01 Introduction to Gate Valves
1. Working Principle and Functions
Working Principle: Gate valves are classified as shut-off valves. The gate moves vertically to either block or allow the flow of media in the pipeline. The valve closes by lowering the gate, which relies on the precise contact of sealing surfaces to prevent leakage. When fully open, the flow path is unobstructed.
Features:
Advantages: Low flow resistance, easy to operate, ideal for large-diameter pipelines (usually ≥100mm).
Disadvantages: Susceptible to seal wear and leakage, difficult to repair, and not suitable for flow regulation. It must be either fully open or fully closed.
2. Structure
Main Components: Valve body, bonnet, stem, gate (single or double), and packing seal.
Materials:
Carbon steel: Suitable for pipelines with media temperatures ≤450°C, such as water supply systems.
Alloy steel: Suitable for high-temperature conditions >450°C, such as boiler exhaust systems.
02 Gate Valve Maintenance Procedures
1. Valve Disassembly
Removing the Bonnet and Frame
Loosen the bolts and nuts securing the frame to the bonnet. Rotate the stem nut counterclockwise to separate the bonnet from the valve body.
Use lifting tools to remove the bonnet along with the stem and gate. Place them in a safe, designated area.
Cleaning the Valve Body and Seals
Clean the interior of the valve body, removing any debris or rust. Inspect the valve seat for pitting, scratches, or cracks.
Label the removed seals and packing for proper reassembly later.
2. Repairing Components
Valve Seat and Gate
Valve Seat: Use specialized grinding tools (e.g., grinding guns) to polish the sealing surface from coarse to fine, finishing with a polished surface free of grooves.
Gate: For deep pits or scratches, machine the surface with a lathe or grinder, then polish to ensure smoothness.
Packing and Sealing System
Replace packing with materials that meet pressure and temperature requirements. Cut packing ends at a 45° angle, stagger joints by 90° to 180°, and ensure tight, gap-free installation.
Clean the packing box and ensure smooth inner walls. The packing gland should compress the packing to about one-third of its height.
Stem and Nut
Stem: Remove rust and ensure it is straight and smooth. Replace if it shows significant wear (e.g., thread wear exceeding one-third of its original thickness).
Stem Nut and Bearings: Clean and inspect for wear. Ensure the threads are intact and securely fixed to the housing. Lubricate bearings with grease and check for smooth operation.
Fasteners
Clean all bolts and nuts, lubricate threads with lead powder, and ensure proper tightening.
3. Reassembling the Valve
Adjusting the Seal
Install the repaired gate onto the stem holder. Insert the assembly into the valve seat and ensure the sealing surfaces make complete contact. Adjust the shim thickness at the universal joint to ensure the gate seal is 5-7mm higher than the valve seat.
Installing the Bonnet
Place the stem and gate assembly into the valve body and secure the bonnet. Install packing into the packing box, compress with the gland, and ensure a reliable seal.
Final Inspection
Reassemble the bonnet frame and tightening bolts. Connect the actuator and perform a manual operation test to ensure smooth opening and closing.
Confirm all components are properly marked and that the valve meets inspection standards before restoring pipeline insulation.
03 Quality Standards for Gate Valve Maintenance
1. Valve Body
No casting defects such as sand holes or cracks.
Free from debris, with clear and unobstructed flow paths.
Bottom plugs sealed securely, with no leakage.
2. Valve Stem
Bending must not exceed 1/1000 of the total length; otherwise, straighten or replace.
Trapezoidal threads should be intact, with wear limited to one-third of the original thickness.
Smooth surface free of layered corrosion or rust. The surface finish should meet a standard of ▽6 or higher.
3. Packing and Seals
Packing material must match the medium’s pressure and temperature requirements, with product certificates or test reports available.
Packing dimensions must fit the packing box precisely, leaving adequate compression space. No oversized or undersized packing is allowed.
The packing gland must compress evenly, ensuring a tight and consistent seal.
4. Sealing Surfaces
After repair, sealing surfaces should have no pits or grooves. The contact area should cover at least two-thirds of the sealing width, with a surface finish of ▽10 or higher.
The gate seal should protrude 5-7mm above the valve seat to ensure a tight closure.
5. Other Components
Fasteners, bearings, and spring components must be intact and operate smoothly.
The valve should pass manual operation tests, ensuring flexibility and smooth functionality.
