Manufacturing of Large Valve | JRVAL - Source Manufacturer

1.1 Raw Casting Incoming Inspection

This initial step involves a thorough inspection of the raw castings (valve body, disc, etc.) upon their arrival from the foundry. Inspectors use gauges to verify basic dimensional conformity and employ non-destructive testing (NDT) methods, such as visual inspection, magnetic particle, or penetrant testing, to check for severe casting defects like cracks, shrinkage, or porosity that could compromise structural integrity.

Purpose: To prevent defective parts from entering the subsequent, more expensive machining stages, thus saving time and resources.

1.2 Casting Cleaning and Aging Treatment

Workers use pneumatic tools, grinding machines, and other equipment to remove casting remnants like gates and risers, as well as fins, burrs, and adhered sand from the casting surface. This process, often characterized by a shower of sparks from high-power angle grinders, is a seemingly rough but crucial step. It ensures the casting is clean and has stable surfaces for secure clamping in the subsequent machining operations.

For large valves with high precision requirements or those intended for severe service conditions, an "aging" or heat treatment process (such as annealing or normalizing) is performed after cleaning.

Purpose: While cleaning is for aesthetics and safety, the aging treatment is critical for relieving internal stresses developed during the casting's cooling phase. This prevents deformation during machining and ensures the dimensional stability of the final product.

2.1 Body Machining

This is the most complex and critical part of the process, typically performed on large Vertical Turret Lathes (VTL) or Horizontal Boring and Milling Centers. Top-tier manufacturers aim to complete as many operations as possible in a single setup to guarantee positional accuracy.

(A) Machining of Internal/External Contours and Flange Faces
This operation has two primary tasks, often performed on a VTL. First, the cutting tool reaches deep inside the valve body to precisely machine the seat surface, where the disc will seal. The dimensional accuracy and surface finish here are paramount for the valve's sealing performance.

Second, the tool machines the flange face at the end of the valve body. The mist seen is cutting fluid, used for cooling and lubrication. A perfectly flat and smooth flange face is essential for a leak-proof connection to the pipeline. Performing these two cuts in the same setup ensures perfect perpendicularity between the seat and the flange face.

(B) Machining of the Stem Holes
The valve body is then moved to a boring machine or machining center. Here, a long boring bar extends into the body to precision-machine the upper and lower stem holes. The ultimate goal of this operation is to achieve perfect concentricity—the upper and lower holes must align on a single, perfectly straight centerline. Any deviation can cause the stem to bind and lead to seal failure. The top mounting flange for the actuator is also machined during this step.

(C) Machining of Flange Holes
After the flange faces are machined flat, the bolt holes are created. Typically using a radial drill press or the machining center itself, holes are drilled on the flange face according to standards like ASME or DIN. The position, number, and diameter of these holes must strictly comply with the standard to ensure interchangeability with any standard pipeline flange.

 

2.2 Disc Machining

The raw disc casting is also machined on a lathe or milling machine. The main areas of focus are the outer edge, which forms the sealing surface (machined to extremely high tolerances for metal-seated valves), and the central bore for the stem.

Purpose: To ensure the disc edge can perfectly mate with the seat surface inside the body and to prepare it for connection to the stem.

2.3 Stem (Shaft) Machining

High-strength alloy steel bars are turned on a lathe to create various diameters to fit bearings, seals, and the disc. Subsequent milling operations may be required to create keyways or pin holes.

Purpose: To create the "backbone" that supports the disc and transmits torque. Its straightness and surface finish are crucial for smooth operation and long seal life.

3.1 Pre-assembly and Match Drilling

This advanced technique begins by placing the machined disc inside the valve body and temporarily positioning it with the stem. Then, using the already precision-bored stem holes in the body as a natural guide, a special long drill bit or reamer is passed through the body and the disc. This action machines the final connecting pin holes in the disc in perfect alignment with the body.

Purpose: This "match drilling" or "match reaming" process masterfully eliminates any cumulative dimensional errors from individual parts. It fundamentally guarantees that when the connecting pins or keys are installed, the rotational centers of the body, stem, and disc are perfectly unified. This is the finishing touch for achieving smooth actuation and a uniform seal.

 

3.2 Final Assembly

This stage involves installing all sealing and support components, such as stem bearings, packing seals, and O-rings. The stem is then permanently fixed to the disc using keys or pins. Finally, the top cover or actuator mounting bracket is installed.

Purpose: To assemble all individual components into a complete valve unit.

4.1 Pressure Testing

This is the most critical assessment before the valve can leave the factory, consisting of two main tests:

(A) Body Hydrostatic Strength Test
This is a critical test. With the valve in the fully open position, the two flange ends are sealed with blind flanges. The body cavity is then filled with water and pressurized to 1.5 times the valve's nominal pressure rating. For a specified duration, workers meticulously inspect the valve body casting, bonnet joints, and stem penetration points for any signs of leakage or structural deformation. This test assesses the valve's "bones," ensuring it is strong enough to withstand high pressure.

(B) Seat Leakage Test
With the valve in the fully closed position, pressure is applied to one side (both high-pressure and low-pressure tests are conducted). Inspectors check for any media leakage to the other side of the disc over a specified time.

Purpose: This test evaluates the valve's "internal capability"—its ability to seal tightly when closed. This is a core metric of valve quality.

4.2 Coating and Marking

Valves that pass the tests are moved to a blasting room for thorough surface cleaning, followed by the application of primer and topcoats. Heavy-duty anti-corrosion coatings, such as epoxy paint, are commonly used. Finally, a nameplate is affixed or engraved onto the body, detailing information like nominal pressure, nominal diameter, material, and manufacturer, as required by standards.

Purpose: To provide long-term corrosion protection and to supply users with all necessary product information.

4.3 Final Inspection and Packaging

A final check is conducted on the valve's appearance, paint job, nameplate, and flange protectors. A certificate of conformity is then issued. The valve is subsequently packed securely in a wooden crate or on a pallet to prevent damage during transportation.