Transients in water distribution systems are a major concern for pipeline analysis, design and operation as they have the potential to wreck or damage pipeline systems and equipment, induce adverse water quality conditions, reduce system efficiency, and threaten the integrity and quality of supply as well as public safety. These impulse loads normally develop when the steady-state velocity is suddenly altered following rapid valve closure or opening, vapor pocket collapse, safety valve blow down, and starting or stopping of a centrifugal pump. They are also known as pressure surges and, in water applications, waterhammer. The magnitude of the pressure changes is proportional to the change in flow velocity. These events typically occur over a millisecond time frame, but may span several seconds in large systems.
Transient analysis of the performance of piping systems is often more important to the engineer than the analysis of the steady state operating conditions which we normally use as the design basis. Pressure surges (waterhammer) developed during startup and shutdown, and/or under accident conditions such as loss of power to the pumps or inadvertent valve closure, can easily exceed steady state values. Cavitation or excessive pressure surging during transient operation can lead to pipeline or component failure. Surge control devices are often required to prevent the development of such conditions. The proper selection and evaluation of these devices requires a reliable transient flow analysis.
InfoSurge was developed to supplement InfoWater Pro's powerful steady-state and dynamic (EPS) modeling capabilities with the ability to model a broad range of waterhammer events, including transient cavitation and various surge suppression devices. Vapor cavitation and liquid column separation can be explicitly modeled, allowing the effect of pressure surges due to vapor cavity collapse to be properly evaluated and sound preventive measures adequately designed. The best design is one that prevents formation of a vapor cavity.
Hydraulic transient analysis involves the determination of transient pressures and flows, which occur when a disturbance is introduced into the flow system initially operating under steady-state conditions. The principle components of the disturbances are abnormal pressure changes caused by the propagation of pressure waves throughout the distribution system. Pressure waves are generated at locations where the pressure-flow characteristics or other conditions are time dependent. The pipeline sections transmit the pressure waves at sonic speed and also modify (attenuate) the waves to account for the effects of line friction. Energy-transformation losses cause the pressure wave to gradually decay until normal (steady) pressures and velocities are once again restored.
Waves are altered at network elements which consist of:
- valves, orifices, or other resistive elements.
Disturbances causing transients may be generated by the following conditions in InfoSurge:
- valve opening or closing (variation in flow area); pump start up or shutdown (variation in pump speed-pump trip); change in specified inflow (or outflow).
- closed ends,
- pipe junctions,
- open surge tanks and feed tanks,
- closed surge tanks and bladder tanks,
- pressure relief valves,
- surge anticipation valves,
- air release/vacuum valves,
- check valves,
- bypass lines.
Disturbances also occur when check valves, feed tank connections, bypass lines, pressure relief valves and air relief valves undergo an operational status change (open or closed) due to conditions encountered during transient operation. Out of balance initial conditions can also be used to simulate other kind of disturbances, such as a sudden rupture of a pipe.
The means of controlling pressure transients will in general vary depending upon whether the initiating event results in an upsurge or downsurge. Water distribution systems must be designed to accommodate both normal and abnormal operations and safeguarded to handle adverse external events such as power failure, pipeline fracture, pump failure, etc. To combat potential transient in a system there are a number of options, mostly involving the design and installation of one or more surge protection devices. InfoSurge can accommodate a wide range of commonly employed surge protection devices including: open surge tanks, closed surge tanks, discharge tanks, pressure relief valves, surge anticipation valves, air release/vacuum valves, and pump bypass lines. The relative merits and shortcomings of all the protection devices must be carefully evaluated. This is normally accomplished by performing a transient analysis on the basic system, and then changing this system - by adding surge protection devices such as relief valves or other devices - until the system hydraulic response is considered acceptable. Usually the best solution is a combination of several devices and/or improved operational strategies.
One of recent challenging problem associated with pipeline transients is the associated water quality risk posed by transient intrusion. Since all pipeline systems leak and hydraulic transients occur continuously in most distribution systems, low pressure transient waves present considerable risk of drawing untreated and possibly hazardous water into a pipeline system. Therefore, the designer should never overlook the effect of waterhammer or surge pressures in the design of WDS and the determination of ultimate system cost. InfoSurge automatically calculates the volume of intrusion due to negative system pressures. This will help to determine the extent of contamination and evaluate the most effective response strategies.
Creating a Transient
InfoSurge allows you to create a transient by providing a curve which will alter pump and valve operations as well as junction demands (inflows or outflows). The example below describes how to assign a curve to a pump:
- Click on the Select icon of the Attribute Browser window and then click on the desired pump.
- Click on the Pump Operation Change
- Select the Pump Disturbance Type and choose either an existing Speed Curve from the drop-down list or click on the Browse
Creating a Transient – Pumps
Operating speed changes for pumps produce transients. A time dependent change in the speed ratio(s) (operating speed/rated speed) may be defined for all types of pumps. Pumps can also be tripped (lose power) and the resulting rundown calculated based on the pump and motor inertia and dynamic characteristics described in the Pump File. A trip can be specified with a ratchet to prevent reverse rotation. Three example scenarios are described below.
Example 1 – The pump is running at full speed (1.0) for 1 second and then the speed ramps down to zero over the next 2 seconds. Select Speed as the Disturbance Type in the Pump Operation Change Data dialog box.
Example 2 – The pump trips (loses power) at 1 second. Select Trip as the Disturbance Type in the Pump Operation Change Data dialog box and enter the time when the Trip Start Time is initiated. This pump trip simulation requires a pump file and its setting in Pump Surge Data.