Valve Types

There are many types of valves available to accommodate a wide range of applications. GlobalSpec offers industrial buyers over 30 valve varieties, each with a unique method of control, function, or media application.

Classification

There are two major methods used to classify valves. These include method of control and valve function. Most valves fall into one or more of the classifications listed below. It is important to understand how the valve operates, as well as how it is best used, to reduce the need for valve maintenance and extra wear on the valve parts.

Methods of Control

The mechanism that the valve uses to control the flow depends on the type and situation of the closure element in the valve body. In general, there are two methods of controlling flow through a valve.

• Linear motion valves use a closure member that moves in a straight line to start, stop, or throttle the flow. The closure device could be a disc, slat or flexible material, like a diaphragm. Linear motion valves tend to be slower in operation, but they have a higher level of accuracy and stability in the position of the closure member.

  The closure device can be used to:

  • Move a disc, or plug into or against an orifice
  • Slide a slat, cylindrical, or spherical surface across an orifice
  • Move a flexible material into the flow passage
  
  
  
  The most common example of linear motion valves includes gate valves, globe valves, fixed cone valves, needle valves, and pinch valves.

• Rotary motion valves rotate a disc or ellipse about an angular or circular shaft extending across the diameter of an orifice. They are also known as quarter turn valves. Quarter turn valves will be in their fully open or fully closed state (0°) after a 90° turn of the stem. Their operation is much quicker than linear motion valves.
  
  Image Credit: McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
  
  

  The most common examples of rotary motion valves include ball valves, butterfly valves, plug valves, and spherical valves.

Valve Function

Valves are a part of many daily-used machines and perform a variety of functions. The three common valve functions include stopping and starting flow, throttling (control) flow, and acting as a non-return check for flow (check).

• Start/stop valves are used for systems that do not need the flow throttled. The valve opens to allow flow and closes to stop flow. They are also known as on/off service or block valves. These valves are often used with a manual hand wheel, but can be automated with an actuator. Start/stop valves are used when flow needs to be diverted or for mixing applications when exact measurements are not required. Automated start/stop valves are used in safety management systems to immediately shut off the system in an emergency situation. Some start/stop valves can be considered self-actuated valves because their mechanisms operate based on system flow without direct input from the operator. Relief valves are also used to protect the system from over-speed flow by immediately closing if a downstream pipeline is broken or blocked. For systems with air or gas, on/off valves can be used to regulate the pressure through the piping and protect against overpressure by allowing gas out of the piping.

• Throttle or control valves control the speed and capacity of flow, temperature, or pressure, through a system. Control valves can be moved into any position, including fully opened and fully closed. They can be operated with a manual hand wheel or can be equipped with an automated actuator. Actuation systems provide greater thrust and positioning capacity. Pressure regulators are responsible for varying the valve’s position to maintain constant pressure downstream. The regulator opens and closes to increase or decrease the pressure in the system.

  
  Video Credit: iecSimulations
  
  

• Non-return or check valves control the direction of flow. Flow in the desired direction opens the valve, while flow in the opposite direction forces the valve closed. These valves are important for preventing backflow to systems in applications such as wastewater management and in process systems that have varying pressures and must be kept separate.

  
  Video Credit: iecSimulations
  

Some valve types, depending on their design, may be used for one or more of the functions listed above.

Type	Description	Advantage	Disadvantage
Gate valves	Linear motion valves used to start and stop flow. The valve is completely opened when the disk is removed from the flow stream. Classification of gate valves is made by the type of disk used: solid wedge, flexible wedge, split wedge, or parallel disk. Image Credit: Cameron	No resistance to flow when open	Flow changes nonlinearly with stem travel
		Little pressure drop	Vibration/cavitation when partially open
		Good sealing when closed	Subject to wear
		Little/no leakage	Repair work is difficult
Globe valves	Linear motion valves used to start, stop and regulate flow. The disk moves perpendicular to the seat to open or close the flow so the annular space between the disk and seat ring gradually changes. There are three body designs for globe valves: Z-body, Y-body, and angle and three designs for the disk: ball disk, composition disk, and plug disk. Image Credit: Flowserve	Throttling and regulating flow	High head loss due to flow direction changes
		Less seat leakage than Gate Valve	Dynamics can create pulsation and damage trim/packing/actuators
			Noisy in high pressure applications
			Valves can be very heavy/large in size for a given application
Ball valves	Rotational motion valves used to start, stop or throttle flow. The valve uses a ball shaped disk with a hole in it. When the valve is opened the hole of disk is turned in-line with the direction of the flow. When the valve is shut, the ball is rotated so that the hole is perpendicular to flow direction. Image Credit: Cameron	Less expensive	Relatively poor for throttling
		Low maintenance costs	Throttling leads to seat erosion
		Low torque
		Quick action on/off
		Compact
		No lubrication
		Tight sealing
Plug valves	Rotation motion valves used to stop and start fluid flow. The disk is a solid tapered or cylindrical plug with a bored passage at the right angle to the longitudinal axis of the plug. When open, the plug lines up with the inlet and outlet port of the valve body. The plugs are either round or cylindrical with a taper. Plug valves are easy to adapt Image Credit: Cameron	Rotational motion	Typically NOT for throttling
		Suitable for multi-port designs
Diaphragm valves	Linear motion valves that are used to start, regulate, and stop fluid flow. The disk is flexible and seals with the seat in an open area at the top of the valve body. Image Credit: Dharmi engineers	Well-suited for difficult environments (corrosive chemicals, slurries, radioactive fluids)
		Ability to throttle
Reducing Valves	Automatic valves that reduce supply pressure to a preselected pressure. The supply pressure must remain at least as high as the selected pressure.	Automatically reduces supply pressure to preselected pressure
Pinch valves	The simplest of any valve design. Pinch valves consist of a sleeve molded of rubber or other synthetic material and a pinching mechanism. The pinching mechanism, a bar or gate, is lowered onto the valve body to cut off the flow through the system. Image Credit: Flowrox	Relatively inexpensive
		On/off as well as throttling/regulating
		Good for slurries, solids
		Well-suited for difficult environments
Butterfly valve	Rotary motion valves that can be used in on-off and throttling systems. They are quick and easy to operate. The flow control element is on either a vertical or horizontal axis and is opened when the handle is rotated 90 degrees and closed when the valve is turned an additional 90 degrees. Image Credit: Cameron	On/off as well as throttle/regulate
		Easily/quickly operated
		Good for large flow/low pressure applications due to saving in weight/size/cost
		Good for slurries/suspended solids.
Needle valves	Needle valves have a long, tapered, needle-like point that is used to make relatively fine adjustments in the amount of fluid flow. They are sometimes used as component parts for other valves because the needle allows for a gradual change in the size of the fluid flow opening.	Good for fine adjustment throttling
Check valves	Check valves are used in systems employing gate valves because they prevent the reversal of flow in the piping system and there is a low pressure drop across the valve. The pressure of the fluid through the system opens the system, while the weight of the check mechanism will close the valve if the flow is reversed. Image Credit: Cameron	Prevents reverse flow
Relief and safety valves	A relief valve opens slowly as the pressure increases about the set-point and only opens as necessary. A safety valve rapidly opens as the pressure setting is reached and will stay open until the pressure is lower than the actuating pressure set-point. Both valves are used to prevent damage by relieving accidental over- pressurized fluid systems. Image Credit: Watts	Prevents over-pressurization
		Relief valves are used for incompressible fluids such as water or oil
		Safety valves are used for compressible fluids such as steam.

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