A valve as a relatively complex monomer pipeline component, involves a variety of properties and description parameters which include the valves' types, the basic parameters such as the application standard, the nominal diameter, the nominal pressure level, the main material, etc., structural parameters such as the valves' body structure, the type of the internal part, the type of the end connection, the bonnet type, the mode of operation, etc., the main parts' material such as the valve seats' sealing material, the valve sealing surfaces' material, the valve bonnets bolts' material, the gaskets' material, etc., and other special requirements including inspection, structural and manufacturing requirements. Most of these attribute parameters are determined by the engineering designer in order to eventually get the valve to meet the application requirements of the industrial production plant. The general valve selection is based on the same process, or learning from the experience of selecting similar devices.

For the valve applied to conditions with stringent requirements, just determining the above-mentioned valves' attribute parameters may not be enough. Sometimes, we should also take control of or have requirements on other factors which can affect the quality of the valve in accordance with the specific medium condition. These influence factors include the processing requirement such as the materials' smelting requirement, the forming method, the heat treatment, etc., and the inspection test requirement such as the non-destructive testing, the strength test, the seal test, the corrosion test, etc. These requests can not be described by just simple phrases or acronyms. Thus the engineering designer will prepare the order specification to describe the above requirements.

The valve is an important component of petroleum and chemical plants, and the successful operation of any device is inseparable from the correct selection of valves. Therefore, the valve selection is neither just considering the characteristics of the valve itself, nor just the professional design of pipes. In reasonable selection of valves, not only professional design work of the pipeline and equipment layout including the chemical process, the process system, the chemical safety, environment protection and energy saving are supposed to be considered, other factors such as the fluid property, the operating condition, the external environment, economical efficiency and related laws and regulations should also be taken into account.