Valve body cover

Detailed explanation of types, specifications and styles of valve body and bonnet.

I. Material types and characteristics of valve body/bonnet

II. Structure Type and Style of Valve Body

(A) Classification by structural form

1. Integral valve body
   • Features: The valve body and bonnet are cast as a whole, which has good sealing performance and no risk of leakage at the joint, but it is inconvenient to maintain.
   • Style: It is often used for small valves (DN≤50), such as integrated valve body of ball valve and globe valve.
2. Split valve body

• Features: The valve body and bonnet are manufactured separately, and connected by bolts, welding or threads, which is convenient for maintenance and replacement of internal components.
• Subtype:
  • Flanged valve body: the valve cover is connected by flange bolts, which is the most widely used (such as DN15-DN2000 gate valve).
  • Clip-on valve body: it can’t be flanged, and it is clamped and fixed by flanges on both sides (such as butterfly valve), with compact structure and light weight.
  • Welded valve body: the valve body is directly welded to the bonnet (such as high-pressure forged steel valve), which has excellent sealing performance and is suitable for PN≥160MPa.

(B) by channel form classification

(C) Special valve body design

• Forged steel valve body: formed by forging process, its strength is higher than that of cast steel (for example, the tensile strength of A105 forged steel is ≥485MPa), and it is suitable for small-caliber valves with high pressure (PN≥100) (DN≤150).
• All-welded valve body: The valve body and valve cover are connected without bolts, and welded integrally (such as buried gas valve). The leakage prevention level meets the requirements of API 6D, and it is suitable for flammable and explosive media.

III. Valve cover type and connection style

(A) Classification by connection mode

1. Bolted Bonnet
   • Structure: the valve cover is fixed by bolt group, which is convenient for disassembly and maintenance, and is the most widely used (such as gate valve and globe valve).
   • Sealing form: spiral wound gaskets+bolt pre-tightening, pressure grade PN16-PN250.
2. Welded Bonnet
   • Type:
     • Socket welding (SW): suitable for DN≤80, such as high-pressure globe valve;
     • Butt welding (BW): suitable for DN≥100, such as valves in high temperature and high pressure power stations.
   • Features: no leakage point, pressure resistance can reach PN420, but maintenance needs to be cut and replaced.
3. Threaded Bonnet
   • Structure: The valve cover and the valve body are connected by threads (such as NPT threads), and the sealing depends on PTFE tape or metal gasket.
   • Application scenario: small-caliber (DN≤50) and low-pressure (PN≤10) valves, such as instrument valves and sampling valves.

(B) Valve cover with special function

• Bellows sealing valve cover: metal bellows is built in to prevent medium leakage (such as toxic gas valve), and the sealing level reaches ISO 15848-1 zero leakage standard.
• Extended Bonnet: The bonnet is designed to be lengthened to isolate the low-temperature medium from the outside (such as liquid nitrogen valve) and prevent the valve stem from frosting. The extended length is usually 150-300mm.

IV. Specification System and Standard Parameters

(A) nominal diameter (DN) and inch (in)

(B) pressure grade standard comparison

(C) Temperature grade range

• Low temperature valve: -196℃ (liquid nitrogen) ~-46℃, and the valve body needs 304L and LCB (low temperature cast steel);
• Normal temperature valve: -46℃~121℃, commonly used cast iron and WCB；;
• High-temperature valve: 121℃~650℃, WC9 and Inconel alloy are needed, and the valve cover should be designed with cooling ribs.

V. Key Factors and Cases of Selection

1. Medium characteristics are preferred:
   • Example: To transport 98% sulfuric acid, it is necessary to select 316L stainless steel valve body and bolted valve cover (with PTFE gasket) to prevent cast iron from being corroded;
   • Example: High pressure steam (10MPa, 540℃) requires A182 F91 forged steel valve body and welded valve cover to prevent bolts from loosening and leaking at high temperature.
2. Weight and installation restrictions:
   • For water supply and drainage of high-rise buildings, if cast iron valve body (weighing about 200kg) is selected for DN300 valve, it is easier to hoist than cast steel (350kg) and the cost is 30% lower.
   • For offshore platform valves, aluminum alloy body (such as 5083 aluminum alloy, density 2.6g/cm³) is required, which is 60% lighter than steel and meets the load limit of ships.
3. Maintenance convenience:
   • It is recommended to connect the valve cover of chemical pipeline with bolts. When the internal sealing surface is worn, the valve cover can be opened for maintenance without disassembling the pipeline, which saves 50% maintenance time compared with welding the valve cover.

VI. Summary: Accurate matching from structure to function

• Materials determine the basic properties of temperature resistance, pressure resistance and corrosion resistance;
• Structural forms (such as flange/welding, straight-through/angle type) affect fluid resistance and installation space;
• Specification parameters (DN, PN) shall be strictly matched with the pipeline system.

For example, for LNG cryogenic pipeline, LC3 low-temperature cast steel valve body (-196℃ impact energy ≥47J)+ extended valve cover (200mm in length) should be selected, and all-welded structure should be adopted to avoid low-temperature leakage, while gray cast iron valve body+bolted valve cover for ordinary tap water pipeline can meet the demand. Through the above classification and parameter comparison, the efficient selection of valve components can be realized.