ZGB 150 Slurry Pump Discharge Gate Valve Lost Foam Casting Defect Identification and Field Repair

Subtitle: Sand holes, shrinkage porosity, slag inclusion – identification and metal epoxy repair of three common casting defects

Introduction

The ZGB 150 slurry pump (150mm discharge) is widely used in power plant ash handling and mining tailings transport. A gate valve is typically installed on its discharge pipeline for flow control or pump isolation. Leakage from the valve body is a common field problem, often caused by lost foam casting defects – sand holes, shrinkage porosity, or slag inclusion. These defects may not be detected during factory inspection but develop into through‑leakage under pressure fluctuation and slurry erosion. Replacing the entire valve is expensive, time‑consuming, and requires pipeline shutdown.

Hebei Xingou Machinery Equipment Co., Ltd. has developed a fast identification method for lost foam casting defects and a field repair procedure using metal epoxy. This article describes the characteristics of three typical defects, step‑by‑step repair procedures, and quality control points, helping users solve valve leakage at low cost and extend valve service life.

1. Three Typical Lost Foam Casting Defects

Lost foam casting uses a foam pattern that vaporizes to form the mold cavity. This process is prone to three common defects:

These defects can occur on the valve body, bonnet, or flange of ZGB 150 gate valves. Small defects may be covered by paint and only become apparent under pressure after some operation time.

2. Field Defect Identification Methods

2.1 Visual Inspection

2.2 Leak Location

For valves already leaking, the simplest method is to dry the surface and observe the leak pattern (point, strip, or area).

3. Field Repair Materials and Tools

3.1 Metal Epoxy Selection

Hebei Xingou Machinery recommends Belzona or equivalent domestic metal epoxy. For ZGB 150 gate valves (typically ≤120°C, ≤2.5 MPa), standard metal epoxy is sufficient.

3.2 Tool List

4. Field Repair Procedure

4.1 Preparation

4.2 Epoxy Application

For large shrinkage porosity areas, the entire affected area should be ground down and coated with epoxy, not spot‑repaired.

4.3 Quality Inspection

5. Repair Tips for Different Defects

6. Case Study: Sand Hole Repair on a Power Plant ZGB 150 Gate Valve

Background: A gate valve on the discharge of a ZGB 150 pump at a power plant had a point leak of 20‑30 drops per minute on the valve body. Inspection revealed a sand hole about 2 mm in diameter and 3 mm deep.

Repair process:

• Stopped pump, depressurized, drained pipeline

• Ground off paint around the sand hole

• Used rotary burr to enlarge the sand hole into a V‑groove (6 mm diameter, 4 mm deep)

• Cleaned with acetone and dried

• Mixed Belzona 1111 metal epoxy and filled the groove

• Cured at room temperature for 8 hours

• Sanded smooth

Results:

• Hydrostatic test at 1.6 MPa showed no leakage

• The valve has been in service for 2 years without further leakage

• Repair cost ~$40, compared to $400 for a new valve

Lesson: With timely detection and proper procedure, metal epoxy repair can meet industrial valve service requirements. For high pressure, high temperature, or vibrating applications, new valve replacement is still recommended.

7. Precautions

Conclusion

Lost foam casting defects (sand holes, shrinkage porosity, slag inclusion) are common causes of leakage in ZGB 150 slurry pump discharge gate valves. Visual inspection and leak locating can quickly identify the defect type. Field repair using metal epoxy – cleaning, grooving, applying, curing, and sanding – can solve leakage problems without valve replacement at only 10%-20% of new valve cost. For through cracks or high‑pressure applications, however, new valve replacement is still recommended for safety. Hebei Xingou Machinery Equipment Co., Ltd. offers on‑site defect evaluation and repair technical support. Please contact us.