Hazards from overpressure from a ruptured vessel or piping system

The atmospheric boiling point of LNG is approximately -160°C, any residual water left in the equipment, such as by hydraulic pressure testing, is undesirable. Pneumatic pressure testing is therefore frequently used for LNG and other piping and vessels in which moisture must be avoided. As mentioned, the stored energy of compressed gas is very high, so rupture of a piping test system during a pneumatic pressure test can release much energy. Damage due to rupture can result from shock waves, flying projectile fragments from the ruptured piping, and unrestrained movement of piping and equipment propelled by escaping gas. In fact, the cryogenic gas industries have experienced pneumatic pressure testing incidents in the past, sometimes resulting in serious injuries and major equipment damage.

 

To address the risks associated with pneumatic pressure testing, many companies attempt to limit the amount of stored energy in the test system to a prescribed maximum value by limiting the size of each test system. This approach is often impractical for high pressure piping of typical diameters because of the severe limitation that it places on the size of each test system. Consequently, this approach may result in an unreasonably large number of test systems. Attempting to isolate and test a large number of test systems may prove to be impractical. When the approach of limiting the amount of stored energy becomes impractical, an alternative approach, such as that described herein, may offer the best option. Regardless of which approach is taken, many of the considerations outlined in this article must be taken into account to safely execute high-pressure, pneumatic pressure testing.

A variety of measures can increase the safety of pneumatic testing. Of first importance are measures to ensure the mechanical integrity of the vessels and piping systems undergoing testing. These measures include design, fabrication, and inspection methods.

It is also necessary to bar personnel from exclusion zones (an area where personnel are prohibited to enter) surrounding the vessel or piping system under test, and to conduct tests at night or on weekends when few people are in the vicinity of the test site.

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