Pressure Vessel Testing: Common Failure Scenarios and Prevention Strategies

At TotalShield, where we specialize in crafting shielded enclosures, understanding the intricacies of pressure testing is crucial to ensuring safety and reliability. That’s why this time we reached Syed Adil Hasan, Senior Inspection and Integrity Engineer, who shared valuable insights with us.

In this blog post, we’ll explore common failure scenarios and offer practical strategies to prevent them so you can maintain the highest standards in your operations. 

Without further ado, let’s get started!

TABLE OF CONTENTS

• Pressure vessel regulatory codes
  • Code Compliance
  • When to use pneumatic Vs hydrostatic pressure testing
  • Non-Destructive Examination
• Pressure vessel testing safety considerations
• Failure scenarios, causes, and prevention
• Key takeaways for safe pressure testing

Pressure testing is an essential procedure to ensure the integrity and safety of pressure vessels and associated components. They help detect leaks, deformations, and weaknesses, thereby preventing catastrophic failures during operation. However, conducting these tests requires stringent adherence to safety protocols to protect personnel and equipment.

Before we begin, it’s crucial to define what a “pressure vessel” is. The term “pressure vessel” encompasses various storage containers used in industrial settings, including storage tanks, boilers, heat exchangers, and process vessels. Legally, a vessel is considered a pressure vessel when it holds vapors, gases, or liquids at 15 psig or above pressures. 

Pressure vessel regulatory codes

Regulatory design codes for pressure vessels vary by country, such as the ASME Boiler and Pressure Vessel Code in North America, Australian Standards, CSA B51 in Canada, the Pressure Equipment Directive of the EU, and the Japanese Industrial Standard (JIS). Worldwide standards include ASME Boiler and Pressure Vessel Code Section VIII, API 510, B51-09, BS 5500, CODAP, EN 13445, and EN 286.

ASME Section VIII is widely used for pressure vessel fabrication and testing. It outlines standards for the design, fabrication, inspection, testing, and certification of pressure vessels operating at pressures exceeding 15 psig. Section VIII is divided into three sections:

• Division 1 covers general requirements for vessels up to 3000 psig, excluding internals (except weld attachments), fired process heaters, and certain integrated pressure containers. 
• Division 2 offers alternative rules for similar pressure ranges, focusing on different criteria, such as allowable stress, design, and fabrication methods, chosen based on material economics. 
• Division 3 addresses the construction of high-pressure vessels generally above 10,000 psi, considering applications involving external pressures, process reactions, or thermal effects, without specific upper or lower pressure limits set by Divisions 1 or 2.

Code Compliance

After fabricating any pressure vessel, or sometimes after any alteration or repair, evaluating the strength, integrity, and reliability of pipework, vessels, and other components designed to contain fluids or gases becomes mandatory as part of various safety regulations. 

Hydrostatic (or hydraulic) pressure testing evaluates these aspects. This non-destructive testing method uses water introduced to the system before being pressurized via a hand pump to a calculated or specified pressure. 

According to ASME Section VIII-1, UG-99, pressure vessels designed for internal pressure must undergo a hydrostatic test pressure that is at least 1.3 times the Maximum Allowable Working Pressure (MAWP) multiplied by the lowest ratio of the stress value for the test temperature to the stress value for the design temperature (Ph = 1.3 MAWP x (S at test temp. / S at design temp.)). 

As per UG-100, a pneumatic test may be used instead of the standard hydrostatic test under specific conditions, such as when the vessel cannot be safely filled with water, cannot be readily dried, or will be used in services where traces of the testing liquid cannot be tolerated. In these cases, the pneumatic test pressure must be at least 1.1 times the MAWP multiplied by the lowest ratio of the stress value for the test temperature to the stress value for the design temperature (Pn = 1.1 MAWP x (S at test temp. / S at design temp.)).

Various types of pressure tests are used in the industry:

• Pre Start-Up Leak Tests ensure the tightness of joints before initial startup or during commissioning. 
• Revalidation Tests verify the integrity of existing piping or equipment. 
• Service Tests are conducted at operating pressures using the service fluid. 
• Strength Tests validate the integrity of piping systems or equipment at design pressures. System Tests apply pressure to a group of piping and equipment as a system. 
• Tightness Tests ensure the piping system remains leak-free at the test pressure.

Hydrostatic tests use water or approved liquids as the test medium, while pneumatic tests use air or approved gases (such as air, nitrogen, or argon) either alone or in combination with liquids. Industry codes permit pneumatic tests under certain conditions. 

While hydrostatic testing with water is mostly used, pneumatic tests are often chosen for their minimal downtime and cost-effectiveness compared to hydrostatic tests. 

Pneumatic testing is particularly useful for pinpointing very fine leaks undetectable in hydrostatic tests, and it’s typically recommended for already proven-safe equipment and low-pressure applications, with test pressures lower than those in hydrostatic tests (per ASME Sec. VIII Div. 1). 

However, pneumatic testing is considered less safe than hydrostatic due to specific hazards. 

