Design and construction of municipal water systems must be done in a way that considers the effects of hydraulic transients, also known as “water hammer.” Photo: Golder Associates Ltd.
Many public utility systems use complex, interdependent networks of pipelines, valves, pumps, and other equipment to move water around, making them vulnerable to damage from hydraulic transients, which can occur whenever the flow of a liquid in a pipeline changes quickly — as when a valve rapidly opens or closes or a pump starts or stops.
Flowing liquids have kinetic energy. When they run into a valve that closes or a pump that stops suddenly, the resulting transient shock wave travels back and forth in the pipe until friction or damping controls dissipate it. The pressure also might spike high enough — even for an instant — to weaken and eventually burst the pipe. Similarly, opening a valve or stopping a pump can cause a low-pressure wave that can vaporize the liquid in part of the line, potentially causing negative pressure that can also result in pipeline collapse. The same forces can occur in an unplanned event such as a power outage, so that a pump stops unexpectedly and normal operational controls may not function.
The problem has come to be called “water hammer,” since it often results in a loud banging of the pipe. This sound can occur far from the actual source of the original pressure change, such as a valve.
Even when transient pressures are small or change gradually, regularly occurring fluctuations in pressure can eventually wear out pipeline systems and components. The result can be a costly shutdown while repairs are made. Serious amounts of water may be lost, or sewage can seep into the soil and groundwater. In systems in which water supply lines and sewer lines lie close to each other, breaks caused by hydraulic transients can cause sewage or stormwater to migrate into the water supply.TO DESIGN OR NOT
Many utilities believe that they can deal sufficiently with the hydraulic transient problem in the design stage. Using complex formulas and computer models, designers determine whether a given design will successfully manage hydraulic transients. Their calculations include the metallurgy or material properties; pipe diameter and wall thickness; the pumps, valves, and other equipment; and the nature of the fluid being pumped.
That type of computer-based analysis can go a long way in managing the hydraulic-transient problem, and it is far less costly to fix a problem at the design stage than it is to fix it after the system is operating.
However, because most utility systems have valves and pumps interacting with each other it is often not practical or possible to foresee all possible pressures that may result. System failures may occur due to incorrect supply or installation of control devices; improper operation of the system so that resulting surge pressures are not adequately limited by the transient control system; or inadequacies in supplied components.
Typically, systems for managing transient pressures and surges are designed by a transient specialist engineer. Design codes offer little explicit guidance about how to appropriately involve the transient specialist.
In many cases, that work is subcontracted out and is performed only after the basic design is complete. So the transient specialist is expected to work within the broad strokes of the existing design, including pipeline material, diameter, strength, pump specifications, and other details. Accordingly, the system may not be optimized for hydraulic transient management.
It would be better to bring the specialist engineer into the process earlier — at the conceptual design, or even the permitting stage — to assess the relative merits of a given facility design.