How to Select the Right Valve for Your ApplicationGary Kardys | April 13, 2017
Many factors are involved in selecting the correct valve for your application. While valve sizing and selection can be a complex process, an engineer can start the valve selection process by understanding their application requirements and finding suitable valve specifications to meet these needs. The first step in this process is to consider the most important valve attributes.
What valve specifications are most important to engineers searching for valves?
In a review of the 12,612 search selections made by engineers and other technical users looking to discover valves for their applications on the Industrial Valve SpecSearch® form from 2012 to 2016, we found:
- 37 percent of specification selections were for numerical performance specs, such as size or pressure
- 34 percent of spec selections were for valve type (ball, gate, globe, diaphragm, butterfly, needle, etc.)
- 14 percent of spec selections were made for valve construction materials
- 8 percent of spec selections were made for actuation type
- 7 percent of spec selections were for media handled by the valve
The type of valve required for your application will depend on the flow control, function and valve operation. If your application only requires flow control, then a valve type with good throttling performance is required. Ideally, valve size and type should be selected to provide flow control over the range of opening of the valve plug. Inherent in the design of certain valves is a change in flow rate during opening. Valves with characterized plugs can be selected to alter the flow or gain of the valve. For on/off or shutoff applications, ball, plug, gate, diaphragm and butterfly valves provide good performance. Globe, ball, diaphragm and needle valves have good throttling capabilities. Plug and ball valves are not good choices for flow control applications.
If your application requires the prevention of back-flow, then selection of a check valve would be your best option. Applications requiring pressure control would need a pressure regulator or pressure regulating valve. A mixing valve might be the appropriate choice for tempering of water stream or blending two process streams. In comparison, if you are feeding chemicals or additives into a formulation or compounding process, then a metering valve can often provide the control capabilities needed.
The media flowing through the system will also impact valve selection. Applications with powders, granules or other bulk materials necessitate a solids valve. Vacuum chambers, vacuum pumps and other vacuum equipment without any media utilize vacuum valves. Selection of solids valves and vacuum valves demands separate treatment because their configuration and technology is so different compared to liquid flow control valves.
The media flowing through the valve often defines a category of valve such as hydraulic valves, water valves, gas valves, steam valves, air valves, solids valves or vacuum valves. In these cases, manufacturers have tailored the performance, materials and attributes for specific media types. If your media is a food, beverage, drug or active pharmaceutical product, then you will most likely require sanitary valve.
Valve Sizing and Performance Specifications
The selection of the proper valve size or correct valve sizing is the most important step in selecting the right valve for your application. Engineers on our site selected valve size (in inches) most frequently during valve searches, according to a 2009 to 2015 review of valve performance of numerical specification usage during SpecSearch® parametric searches. After valve size, maximum pressure or pressure rating (psi) had the next highest usage, followed by number of ports, regulating pressure and media temperature, respectively.
A valve improperly sized for the application can cause operating problems within the valve or elsewhere in the fluid transfer system. For example, an undersized valve can choke flow downstream and create upstream back pressure. Highly oversized flow control valves only regulate flow over a limited range near the closed position.
Operation of the valve in an almost closed position results in cavitation, flashing and flushing. The valve plug can be pulled in the seat, shutting off the flow in cases with severe oversizing. In addition to performance problems, an oversized valve along with adapter fittings adds unnecessary cost additions to your project.
If your application requires an on/off, block or isolating valve, or a check valve, then the valve typically can be sized to the existing pipe, tube or hose sizes. In the open condition, flow should proceed as if there is no valve in the line. The sizing of flow control valve is more complicated because the flow constriction changes with the changing gap between the plug and seat.
The valve flow coefficient (Cv), or valve characteristic, is used to select the proper valve size while maintaining stable flow. The simple equation to calculate Cv is:
Cv = (Flow) x m (Specific gravity of the media at flowing temperature/pressure drop)1/2
A common mistake is calculating too high of a Cv by using the maximum flow rate, which results in selection of an oversize valve. The range of flow (min, max and mean flow rates) should be utilized to properly size the valve. More information on determining the best valve size is available on GlobalSpec’s Valve Flow and Sizing page.
