Piping Valve Classifications

Valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid. Valves can be designed to handle either liquid or gas. By nature of their design, function, and application, valves come in a wide variety of styles, sizes, and pressure rating classes. We will try to classify valves according to their function, application, motion, port size etc. One confusing aspect of valve classification is that a specific valve my fit into more than one classifications. Therefore, the user should be careful when equating a particular valve with a particular classification.

Valve Classification According to Function

By the nature of their design and function in handling process fluids, valves can be categorized into three areas.

1. On-off Valves (also known as Block Valves or Shut off Valves)
2. Non Return Valves
3. Throttling Valves

On-off Valves / Block Valves / Shut off Valves

On-off valves are used to start or stop the flow of the medium through the process. A majority of on-off valves are hand operated, although they can be automated with the addition of an actuator. Although the functioning of an on-off valve is simple, providing for highly reliable and repeatable tight shutoff can be equally challenging.

On-off valves are commonly used in applications where the flow must be stopped completely and quickly. Reasons can be maintenance, any potential safety hazard like leakage, or shut down in down stream process piping / equipment. On-off valves are also used in mixing applications where exact measurement of fluids being mixed does not matter. Safety management systems also require automated on-off valves to immediately shut off the system when an emergency situation occurs.

Common on-off valves include gate, plug, ball, pressure-relief, and tank bottom valves. Pressure relief valves are self actuated on-off valves that open only when a preset pressure is surpassed.

Non Return Valves

Non return valves allow the fluid to flow only in one desired direction. Any flow in the opposite direction is mechanically restricted from occurring. A non-return valve is fitted to ensure that a medium flows through a pipe in the right direction, where pressure conditions may otherwise cause reversed flow.

Non return valves are used to prevent back flow of fluid, which could damage equipment or upset the process. Such valves are especially useful in protecting a pump in liquid applications or a compressor in gas applications from back flow when the pump or compressor is shut down. Non return valves are also used in process systems that have varying pressures, which must be kept separate.

All check valves are non return valves.

Throttling Valves

Throttling valves are used to regulate the flow, temperature, or pressure of the service. These valves can move to any position within the stroke of the valve and hold that position, including the full-open and full-closed positions. Therefore they can act as on-off valves also.

Throttling valves are equipped with actuators or actuation systems, which provide greater thrust and positioning capability, as well as automatic control. Few throttling valve designs are also provided with a hand operated manual handwheel but accuracy is a concern there.

Pressure regulators are throttling valves that vary the valves position to maintain constant pressure downstream. Control valves, also part of the family of throttling valves, are capable of varying the floiw conditions to match the process requirements. To achieve automatic control, these valves are always equipped with actuators.

Valve Classification According to Application

By the nature of their applications, valves can be categorized into three areas.

1. General Service Valves
2. Special Service Valves
3. Severe Service Valves

General Service Valves

General service valves are those valves that are designed for the majority of commonplace applications that have low or moderate pressure and temperature ratings (pressure rating between class 150 to 600 and temperature between -50 to 350 degree C), non corrosive fluids, and common pressure drops that do not result in cavitation or flashing. General service valves have some degree of interchangeability and flexibility built into the design to allow them to be used in a wider range of applications. Their body material are usually carbon steel or stainless steel.

Special Service Valve

Special service valves are those valves that are custom engineered or designed for a single application that is outside the preview of normal process applications. Because of its unique design and engineering, it will only function inside the parameters and service conditions of that particular process application. Such valves usually handle very high temperature, high pressure, or a corrosive medium.

Severe Service Valve

Severe service valves are special service valves designed with special features to handle extremely severe process  applications, such as high pressure drops that results in severe cavitation, flashing, chocked flow, or high noise levels. Such valves may have highly engineered trims or special disks to either minimize or prevent the negative effects of the application on the valve. In addition to highly engineered valve body parts, severe service valves may require special actuation control to overcome the forces of the process application.

Valve Classification According to Motion

By the nature of their mechanical motion, valves can be categorized into two areas.

1. Linear Motion Valves (also known as Multi Turn Valves)
2. Rotary Motion Valves (also known as Quarter Turn Valves)

Linear Motion Valves / Multi Turn Valves

Linear motion valves are those valves that have a sliding stem design that pushes a closure element into the valve body which further regulates the open and close positions. 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.

Linear valves are known for their simple design, easy maintenance, and versatility with more sizes, pressure classes, and design options. These types of valves are extremely versatile with many different trim sizes and design options available. A linear-motion valve is also more resistant to cavitation compared to a rotary valve. Although linear valves are typically more expensive.

Gate valves, Globe valves, pinch valves, diaphragm valves, three way valves, angle valves all belongs to linear motion valves category.

Rotary Motion Valves / Quarter Turn Valves

Rotary motion valves are those valves that use a closure element that rotates through a quarter turn (45 degree) to open or close the flow. Rotary valves are usually smaller in size and weigh less than comparable linear motion valves, size for size. It should be noted that rotary valves are limited to certain pressure drops and are known to have cavitation and flashing problems. However, as technology continues to advance, these problems are becoming less common.

Ball valves, Butterfly valves, Plug valves, and Spherical valves all belong to rotary motion valves.

Valve Classification According to Port Size

By the nature of their flow passageway, valves can be categorized into two areas.

1. Full Port Valves
2. Reduced Port Valves

Full Port Valves

Full port valves are those valves whose closure element does not restrict the flow. Valve internal flow passageways are large enough to pass the flow without any significant restriction.

Full port valves are used primarily with on-off and blocking services, where the flow must be stopped or diverted. Full port valves also allows for the use of pig in the pipeline to remove any process buildup or scale.

Reduced Port Valves

Reduced port valves are those valves whose closure element restrict the flow. The flow area of the port of the closure element is less than the area of the inside diameter of the pipeline. This restriction allows the valve to take a pressure drop as flow moves through the closure element, allowing a partial pressure recovery after the flow moves past the restriction.

The primary purpose of reduced port valves is to control the flow through reduced flow or through throttling, which is defined as regulating the closure element to provide varying levels of flow at a certain opening of the valve.