When it comes to rotodynamic pumping systems, piping requirements are imperative for an energy-efficient and reliable pumping system design. The function of pump piping is to provide a conduit for the flow of liquid to and from a pump without adversely affecting the performance or reliability of the pump. Ideal piping designs provide uniform and symmetric fluid velocity profiles to pumps with sufficient pressure to avoid cavitation. Poor piping design may adversely affect the performance and reliability of the pumping system by creating strong local currents and swirling near the pump. Effects include reduced hydraulic performance, noisy pump operation, random axial load oscillations, premature bearing and/or seal failure, cavitation damage and possible damage caused by liquid separation on the discharge side.
Piping systems must consider the piping loads transmitted to the pump nozzle. The piping system must have the necessary flexibility and restraint to absorb piping misalignment, vibration and expansion forces so the piping does not fail and so excessive loads are not transmitted to the pump.
ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping was first published in 2009 to provide these practical piping requirements to system designers, engineering procurement contractors, industry consultants and pump end users. The purpose of the standard is to:
- Understand the physical arrangement of piping systems and state the required straight piping length to ensure equal and sufficient flow to the pump impeller for reliable performance.
- Understand what piping configurations require a physical model.
- Understand the effects that excessive piping loads can have on pumps such as misalignment and distortion.
- Learn about the recommended procedures that can reduce the risk of pump failure because of interaction with piping systems.
The standard dives into the most common detrimental effects caused by poor pump piping designs, including excessive loads that the piping can place on a pump, failure to properly restrain the pump to the foundation support structure, excessive loads resulting from unsupported or poorly supported valves or fittings, and uneven or swirling flow entering the pump.
Suction piping requirements
The function of suction piping is to provide a uniform velocity profile or symmetric approaching flow to the pump suction connection with sufficient pressure to avoid pump cavitation. An uneven distribution of flow is characterized by strong local currents and swirls. This detrimental effect is shown in Figure 1. The ideal approach is a straight pipe coming directly to the pump with no turns or flow-disturbing fittings close to the pump. Figure 2 provides an example of an ideal pipe design with the suction line at a slope constantly upward toward the pump. Note that for a suction lift system as illustrated, a foot valve or other method of priming may be required when the pump is started.
The recently released 2016 edition of ANSI/HI 9.6.6 goes more in-depth regarding fitting types, including several visuals revolving around the minimum length of straight pipe required immediately upstream of the pump suction nozzle for various fitting types. The standard covers industry technologies used to validate pump piping configurations such as physical hydraulic model studies and the acceptable use and limits of computational fluid dynamics (CFD) modeling. A properly conducted physical model study is a reliable method to identify unacceptable flow patterns at the pump suction for given sump or suction piping designs and to derive acceptable intake sump or piping designs. CFD is an analysis method used in fluid mechanics that deals with numerical solutions of the general flow equations for mass, momentum and heat transfer. Today, CFD is used within many sciences and industries that involve fluid flow and transport phenomena, including the pump industry. The advances in CFD indicate that further uses of these computational simulations may be possible in the future, and the Hydraulic Institute (HI) may consider additional applications of CFD in future standards or guidelines.
Discharge piping requirements
Discharge piping flow characteristics normally will not affect the performance and reliability of a rotodynamic pump, with a few exceptions. Sudden valve closures can cause excessively high water-hammer-generated pressure spikes to be reflected back to the pump, possibly causing damage. Where there may be a sudden closure of a check valve or sudden stopping of the pump, the standard recommends a transient flow analysis.
Discharge piping can affect the starting, stopping and priming of the pump. The discharge piping configuration can also alter any discharge flow recirculation that might extend into the discharge piping at low flow rates. System pipe friction losses, life cycle costs, and process considerations normally dictate the size of the discharge piping and fittings.
Pipe fittings mounted close to the discharge flange will normally have a minimal effect on the performance or reliability of rotodynamic pumps. However, some pumps can be sensitive to flow-disturbing fittings mounted close to the pump discharge. This can result in increased noise, vibration and hydraulic loads. HI recommends direct communication with the pump manufacturer for independent requirements.
Nozzle load requirements
Nozzle loads affect pump operation in several ways. At low levels, the effects may be insignificant. At high levels, nozzle loads cause serious reliability problems such as pump case distortion and coupling misalignment, which can lead to heat buildup in bearings; reduced bearing life; decreased coupling life; increased noise and vibration levels; mechanical seal or packing failures; and in some cases, failure of the pump casing, foundation or shaft. HI, the American Petroleum Institute and the Committee European for Normalization gathered resources to provide methods for evaluating the loads applied to pump nozzles.
Apart from piping design requirements, ANSI/HI 9.6.6 provides informative content regarding system curves; water hammer; pipe supports and restraints; expansion joints and couplings; specialty piping components and applications; and pressure pulsations and acoustic resonance. The standard was drafted to serve as a leading resource for technical experts in the field. HI standards are adopted in the public interest and are designed to help eliminate misunderstandings between the manufacturer, the purchaser and/or the user and to assist the purchaser in selecting and obtaining the proper product for a particular need.
The latest version of ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping was approved by ANSI on March 23, 2016, and is available for purchase directly from HI’s eStore: pumps.org.
Peter Gaydon is the director of technical affairs for the Hydraulic Institute. Gaydon has a mechanical engineering degree from Alfred University and has more than 10 years of experience doing design, development and testing with major pump manufacturers. He joined the Hydraulic Institute in 2014 with technical responsibility for all standards, guidebooks and program guides. Darcy Chiriboga received her mechanical engineering degree from Rutgers University and works with HI standards committees as a standards engineer.
Founded in 1917, the Hydraulic Institute represents the pump manufacturing industry in North America. It is the recognized authority on pumps and pumping systems through its standards, guidebooks and technical and educational resources. The Institute represents 120 member organizations consisting of pump manufacturers and their suppliers and is a value-added resource for engineering consulting firms and pump users around the globe. For more information, visit pumps.org.