The means of controlling pressure transients in water distribution systems will in general vary depending upon whether the initiating event results in an upsurge (a high pressure event caused by a closure of a downstream valve) or a downsurge (low pressure event caused by a pump power failure). The latter event can lead to the undesirable occurrence of water-column separation, which itself can result in severe pressure surges following the collapse of a vapor cavity. InfoSurge can accommodate a number of surge protection devices that are used to help control starting and stopping transients in pipe systems. No two systems are completely identical and hence the ultimate choice of surge protection devices and operating strategies will usually differ. Of course, it is always best whenever possible to avoid rapid flow changes. A transient analysis should be carried out to predict the effect of each individually selected device. Due to the complex nature of the transient behavior, a device intended to suppress or fix a transient condition could result in a worsening of the condition if the device is not properly selected or located in the system. A combination of devices may prove to be the most desirable and most economical. A brief overview of various surge protection devices, as shown in Figure 5, and their functions follow.
Simple Surge Tanks (Open)
Open Surge Tanks or Stand-Pipes can be an excellent solution to both upsurge and downsurge problems. These tanks can be installed only at locations where normal static pressure heads are small. Open Surge Tanks mainly serve two purposes:
- prevent high pressures following shutdowns by receiving liquid; and
- prevent cavitation during starting conditions by providing liquid to a low-pressure region.
Surge Vessels (Closed Surge Tanks – Bladder Surge Tanks – Hybrid Tanks)
Surge Vessels can be installed anywhere along a line regardless of normal pressure head. They serve the same function as an Open Surge Tank but respond faster and allow a wider range of pressure fluctuation. Their effect depends primarily on location, vessel size, entrance resistance and initial gas volume and pressure. Closed Surge Vessels are normally equipped with an air compressor to control the initial gas volume. Some Closed Surge Tanks are equipped with a pre-charged pressurized bladder (Bladder Surge Tanks) which eliminate the need for an air compressor. Hybrid Tanks are equipped with an air vent which admits air when the pressure goes below atmospheric pressure.
Surge vessels provide very effective protection against pressure surges in piping systems. These vessels are normally positioned at pump stations to provide protection against a loss of power to the pump. Several types of surge vessels with various features can be modeled with InfoSurge.
- Compressor Vessels: These vessels are equipped with a compressor to maintain the desired initial water level (and air volume) under normal operating conditions.
- Bladder Tanks: These vessels have a bladder which is pre-charged to a pre-determined pressure to maintain the desired air volume under normal operating conditions.
- Hybrid Tank with Air Compressor: This vessel behaves the same as the compressor vessel until the air pressure drops to atmospheric pressure. At this time, air is admitted through a vent at the top of the tank. The compressor is required to maintain the desired air volume under normal operating conditions.
- Hybrid Tank with Dipping Tube: This vessel provides the same functionality as the one equipped with the air compressor without requiring the compressor. The dipping tube controls the closure of the air vent when the tank is filling and the length of the dipping tube is varied to maintain the desired air volume under normal operating conditions.
Feed Tanks (One Way Surge Tanks)
The purpose of Feed Tanks is to prevent initial low pressures and potential water-column separation by admitting water into the pipe subsequent to a downsurge. They can be either open or closed tanks that are equipped with a check valve to allow flow only into the pipe system. They can be installed anywhere on the line and are used to avoid cavitation. When low pressures develop, water is discharged from the Feed Tank into the pipe system.
Pressure Relief Valves
These are valves that eject water out a side orifice to prevent excessive high-pressure surges. They are activated when the line pressure of a specified location (not necessarily at the valve) reaches a preset value. Valve closure is initiated at a second prescribed head that is normally around 20% lower than the activating head. The valve opens and closes at prescribed rates. The valves can eject liquid into the atmosphere or a pressurized region, or into an Open or Closed Surge Tank.
Surge Anticipation Valves
These are valves that open on a downsurge in pressure (sensed at a specified location) in anticipation of an upsurge to follow. This valve, when activated, follows and completes a cycle of opening and closing based on valve opening and closing rates. For systems for which water-column separation will not occur, the Surge Anticipation Valve can solve the problem of upsurge at the pump due to reverse flow or wave reflection.
Air Release/Vacuum Valves
These are Surge Protection Valves installed at high points in a pipeline that are intended to prevent low pressure (cavitation) by emitting air into the pipe when the line pressure drops below atmospheric. The air is then expelled (usually at a lower rate) when the line pressure exceeds atmospheric. Two Stage Air Valves release the air through a smaller orifice to prevent the “air slam” which occurs when all the air is released and the water column rejoins. A Three-Stage Air Valve is designed to release the air through a second (smaller) orifice to further reduce the “air slam”.
These are devices that allow flow only in one direction and close when flow reversal is impending. For transient control they are usually installed with other devices such as a Pump Bypass Line as described below. Pumps are also often equipped with a Check Valve to prevent flow reversal. Because Check Valves do not close instantaneously it is possible that a substantial backflow may occur before closure. This can produce additional and sometimes large surges in the system. Check valve modeling includes a time delay between check valve activation and complete closure of the Check Valve. The Check Valve is thus treated as a valve closing in a linear fashion that is activated by flow reversal and closes completely over the delay period.
Pump Bypass Line
In low-head pumping systems having a positive suction head, a Bypass Line around the pumps can be installed to allow water to be drawn into the discharge line following power failure and a downsurge. Bypass lines are generally short line segments equipped with a check valve (non-return valve) preventing back flow (from the pump discharge to the suction side) and installed parallel to the pump in the normal flow direction. They are activated when the pump suction head exceeds the discharge head and are useful for two reasons:
- prevent high-pressure buildup on the pump suction side; and
- prevent cavitation on the pump discharge side.
Resistive Component Bypass Lines
These Bypass Lines cause a change in resistance if flow reverses direction in a line and may be utilized to greatly increase resistance when flow reversal occurs retarding the momentum of the fluid in the reverse direction. The principal objective is to limit the momentum of reverse direction water column in a rising main system following a pump trip.
In addition to these devices, pressure surge control can be accomplished (and modeled) by utilizing component devices that require only that the device data be specified to represent the desired conditions. For example, added pump inertia or valve stroking is modeled by using appropriate data for the pump inertia or the closure characteristics for the valve.