What is a pump minimum flow BYPASS line?

Canned motor pumps rely on fluid to lubricated while operating. They also rely on the fact that discharged fluid has somewhere to go. But what if the process dictates it needs only a fraction of the pump’s minimum flow requirement? What if there’s a closed valve downstream? What happens when a pump’s basic hydraulic needs are no longer met?

This is where a minimum flow bypass line is best applied. A bypass line is most commonly used when there’s an issue meeting the minimum flow requirements, and/or for protection against deadheading the pump.

A minimum flow bypass, or recirculation, line can be configured many ways. It could be as simple as a continuous bypass, where the requirements are only piping and an orifice. A more complex line could be set up to use a series of valves.

Below is an illustration of a simple minimum flow bypass line.

Meeting Minimum Flow Conditions

Every pump has a minimum flow requirement. Operating pumps at below minimum flow can result in the following consequences:

Pitted/worn impeller vanes
Overheated casing/bearings
Excessive noise or vibration
Broken shafts
Mechanical seal failures
Poor performance efficiency

Adding a bypass line will allow the pump to sustain the minimum flow requirement, even when the process requires less flow.

Protection Against Deadheading

Our Application Engineers always recommend installing a recirculation line when high pressure pumps are in use. If a high pressure pump is deadheaded, significant damage results for the pump or the system. Not only can this be an expensive mistake, but also a cause for safety concern.

Recirculation lines provide a means of relief for high pressure pumps when operated against a closed valve or other system obstruction.

Process Liquid Impact Consideration

Meeting minimum flow and protecting the pump against deadheading conditions are the main reasons why recirculation loops are installed. But, they can also help keep product in a desired state.

For instance, they can help turn a tank over and keep solids in suspension. They can also keep fluids susceptible to change, due to temperature or shear thinning/thickening, in the state preferred for process.

Some engineers have concerns about the inevitable wasted energy that comes from using minimum flow bypass lines. It’s important that the system has been evaluated to ensure the pump is properly sized for the application and the bypass line is absolutely necessary.

If you’re not sure that a bypass line is right for you, consult an engineer who is well versed in pump selection and pipe design.

This article is from What Is a Centrifugal Pump Minimum Flow Bypass Line? (craneengineering.net)