🧐 What essential information should be understood regarding Pneumatic Testing?

UG-100(a):  Pneumatic Test :

When we may opt for Pneumatic testing?

• Vessels that are so designed and/or supported that they cannot safely be filled with water

When considering the criteria for pneumatic testing, certain factors are important to note. Opting for this test instead of the more common hydro test is recommended when liquid residue cannot be sustained within the pressure vessel due to its construction. Like, If the vessel contains numerous internals that are difficult to dry completely, pneumatic testing is usually preferred. Furthermore, if the vessel is designed for a non-liquid application and requires supports specifically designed for that purpose, choosing pneumatic testing can prevent undue strain on the supports, ensuring they bear the load they were originally designed for. Thus, the pneumatic test serves as a suitable alternative in these scenarios.

• In vessels where traces of the testing liquid cannot be tolerated

If we are designing a pressure vessel in which even small traces of water remaining after hydro test can be dangerous may due to reaction between operating fluid and water or due to some other reasons.

UG-100(a): Examination before Pneumatic

Full length of following weld shall be examined (UW-50) by either LPE or MPE:

• UW-50(a) (1):   All welds around openings

• UW-50(a) (2):   All attachment welds having Throat thickness >1/4 in. (6 mm), including welds attaching non pressure parts to pressure parts

Prior to conducting the pneumatic test, it is necessary to carry out specific examinations based on UW-50, such as LPE (Liquid Penetrant Examination) or MPE (Magnetic Particle Examination). In terms of surface examination, magnetic particle examination is superior to LPE but it can be done only on ferrous material.

It is essential to perform these examinations as a prerequisite for proceeding with the pneumatic test. These examinations encompass various types of welds, including those around openings, as well as welds on attachments, even if they are non-pressure parts attached to pressure parts, such as lugs. LPE or MPE must be employed to examine all these welds thoroughly.

There are specific exemptions applicable to certain vessels that allow for the waiver of examinations.

UG-100(b): Test Pressure

Instead of using the multiplier of 1.3, as done in the hydrostatic test; the pneumatic test pressure is determined by multiplier of 1.1.

Which test we should use? Pneumatic or Hydro?

The decision between pneumatic and hydro testing arises frequently. In pneumatic testing, the applied pressure is lower, compared to hydro testing. The lower pressure requirement in pneumatic testing is favourable. Also, Pneumatic test eliminates water wastage.

Thus, at first glance, pneumatic testing seems like a better option.

However, you might wonder why we generally choose hydro testing over pneumatic testing if water is readily available. The reason is that, water being a liquid, is not as compressible as gas. When a small amount of water is injected into a vessel filled with water, the pressure increases immediately due to the limited compressibility of the liquid. Similarly, if there is any leakage, even a small amount of water, the pressure will drop rapidly. This immediate response allows for quick detection of pressure fluctuations.

In pneumatic testing, the gas used is highly compressible, which means it can store a significant amount of energy when compressed. This high energy storage makes pneumatic testing potentially dangerous, similar to a bomb with substantial stored energy. Consequently, extreme caution must be exercised during pneumatic testing due to the inherent risks involved.

If a leak or crack occurs during pneumatic testing, the pressure does not immediately drop like it does in hydrostatic testing. Since gas is involved, the pressure decreases gradually over time. The consequence of this behaviour is that if a leak or crack is present during pneumatic testing and the pressure is does not decrease immediately. This situation can lead to a catastrophic failure of the vessel before the pressure is reduced. This inherent characteristic of pneumatic testing highlights its potential danger.

In contrast, hydrostatic testing involving water would result in an immediate pressure drop if there is a small leakage from a crack. The pressure would quickly decrease, preventing further escalation or complete failure of the vessel.

These distinctions emphasize the importance of understanding the risks associated with pneumatic testing and why precautions are crucial. Hydrostatic testing provides a safer alternative due to the immediate response of water in the event of a leak or crack.

Comparison

Thank you…

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