Protection Devices
Using a transient model, the engineer can try different valve operating speeds, pipe sizes, and pump controls to see if the transient effects can be controlled to acceptable levels. If transients cannot be prevented, specific devices to control transients may be needed.
Some methods of transient prevention include:
- Slow opening and closing of valves—Generally, slower valve-operating times are required for longer pipeline systems. Operations personnel should be trained in proper valve operation to avoid causing transients.
- Proper hydrant operation—Closing fire hydrants too quickly is the leading cause of transients in smaller distribution piping. Fire and water personnel need to be trained on proper hydrant operation.
- Proper pump controls—Except for power failures, pump flow can be slowly controlled using various techniques. Ramping pump speeds up and down with soft-start or variable-speed drives can minimize transients, although slow opening and closing of pump-control valves downstream of the pumps can accomplish a similar effect, often at lower cost. The control valve should be opened slowly after the pump is started and closed slowly prior to shutting down the pump.
- Lower pipeline velocity—Pipeline size and thus cost can be reduced by allowing higher velocities. However, the potential for serious transients increases with decreasing pipe size. It is usually not cost effective to significantly increase pipe size to minimize transients, but the effect of transients on pipe sizing should not be ignored in the design process.
- Stronger pipe—For long-term reliability, pipes and joints should be strong enough to resist both high and subatmospheric, or even vacuum, pressures.
To control minimum pressures, the following can be adjusted or implemented:
To control maximum pressures, the following can be implemented:
The items in the preceding lists are discussed in the sections that follow. These items can be used singly or in combination with other devices.