Design and Selection Criteria for Check Valves

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Basic information relating to check valve selection.
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    Design and Selection Criteria of Check Valves VAL-MATIC VALVE AND MANUFACTURING CORP. 905 RIVERSIDE DRIVE, ELMHURST, IL 60126 TEL. (630) 941-7600 FAX. (630) 941-8042 www.valmatic.com  Copyright © 2011 Val-Matic Valve and Manufacturing Corp.    1  FOREWORD Design and Selection Criteria of Check Valves was written to assist design engineers in understanding basic valve principles and the functions of various types of check valves associated with pumping systems. It is not intended to provide all of the information necessary for selecting valves but rather to explain the engineering parameters associated with check valve selection and use. Successful pumping system design should consider the combined characteristics of the pump, check valve, air valve, control valve, and surge equipment. With this knowledge, the design engineer can better select check valves and understand some of the  pitfalls common to valve selection. When selecting valves their flow characteristics are important,  but other design issues such as head loss, reliability, and cost are equally important factors that should be considered in making the final valve selection. The test data presented offers valuable information for predicting valve performance. It is based on independent tests conducted at the Utah Water Research Laboratory and remains the property of Val-Matic Valve & Mfg. Corp. Any use of this information in other public disclosures requires written permission from Val-Matic. Val-Matic offers no warranty or representation as to design information and methodologies in this  paper. Use of this paper should be made under the direction of trained engineers exercising independent judgment regarding the suggested use of the valves in fluid systems.    2 DESIGN AND SELECTION CRITERIA OF CHECK VALVES INTRODUCTION An essential element in the design of water and wastewater pumping systems is the proper selection of the pump discharge check valve, whose purpose is to automatically open to allow forward flow and automatically return to the closed position to prevent reverse flow when the pump is not in operation. Another function that is often overlooked is the valve’s ability to minimize energy consumption. Patton estimated that water and wastewater plants in the United States consume 75  billion kW·h of energy annually and nearly 80% of that energy is consumed for high service  pumping costs to overcome the static head and friction losses. But just as important, the valve should protect the pumping system and piping from pressure surges caused by sudden closure. Every pump station designer has witnessed check valve slam, which is caused by the sudden stoppage of reverse flow through a closing check valve. To prevent slam, an automatic check valve must close very quickly or a pump control valve must close very slowly; anything in the middle will likely cause havoc in the pumping system. FIGURE 1. Typical Pumping System with Swing Check Valves   Three general categories of check valves will be presented in detail. First, Lift Check Valves such as the fast-closing silent and nozzle check valves, have spring-loaded discs, which move along the  pipe axis over a short distance to close automatically in a fraction of a second. Because of their fast closure, these check valves rarely slam and hence have earned the name “silent”. The second category of check valve is the Swing Check Valve such as the traditional swing check valve, which has a flat disc that pivots or swings about a hinge pin. Traditional swing check valves are by far the most common, can be equipped with various accessories such as a lever and weight, and unfortunately may be the most likely to slam. Lastly, Pump Control Valves including power-operated ball or plug valves are electrically wired to the pump control circuit to provide synchronized functions with the pump to systematically control the changes in pipeline fluid velocity over a long period of time (i.e. 60 to 300 seconds) to prevent surges in distribution systems. These    3 Silent Check Valve Ball Check Valve three categories of check valves, Lift, Swing, and Pump Control are each designed with unique features for specific applications and each contribute differently to the system response and costs. So there is no “universal” check valve for all applications. Even when all of the various categories and types of check valves are understood, it is still difficult to make a rational decision about which type of check valve is best for a given application. Buying a check valve is similar to buying a car. There are many to choose from because every model is designed to meet different needs. The best car is not necessarily the fastest one. You may be looking for compactness, high performance, low cost, or advanced features; whatever the case, just as there is a car that best meets your requirements, there will similarly be a check valve that best meets your requirements. This paper will therefore describe the various types of check valves and discuss the common selection criteria such as cost and reliability that can be used to narrow down the field of selection. Finally the check valves will be rated on every criteria so that a methodical decision  process can be used to identify the best valve solution to meet a given application. LIFT CHECK VALVES Lift Check Valves are simple, automatic, and cost effective but can result in high energy costs in the long run. Examples of lift checks include nozzle check, silent check, and ball check valves. These valves have no external moving parts and can be economical to produce and reliable in operation. Unfortunately, they do not provide indication as to whether they are open or closed, which may be an important feature in a pumping system. Silent Check Valves are commonly used in high-rise buildings and high head applications because of their quiet closure. They consist of a threaded, wafer, or flanged body; a corrosion resistant seat; and a disc with integral stems. When the flow is initiated, the disc is pushed to the left to allow forward flow. When the pump is stopped, the compression spring in the valve forces the valve closed before the flow reverses, which provides silent closure. They close very quickly (in about one tenth of a second) because of a short linear stroke, which is equal to one fourth of their diameter. It is interesting to note that even though the stroke is short at D/4, the cylindrical area between the open disc and the seat ( π · D · D/4) equals the full port area ( π · D 2 /4) where D equals the port diameter.  Nevertheless, because the disc remains in the flow stream, a Silent Check Valve has high head loss and is mostly used for clean water applications with high head.  Nozzle check valves operate similar to Silent Check Valves but have a smooth venturi-shaped flow  path and annular disc with lower head loss than the Silent Check Valve, but with a longer laying length. Like the Silent Check Valve, the nozzle check has a spring-assisted, short linear stroke, which provides the best non-slam characteristic of check valves. Nozzle Check Valves are commonly made in steel for high pressure classes to meet the rigors of industrial and power plant applications. Ball Check Valves are simple and compact and commonly used on small water or wastewater pumps where economy is important. A Ball Check Valve ◄ Flow   ◄ Flow
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