The Bathtub Curve as Applied to Pumping Systems

Those of you who are involved in reliability are most likely familiar with the “bathtub curve.” For those of you who are not, here is a brief explanation. The bathtub curve has been used extensively within the reliability engineering segment. The curve depicts life cycle reliability in three parts:

• early failures or startup and commissioning
• random failures or useful life period
• increasing failure rate or wear out failures or end of life.

Click here to listen to a Pumps & Systems podcast with Livoti on common reasons why pump systems are poorly designed The name is derived from the cross-sectional shape of a bathtub (see Image 1). The theory behind the bathtub curve and its shape is generated by tracking the number of early failures during the commissioning and startup process, the number of random failures relative to constant failures during the equipment’s useful life period and the rate of normal wear failures as the product reaches or exceeds its design lifetime. The thought process behind the bathtub theory is that in the early life of a piece of equipment (in our case, a pump system), the failure rate is high but rapidly decreases as defective products, installation, commissioning, operation and startup issues are corrected. In the useful life period, once all the initial startup or commissioning issues have been addressed, the failure rate is low and constant. In the late life of the system, the failure rate increases as age and wear take their toll on the pump system. This theory has been around for as long as I have been in the industry, and for that same period I have had an issue with this theory being applied to pumping systems. The bathtub theory may apply to automobiles, electronic devices, etc., but I cannot accept, nor can I endorse, using this theory on pumping systems for one simple reason. We are accepting the status quo! I have interacted with many reliability engineers over the course of my career who accept this theory as the norm. This is not the case. Have you ever investigated why you experience so many failures during installation, startup and commissioning? I have. The primary root cause of the high rate of early failures begins with the end user’s failure to write a comprehensive General Scope Document/Specification, as well as a clearly defined, detailed owners acceptance criteria. The pumps are incorrectly specified or sized and installed poorly. In many cases, the operators were not properly trained on how to operate the pumps. A well-defined pump system specification and properly formulated and executed owners acceptance criteria will result in a curve shown in Image 2. The “shift” in approval begins with the end user taking responsibility for writing the scope document and for the pumping system specification. I have written a series of articles regarding pump system standards, beginning with the all-important General Scope Document. The first article of the series was published in the February 2018 issue of Pumps & Systems. The series goes into the development and thorough process required (by the end user) to prepare a pump system specification for submittal to the engineering, procurement and construction (EPC) or design firm. We will continue the ongoing Pump System Standards column in the August issue of Pumps & Systems.