How to choose a valve:
When in doubt you should always check with an expert, Amrutha Technologies can always assist in the valve selection process, with our wealth of experience.But first of all you should know the service conditions.
1. Where the valve is to be used, what function you want to perform?
2. What are the pressure/temperature requirements?
3. What kind of fluids will be in line?
4. What size is the line?
5. Are the conditions moderate or extreme?
6. How much room have you got for valve stems/handles?
What are the service requirements, are the valves to be regularly dismantled for cleaning? Is the installation permanent, or likely to be cut into frequently?
If you can answer these questions the choice of valve becomes evident very quickly, especially to one of our experienced valve sales staff.
Here are some basic guidelines for valve selection:
The principle function of a valve
1. Starting and Stopping flow: Gate, Ball and Butterfly Valves are the most common form of shut-off valve, 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.
Common materials valves are made with are
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.
In addition to the maker’s brand and valve size, is a basic service rating which appears on the body of most valves. Pressure and Temp ratings are normally expressed in pounds and are in terms of steam, unless otherwise indicated.Steel ratings bear a mark such as 150, 300, 600, 900 etc. The figures denote a maximum pressure (in pounds) at a certain maximum temperature, for which the valve is suited. And will also reflect on any ANSI flanges required for the end connections.The important effect temperature has on pressure ratings means it is imperative that manufacturer’s service tables are referred to, and that is where a call to Amrutha Technologies will ease your mind and make your valve selection process easy
Sunday, October 25, 2009
How to choose a valve: