Sealing Sense
Why Pump Engineers Don’t Like Expansion Joints
Lessons from participating in the Hydraulic Institute standards process.
Marty Rogin | Fluid Sealing Association member, Metraflex
Fluid Sealing Association (FSA) members from the piping expansion joint division had the opportunity to participate in revisions to Hydraulic Institute (HI) Standard 9.6.6, Pump Piping. Specifically, Appendix D of HI Standard 9.6.6 addresses expansion joints when installed as pump connectors. The process was revealing and provided important information on how the pump industry perceives expansion joints. The process also revealed conflicts between piping standards in the heating, ventilation and air conditioning (HVAC)/plumbing and process industries. Piping standards for one industry may not work well when applied to another.
The pump industry and consulting engineers in areas that regularly specify large, high-capacity pumps (process and water/wastewater, for example) make up a majority of the committee developing and maintaining Standard 9.6.6. The scope of this standard covers pump piping requirements for rotodynamic pumps and the effects of inlet/outlet piping on pump performance.
The purpose of appendix D is to explain the types of pipe couplings used with pumps, their typical uses and how to apply them without causing unacceptable loads on the pump and pump base. The term “pipe coupling” is being used generically to describe a spool piece, expansion joint, dismantling joint or any other connector used to join pipes. This appendix further describes pipe couplings and their application, installation and restraint; however, it is not intended to cover broader use of pipe couplings in piping systems.
The specific text in the revised appendix D emphasizes the main concerns of the pump industry:
“Pump nozzle loading should be limited as required by the pump manufacturer. Failure to do so may result in excessive pump nozzle stress, foundation stress, anchor bolt stress, casing distortion, pump/driver misalignment and other excessive stresses that can result in various related failure modes.”
The objective of the standard is to minimize the loading on the pump and the pump base by stiffening the connected piping and components. Therefore, the recommendations in HI 9.6.6 Appendix D will contrast with more familiar standards such as the FSA Piping Expansion Joint Handbook and other standards based on American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA). These differences are reflected in the system and component levels.
Restrained vs. Unrestrained Pump Connectors
An important distinction in connector design is the concept of restrained and unrestrained connectors. An unrestrained connector is defined as not having tie rods, or may include limit rods, either of which allow thrust loads to be transferred to the pump nozzle.
A restrained connector does not allow the device to transfer loads to the pump nozzle while under pressure. A typical pipe expansion joint may be considered restrained if the limit rods are converted to tie rods, with nuts on both sides of the lugs.
| IMAGE 1: Examples of unrestrained connectors according to HI 9.6.6. Note that limit rods will prevent overextension but will not prevent compression of the connector. This is why the connectors are considered unrestrained. (Images courtesy of the Hydraulic Institute)
Tie Rod Sizing
The more familiar criteria for tie rod sizing are based on maximum allowable stresses. This is still the case for the revised HI 9.6.6, but there are additional requirements for the restraints.
The familiar first step is to evaluate and design to not exceed allowable tie rod stress. This evaluation may use American Society of Mechanical Engineers B31.3, American Water Works Association M11 or a similar method.
The additional step is to evaluate axial stiffness and design to limit deflection so that pump maximum-allowable nozzle loads are not exceeded. This evaluation should be done in concert with piping system movements and restraint stiffness. As a rule of thumb, deflection under maximum system working pressure should be limited to the deflection of an equivalent length of pipe plus 0.010 inches (0.25 millimeters [mm]), including deflection of any supporting brackets (tie rod lugs), and the system and pump should be evaluated to ensure the deflection is acceptable for all components.
| IMAGE 2: Examples of restrained connectors according to HI 9.6.6. Note that a typical pipe expansion joint can be converted into a restrained connector by adding nuts to the inside of each lug.
Tie Rod Installation
Tie rods are installed to prevent axial movement of an expansion joint used as a pump connector—double nut tie rods on both sides of the lugs to prevent deflection in either direction and to prevent loosening of the nuts. Install with no gap between the nuts and lugs and finger-tighten the nuts, as overtorquing nuts can create undue stress in the system.
| IMAGE 3: A typical HVAC/plumping pump installation with expansion joints
System-Level Installation: 2 Strategies
By now, anyone familiar with pipe expansion joints may be bewildered. Why stiffen a perfectly good expansion joint when it is used as a pump connector? The answer (and it is tentative at present) is to consider the application.
This installation is focused on noise and vibration isolation, and it includes a vibration isolation base and flexible connectors at the pump inlet and outlet. Pipes are supported with flexible spring hangers. This configuration is effective at isolating the pump from the connected piping and building structure, but little consideration is generally given to the loads at the pump nozzles. It is rarely an issue for an HVAC or plumbing piping installation.
The pump base is anchored to the structure with no isolation base, and anchors are near the pump connections. If unrestrained connectors are required, they are installed outside of the anchor. Any connectors (not shown in the illustration) must be restrained and will be installed between the pump nozzle and the anchor.
This installation is different from the ASHRAE/SMACNA noise and vibration isolation strategy, but it is the correct solution for certain design conditions. A 2019 article in Water Environment & Technology, the Water Environment Federation monthly journal, described a pump installation in a wastewater plant that had serious vibration problems. The solution was to replace the elastomeric joints with flanged stainless steel reducer fittings. And it worked. The author had this devastating conclusion:
“Close examination of two such facilities led to some important revelations. These revelations validate the recommendation that rubber expansion joints should not be used in conventional process wastewater and water pumping systems.”
| IMAGE 4: Recommended connections as shown in HI 9.6.6 Appendix D
Through the process of editing HI Standard 9.6.6, the participants from the expansion joint industry learned some valuable insights. The most important was that the pump industry does not like expansion joints because they can damage pumps if misapplied. Hopefully participation in the standards committees can change this perception and provide technical guidance to the pump manufacturing and consulting engineering communities.
Another valuable insight is that the pump and connected piping must be considered as a complete system. Standards from one industry may not be appropriate for all installations. A working theory on why some systems have problems with expansion joint connectors and many others do not is simply the attached piping and supports. A typical HVAC/plumbing system will have small-diameter piping with flexible pipe supports. Any connector loads will be directed away from the pump and into the attached flexible piping system.
There are other piping systems, commonly found in water and wastewater plants, where the connected piping is large-diameter and normally running full, and the pipe supports act as rigid support points. With no thermal movement, little flexibility in the piping and support system and heavy connected pipe, any vibration in the pump will be directed back at the more flexible pump and base.
A new edition of HI Standard 9.6.6 is being revised with the committee, incorporating several public comments. The new edition is expected to be published in 2027
References
- “Assessing Rubber Expansion Joints,” John E. Koch and Greta Gilman, Water Environment & Technology (WE&T), June 2019.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.
Marty Rogin, P.E., is engineering manager at The Metraflex Company. For more information, visit metraflex.com.
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