In fluid piping systems, valves are control elements. Their primary functions are to isolate equipment and piping systems, regulate flow, prevent backflow, and regulate and relieve pressure. Selecting the most suitable valve for a piping system is critically important. Therefore, understanding valve characteristics and the steps and basis for selecting valves becomes paramount.

Up to now, the valve industry can produce a comprehensive range of products including 12 major categories such as butterfly valves, gate valves, globe valves, ball valves, check valves, hydraulic control valves, diaphragm valves, control valves, pressure reducing valves, plug valves, piston valves, steam traps, power station valves, American standard valves, and hydraulic control valves. This encompasses over 3000 models and more than 4000 specifications. Valves are available for maximum working pressures of 600 MPa, maximum nominal diameters up to 5350 mm, maximum working temperatures of 1200°C, and minimum working temperatures of -196°C. They are suitable for media including water, steam, oil, natural gas, highly corrosive media (such as concentrated nitric acid, medium-concentration sulfuric acid), flammable media (such as benzene, ethylene), toxic media (such as hydrogen sulfide), explosive media, and radioactive media (such as sodium metal, primary loop pure water, etc.). Pressure-bearing components can be made from materials like cast copper, cast iron, ductile iron, high-silicon cast iron, cast steel, forged steel, high/low alloy steel, stainless acid-resistant steel, Hastelloy, Inconel, Monel alloy, duplex stainless steel, titanium alloy, etc. The industry is also capable of producing various electric, pneumatic, and hydraulic valve actuators. Faced with such a vast array of valve types and complex operating conditions, selecting the most suitable valve product for installation in a piping system requires, in my view, first understanding the characteristics of valves; second, mastering the steps and basis for selection; and third, adhering to the principles of valve selection.

1. Valve Characteristics
Valves generally have two types of characteristics: Service Characteristics and Structural Characteristics.

Service Characteristics: These determine the main performance and application scope of the valve. Service characteristics include:

Valve category (isolation valves, control valves, safety valves, etc.).

Product type (gate valve, globe valve, butterfly valve, ball valve, etc.).

Material of main components (valve body, bonnet, stem, disc, sealing surfaces).

Valve actuation method.

Structural Characteristics: These determine aspects related to valve installation, maintenance, and upkeep. Structural characteristics include:

Face-to-face and overall dimensions.

Connection type to piping (flanged, threaded, clamped, external thread, weld end, etc.).

Sealing surface form (inserted ring, threaded ring, hardfaced, spray welded, integral with body).

Stem design (rotating stem, rising stem).

2. Steps and Basis for Valve Selection

The general steps and basis for selecting valves are as follows:

(A) Selection Steps

Define Purpose: Clarify the valve’s purpose within the equipment or system and determine its operating conditions: applicable media, working pressure, working temperature, etc.

Determine Connection: Establish the nominal diameter (DN) of the connecting pipe and the connection method: flanged, threaded, welded, etc.

Determine Operation Method: Specify the actuation method: manual, electric, solenoid, pneumatic, hydraulic, electro-pneumatic, or electro-hydraulic linkage.

Select Materials: Based on the pipeline medium, working pressure, and working temperature, select the material for the valve body and internals: grey cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.

Choose Valve Category: Select the valve category: isolation valve, control valve, safety valve, etc.

Determine Valve Type: Specify the valve type: gate valve, globe valve, ball valve, butterfly valve, needle valve, safety valve, pressure reducing valve, steam trap, etc.

(B) Basis for Valve Selection

While understanding the selection steps, it’s also essential to grasp the basis for selection:

Intended Use & Conditions: The purpose, service conditions, and control method of the selected valve.

Medium Properties: Characteristics of the working medium: pressure, temperature, corrosiveness, presence of solid particles, toxicity, flammability, explosiveness, viscosity, etc.

Fluid Performance Requirements: Flow resistance, flow capacity, flow characteristics, sealing class, etc.

Size Constraints: Installation and dimensional requirements: nominal diameter, connection type and dimensions to piping, overall dimensions, or weight limitations.

Additional Requirements: Requirements for product reliability, service life, explosion-proof performance of electric actuators, etc.

(Note: For control valves, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, normal flow pressure drop, shut-off pressure drop, maximum and minimum inlet pressure.)

Based on the above selection basis and steps, rational and correct valve selection also requires a detailed understanding of the internal structure of various valve types to make an informed choice for the preferred valve.

Valve Selection Principles

The final control element in a pipeline is the valve. The valve’s closure member controls the flow pattern of the medium within the pipe, and the shape of the valve flow passage determines its flow characteristics. This must be considered when selecting the most suitable valve for a piping system.

The following principles should be adhered to when selecting valves:

Valves for Shut-off and Start-up (On/Off Service):

Valves with straight-through flow paths (e.g., gate, ball, plug) offer low flow resistance and are typically chosen for on/off service. Downward-closing valves (e.g., globe, piston valves) have more tortuous paths and higher resistance, making them less common for pure on/off. They may be used where higher resistance is acceptable.

Valves for Flow Control (Throttling Service):

Valves suitable for easy flow regulation are typically chosen for control. Downward-closing valves (like globe valves) are well-suited as the relationship between seat size and closure member travel is proportional. Rotary valves (plug, butterfly, ball) and flex-body valves (pinch, diaphragm) can also be used for throttling, but usually within limited size ranges. Gate valves, where a disc moves transversely across a circular seat, only offer good control near the closed position and are generally not recommended for flow control.

Valves for Flow Diversion:

For diverting or changing flow direction, valves with three or more ports are needed. Plug valves and ball valves are particularly suitable for this purpose. However, other valve types can sometimes be interconnected appropriately to achieve diversion.

Valves for Media with Suspended Solids:

When the medium contains suspended particles, valves whose closure member slides across the sealing surface with a wiping action are best. If the closure member moves perpendicularly to the seat (like a globe valve), it may trap particles. Therefore, unless the sealing material allows particle embedding, such valves are only suitable for essentially clean media. Ball valves and plug valves provide a wiping action during opening and closing, making them well-suited for media with suspended solids.

Currty, whether in petroleum, chemical, or other industries, valve applications, operating frequencies, and service conditions vary immensely. Valves remain the most important and critical equipment for controlling or eliminating even minor leaks. The final control element in a pipeline is the valve, and its service and reliable performance across various fields are unique.