December 1, 2014

Service: Sahara 
Transient Pressure Monitoring
Client: City of Sault Ste. Marie
Type of Pipeline: Sewer Force Main
Diameter: 30 to 36-inch
Pipe Material: PCCP

In October 2008, the City of Sault Ste. Marie conducted a condition assessment program on two of its critical wastewater force mains: the 36-inch Clark Creek force main, and the 30-inch Pim Street force main. These force mains are the two pumped portions of the pipeline transporting wastewater from the city’s downtown to the treatment plant.

The condition assessment project consisted of a Sahara pipeline inspection to detect any leaks or pockets of trapped gas on the lines, as well as a period of pressure transient monitoring on each pipeline to check for any sudden peaks or drops in the pressures which could be damaging to the lines. Pure's pressure transient monitoring system records the variation of pressures within a pipeline caused by sudden changes in velocity of the liquid within the pipeline.

Known as a “surge” or “water hammer”, these events can exceed the structural strength of the pipeline and may cause damage in the form of a rupture or, more frequently, in the form of latent damage that can result in failure at a later date.

The Sahara inspection showed both force mains to be in good condition, however the pressure transient monitoring gave widely different results for the two mains. While both lines operated at a pressure of around 10 psi, the Clark Creek force main held a steady pressure, while the Pim Street force main experienced several severe transients, ranging from surges as high as 54 psi, to a vacuum of -8 psi.

Concerned that these transients may pose a threat to the integrity of the pipeline, Sault Ste. Marie extended the monitoring period, and employed a consultant to diagnose the cause of the transients. By comparing the times of the transients to the records of pump and valve operation, the city and their consultants were able to identify the source of the transients. They have since repaired the source of the transients before they were able to cause significant damage to the pipeline, restoring the city’s confidence in these critical assets.