Risks include the high compressibility and stored potential energy of gases, which can lead to sudden, uncontrollable explosions upon rupture. In contrast, hydrostatic tests with incompressible liquids like water are safer, as they store less energy and allow for easier detection and mitigation of leaks before catastrophic failure.

Read our Pneumatic Pressure Testing Handbook.

When to use pneumatic vs. hydrostatic testing for pressure vessels

Various construction codes for pressure vessels and piping allow pneumatic testing as a final integrity check before equipment is commissioned. 

As mentioned before, this type of pressure testing is an alternative to hydrotesting under specific conditions, as recommended by the engineering team for new installations or the Asset Integrity Department of the owner/operator for existing systems. These conditions include: 

• Inadequate support for liquid weight
• Inability to fully dry the system post-test
• Presence of linings vulnerable to damage from hydraulic mediums, and compliance with standards such as ASME Section VIII-Div -1 for newly fabricated vessels (UG-100). 

Requirements stipulate that enameled vessels must withstand pressures equal to or above the design pressure, while other vessels must meet pressure criteria adjusted for stress and temperature factors. Welded vessels subject to pneumatic tests must undergo comprehensive weld inspections. 

For in-service vessels, per API 510 guidelines, pressure tests are mandated at the discretion of authorized inspectors following repairs or alterations. ASME Section VIII-Div -2 outlines similar requirements, specifying test pressures based on material strength ratios. Nonflammable, non-toxic mediums are standard for pneumatic tests, with compressed air usage requiring careful consideration of operational safety.

Hydrostatic tests are recommended for high-pressure applications. The stored energy per unit volume of water under pressure is minimal, reducing the risk of equipment failures. They’re suitable for proving equipment strength and cleaning afterward, though they require pressure relief valves and a larger safety cordon due to the potential energy release.

In contrast, pneumatic tests are advised for low-pressure applications. The energy stored per unit volume of compressed air is high, posing a greater risk of equipment or test apparatus failures. These tests typically involve higher risks and demand meticulous supervision by experienced personnel. They require specific safety precautions, such as careful joint inspection and thorough cleaning to remove moisture after testing. 

Despite being suitable for leak tests on equipment previously validated by hydrostatic testing, pneumatic tests necessitate a more extensive safety area due to the potential for significant damage upon failure. Both methods require well-documented procedures before conducting the test to ensure safety and efficacy.

Non-Destructive Examination (NDE)

Interestingly, API and ASME in-service inspection codes allow for NDE (Non-Destructive Examination) to substitute pressure testing after repairs or alterations when a pressure test is impractical or unnecessary. 

The key question in such scenarios is when an NDE can replace a pressure test and which specific NDE methods should be employed. While it’s impossible to fully address this in a brief discussion like this, it’s crucial to highlight that we shouldn’t readily dismiss the necessity of a pressure test. 

There are certainly situations where pressure tests are not feasible or advisable, such as with refractory-lined vessels, equipment not structurally designed for full water weight, or where water contamination risks exist. 

However, there are also instances where project engineers may seek to expedite projects by avoiding pressure tests. In these cases, conducting a risk analysis can help weigh the advantages and disadvantages of substituting NDE for pressure tests. One downside of forgoing pressure testing is the missed opportunity for rigorous stress and leak testing inherent in direct pressure tests.

Pressure vessel testing safety considerations  

Once a pressure vessel is determined to go for testing, the system is filled with a liquid, typically water, and pressurized above its maximum operating pressure to detect leaks, deformations, or weaknesses. The pressure is maintained for a specified period to identify any issues that are addressed before the system goes into service. This testing serves as a crucial safety measure to ensure that components can safely withstand the pressures encountered during normal operations. 

However, while the testing ensures safety, the pressure in the system itself becomes a safety hazard, requiring surplus safety precautions, such as:

• A thorough risk assessment must be conducted before pressure testing
• Supervisors must be trained, competent, and qualified, and effective communication should be established. 
• Limit access to the test area to essential personnel only, using barriers, warning signs, announcements, and patrolling. 
• Testing should be performed under a Permit to Work (PTW) for each test. 
• Pressuring equipment must have calibrated pressure control/regulator devices visible to the operator and should never be left unattended. 

When the test pressure is reached, isolate and, if possible, disconnect the pressuring equipment from the test equipment/piping, and lock the pressuring valve in a closed position. Consider ambient temperature changes, such as the thermal expansion of liquids in a closed system, and monitor pressure in adjacent systems not positively isolated. 

Equipment should not be closely examined above the maximum allowable working pressure until the pressure has been held for 30 minutes. If inspection is required during the hold period, reduce the pressure to the design pressure. Proper area barricading is essential for both hydrostatic and pneumatic testing. 

Given the high stored energy, pneumatic tests require utmost care and written authorization from the competent Project Manager or delegate. Avoid standing in the line of fire during tests, and perform hydro tests preferably during the day or ensure adequate illumination at night. Maintain proper communication between the pump operator and hydrotesting engineer during the test procedure, and wear all required PPE, such as safety goggles, helmets, safety shoes, and gloves, at all times. 

After completing the hydro test, release pressure first from the highest vent point, then open the bottom vent to drain piping and equipment to avoid vacuum generation.

