Proof pressure testing your process pipes and other components of your pressurized vessel is critical to keeping everyone in your workspace safe. Without this step, you risk installing materials that can’t withstand the pressure levels you need, which could lead to the materials warping or, even worse, bursting.
Proof pressure testing is one of the many services that Precision Fabrication & Cleaning (PFC) offers for cleaning and testing facilities. In this article, we’ll guide you through the three different types of pressure, what proof pressure testing is, and the two main methods of proof pressure testing, to show you what this process entails and how important it is for your safety.
There are three main types of pressure that a material or item, such as process piping, should be tested for.
The working pressure refers to the pressure of the item in the working conditions range, which is normally called the pressure range.
Burst pressure is the maximum pressure that can be applied to the item without physical damage. It can also be defined as the maximum pressure an item can withstand before breaking. When the pressure exceeds the burst pressure, the item will explode.
Proof pressure refers to the maximum pressure that may be applied to the pipe fittings or flanges without changing its quality.
As evidenced by its name, proof pressure testing is the process of testing an item, such as piping, for its proof pressure. This type of test is non-destructive; the point is not to find out at what pressure the item will burst, but at what pressure the item will start to deform.
During the test, the item is exposed to various pressure levels under controlled conditions. This procedure tests to a certain level rather than failure. In a successful proof pressure test, the item endures pressures above normal operating conditions without bursting, deforming, or leaking.
At PFC, we adhere to an industry standard of proof test pressures for piping called IAW ASME B31.3. This helps us ensure that our proof pressure tests are successful.
For example, IAW ASME B31.3 requires that in section 345.5.4 that,
“The test pressure shall be not less than 1.1 times the design pressure and shall not exceed the lesser of
- 1.33 times the design pressures
- the pressure that would exceed 90% of the pressure described in para. 345.2.1 (a)”
In addition, section 345.4.1 requires that “the hydrostatic test pressure at every point in a metallic piping system shall be as follows:
- not less than 1.5 times the design pressure.”
There are two main types of proof pressure testing: hydrostatic and pneumatic.
Hydrostatic testing is the most common method employed for proof testing.
A hydrostatic test uses water as the test medium and tests an item for strength and leaky spots. The test involves filling the item with water and pressurizing it to the specified test pressure. At PFC, we use a top-of-the-line Sprague air-driven pump to deliver the high-pressure liquids required for hydrostatic testing. We test pressure tightness by shutting off the supply valve and observing whether there is a pressure loss. Strength is usually tested by measuring permanent deformation in the test item.
Furthermore, there are two methods of hydrostatic testing.
- The Water Jacket Method
The Water Jacket Method involves filling the vessel with water and loading it into a “test jacket,” a sealed chamber filled with water. Next, the vessel is pressurized inside the test jacket for a set period of time. As a result, the vessel will expand within the test jacket, forcing water out of the test jacket and into a glass tube, which measures precisely how much the vessel expanded.
The vessel is then deflated, returning it to its original size. As it shrinks, the water flows back into the test jacket. In some cases, the vessel will only partially deflate, and will not go back to its original size. Its new size is called a permanent expansion.
Depending on the difference between the total expansion and the permanent expansion, the vessel may or may not be considered fit for service.
- Direct Expansion Method
The direct expansion method involves filling a vessel with a specific amount of water and pressurizing the system. The pressure is then released, and the amount of water that leaves the vessel is measured. The amount of water that leaves the vessel is then compared to the amount of water originally forced into the vessel, and the test pressure is also considered.
A pneumatic test uses air, such as nitrogen or any other form of non-toxic and non-flammable gas as the test medium.
This type of proof pressure test is considered more dangerous than the hydrostatic version. The reason for this is it involves storing a large amount of compressed gas, which creates a lot of pent-up energy. Therefore, there’s a bigger danger that the system will rupture and cause serious injuries in humans, as well as significant damage to any equipment located nearby.
For example, an explosion of 200 feet of 36 inch-pipe containing 500 psi can create a blast wave nearly equivalent to 80 pounds of dynamite.
We use pneumatic testing at PFC, but we do so in a carefully controlled environment and utilizing high quality equipment. This is why it’s important for you to hire a professional testing and cleaning service like PFC, to ensure that proper testing equipment, procedures, and standards are adhered to so that safety is never compromised.
At PFC, we’re dedicated to making sure that our proof pressure tests are performed safely and successfully. By proof pressure testing your process piping and other parts of your high-pressure system, you’ll gain peace of mind for the future safety of your team members and the long-term functionality of your pressurized vessel.
Contact us today!