The Pneumatic Pressure Testing Handbook [Part 1]

At TotalShield, we are committed to providing our customers with the best blast shielding solution by conducting thorough research, tests, and gathering information. This time, we've teamed up with Karl Kolmetz, a Technical Engineering Professional and an expert in engineering documentation, to develop a comprehensive guide on pneumatic pressure testing. 

Goto MetalsKingdom to know more.

Throughout a three-part blog series, you will gain a comprehensive understanding of this pressure test method, including procedures, challenges, safety requirements, and best practices.

Let's begin with an exploration of pressure testing methods.

TABLE OF CONTENTS

1. Pressure testing methods
2. Comparison of hydrostatic and pneumatic testing
3. General requirements of pressure testing procedures
4. When to use pneumatic testing 
5. Benefits of pneumatic testing
6. Limitations of pneumatic testing
7. Glossary & References

Pressure Testing Methods

Across different industries, equipment such as pressure vessels, heat exchangers, columns, pipelines, gas cylinders, fuel tanks, etc., need to be tested for leak tightness at various stages, such as: 

1. completion of fabrication
2. before commissioning
3. at regular intervals during plant operation to ensure adherence to Statutory regulations and Safe Operation. 

Pressure tests are a nondestructive and reliable way needed to ensure equipment safety, reliability, and leak tightness. They are required before utilizing newly installed and recently repaired pressure systems, helping to understand their limits and capabilities, which is crucial to know before putting them into service. Of course, they also help to prove that the equipment meets industry qualifications and requirements.

There are two commonly employed pressure testing methods: hydrostatic and pneumatic. A hydrostatic test uses water as the test medium, whereas a pneumatic test uses air, nitrogen, or any non-flammable and non-toxic gas. 

In general, pressure testing introduces hazards that must be identified and understood so that appropriate measures can be considered to manage the risk of a potential failure. 

Comparison of hydrostatic and pneumatic testing 

Pneumatic tests are widely used to minimize downtime and testing costs while providing convenience compared to hydrostatic tests. They help detect very fine leak paths that may not be found in hydrostatic tests. However, pneumatic testing is inherently more hazardous than hydrostatic testing in the same volume, pressure, and temperature conditions.

While pressure tests are mandatory, it's important to emphasize that pneumatic testing is not the first option. Most services will attempt a standard hydrostatic test before considering pneumatic testing. Both are viable options, but as mentioned before, pneumatic testing is potentially more dangerous. 

Compressed air or nitrogen can contain 200 times more stored energy for the same free volume and pressure conditions than water used in hydrostatic tests. With much higher amounts of stored energy, it is more likely to cause damage if mishandled. This is why, although pneumatic testing is convenient and more accurate, the industry requires hydrostatic testing to be considered beforehand. 

Pneumatic testing is mainly recommended only for equipment already tested and proved safe by hydrostatic pressure tests and is preferably done only for low-pressure applications and vessels having low-volume capacity.

In specific scenarios, though, pneumatic testing becomes the only available option. An experienced service will follow all guidelines and ensure that the equipment is not damaged.

Below, you can find a comparison of these test methods.

General requirements of pressure testing procedures

1. Stress exceeding yield strength. The test pressure may be reduced to the maximum pressure that will not exceed the yield strength at test temperature.
2. Test fluid expansion. If the test pressure is to be maintained for a period of time and the fluid in the system is subject to thermal expansion, precautions shall be taken to avoid excessive pressure.
3. Preliminary pneumatic test. A preliminary test using air at no more than 170 kPa (25 psi) gage pressure may be made before hydrostatic or pneumatic testing to locate significant leaks.
4. Examination for leaks. A leak test shall be maintained for at least 10 minutes, and all joints and connections shall be examined for leaks.
5. Heat treatment: Leak tests shall be conducted after any heat treatment has been completed.
6. Low-test temperature. The possibility of brittle fracture shall be considered when conducting leak tests at metal temperatures near the ductile-brittle transition temperature.
7. Personnel protection. Suitable precautions in the event of piping system rupture shall be taken to eliminate hazards to personnel near the lines being tested.
8. Repairs or additions after leak testing. If repairs or additions are made after the leak test, the affected piping shall be retested.
9. Test records. Records shall be made of each piping system during the testing, including:
   • Date of test
   • Identification of piping system tested
   • Test fluid
   • Test pressure
   • Certification of results by examiner

All pressure tests must be conducted using a gauge that has been calibrated within the previous 12 months. The gauge should be sized so that the test pressure is in the middle third of its pressure range. Gauge materials and fluids must be compatible with the test fluid. 

