Fluid handling characteristics insights

• Fluid handling characteristics matter because the piping material significantly impact productivity and can increase or decrease the frequency of necessary maintenance depending on how the fluid handling characteristics are.
• Knowing the viscosity of the material that will flow through pipes is one factor to help determine the appropriate pipe diameter size.
• The flow characteristics of water flowing through piping systems are affected by several factors including system configuration, pipe size and length and friction at the pipe and fitting surfaces, which can cause head loss or pressure drop.

Galloping inflation, interest rate hikes and interminable supply chain issues aren’t stopping the construction of new business locations and manufacturing facilities. Continuous improvement is key to staying on the cutting edge – and this doesn’t just mean having the best equipment and workers on the production floor. Your choice of behind-the-scenes elements like piping materials can significantly impact productivity, uptime and bottom lines. When selecting piping materials for a new installation or retrofit, don’t forget an easy-to-overlook quality: the material’s fluid handling characteristics.

Why do fluid handling characteristics matter?

Especially for industrial applications, the carrying capacity and friction loss rates of a piping material significantly impact productivity and can increase or decrease the frequency of necessary maintenance. Defaulting to traditional metallic materials may decrease efficiency and increase necessary maintenance due to metal’s fluid handling characteristics.

While no single piping material is appropriate for all applications – for example, some chemicals cannot run through a plastic pipe without causing failure while other plants handle materials that can corrode traditional metallic pipes – plastics like CPVC bring superior fluid handling capabilities to plants that must prioritize uptime and minimize maintenance.

Seek information on the following characteristics to determine a piping material’s suitability for your application’s needs and your expectations.

Linear velocity of fluid flow

The linear velocity (in feet per second) of a flowing fluid in a pipe is calculated from the flow rate (in gallons) per minute divided by the inside diameter of the pipe (in inches). (See figure 1)

Figure 1: A visualization of the formula for calculating linear velocity. Courtesy: Corzan Material and Piping Solutions.

Knowing the viscosity of the material that will flow through pipes is one factor to help determine the appropriate pipe diameter. Linear fluid flow velocity in a system should generally be limited to five feet per second for industrial applications, particularly for pipe sizes six inches or greater. Too great a linear fluid flow velocity can create turbulence that agitates the material within. This turbulence can also be exacerbated by the roughness of the internal surface of the piping material.

Hazen-Williams C factor

When choosing a piping material, check its C Factor to help anticipate both potential maintenance and how the material should perform over time. The Hazen-Williams formula is generally accepted for calculating friction head losses in piping systems: the lower the C Factor, the higher the friction, and the higher the friction, the worse the performance. (Figure 2)

Figure 2: The surface roughness constants for both new and old and varied piping materials. Courtesy: Corzan Material and Piping Solutions.

As the table above shows, the texture of a metal pipe’s inner surface is naturally rougher than that of CPVC and other plastic piping, even when CPVC is decades old. This rough texture allows fluid to build up in these niches and degrade material quickly, especially at welds and joints. Furthermore, metals “pit,” where tiny divots form along the length of the pipe. This diminishes the pipe’s wall thickness, which reduces its pressure-bearing capability and can introduce corrosion chips into the fluid traveling through the pipes. Corrosion can also cause pipes to fail. By contrast, the naturally smooth, innately corrosion-resistant interior surface of CPVC is resistant to scaling, pitting and fouling, keeping its C Factor high over the service life. (Figure 3)

Figure 3: A metallic pipe (left) and a CPVC pipe (right) after the same length of service. The metallic pipe has significant fouling inside, while the CPVC pipe remains in like-new condition. Courtesy: Corzan Material and Piping Solutions.

Friction head loss

The flow characteristics of water flowing through piping systems are affected by several factors including system configuration, pipe size and length and friction at the pipe and fitting surfaces. These and other factors cause a reduction in pressure over the length of the system; this is called head loss or pressure drop. Significant head loss means a piping system isn’t performing optimally, which reduces efficiency and slows processes.

A piping material provider should provide numbers on head loss. If they don’t, one can calculate water velocities, head losses and pressure drops as a function of flow rates with the following formula: (Figure 4)

Figure 4: A visualization of the formula for calculating friction head loss. Courtesy: Corzan Material and Piping Solutions.

Friction losses through fittings are calculated from the equivalent length of straight pipe that would produce the same friction loss in the fluid. Similar to head loss in the pipes themselves, friction at fittings reduces performance efficiency.

Equivalent Length of Pipe (feet) (Figure 5)

Figure 5: The data provided in this table is for reference only. Consult the fitting manufacturer’s literature for additional information. Courtesy: Corzan Material and Piping Solutions.

The most common cause of head loss from friction is a rough inside diameter of pipe. The naturally rougher interior surface of metallic materials compared to plastics like CPVC becomes more uneven as corrosion and scale accumulate over time. The more bumps and divots a fluid must navigate as it flows through a pipe, the greater the friction head loss, the lesser the efficiency and the sooner the necessary downtime for maintenance.

Maximum surge pressure

A change in the flow rate of a fluid results in a pressure surge known as water hammer. Water hammer may be caused by opening or closing a valve, starting or stopping a pump or trapped air moving through the pipe. The longer the line and faster the fluid is moving, the greater this hydraulic shock will be. Persistent and/or significant water hammer can rupture fittings, breaking pipe supports and more – in short, introducing potential points of failure into your piping system.

The maximum water hammer surge pressure may be calculated using the following formula: (Figure 6)

Figure 6: A visualization of the formula for calculating maximum water hammer surge pressure. Courtesy: Corzan Material and Piping Solutions.

The water hammer surge pressure added to the system operating pressure should not exceed 1.5-times the recommended working pressure rating of your piping system. While piping system materials play a role in how much hammer will occur, the design and use of the system will determine if and how much hammer occurs.

Limiting linear fluid flow velocity to five feet per second in pipe sizes six inches or larger can minimize hydraulic shock due to water hammer. Velocity at system start-up should be limited to one foot per second during filling until all air has been flushed from the system and the pressure has been raised to operating conditions.

Some additional tips for avoiding water hammer include: not allowing air to accumulate in an operating system; not allowing pumps to draw in air; and adding extra protection like pressure relief valves, surge arrestors and more.

Make an informed decision based on fluid handling characteristics

A piping system must remain reliable over time. Fluid handling is an important metric to help anticipate the expected performance and annual maintenance of any piping material. Making the best decisions for behind-the-scenes materials like piping can make a significant difference for your plant’s performance, whether building a new facility or retrofitting an existing one. In many cases, CPVC material has proven to be more effective in handling fluids in harsh conditions than metals in industrial plants.

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