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.
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).
Figure 3 is a CFD image of a suction diffuser with a clean cylindrical screen. The color variation indicates different fluid velocities or turbulence. The left side of the figure shows the velocity profile presented to the pump impeller. As illustrated, the flow is not well-developed. In Figure 4, a suction diffuser and conical screen have created a similar uneven flow.
WHAT SUCTION DIFFUSERS DO
A typical suction diffuser “diffuses” flow with an “orifice cylinder” or “diffuser screen.” Flow-straightening vanes then recondition the diffused flow to a pump. In many applications, however, a suction diffuser is used incorrectly as a strainer. Although a diffuser screen may look like a Y-strainer screen, it is much smaller and has significantly less capacity. As a result, it clogs quickly and increases pressure drop, lowering the net pump suction head available (NPSHA). Installations in which a suction diffuser's screen mistakenly is intended to prevent debris from reaching a pump but is neglected until noticeable flow problems occur further complicate the problem. In such cases, the system needs to be shut down and the screen cleaned or replaced. On the other hand, Y strainers are designed to protect pumps from debris and are blown down routinely as part of a normal maintenance schedule.
CONDITIONING FLOW FOR INTAKE
With suction-diffuser shortfalls and limited space for additional lengths of piping, there needs to be a way to condition flow immediately after an elbow. The ideal solution is to install custom-engineered curved stationary vanes just before an elbow, which will cause fluid to “twist” as it enters the elbow. Fluid then can exit the elbow with a flat velocity profile and enter the pump intake without turbulence (Figure 5).
Figure 6 shows an improved velocity profile resulting from the use of curved stationary vanes in a standard 90-degree elbow. In the upper left corner is a CFD image of a 90-degree elbow that displays the eddy currents that develop as fluid changes direction. In the lower left corner is the velocity profile of water passing through a 90-degree elbow. The uneven line indicates turbulent flow coming out of the elbow and presenting itself to a pump impeller. The upper right corner shows how curved stationary vanes rifle fluid around a 90-degree elbow. The diagram in the lower right corner shows how an elbow with curved stationary vanes provides a velocity profile much flatter than an elbow without vanes. Using the vanes reduces or eliminates pump cavitation.
Figure 7 shows a side-by-side CFD comparison of a standard 90-degree elbow and an elbow with vanes installed. Again, the difference in fluid flow is indicated by color variation. The elbow with the vanes uses the entire cross-section of the pipe, resulting in more even fluid flow. Therefore, the pressure drop of an elbow with vanes is no different than that of a straight section of pipe. Additionally, a 90-degree elbow with vanes installed provides a better velocity profile to a pump than a suction diffuser.
In Figure 8, a die stream has been injected into fluid flow. The die stream bled out quickly when vanes were not installed (left). When vanes were installed (right), the die stream remained cohesive, showing how flow tracks smoothly through an elbow and exits without turbulence or significant pressure drop.
CASE STUDY: INDIANA UNIVERSITY
A pump was installed in a chilled-water extension used to cool a nuclear accelerator at the Indiana University (IU) Cyclotron Facility (Photo A). The pump had to be installed in an area with insufficient space for a proper length of pipe to be installed at the intake. The uneven velocity profile entering the pump caused turbulence on one side of the impeller, prematurely wearing the pump bearings and requiring pump-bearing replacement every six months.
Faced with either continued bearing replacement or a costly system reconfiguration, IU engineers installed pre-engineered curved stationary vanes just in front of the elbow. As a result, the pump has been running trouble-free for three years. The improved pump performance and NPSHA are illustrated in Table 1.
TURBULENCE AT PUMP DISCHARGE
Either a multipurpose or a check valve usually is installed on the discharge side of a pump. However, as previously mentioned, the increased use of variable-speed drives with pumps has resulted in the installation of more check valves. To decrease costs, wafer-style check valves are being chosen over globe-style check valves. However, a wafer-style check valve has a higher pressure drop than a bulbous-shaped globe-style check valve.
Valve manufacturers recommend five to 10 pipe diameters of straight pipe between a pump and valve so fluid turbulence off the pump will dissipate. However, space constraints or a poor understanding of proper installation have resulted in check valves regularly being installed directly on pump discharges without spool pipe. The effects of this can be disastrous. Photo B shows how turbulence from a pump nearly vibrated the hinge pin out of a valve body. The valve “chatter” that would have occurred as the pin continued to vibrate would have been an early warning of premature valve failure.
One solution is to install straightening vanes, which condition fluid flow yet require significantly less space than the recommended length of spool pipe. Figure 9 is a CFD image of straightening vanes installed in a pipe. The images show the vanes' effectiveness in eliminating turbulence while improving flow-meter reading accuracy.
Straightening vanes can be utilized as part of a standard flexible connector. This configuration provides the flow-straightening effectiveness of 10 pipe diameters of straight pipe in a fraction of the length. Additionally, a valve can be installed directly on the connector, eliminating the need for a spool of pipe.
Sometimes, solutions can be simple. For example, turbulent flow from a pump and an eccentric reducer caused a check valve to chatter from the day the system was put online (Photo C). Because the eccentric reducer was a source of its own turbulence, it was relocated and connected directly to the pump (Photo D). The double-sphere connector then was replaced by a flexible connector with straightening vanes installed between the eccentric reducer and check valve. As a result, the straightening vanes eliminated turbulence coming off the pump/reducer configuration, and the chattering check valve was silenced.
Ultimately, an engineer must review the entire system and develop a design that will achieve the owner's goals. CFD analyses show how designs and practices that have become industry standards can be advanced with vane technologies. Pre-engineered vanes reduce pressure drop and protect components by eliminating unwanted turbulence in ever-tightening engineered-piping installations.
Regional sales manager for The Metraflex Co., Jim Clauss has more than 27 years of practical piping experience, holding various positions, including technical-service-staff member, project engineer, and production manager. He has written and co-written standards and codes for pipe and fittings and been an ASTM International task-group chairman.