Turbulent flow churns, spins, tumbles, and creates unexpected havoc in piping systems. If left unchecked, it can shorten the life of critical piping components, create noise and vibration, and make maintaining a balanced piping system virtually impossible. Building owners and architects complicate matters by attempting to reclaim valuable, rentable space, forcing engineers to squeeze systems into tighter and tighter spaces. Additionally, equipment manufacturers require service space. When construction begins, contractors need to problem-solve on the fly as unexpected issues arise, impacting even the most carefully balanced engineered piping designs. Even with these challenges, however, the biggest turbulence culprits often are the very components in engineered piping systems.

PROPER DESIGN HURTS PUMP EFFICIENCY

There are several commonalities in every engineered-piping-system design. Every centrifugal pump needs a strainer upstream to remove debris. Each side of the pump has a shutoff valve, and the discharge side has a check valve. Sometimes, the check valve and discharge-side shutoff valve are combined with a metering valve, creating a single multipurpose valve. However, the increased use of variable-speed pumps eliminates the need for a control valve, making multipurpose valves unnecessary. In these instances, a center-guided spring-loaded check valve is used.

Ideally, a section of straight pipe approximately five to 10 pipe diameters long — depending on the recommendations of the pump manufacturer — should be connected to a pump intake. A suction diffuser or guide could be used as an alternative to the pipe (Figure 1).

Increasingly, mechanical rooms are constructed with insufficient space to install a pump with enough straight pipe to dissipate turbulence. Meanwhile, using an elbow alone can create turbulent flow. Without the proper length of pipe, turbulent flow can impact pump efficiency and cause cavitation. Placing a suction diffuser upstream of a pump has become the default design to condition flow in an attempt to improve pump efficiency, but suction diffusers can cause significant pressure drops.

CFD STUDIES

Computational-fluid-dynamics (CFD) studies of a standard long-radius 90-degree elbow show how fluid turning around a fitting results in turbulent flow and causes pressure drop in a piping system (Figure 2).