Hydraulic Test Operation Guide for Pressure Vessels

Hydraulic testing is a critical part of pressure vessel safety inspection. It primarily uses liquid as the medium to assess the structural strength and sealing performance of the vessel. This test plays an essential role in checking the pressure-bearing capacity, leak-tightness, and durability of the pressure vessel, ensuring that it operates safely during actual use. This guide will cover the selection of test media, temperature control, test preparation, operating procedures, post-test treatment, and acceptance criteria to ensure the safety and effectiveness of hydraulic testing.

Basic Principles and Features of Hydraulic Testing

The principle of hydraulic testing is based on the low compressibility of liquids. Unlike gas tests, a liquid test can quickly reduce internal pressure and release energy when a pressure vessel leaks or ruptures. This minimizes the risk of accidents and ensures the safety of personnel. Therefore, hydraulic testing is widely used for testing and verification of pressure vessels. Due to the stability of the liquid medium, hydraulic tests are generally safer and more reliable than air tests.

Selection and Requirements of Test Media

The choice of test medium is one of the key factors that affects the test results and safety. Selecting the right liquid not only ensures accuracy but also avoids safety hazards during the test. Below are the principles and specific requirements for choosing the test medium:

1. Common Test Media

Clean Water: Water is the most commonly used test medium because it is easy to obtain, cost-effective, and safe. It is easy to handle and unlikely to cause fire hazards. To ensure accuracy and safety, the test water should be clean and free from impurities that could affect the sealing and test results.
Petroleum Distillates: In some cases, especially for pressure vessels in high-temperature environments, petroleum distillates may be used. However, the test temperature should always be well below the flash point of the petroleum product to prevent fire or explosion risks.
Other Liquids: If water is not suitable, other liquids, such as specialized chemicals, can be used according to design requirements. These liquids must meet safety standards and must not corrode or chemically react with the pressure vessel materials.

2. Special Material Requirements

Austenitic Stainless Steel: Austenitic stainless steel is sensitive to chloride ions, so the concentration of chloride ions in the test water must be strictly controlled, usually not exceeding 25 mg/L. After testing, water must be thoroughly removed, and steps should be taken to prevent corrosion from fluoride ions.
Carbon Steel and Low-Alloy Steel: For steel pressure vessels made of materials like Q345R, the liquid temperature should not be lower than 5°C to ensure the material's ductility. For other low-alloy steel pressure vessels, the liquid temperature should be maintained above 15°C to prevent brittle fracture due to low temperatures.

3. Liquid Performance Requirements

The temperature should be below the boiling point of the liquid to prevent it from vaporizing and creating unsafe conditions.
The temperature should be above the brittle transition temperature of the pressure vessel material to ensure the material does not become brittle during the test, maintaining good toughness.

Temperature Control in Hydraulic Testing

Temperature control is crucial to the accuracy of the test results and the safety of the pressure vessel. The temperature of the test liquid affects not only its physical properties but also the mechanical performance of the material and the overall reliability of the pressure vessel.

1. Temperature Requirements and Control

The temperature of the test liquid is a critical factor. Too high or too low a temperature can affect both the test results and the performance of the pressure vessel materials. The temperature should be below the flash point of the liquid to avoid fire hazards, and also below the boiling point to prevent gas formation, which could cause instability.

2. Material Characteristics and Temperature Control

Austenitic Stainless Steel Pressure Vessels: For water pressure tests, the test temperature should be below the boiling point of water and not lower than its brittle transition temperature.
Low-Alloy Steel and Carbon Steel Pressure Vessels: These materials have strict temperature requirements. For low-alloy steel, the test temperature should generally not be lower than 15°C, while for carbon steel, the minimum test temperature should be 5°C to avoid brittle fracture at low temperatures.

3. Consistency with Design Drawings

The test temperature must be adjusted according to the design drawings to ensure it aligns with the material characteristics and safety requirements.

Preparation for Hydraulic Testing

The quality of preparation directly impacts the safety, accuracy, and efficiency of the test. Proper preparation is essential to ensure a successful hydraulic test.

1. Equipment Inspection

Bolts and Seals: Ensure that all bolts and fasteners are complete and tightly secured to prevent leaks.
Pressure Gauge Calibration: Use two calibrated pressure gauges to ensure accurate measurements.
External Inspection: Check the pressure vessel's exterior for any damage and ensure there are no water stains, oil stains, or other factors that could affect the test.

2. Safety Measures

Set up safety barriers and warning signs, and ensure workers maintain a safe distance during the test.
Provide appropriate protective devices, such as pressure relief and explosion-proof devices, to avoid accidents.

3. Preparation of the Test Environment

Ensure the pressure vessel's exterior is dry so that any leaks or cracks can be quickly detected.
Confirm that all instruments and equipment are in normal working condition, including pressurizing equipment, liquid pumps, and safety valves.

Hydraulic Test Procedure

The procedure for hydraulic testing is critical to ensuring the test is safe and accurate. Following the correct steps allows for effective testing of the pressure vessel's strength and sealing ability while detecting potential issues before they cause accidents.

1. Filling the Pressure Vessel and Venting Air

During the test, liquid should first be injected from the top of the pressure vessel, ensuring that it is completely filled. All air must be vented from the pressure vessel to prevent any danger from gas expansion during pressurization.

2. Pressurization and Holding Pressure

Initial Pressurization: Gradually pressurize the pressure vessel to the design pressure and hold for a period, checking for leaks.
Test Pressure Phase: Once the test pressure is reached, maintain it for at least 30 minutes to ensure the pressure vessel's sealing performance, observing for any abnormalities.
Depressurization and Inspection: Gradually lower the pressure to 80% of the test pressure and hold for a period, inspecting critical areas such as welds and joints for leaks.

3. Handling Abnormal Situations

If any abnormalities occur, such as leaks, paint peeling, or pressure drops, immediately stop the test. Identify the cause, repair it, and resume the test.
For leaks, welding repairs must be made, and the test should be repeated.

Post-Test Treatment

After the test, the pressure vessel's interior should be drained of all liquid. Then, use compressed air or other inert gases to dry the pressure vessel. For stainless steel or other corrosion-prone materials, water stains should be removed to prevent long-term damage from chloride ions and other corrosive substances.

Hydraulic Test Acceptance Criteria

The test is considered successful not only if the pressure vessel can withstand the test pressure, but also if the following criteria are met.

No Leaks: Check welds, joints, and other sealing points for leaks.
No Deformation: The pressure vessel should not deform during the test and should maintain structural integrity.
No Abnormal Phenomena: No unusual noises, pressure drops, or liquid overflow should occur during the test.
No Surface Cracks: For materials with tensile strength above 540 MPa, surface inspections should show no cracks or other defects after non-destructive testing.

Importance of Hydraulic Testing

Hydraulic testing is a vital method for verifying the design and manufacturing quality of pressure vessels, ensuring their safety and reliability in actual use. By following a strict testing procedure, accurate operational steps, and suitable acceptance standards, hydraulic testing can not only assess the pressure-bearing capacity of the pressure vessel but also effectively prevent potential safety hazards. Whether during the design, manufacture, or maintenance of the pressure vessel, hydraulic testing plays a crucial role in ensuring industrial safety and the long-term stability of equipment.