Episode 03. Valve Selection Criteria and Classification
Summary
Valves are essential components of piping systems. When selecting a valve, we need to determine various specifications. This episode covers the main criteria for valve selection. In particular, we discuss valve types according to their function.
1. Introduction
It is necessary to stop or control the flow rate during operation for proper fluid transfer, equipment maintenance, or safety. To achieve this, we commonly use valves. Valves are one of the main components of piping systems and come in a wide variety of types. We can classify valves based on various criteria, such as mechanical motion or function. Additionally, we can further subdivide even valves of the same type based on their components. Besides the valve type, we must select various specifications when ordering a valve from the manufacturer. In this episode, we discuss criteria when selecting valves and classify valves by their functions.
(Figure 3.1)
2. Valve selection criteria
Valves must function smoothly under complex conditions such as fluid type, temperature, pressure, surrounding environment, and cost. Therefore, we check several criteria when selecting valves. Here’s some information we typically need to decide on.
- Valve type
We need to identify the required function, such as whether it needs to block or regulate flow. Then, consider the pros and cons of the valve, such as weight, cost, maintenance, and specific features, to determine which type of valve to use.
- Size
We can match the valve size to the connecting pipe size. However, we often use calculations to determine the appropriate valve size. If the size is too small, it can cause higher pressure drop and noise. On the other hand, if the size is too large, it can make flow regulation difficult and increase both weight and cost.
- Rating
The valve must be able to withstand the system’s pressure and temperature. Therefore, we select the appropriate “Class” from the pressure-temperature rating based on the given conditions. Generally, the higher class provides greater minimum wall thickness, enabling the valve to withstand higher working pressure at the same temperature. These ratings vary depending on the material.
- Material
Valves are damaged by fluid, pressure and temperature. Therefore, we must consider required properties such as strength, corrosion resistance, and durability. The material for parts exposed to the fluid can differ from those not exposed, and even among parts exposed to the fluid, we may select different materials based on the specific properties required.
- End connection
End connection methods available to connect valves to piping systems include threaded end connections, welded end connections, flanged end connections, and so on.
- Actuators and accesories
We must decide how to operate the valve. Valves are operated manually or automatically. Some valves are self-acting without external power. We can classify automatic operation according to the type of external power used. And we can choose additional accessories for better functionality.
3. Valve classification according to function
Now, let’s explain the valve functions mentioned above. We can classify these functions into five main categories:
On-Off valves, Regulating valves, Non-return valves, Pressure relief valves, Special purpose valves
(Figure 3.2)
On-Off valves
Description
These valves are used to start/stop the flow. They have low flow resistance when fully open. And they provide excellent shut-off capability when fully closed. Many valves perform this function, but each has its unique characteristics. (Figure 3.2) shows a gate valve with a wedge moving in a linear motion. There are also valve types like the plug valve, which use rotary motion for opening and closing.
Type
Gate valve
Plug valve
Ball valve
Butterfly valve
Diaphragm valve
(Figure 3.3)
Regulating valves
Description
These valves are used to regulate the flow, pressure, or temperature. Regulating valves can be divided into two types: those that regulate without an external signal(regulators) and those that regulate with an external signal (control valves). (Figure 3.3) shows a part of a globe valve, which is one type of regulating valve.
Type
Globe valve
Needle valve
Ball valve
Butterfly valve
(Figure 3.4)
Non-return valves
Description
These valves prevent backflow and allow fluid to flow in one direction. This valve type operates automatically without external power sources. (Figure 3.4) shows a swing check valve, which is one type of non-return valve.
Type
Swing check valve
Lift check valve
Tilting check valve
Stop check valve
(Figure 3.5)
Pressure relief valves
Description
These valves are installed to prevent overpressure or vacuum. When used for overpressure prevention, the valve automatically opens to reduce system pressure when it exceeds the set pressure. Pressure relief valves can be broadly categorized into safety valves, relief valves, and safety-relief valves. Safety valves are used to prevent overpressure in gas services, relief valves are used in liquid services, and safety relief valves can be used in both gas and liquid services. (Figure 3.5) shows a safety valve.
Type
Pressure relief valve
Safety valve
Vacuum relief valve
Additionally, there are valves designed for special purposes, such as multi-port valves, line blind valves, and knife gate valves.
We can see that different valves can perform the same functions. This means that even though they have the same function, we must choose the most suitable type for each specific situation. We will cover details about each valve type in other episodes.