The above-listed common safety aspects must be considered while performing hydrotests. In the industry, even after thorough checks, situations can worsen if these aspects are missed or ignored. 

Failure scenarios, causes, and prevention

During my career, I have witnessed several hydrotests, and fortunately, I am not in the news. However, I would like to list a few unwanted incidents from the past that can help prevent similar or near incidents if diligently followed. 

Incident Case 1

📍Fabrication shop

During a hydrostatic test of a pressure vessel, the pressure exceeded safe levels because a gas cylinder was used to boost the pressure when the pump couldn’t generate the required pressure. This uncontrolled pressure increase caused the bolting on one manway to shear, ejecting the manway cover with force. The cover struck the inspector, resulting in an immediate fatality.

Recommendation: Test pressure should be achieved using only water, ensuring no air remains in the vessel or is used to increase pressure. A relief valve should be provided for the hydrostatic pressure arrangement.

Incident Case 2

📍Large petrochemical plant

A 3.5-meter diameter vertical storage tank with a flat top, featuring a manway and several nozzles, was to be tested with a small amount of air pressure to check some instruments. Due to an error, the pressure increased excessively, causing the entire top cover to shear off from the shell and fly off like a projectile, landing about 130 meters away. While there was considerable damage to other installations, fortunately, no human lives were lost.

Recommendation: For low-pressure tests, use a U-tube manometer instead of pressure gauges. Take extra precautions to prevent overpressure, and install a certified safety relief valve for additional safety.

Incident Case 3

📍 Dairy plant

A used condenser was sent for repairs. The fabrication shop conducted a pneumatic test instead of a hydraulic test on the shell side to detect tube-to-tube sheet leakage. The condenser burst during the test, seriously injuring several workers and damaging some equipment in the shop due to the explosion.

Recommendation: Used equipment should initially be tested with water. If necessary, a pneumatic test at reduced pressure may be conducted afterward.

Incident Case 4

A hydrostatic test was being conducted on a newly installed pipeline. During the test, a section of the pipeline burst because the pressure was increased too rapidly, causing a pressure surge. This sudden burst caused significant water damage to nearby equipment and infrastructure.

Recommendation: Increase test pressure gradually to avoid pressure surges. Use pressure gauges and control devices to monitor and regulate the pressure rise during testing.

Incident Case 5

📍A refinery

Due to an oversight, a pressure vessel was being tested with a mixture of water and air. The presence of air pockets caused uneven pressure distribution, leading to a localized failure of the vessel wall. The resultant explosion caused extensive damage to the surrounding area and injured several workers.

Recommendation: Ensure that only the appropriate test medium is used for hydrostatic tests. Eliminate all air pockets before pressurizing the system.

Incident Case 6

During the commissioning of a new chemical processing plant, a pressure vessel intended for hydrostatic testing was inadvertently pressurized with compressed nitrogen instead of water due to miscommunication between the engineering team and the testing crew. 

The high-pressure nitrogen caused sudden structural failure of the vessel, leading to an explosion that damaged nearby equipment and injured several workers.

Recommendation: Ensure clear communication and strict adherence to testing procedures. Implement stringent verification processes to confirm the correct test medium before pressurization. Conduct thorough safety briefings for all personnel involved in testing operations.

Incident Case 7

📍 Power plant 

A large boiler was undergoing routine inspection and maintenance. During a pneumatic test to check for leaks, the pressure exceeded safe limits due to a faulty pressure relief valve. The sudden overpressure caused a rupture in the boiler shell, releasing high-pressure steam and injuring maintenance personnel nearby.

Recommendation: Regularly inspect and maintain pressure relief valves and safety devices. Implement dual pressure relief systems or backup safety measures to prevent overpressure incidents during tests. Conduct comprehensive safety training for personnel involved in boiler maintenance and testing.

These are just a few examples, but pressure safety incidents occur frequently because pressure testing is an integral part of QA/QC procedures. To ensure safe testing, always follow a qualified procedure and use inspected pressure testing equipment and tools. 

Take responsibility now to ensure every pressure test is incident-free and make the industry safer for everyone.

Key takeaways for safe pressure testing

1. Conduct a thorough risk assessment before testing.
2. Ensure supervisors and personnel are trained, competent, and qualified.
3. Establish effective communication and limit access to the test area.
4. Use calibrated pressure control devices and avoid leaving pressurizing equipment unattended.
5. Gradually increase test pressure and monitor for any irregularities.
6. Properly barricade the test area and use appropriate PPE (Personal Protective Equipment).
7. Release pressure safely after testing and avoid standing in the line of fire.

By implementing these recommendations and maintaining vigilance during pressure tests, we can significantly reduce the risk of accidents and enhance the overall safety of industrial operations. 

Safety is not just a priority but a necessity in ensuring the well-being of workers and the integrity of equipment.

We hope this blog post has shed light on the crucial aspects of pressure vessel testing and provided you with actionable strategies for avoiding common failures. 

Like Syed, we’re all about safety. If you’re looking for shielded enclosures for your testing procedures, contact us, and our engineering team will design the perfect shielding solution for your business applications.