When possible, the use of blind/blank flanges or caps should be considered for test boundaries to prevent damage to valves. Pressure tests must always be performed under controlled conditions, following an approved test plan, and documented in a test record. A single approved test plan may be used for several similar tests, but a separate test record is required for each.

A pressure test plan should, at a minimum, include the following formation: 

• Approved pressure test plan form. 
• Drawings of the system being tested. This includes identifying the location of the test setup and test boundaries, as well as all blank/blind flange locations, if applicable. 
• Drawing showing the exclusion zone with the location of signage, barricades, or other controls. 
• Details of the test setup, including the pressure ratings of all components and the pressure relief valve setting. If needed, provide product data sheets. 
• Pressure gauge calibration sheet. 
• Detailed test procedure.
  

When to use pneumatic testing

Pneumatic testing is recommended when the vessels are designed and/or supported so that they cannot be safely filled with water, i.e., refrigerant systems; not readily dried, or used in services where traces of the testing liquid cannot be tolerated.

To put it simply, pneumatic testing is used when:

• Pressure systems are designed so that they cannot be filled with water
• Traces of water cannot be tolerated when the system is in service

For piping systems that transport primarily gas, like natural gas pipelines, pneumatic testing would also be used. Water or any other liquid would be too heavy and potentially damage the pipelines from their weight.

But, as mentioned before, a leak or sudden collapse of pressure systems can cause tremendous financial damage. That's why it should get a pneumatic test done at least once a year. 

Keep in mind that only a pneumatic or hydrostatic test is required'not both.

The following systems may be considered for this pressure test method:

• Relief or flare systems outside the plant area.
• Piping with internal linings, which may be subject to damage by the hydrotest fluid.
• Piping systems in which moisture is undesirable or cannot be tolerated, such as in instrument air systems or refrigerant systems.
• Large bore piping carrying gas (for example, flare gas) over long runs for which the supporting structure is not designed to handle hydrostatic test loads.

Benefits of Pneumatic Testing

Specific benefits of pneumatic testing should be brought to attention.

• More accurate at detecting leaks. The small atomic structure of gases 'particularly helium' allows them to pass through leaks that liquid cannot. Paired with mass spectrometry, it's easy to tell if gases have leaked from the pressure system.
• No water damage. There's no need to worry about the weight of water collapsing the structure of the pressure system.
• Easy to clean

Accuracy is especially important if piping or other pressure systems are sensitive to leaks. Pinpointing the location of leaks can prevent catastrophic damage before it occurs.

Limitations of Pneumatic Testing

Working with gases is the main cause of limitations in pneumatic testing. 

If anything were to go wrong, compressed gases would have more stored energy than liquid and volatile gases. If an old piping system collapses during the pneumatic test, the energy is released, which may cause fatal damage.

Because of this intensity, consider the following limitations of pneumatic pressure testing:

• Recommended only for low-pressure applications.
• Restrict where chances of equipment or pipe failure are high.
• Only small segments of the piping can be tested at a time.
• The damage tends to be extensive if handled improperly.
• Must be conducted by an experienced service or personnel ' This is not a suggestion.
• Needs special attention and safety regulations: shielding barriers must be installed, and people cannot be working during the entirety of testing.

A standard pneumatic test procedure for pressure piping systems may be used with the following limitations:

1. The stored energy value will not exceed kJ,
2. The test medium is air or nitrogen,
3. Testing will be conducted at a temperature at least 17 °C (30 °F) above the piping system design minimum temperature and
   Note: If the design minimum temperature is not specified, then the owner or their designee must establish the minimum test temperature, but in any case, the testing shall not be conducted at a temperature lower than 16 °C (61 °F),
4. Refer to the appropriate code of construction for possible additional requirements.

Before a pneumatic test can be carried out, the service will need a written justification for the pneumatic testing along with a piping schematic. While the service will handle the schematic and other documentation, it may delay the speed at which pressure systems can be tested.

For more information, please visit Pressure Cycle Test Bench.

In the next blog post, we'll cover all the details about the pneumatic testing procedure, its preparations, reports and records, and more. 

Whether you're new to pressure testing or looking to expand your expertise, you'll learn valuable insights to understanding and applying these crucial procedures.