process.

A pipe that is too small can create excessive velocity and pressure loss. However, simply increasing pipe diameter is not always the answer. Larger piping costs more, increases product hold-up and may not provide the desired velocities for certain process or cleaning conditions.

Proper pipe sizing requires balancing flow, velocity, pressure loss and the requirements of the product and process.

How Pipe Diameter Affects Fluid Velocity

For a fixed flow rate, reducing pipe diameter forces the same volume of fluid through a smaller cross-sectional area. The result is higher fluid velocity. Increasing pipe diameter has the opposite effect.

This relationship is important because even a relatively small change in pipe diameter can create a significant change in velocity.

For example, moving the same flow through a smaller line requires the product to travel faster. That higher velocity affects how much resistance the fluid encounters as it moves through the system.

Higher Velocity Means Greater Friction Loss

As fluid travels through piping, energy is lost to friction. Resistance occurs along the pipe wall and as the fluid passes through elbows, valves, tees, reducers, heat exchangers, filters and other process components.

As velocity increases, these friction losses can rise significantly.

This means an undersized pipe can require the pump to generate considerably more pressure to maintain the desired flow. Increasing pipe diameter generally reduces velocity and friction loss, allowing the same volume of product to move through the system with less resistance.

Pipe length also matters. Every foot of pipe contributes friction loss, so the effect of pipe diameter becomes increasingly important as piping runs become longer or contain numerous fittings and process components.

Pipe Size Can Affect Energy Requirements

Pressure lost to friction represents energy that must be supplied by the pump.

If an unnecessarily small pipe creates excessive system resistance, additional pumping energy is required to achieve the desired flow. Over the operating life of a process system, this can make pipe sizing an energy and operating-cost consideration as well as an engineering decision.

Larger piping can reduce these losses, but bigger is not automatically better. Larger tubing, valves and fittings increase installed cost and the amount of product contained within the piping.

The objective is to select a pipe diameter that provides acceptable velocity and pressure loss without unnecessarily increasing the size of the system.

Product Characteristics Matter

Pipe sizing should not be based on flow rate alone. The characteristics of the fluid being transferred also affect how easily it moves through the system.

Water, cleaning solutions and other low-viscosity fluids generally move through piping with relatively low resistance. More viscous products such as creams, syrups, sauces and concentrates can create substantially greater pressure losses.

Temperature can also change the viscosity of many products. A product that flows readily when warm may behave very differently at a lower temperature.

For this reason, piping calculations should reflect the actual product properties and operating temperatures expected in the process rather than relying exclusively on conditions based on water.

Pay Particular Attention to Suction Piping

Pipe sizing is especially important on the suction side of a pump because friction loss in the suction line reduces the pressure available at the pump inlet.

A suction line that is too small, excessively long or contains unnecessary restrictions can create unfavorable inlet conditions before the fluid reaches the pump. Elbows, valves, strainers and other components add additional losses, particularly at higher fluid velocities.

Good system design therefore considers not only how easily the pump can move product through the discharge piping, but also how effectively product can reach the pump.

Production and CIP May Require Different Conditions

Hygienic processing adds another consideration because the same piping frequently handles both product and cleaning solutions.

During production, a line may carry a viscous product at one flow rate and temperature. During Clean-in-Place (CIP), the same line may carry a low-viscosity cleaning solution at a considerably different flow rate.

Pipe diameter must accommodate both operating conditions. A line sized only around production requirements may not provide the desired conditions during cleaning, while a system designed solely around CIP may not be optimal for product transfer.

Evaluating both duties helps ensure the piping supports the complete operating cycle.

Finding the Right Pipe Size

Proper pipe sizing is ultimately about balance. The pipe must accommodate the required flow without creating excessive velocity and friction loss, while also accounting for product characteristics, suction conditions, cleaning requirements and system configuration.

Even relatively small changes in pipe diameter can have a meaningful effect on system resistance. Understanding that relationship can help process engineers make better decisions when designing a new system, increasing production capacity or modifying an existing process.

Fristam’s Friction Loss Calculator can help evaluate these conditions using actual flow rate, viscosity, specific gravity, temperature, tubing size and length, along with valves, elbows, tees and other system losses.

Understanding the relationship between pipe diameter, velocity and friction loss helps ensure the pump and piping are evaluated as parts of the same system. Fristam application engineers and authorized distributors can also assist in evaluating process conditions and selecting the appropriate pumping solution.