M ÀSTER U NIVERSITARI EN E MPRENEDORIA I I NNOVACIÓ

Automatically controlled valves are typically used in lieu of the conventional check valve when the surge analysis requires controlled opening and closing in order to alleviate surge pressures. It functions like a check valve but it offers an adjustable opening and closing periods. These are known as pump control valves. They are designed to be fully closed when the pump starts and, through an operator, slowly open over a pre-determined time period. When the pump is required to stop, the controller calls for the valve to slowly close before the pump actually stops. Control valves shall be capable of closing automatically after a power failure event using stored energy such as an air or nitrogen accumulator. Thus, through the control valves, the system transitions from no flow to full

flow and back. Pump control valves may be actuated by diaphragm, pneumatic or hydraulic piston operators. Valves used for pump control should have good throttling characteristics, such as globe, plug, and cone or ball configurations. Typically valves on discharge piping are designed for a velocity of approximately 8 ft/sec. When required, globe pattern valves, globe pattern valves may be utilized as combined pump control and check valves. Under such applications, normal velocities through such valves should be less than 12 ft/sec.

Figure 2-33: Example Control Valve Installation 2.5.3.1. Butterfly Valve

The butterfly valve is a quarter turn rotary valve that controls flow and pressure by rotating a circular disc supported on a shaft to a circular seat. The AWWA C504 valve is equipped with a rubber seat.

For pressures higher than 250 psi and beyond the rating of AWWA butterfly valves, triple offset butterfly valves are recommended. The rubber seat is not conducive to high velocities and sustained throttling and may fail in a low angled throttled position (<20º). Similar to the ball and cone valve, if a butterfly valve is being used for flow and pressure control the seat should be metal to metal to mitigate damage from high velocities occurring during the throttling process. The butterfly valve has a limited throttling range of 20º or higher. When throttled at lower angles the velocity through the butterfly valve becomes high on two sides of the disc and may induce vibration due to high unbalanced dynamic torque on the disc. Valve cavitation may also occur at a low angled throttled position. Below is a list of the valve characteristics of the butterfly valve.

 Lightweight, compact, and relatively inexpensive

 Moderate head-loss across the valve when in a fully open position and properly sized

 Susceptible to cavitation in a throttled position and if severe cavitation persist the valve is subject to damage

 Limited pressure control characteristics in low flow high pressure drop conditions

 Susceptible to cavitation and choking in low back pressure conditions

Figure 2-34: Example Butterfly Valve Details 2.5.3.2. Cone Valve

The cone valve is representatively named. The valve controls flow and pressure by seating a cone into a conical shaped valve body through a rotary (90º) movement. Because cone valves are commonly used for throttling, the seat is typically metal to metal to mitigate damage from high velocities occurring during the throttling process. The cone valve is capable of precise throttling under high head and high flow velocity conditions. Below is a list of the valve characteristics of the cone valve.

 Low head-loss across the valve when the valve is in a full open position and properly sized

 Good throttling characteristics in low flow conditions

 With metal seating the valve is suitable for continuous throttling

 Has large dimensions requiring adequate space in a vault or dry pit

 The actuator associated with the valve is tall

Figure 2-35: Example Cone Valve Details

2.5.3.3. Ball Valve

The ball valve is a rotary (90º) valve which controls flow and pressure by rotating a spherical shape with a hollow cylindrical core. Ball valves are typically equipped with either a metal to metal seat or a rubber to metal seat. Resilient seated ball valves are normally used for bubble tight application.

When ball valves are being used for flow and pressure control the seat should be either reliant seated or metal seated. When being used as a throttling valve, the ball valve actuator can be made to provide a direct 1:1 ratio to open and closing position allowing for precise control of the valve.

Below is a list of the valve characteristics of the ball valve.

