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Pneumatic test vs. Hydrostatic Test: Differences and Safety Protocols

Apr 7, 2022 | Pressure Testing

Pressure testing procedures are key to ensuring the integrity of a wide variety of industrial components and equipment – including pipes, tanks, pressure vessels, fittings, and more. These tests are vital in both new component manufacturing, during routine maintenance, and quality verification of a component. 

Pressure testing can be done using two different methodologies:: hydrostatic and pneumatic. Hydrostatic tests utilize water as a medium, while the latter uses gas. There are important differences between both, most notably the level of energy released at failure. Let’s explore those differences and how we can keep your staff safe under both scenarios.

When to use pneumatic testing and hydrostatic testing

Hydrotest or hydrostatic testing is typically used for higher pressure components. Although water is most commonly used, other fluids such as oil or hydraulic fluid are sometimes used based on the materials under test and corrosion considerations.

On the other hand, pneumatic testing is regularly used for lower pressure components. Air is commonly used as the test medium, although another gas such as nitrogen can also be used in particular circumstances where low chemical reactivity is desired.

It is important to understand that water or oil are considered to be incompressible under the most reasonable pressures used in pressure testing. This is beneficial in that the test requires less energy to exert the required pressure than if a compressible medium is used. Less energy in means less energy out in the case of a failure

Why is pneumatic test more dangerous than hydrotest?

Because pneumatic tests use a compressible gas, more energy is required to reach the required pressure. This energy is stored in the compressed gas and can be released explosively on failure. For this reason, pneumatic testing represents a significant hazard above and beyond hydrostatic testing. How much of a hazard?

An explosion of 200 ft. of 36 in. pipe containing 500 psi of pressure can create a blast wave nearly equivalent to 80 lb of TNT.

Pneumatic testing is much more dangerous than hydrostatic testing. Pneumatic testing requires higher levels of training, a pressure relief device is mandatory, and a higher safety rating of an enclosure such as TotalShield’s Shielding Rooms or Machine Enclosure is needed.

Based on the higher risk associated with pneumatic pressure testing, should hydrostatic testing always be used? Well, not necessarily. Let’s look at some of the considerations.

Pneumatic Testing Advantages and Disadvantages

Graphic showing the advantages and disadvantages of pneumatic testing.

Hydrostatic Advantages and Disadvantages

Graphic showing the advantages and disadvantages of hydrostatic testing.

No matter which pressure testing process is used, the safety of your staff should be a top priority. Let’s discuss now how to ensure safety during both pressure testing processes. 

Guide to Pressure Testing Safety

Hydrostatic Testing Safety Procedures

In the case of hydrostatic pressure testing, there is a risk of injury from a high-pressure stream of liquid hitting the operator or the possible expulsion of a subcomponent such as a bolt or flange at high velocity. To prevent injury, all test components should be secured to avoid movement under pressure.

These are some hydrotest pressure safety protocols you should follow:

  • All fittings and connectors are installed and tensioned properly 
  • The test operator should wear proper safety equipment, including eye protection
  • When liquids other than water are used, additional safety and environmental factors must be observed
  • Unauthorized personnel should not be allowed in the testing area

TotalShield manufactures a range of pressure testing blast containment products to add a final level of protection for hydrostatic testing. We can provide blast and impact-resistant Shielding Rooms for testing on larger items or smaller benchtop mounted enclosures for small components. 

In either case, we work with the operator of the hydrostatic test to determine likely modes of failure and then calculate the energy associated with the ballistic impact of an ejected subcomponent. 

It is critical to analyze the size and mass of every component that might be ejected and then design an enclosure with a margin of safety to contain those ballistic objects. Our engineers can also work with your team to create inlet and outlet ports in the enclosure for test fittings such as fluid hoses and drains.

Pneumatic Testing Safety Procedures

As we’ve mentioned before, the risks associated with pneumatic testing are much greater than in a hydrostatic scenario. Like hydrostatic testing, there is the threat that an object will be ejected ballistically. There is also a risk of explosive blast. Put simply, an object under pneumatic pressure becomes a bomb, and at higher pressures, the explosive energies associated with a failure can be catastrophic. 

Remember, we are adding significant amounts of energy to the system to compress the gas enough to reach the desired pressure. Upon rupture, that energy is released. This can result in an explosive event that releases a pressure wave and shrapnel and can result in grievous injury or death.

TotalShield’s pressure testing blast containment enclosure for pneumatic pressure tests

All of the safety precautions mentioned above for hydrostatic testing also apply to pneumatic testing. In addition, a pressure relief device should be considered to any pneumatic testing. These devices can take many forms:  a simple plug designed to be ejected above a certain pressure, safety valves, or relief valves can be used.

The training of personnel is also vitally important in pneumatic testing. Only employees who have the required training and experience should be allowed to conduct pneumatic testing.

TotalShield’s shield pneumatic pressure testing enclosures are designed with a margin of safety for both ballistic failure and stored energy release in the pneumatic testing scenario. We work with your team to determine the total pressurized volume and total mass of the Unit Under Test (UUT) to ensure that you receive an enclosure certified to keep your staff safe in the event of failure.

Pneumatic and hydrostatic pressure test safety

Both pneumatic and hydrostatic testing play a vital role in manufacturing and maintenance. When done properly, these are safe activities. 

Our engineers can help you design a shielding room, shield pressure testing enclosure, or ballistic shielding blanket that will keep your people safe while doing pneumatic and hydrostatic testing. 

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