Design Challenges for Pump Applications

From an overlooked stress concentration in the hull plating on a Liberty ship to the aero-elastic flutter of the original Tacoma Narrows Bridge, miscalculating small details has led to public disasters. Similarly, small errors have led to the failure of innumerable projects and untold amounts of wasted money. The same attention and design diligence that goes into planning for large design challenges is needed in smaller projects as well. In particular, the seemingly innocuous area of hermetic seals for pump applications warrants attention.

During the course of their careers, design and test engineers will likely encounter an application with a pressure or material exposure design challenge. In applications such as vacuum systems, manufacturing equipment, process monitoring, fluid-filled devices and pressurized equipment, penetrations are needed to get power, electrical signal or optical feeds in and/or out of sealed enclosures.

Missteps often occur when engineers and pump manufacturers:

• Do not make material selection and application design paramount
• Neglect to address all of the environmental concerns that affect material selection, including temperature, chemistry and pressures
• Accept OEM components, and force themselves to solve hermetic feedthrough challenges around existing design
• Underestimate the importance of the feedthrough in their final design or product
• Do not communicate with their component supplier

Although an off-the-shelf feedthrough may suffice in straightforward applications, many design, material or envelope considerations require a customized solution. For example, if a point-to-point design is available in a material that is not ideal for the application environment, a custom connector sealed with a suitable material will likely be more successful for the same cost.

An application often requires a more rugged design than an off-the-shelf feedthrough would provide. Engineers should be aware that working around a feedthrough challenge is not a necessary evil. Today, feedthroughs and connectors can be made to fit the application's requirements rather than the other way around.

Heavy-Duty Applications: Municipal Waste Water Pumps

There are several considerations to keep in mind for these applications, including sealing requirements, the ability to troubleshoot or disconnect and cost concerns.

The grommet seal/gland seal is one of the most common sealing methods: two pieces of metal are squeezed around the cable and "sealed." This solution is the lowest cost option for applications that do not require a hermetic seal.

However, the drawbacks for this solution potentially outweigh the initial cost savings. Grommet seals are never truly hermetically sealed as they are only sealed around the jacket. They often allow leakage through the cables and wires and allow fluids into the pump or control box. Many engineers are so accustomed to these leaks that they have reengineered the electronics box to tolerate higher moisture levels-a "bubblegum" answer that negates the low cost of the grommet seal. Upkeep is a nagging constant for this solution, as grommet seals need to be routinely retightened, specifically in high-vibration environments, or when the elastomer is replaced.

If the solution requires a hermetic seal and the application requires the ability to disconnect, consider a mated connector solution. If the project will eventually require reengineering, retesting or troubleshooting, the benefits of a mated connector solution might outweigh the higher cost and longer installation time associated with this option. If disconnecting the seal is not important, however, the mated connector solution may also introduce a potential failure point-a risk could be avoided with a few more questions for the sealing provider.

The waterproof connector or hermetic feedthrough is a point-to-point wiring solution that is cheaper than the more complex mated connector but still offers the benefits of a hermetic seal. These solutions come pretested, prewired and ready-to-install, but if troubleshooting is needed, retesting and reconnecting would be necessary.

Submersible Sump and Water Well Applications

For this application, the most important question when considering a sealing option is whether leaks are allowable.

If the answer is yes, ask how much leak is allowable. This is not a simple answer, because even if a significant leak is allowed, forecast six months, one year or five years in the future to determine if an extended leak is acceptable. This technology is prone to leakage by design as it only seals to the outside of the cable, allowing gas/liquid to flow through the cable. For some applications, these leaks are tolerable, so the low-cost clamp or jacket seal is perfectly workable.

If an extended leak or any leakage cannot be tolerated-whether due to the pump's design or the nature of the environment-consider only fully hermetic solutions. These solutions stop all ingress, blocking the gas and/or water from entering the insulation jacket and travelling up the wires. Too often engineers may take it for granted that a "seal" means hermetically sealed. Fully discussing the application requirements with the feedthrough and connector supplier will help provide the best solution.

Chemical Applications

Chemical compatibility is a real issue, and not only for hazardous materials. Everything from toothpaste to hot sulfuric acid to hydrofluoric acid can potentially react with the pump's components. Therefore, especially for these applications, consider material choice, component selection, the application and the unique environment.

Material selection is critical. Harsh chemicals that come into contact with wire and cable insulations-O-rings are particularly prone to exposure-can affect the electrical and mechanical abilities of the feedthrough.

For these applications, communication with the feedthrough designer is paramount. In the past, materials have caused problems with commonly used silica filler in epoxies. For example, if the feedthrough designer is not aware that the chemical application will pump hydrofluoric acid, which eats silica, the connector will disintegrate.

The connector supplier should ask about the environment to which the seal may be exposed, but the user should also provide all the details. The supplier should be willing to make samples and prototypes for evaluation, have a solid knowledge of chemical compatibility and be prepared for unknown variables that need to be tested prior to development.

Conclusion

Whoever coined the phrase "Don't sweat the small stuff" was likely not an engineer. Everything engineers do is ultimately defined by the success of "the small stuff" that comprises the larger design projects.

No matter how small or seemingly insignificant the detail, failure to incorporate the best solution for any given application can result in production delays, cost-overruns, frustration and, in the worst-case scenario, catastrophic failure.

Most component and subassembly providers will happily provide input during the design stage to help avoid such issues. If the supplier cannot provide a solution that meets the needs, budget and delivery timetable, consider working with one who can. The new provider should know how important the small stuff is.

Pumps & Systems, November 2009