 Low head-loss across the valve is in a full open and properly sized

 Good pressure control characteristics in low flow conditions

 Durable valve suitable for continuous throttling

 The actuator associated with the valve is tall

 Susceptible to cavitation and choking in low back pressure conditions

 The metal seated ball valve may experience a small amount of leakage even when fully closed

Figure 2-36: Example Ball Valve Details 2.5.3.4. In-Line Sleeve Valve

The in-line sleeve valve has a linear axial action that controls flow and pressure by advancing or retracting a valve gate along a fixed perforated sleeve with a hollow cylindrical core. Sleeve valves are typically equipped with a metal to metal seat consisting of the sleeve and gate. The sleeve valve is specifically engineered to provide a range of flow and pressure control and is ideally suited for low flow and high pressure reduction applications. The valve design is engineered to minimize damage to valve components during cavitation by directing the cavitation to the center of the fixed sleeve and away from metal components. The sleeve valve actuator can be made to provide a direct 1:1 ratio for opening and closing position or made to follow a proportional curve. Below is a list of the valve characteristics of the Henry Pratt sleeve valve.

 Excellent pressure control characteristics in low flow conditions

 Usually less prone to cavitation as compared to other types of valves

 Durable valve suitable for continuous throttling

 High head-loss across the valve

 Has large dimensions requiring adequate space in a vault

 Relatively expensive

Figure 2-37: Example Sleeve Valve Details 2.5.4. Specialty Valve Considerations

2.5.4.1. Pressure Relief Valves

Pressure relief valves are provided when it is necessary to protect the system against excessively high pressures. Protection is provided by relieving the high pressure to the suction side of a pump station or to atmospheric pressure in the wet well. These types of valves are not typically provided adjacent to the pump because of the other safety features normally provided, such as pump control valves and pressure switches. Pressure relief valves are usually of the globe pattern or diaphragm-type. With regards to design criteria, the flow velocities should not exceed 20 ft/sec based on the nominal diameter.

MWH does not recommend the use pressure relief valve in lieu of surge tank for surge protection.

Relief valves do not activate fast enough to relieve surge pressure in the pipeline.

Figure 2-38: Pressure Relief Valve

2.5.4.2. Pressure Sustaining Valves

Pressure sustaining valves are utilized where it is necessary to maintain a minimum pressure in the system upstream of the valve. They are seldom used at pumping stations because it burns energy but may be provided in distribution systems. They are usually of a similar design as valves used for pressure reduction.

Figure 2-39: Pressure Sustaining Valve 2.5.4.3. Pressure Reducing Valves

Pressure reducing valves are frequently installed within the distribution system to reduce pressure and maintain constant pressure downstream into a sub-system. They are rarely located at the pumping station unless the sub-system is so small the cost of providing separate pumps is

unwarranted. Pressure reducing valves may be of several types. However, globe-pattern, diaphragm or piston type is usually the valve of choice. With regards to design criteria, velocities across globe-pattern valves (used for pressure reducing) should be limited to 20 ft/sec (based on nominal diameter). Globe-pattern valves are normally not suitable for raw sewage applications.

Figure 2-40: Pressure Reducing Valve 2.5.4.4. Energy Dissipation Valves

The use of energy dissipation valves at pumping stations is almost always limited to cases where it is necessary to allow a high pressure supply to enter a wet-well and it is impossible to conserve the high pressure by connecting the supply to the suction side of the booster pumps. Dissipation of energy can be achieved through the type of valves used for pressure reduction or pressure relief, i.e., globe-patterned valves, or cylinder sleeve valves. Sleeve valves are essentially two fabricated concentric or eccentric cylinders, one of which is provided with a series of holes. The outer cylinder remains stationary, while the inner cylinder is moved by a motor-operated screw to expose as many holes as necessary to reduce a variable upstream pressure to essentially zero. Sleeves valves may be installed in either the horizontal or vertical position. The most common orientation is vertical, where the valve is located in a stilling well. Unlike other types of energy dissipation valves, the sleeve valve is dynamically balanced for designed flow pressure conditions and there is essentially no vibration. Sleeve valves are not suitable for raw sewage applications or storm water applications.