D IGITAL F ACTORY DEL T ECNOCAMPUS

As previously mentioned, power failure at a pump station is the primary cause of surge problems due to a rapid decrease in the flow velocity. Therefore, the primary method of controlling surge caused by pump failure is to control the rate of change of flow at the pump station. The rate of change of flow needs to consist of a slow decrease and eventual stopping of the flow in an orderly fashion.

Increase the Rotational Moment of Inertia of the Pump/Motor System. The increase in rotational moment of inertia (WR²) in a pump station for surge control is typically accomplished by the addition of a flywheel. A flywheel is a bladed or spoked wheel rotating mass attached to the rotating assembly of a pump and motor providing additional rotating moment of inertia to the pump assembly. If the pump stops suddenly, the flywheel continues to spin, and water continues to be pushed into the system but with decreasing head. This ensures the water supply does not stop instantaneously and the system pressure drops slowly upon power failure. The amount of water pushed, and the length of time the flywheel spins, is based on the rotating system’s WR2, and therefore on the flywheel’s size and weight. Increasing a system’s WR2 by the addition of a flywheel is an effective method of controlling surge in limited situations, typically when the pipeline is relatively short and the profile is flat.

Figure 2-50: Vertical Flywheel Sketch

Figure 2-51: Horizontal Flywheel Sketch

Figure 2-52: Horizontal Internal Motor Flywheel Installation

Utilize a Hydropneumatic Pressure Vessel. Hydropneumatic pressure vessels are normally connected to the pump station discharge header, and each pump is normally equipped with a check valve. The upper portion of the pressurized tank is filled with a volume of compressed air, designed to balance the pump head. For very low lift pump stations, the pressurized tank can be replaced by an open tank or by a standpipe. Upon power failure, pump forward rotation stops rapidly, normally in a matter of a few seconds. The sudden cessation of water flow causes the head downstream of the pump station to decrease sharply. As this occurs, flow is supplied by the surge tank to the discharge header, shutting off the check valves to the pump. The flow supplied by the surge tank continues decreasing slowly as the pressurized air in the tank expands. The surge tank provides sufficient pressure to maintain positive pressure throughout the entire length of the pipeline and so no vapor

pockets form in the pipeline. As time progresses following the surge event, the pressure in the surge tank decreases, the forward flow of water decreases, and the water flow eventually reverses

direction in relation to the static head in the terminal reservoir. This flow then enters the surge tank, re-compressing the air in the tank. The pressure rises as the water volume in the tank increases.

This process repeats itself in cycles until the water’s motion is dampened out to an equilibrium situation due to system friction and energy losses.

Figure 2-53: Horizontal Hydropneumatic Surge Tank

Figure 2-54: Typical Surge Tank Installation

Figure 2-55: Small Capacity Bladder Type Pressure Vessel 2.10.7. Secondary Control

Depending on the pipeline profile downstream of the pump station, mitigation of surge pressure by primary surge control measures at the pump station may be impractical or inadequate. Under these conditions, secondary control devices can be installed on the pipeline at selected locations.

Depending on the situation, secondary control measures can be used alone or to supplement the primary control facilities at the pump station. Commonly employed secondary control devices include (1) combination vacuum relief and air release valves, (2) one-way surge tanks, (3) pressure relief valve, (4) slow closing control or check valves and (5) surge anticipator valves.

Vacuum Relief and Air Release Valves. Vacuum relief valves are used to allow air to enter the pipeline whenever the pipeline pressure falls below atmospheric pressure. The entrained air must later be released slowly through a corresponding air release valve when the pipeline regains pressure. Combination air/vacuum valves for transient control shall be designed for air release or is equipped with surge dampening/check mechanisms to prevent damage to the float or seat. A scheduled maintenance and repair program is required for an air valve to retain it’s ‘like new’ level of performance.

To facilitate maintenance for critical combination air/vacuum valves required for surge control, a tandem valve should be considered to adequately protect the system while valve service and maintenance is performed. In a system where the air valves and air valve vaults are not properly maintained the air valve may not be perform adequately when required. This may result in severe

vacuum conditions causing damage to elastomeric seals and gaskets used at joints and flanges within the pipeline system. This damage leads to potential leakage and under sustained vacuum condition standing ground water may be drawn into the pipeline creating a contaminated water condition. Also, if an air valve vault is not properly drained and storm water runoff is allowed to flood the vault another potential cross connection may occur when the vacuum valve opens.

One-Way Surge Tank. In pipelines operating under pressure, the normal Hydraulic Grade Line (HGL) is often far above the pipeline. Under this condition, it is impractical to install an open standpipe on the pipeline, and a one-way surge tank is often used. A one-way surge tank has a check valve between the tank and the pipeline it is protecting. The water surface elevation in the tank is kept above the pipeline crown elevation but far below the normal HGL. During a surge event, when the HGL in the pipeline at the one-way surge tank location drops below the water surface elevation in the tank, water flows from the tank into the pipeline. This operation provides additional water to the pipeline, preventing water column separation, and thereby reducing the possibility of “waterhammer”

due to separated water columns rejoining.

Slow Closing Check Valve. Another way to provide surge relief is to require a very slow closing check valve. With this type of check valve the surge pressure is translated back through the pumps which act like a surge dampening device as it slows down and begins to spin backwards. The check valve is then closed before a runaway pump condition is achieved.

Surge Anticipator Valve. A surge anticipator valve is installed on a pipeline system to provide relief from high pressures. Instead of operating upon detection of a high pressure, a surge anticipator valve is actuated by detection of a specific occurrence known to cause surge conditions in the pipeline (power failure). Similar to the pressure relief valve, the surge anticipator valve discharges to the atmosphere or a system with a lower HGL than the pipeline. Upon opening, most surge

anticipator valves initiate a slow closure to stop the flow of water from the system.