Gate, Ball and Butterfly Valves are the most common form of shut-off valves, with ball and even butterfly valves rapidly becoming the primary choice due to the ease of automation and fast shut-off abilities, and in the case of ball valves the barest minimum of flow restrictions. We are often supplying ball and butterfly valves fully actuated, as plants of today become more and more automated, and are often now run by computers and the flick of a switch. While you go about selecting the right valve, choosing the right kind of operator is also important. Choosing between manual or motorized actuator (Pneumatic/electric/hydraulic) depends on a few factors. To Learn more in detail about the different types and selection criteria of linear actuators vs rotary actuators, Click Here to read my earlier post.
5 Questions you must ask before you buy Industrial Valves.
Having spent quite many years in the valves industry, I have
observed that, there has been a large displeasure among plant personnel every time a check valve was
recommended. In nutshell a check valve
allows for the flow in one direction and automatically prevents backflow when
the flow reverses direction. It is a self regulated valve unlike other valves
that do not need the valve to be open or close.One of the common wrong practices I have observed are, check valves are
selected the same way on/off valves are selected based on line size and opting
for largest Cv available. What is not understood here is unlike on/off valves,
check valves function largely on the internal flow conditions as the disc is
always subject to flow pressure and responding accordingly. In most cases Cv is
calculated considering 100% open valve position however a reduced flow pressure
in the line results in wear and tear of moving parts as well as an increase in
pressure drop which is not desired resulting in the valve being a failure. To learn more about "The Basic guidelines and applications of Control Valves" [Click Here] Well i decided to write this article to clear some misconceptions and narrow down the criteria for selecting the right valve for your application. First of all you should know the service conditions.
Here are some basic guidelines for valve selection:
A. Where the valve is to be used, what function you want to perform?
1. Starting and Stopping flow:
Gate, Ball and Butterfly Valves are the most common form of shut-off valves, with ball and even butterfly valves rapidly becoming the primary choice due to the ease of automation and fast shut-off abilities, and in the case of ball valves the barest minimum of flow restrictions. We are often supplying ball and butterfly valves fully actuated, as plants of today become more and more automated, and are often now run by computers and the flick of a switch.2. Regulating Flow:
Regulating or throttling flow is most commonly done with the use of a globe valve, and to a much lesser extent an angle valve. The seat design forces the flow evenly around a disc or plug arrangement that enables a closer regulation of flow. Globe valves are often used for the control of steam. Your garden tap is a very common and basic form of a globe valve design.3. Preventing Back Flow:
This is the function of a check valve, it prevents flow reversal in piping, a primary cause of water hammer. Check valves most commonly are swing checks, normally used with gates and ball valves, and lift check or spring checks, which are normally used with Globe and Control type valves.4. Regulating Pressure:
Pressure Regulators and Control Valves are used to reduce incoming pressure and maintain it at a required service pressure. Fluctuations to the inlet pressures of a pressure regulator will not affect the outlet pressure for which it is set at.5. Relieving pressure:
These are known as Safety Valves, normally used for steam, air, and gases, and Relief Valves, usually used for liquids. These are normally spring loaded valves which open automatically when the pressure exceeds a set limit. They can either vent to atmosphere or to other piping. Excessive pressures in something like a Boiler could cause major damage, if a relief valve was not used. To Learn more about "Why Selection and Sizing of Pressure relief valves is critical?" [Click Here] to read my earlier post.B. What is the service medium ? What are the pressure/temperature requirements?
Brass and Bronze: Brass is the stock standard material used in most small bore, standard cold water applications. Bronze an alloy of copper, tin, lead and zinc, is normally used for steam and higher temperatures up to 230°C.
Cast Iron: Normally used for lower pressure applications and in butterfly valves bodies where it provides a cost effective alternative to steel. Common in the fire protection and water industry.
Carbon and Stainless Steel: Steel is recommended for higher pressure and temperature applications and/or in services conditions that may be too harsh for Brass, bronze and Cast iron.
Alloys: Used in severe corrosive or acidic applications, or for use at extreme temperatures. Many different types of alloys are available, but these are normally manufactured and imported as required, as some of the more exotic materials can be quite expensive to have sitting around a shelf gathering dust.
PVCs: Plastics and PVCs are becoming an alternative to brass for use in low pressure/temperature air, water and even chemical applications.
Selection criteria between the different types of valves also depend on various funtionalities and process applications. I have illustrated these selection factors across gate valve, globe valve, ball valve, butterfly valve in the chart below. [lock]
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C. What kind of operator will you need manual/motorized ?
While you go about selecting the right valve, choosing the right kind of operator is also important. Choosing between manual or motorized actuator (Pneumatic/electric/hydraulic) depends on a few factors. To Learn more about "How to select the right actuator for your valve?" [Click Here] to read my earlier post. While majority of the applications prefer pneumatic actuators due to their light weight and rugged structure. Moreover they are cost effective and highly reliable. Pneumatic actuators are used in a potentially explosive environment. The failsafe action of pneumatic actuators is system inherent. Manual shutoff valves are used to isolate or bypass control valves. They are only occasionally operated and used for on/off applications, when the response time is not critical. Electric actuators are mainly used in applications where compressed air is not available viz; in small systems for building automation, or in some areas of power plants. There are many factors that need to be considered while selection of linear actuator vs rotary actuator. However, the implementation of failsafe action (fail open or fail close) upon power supply failure or emergency shutdown in large electric actuators requires a technically complex construction. Finally hydraulic actuators are used in applications where the required force to operate the valve cannot be accommodated by pneumatic or electric actuators.
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