| The Ported Disc Butterfly Valve Design |
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| Written by Laurel Donoho, Frost & Sullivan | |
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Page 1 of 2 Pumps & Systems, March 2008 The difference between the ported disc butterfly valve from Kubota Corporation (Osaka, Japan) and a regular butterfly valve is on the disc. The disc has a fin-shaped disc tail attached to the edge of a butterfly disc, on which an oblong hole-the flow mouth-is made. The hole serves as a bypass valve. An excessively big disc tail increases the valve's pressure loss, so the unit is designed to achieve the least possible pressure loss. Figure 1 shows the valve in the fully-closed position, and in progressive opening states. When the valve is opened, water flows through the flow mouth. The more the valve is opened, the more water flows. The flow mouth is designed so that the flow amount is the same as the flow through a conventional water pipeline bypass system. Water flowing through the flow mouth stays away from the seat, preventing damage to the rubber seat and contributing to valve longevity and reliability.
Because of the unique design of the disc, in the pre-filling stage the flow rate is kept stable regardless of the valve opening. This is significant because with a regular valve (due to its steeply increasing flow characteristics), it is extremely difficult to confine the flow to a stable small volume. With this new design, the volume of water flowing through the valve in the pre-fill stage can be controlled and measured. Key Valve Points of the Ported Disc Butterfly ValveFinancial Benefit In the installation of a new water system, the costs can be sizeable. One part of this complex puzzle that is often overlooked is the initial isolation system. This main isolation valve is the beginning of the entire system and-even though it may be used only once or twice in its lifetime-holds the key to a successful start-up or shutdown of the entire water system it feeds. The typical design for the isolation system is a traditional butterfly valve, which controls the main line, and a secondary smaller line (bypass) which circumvents the main butterfly valve. This secondary line, which has an additional smaller butterfly valve, allows for a "pre-fill" application where pressure is bled off to reduce a pressure head in the water system. The effects are the same during shutdown, but in reverse. In addition to new line commissioning, a system of bypasses are typically needed to divert water away from a particular section for repairs in primary pipe. It is critical that this be done without interruption of overall service to customers so bypass lines become a necessity to maintain water flow to the community. Though an isolation system sees a small degree of use compared to other valves within the system, its presence is important. The construction of these systems, however, can be expensive. This new valve is designed as a standalone replacement to the isolation system. The innovative design of the disc in this valve allows for a pre-fill effect prior to the valve being fully opened. This pre-fill capability allows it to operate as an entire isolation system on its own. On the construction side the valve offers significant cost reduction when considering a traditional isolation system in that there is no longer a need for a vault. It can be buried like a traditional butterfly valve, and there is no need for additional piping and fittings, because the bypass around the valve for pre-filling is no longer needed. The valve is also compatible with newer isolation system configurations that do not have a bypass built in.
Technology Benefits One key potable water issue is water movement characteristics when valves are opened or closed. Both of these activities provide a situation with potential for dangerous and destructive results from water movement, specifically pressure surges (water hammer). On valve opening, the unique shape of the disc makes it easy and safe to pre-fill or refill a pipeline with water without the added expense of a separate bypass system. When a pipeline is filled with water after construction or commissioning, the flow volume should be well below capacity to reduce the risk of water hammer downstream. An average butterfly valve has difficulty controlling the flow volume in low-flow situations. Flow amounts tend to change greatly even though the opening of the butterfly valve changes very little. In the past, a bypass system has been used since the small valve of a bypass system could control the flow to an optimal volume. The disc shape valve solves the problem of controlling small flow amounts. The tailed disc, with its proportional oblong hole relative to the disc size, controls the pre-fill flow to an optimal level, eliminating the need for a bypass system. The unique shape of the disc can reduce the pressure surge in a pipeline when the valve opens. When a valve is closed instantaneously, it may produce a pressure surge that could cause damage to a pipeline. Common mitigation practices have been to close a valve slowly, perform an interrupted close (close slowly, stop, then close slowly) or reinforce the pipeline with a shock absorbing system to transfer the energy of a pressure surge out of the pipeline. Both alternatives can be costly in manpower and other resources.
The new design can reduce a pressure surge with a single continuous closing operation at normal closing speed, eliminating the need to slowly close the valve or perform interrupted closing. The valve maintains the flow rate while narrowing the force transferred. Figure 2 demonstrates the plateau effect generated, which dramatically changes the power curve. This unique advantage can eliminate complex closing operations and maintain the pipeline safely. The unique design of this valve offers a number of system benefits, including:
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