Methods for Transient Analysis
While simpler methods can provide a useful check on computer model results, they can also result in gross over estimates and higher costs (see Figure). Computer model results need to be interpreted by experienced engineers, who must consider all possible operating conditions: normal system operations, emergency shut-downs and breaks (see Table).

Types of Hydraulic Transients
Different causes give rise to different types of hydraulic transients in deep sewers, storage tunnels or water distribution networks such as:

• Short-duration surge pressures several times greater than the pipes' working pressures.
• Sudden or periodic surge forces which may exceed pipe joint or anchor block strength.
• Sub-atmospheric pressures which can flatten pipes or suck in air and groundwater.
• Mass oscillations and flow instabilities due to trapped air, cavitation, vibrations, uneven pipeline or tunnel filling etc...

Complex waterhammer problems can even exceed the in-house expertise of national firms.

EHG's Dr. A. Fok and Dr. S. Zemell (with over 50 years of combined experience) can help your team to overcome the toughest problems. In fact, most of our work involves cooperating with - and learning from - other consulting firms and public utilities.

EHG uses the computer model HAMMER^TM from Haestad Methods Inc., which can analyze any system from single lines to large sewer or water networks with thousands of pipes - whose data it can import from EXTRAN and EPANET or WaterCAD^®, respectively. A variety of operating and protective equipment can also be simulated; as can many fluids (water, oil, fuel, slurry, steam, natural gas) and abnormally high pressure and temperature industrial processing system.

• Pump starts, shifting and stops can result in frequent, low-magnitude transient shocks
• Small-magnitude transient shocks can damage or age systems if they occur often
• Discharge control valves often increase flows non-linearly and may cause transients

• Where possible, use of variable as well as fixed-speed pumps can reduce shocks

• Consider power failures, short-lived brown-outs and back-up power
• Re-start only after allowing sufficient time for transients to decay (time delay circuitry)
• Sub-atmospheric pressure can reduce water quality (groundwater & air intrusion)
• Air accumulation or release from the pipeline can cause large transients
• Transient forces must be checked: they can break even "strong" pipes or anchors

• Friction in large pipe networks can dampen transients & require fewer protection measures

• Break probability maps (based on analysis) can reduce service interruptions in old systems

• Consider malfunctions such as stuck valves or check-valves, operator error, etc...