National standard welded gate valve

1、 Basic concepts

National standard welded gate valveGB Butt Weld Gate Valve is a type of gate valve that uses butt welding connection at both ends of the valve body and complies with Chinese national standards (GB). It is mainly used for cutting off or connecting media in pipeline systems. Its characteristics are compact structure, good sealing performance, high pressure resistance, suitable for high temperature and high pressure working conditions and pipeline systems that require long-term connections (such as petroleum, chemical, power station and other fields).

2、 Core structure

Valve body and valve cover

Valve body: one-piece or split casting, mostly made of cast steel (WCB), stainless steel (CF8/CF8M) or alloy steel (WC6).

Butt welding end: The two ends of the valve body are machined into grooves (V-shaped or U-shaped), which comply with the GB/T 12224 standard and are welded and fixed to the pipeline.

Valve cover: connected to the valve body through bolts or pressure self sealing, including a packing box (sealing valve stem).

Gate and valve seat

Gate type:

Wedge gate: single gate (rigid wedge) or double gate (elastic wedge), suitable for medium and high pressure.

Parallel gate: equipped with guide holes or spring assisted seals, suitable for media containing particles.

Valve seat: welded with hard alloy (such as Stellite 6) or body material, forming a metal hard seal with the gate plate.

Valve stem and transmission components

Valve stem: open stem (visible as the valve stem rises and falls with the gate) or concealed stem (valve stem only rotates without lifting), made of stainless steel (316/17-4PH).

Packing: Flexible graphite, PTFE or graphite braided ring, resistant to high temperature and high pressure.

Welding connection details

Groove form: Designed according to GB/T 985.1 standard (such as V-shaped groove angle of 60 °± 5 °).

Welding process: argon arc welding as the base and electric arc welding as the filling are required, and heat treatment is required after welding (for thick walled valve bodies).

3、 Working principle

Opening and closing action: By rotating the handwheel or actuator to drive the valve stem, the gate is vertically lifted and lowered to achieve full opening or full closing of the flow channel.

Flow characteristics: When fully open, the flow channel is straight through, with minimal flow resistance, suitable for high flow cutoff; But the structure has a large height and a long opening and closing time.

Sealing mechanism:

Metal hard seal: The gate and valve seat are precisely ground and sealed with the assistance of medium pressure.

Elastic seal: The double gate compensates for wear through springs to maintain sealing performance.

4、 Comparison of national welding standards

Typical specifications of GB standard parameters

Welding end dimensions GB/T 12224 Groove forms (V-type/U-type), angles, blunt edges

Structural Length GB/T 12221DN15~DN1200 (Short Series/Long Series)

Pressure temperature rating GB/T 12224PN16~PN420, corresponding to the upper limit of material temperature

Welding Process GB/T 985.1 Groove Processing and Welding Joint Requirements

5、 Key parameters for selection

medium characteristics

Medium containing particles or high viscosity: choose parallel gate or elastic gate.

Corrosive medium: Choose stainless steel (316L) or duplex steel (2205) valve body.

operating conditions parameters

Temperature range: -196 ℃ (deep cold)~550 ℃ (high temperature), matching valve body material (such as WC9 high temperature resistant alloy).

Nominal pressure (PN): Select according to system requirements (e.g. PN100=10MPa).

Operation method

Manual (handwheel/gearbox), electric (multi turn actuator), or hydraulic (high-pressure rapid cut-off).

Special requirements

Fireproof design: The valve stem and packing adopt graphite+metal fireproof structure (in compliance with API 607).

Sulfur resistant treatment: The valve body and internal components comply with NACE MR0175 standard (for use in H-S containing environments).

6、 Typical application scenarios

Oil pipeline: Main shut-off valve for long-distance pipelines (requiring full bore and low flow resistance design).

Chemical plant: High pressure medium isolation at the inlet and outlet of the reactor (hard seal+corrosion-resistant material).

Power station system: Main steam pipeline (under high temperature and high pressure conditions, material selected as 12Cr1MoVG).

Liquefied natural gas (LNG): ultra-low temperature storage and transportation system (cryogenic gate valve, valve stem extension design).

7、 Installation and Maintenance Guide

Installation points:

Before welding, it is necessary to clean the pipeline and valve groove to avoid impurities affecting the welding quality.

Keep the gate valve fully open during installation to prevent the gate from getting stuck due to welding heat deformation.

After welding, radiographic testing (RT) or ultrasonic testing (UT) is required to ensure that the weld seam is defect free.

Maintenance suggestion:

Regularly check for packing leaks and promptly tighten or replace the packing (every 6 months).

Long term storage requires the application of anti rust oil and the installation of protective gaskets between the gate and valve seat.

Regular lubrication of valve stem threads (using molybdenum disulfide grease for high temperature conditions).

8、 Advantages and limitations

Advantages and limitations

Extremely low flow resistance, suitable for high flow cutoff structures with high height and occupying a lot of space

Reliable sealing, high pressure and high temperature resistance. Pipeline cutting is required for maintenance, resulting in high costs

Welding connection has no risk of leakage, slow opening and closing speed, not suitable for frequent operation

9、 Common fault handling

Analysis of the causes of the fault phenomenon and solutions

Valve leakage gate sealing surface erosion or valve seat deformation welding repair sealing surface or replacement of valve seat

The valve stem is stuck and the packing is too tight or the medium is crystallized. Release the packing cover and flush the valve stem

Leakage at the welding point, welding defects or thermal stress cracks, removal of the weld seam, re welding and non-destructive testing

10、 Selection Q&A

Q1: What are the main differences between butt welded gate valves and flanged gate valves?

A1: Welding gate valves are fixed to pipelines through welding, which provides better sealing and higher pressure resistance, but maintenance is difficult; The flange gate valve is detachable and suitable for occasions that require frequent maintenance.

Q2: How to choose valve body material under high temperature conditions?

A2: WCB is optional for temperatures ≤ 425 ℃; WC6/WC9 (chromium molybdenum steel) should be selected for temperatures between 425 ℃ and 550 ℃; For ultra-high temperatures (>550 ℃), it is recommended to choose chromium nickel alloys (such as Inconel 625).

Q3: Can butt welded gate valves be used to regulate flow?

A3: Not possible. Gate valves are designed to be fully open or fully closed. Partial opening can cause vibration of the gate and erosion of the sealing surface. Globe valves or control valves should be used to regulate flow.

Based on the above analysis, users can make reasonable choices according to the characteristics of the medium, pressure and temperature conditions, and installation requirementsNational standard welded gate valveAnd master the key points of installation and maintenance to ensure the long-term stable operation of the system.