Valve Materials of Construction
Valve or valve body material is another widely used search criterion used when engineers look for industrial valves. The media flowing through the valve, the surrounding environment and the future maintenance of the valve are factors impacting valve materials of construction. Corrosive media such as chemicals, acids, saltwater, process waste water or even certain foods and beverages demand corrosion resistant materials of construction. If your media is a liquid-solid mix or slurry with abrasive solids, then erosion resistant materials of construction are required. High pressure media may require a high strength alloy to withstand the internal stresses. A valve passing steam or hot media will require an alloy that maintains strength and oxidation or corrosion resistance at elevated temperatures.
Even if your valves will be carrying relatively non-corrosive media such as air, hydraulic oil or inert gases, a corrosion resistant material is recommended to prevent corrosion from the surrounding industrial environment and reduce future maintenance cost. While a steel or cast iron valve placed in a chemical plant with a corrosive environment may require a protective coating and periodic maintenance or recoating, a corrosion resistant valve body material can provide a maintenance free operation in many applications. The availability of seals, packing, seats or other valve replacement parts can be another factor to consider during valve selection.
Will solvents, fuels or other volatile organic compounds (VOCs) flow through the valve? Does the surrounding environment contain VOCs or explosive vapors? If volatile compounds are involved, then a non-sparking valve material such as brass or bronze alloys might be advised. The availability of seals, packing, seats or other valve replacement parts can be another factor to consider during valve selection, selecting an industrial valve with high quality seals is recommended. Even with a maintenance free valve body, the seal materials will still require leak checking and maintenance or tightening over time – especially for industrial valves in chemical plants, paper mills, oil & gas refineries and power generation facilities where tEPA mandated leak detection and repair (LDAR) programs are in place.
In a review of the selections of 8,061 engineer and other technical user search selections (2009 – 2013) using SpecSearch® valve search forms, we found 80 percent selected corrosion resistant materials such as stainless (27.2 percent), brass or bronze (24.1 percent) or plastic (18.2 percent) – see figure 3. After reviewing the valve materials available from 490 valve suppliers, we found 47 percent offer stainless valves, 34 percent offer brass or bronze valves and 27 percent offer plastic valves.
Valve Actuation, Connections and Certification
How the valve will be operated within your product or plant is another consideration in valve selection. If your valve will be manually operated, then a manual valve, with a hand wheel or handle for opening and closing the valve, is required. A valve installed in a vehicle, chemical plant or aircraft where the valve must turn on and off automatically will require an electric, pneumatic or hydraulic actuator to turn or slide the stem based on control signal inputs. Check valves are operated by a pressure drop within the system and typically do not require any actuation.
The valve end connection or fitting type is another selection factor, which is typically defined by the existing connections in the process or piping system. If the existing chemical plant has flanged fittings, then valves available with flanged ends must be located. Valve connections are not considered primary selection criteria for engineers because suppliers often offer the same valve product with multiple connection options, and adapter fittings can be purchased if the desired connection type is not available.
Certain industries and application have specific codes, standards and regulations that need to be met. In some cases, a valve must meet or be certified to these standards (ASME, API, NACE, etc.) before it can be used in certain boiler, pressure vessel, oil & gas refinery, nuclear or other mission critical applications.
See GlobalSpec’s Valve Standards page for a full list of valve standards and regulations.
A variety of resources are available on our site to aid engineers in finding and selecting valves. Our valve categories provide access to 6,141 valve suppliers with over 192,699 parametrically searchable product data sheets from 446 suppliers, in addition to valve selection resources and guides such as the Industrial Valve Specification Guide, Valve Flow and Sizing and Valve Application. Our 32 valve SpecSearch® categories were viewed by 2 million engineers or other technical users from 2012 to 2016, and over 372,000 valve specification searches were performed by the same parties. Valve selection resources are also available from professional societies and trade organizations such as Valve Manufacturer Association (VMA).