Making Safety Pressure Vessels For Industrial Systems

Have you ever attended a children's party where the balloons started to burst when they were pricked? Sometimes, balloons begin to burst even while they are being filled up. They created a loud noise don't they? And the remains of the balloon simply flew away in any direction.

But balloons contain only low pressure air or gas. How about those huge pressure vessels we see in the factories and other industrial plants? How can we be sure that they will not burst like balloons?

Industrial processes often use pressure vessels at much higher pressures. In compressed air systems, air reservoirs are utilized to store compressed air in large enough volumes so that the pressures in the system do not fluctuate rapidly as the air is consumed. Industrial gases are often stored in gas cylinders at extremely high pressures.

It would be a disaster if any one of these cylinders were to burst or explode. Injury or loss of life may result from such an accident. It is such a safety concern that many government agencies insist that such pressure vessels be inspected regularly. This is because the environment in which the pressure vessel is operated can play a major part in the deterioration in strength of the vessel. Corrosion is a main culprit for the weakening of material of the pressurized vessel.

In order to ensure that pressure vessels are safe for use in the intended service, design engineers usually add a factor of safety for deterioration in their design. The intended service may include parameters like working temperatures, working pressures, type of fluid, weather, vibration and environment. However, they will also have to look at various other aspects like the material used, the welding preparation, and the additional strengthening arrangements in the weak points like openings, the presence of drainage for condensed water, and other structural strength support for the weights. Usually, they will be following some engineering codes and standards like ASME, JIS, BS, etc.

When the vessel is ready to be fabricated, it goes through a sequential series of checks.

Firstly, the steel plates are identified. The steel plates from the steel manufacturers are usually tested for various chemical composition and mechanical strengths. Each steel plate will be identified by their markings and accompanied by their mill certificates.

Once they have been identified, the plates are cut to the sizes required and formed by rolling. The edges of the plates are also prepared for welding by grinding to the required angle.

The welding process will also need to be standardized. Usually, the manufacturer of these pressure vessels will have done tests for the welding positions, electrodes, welding procedure qualifications, welding performance qualifications, welding procedure specifications in their quality control system. Various mechanical tests like tensile and charpy tests should be done during these standardizations.

Non-destructive examinations are carried out on various welding joints to determine the quality of the welds. Some to the methods used are radiography, ultrasonic, magnetic particles, and dye penetrant. In addition, visual examinations are also carried out.

The final test to determine whether the vessel can withstand the required pressure is called the hydrostatic test. This test is conducted by filling the vessel completely with water and pressurizing it to usually 1.3 or 1.5 times the maximum allowable working pressure and holding it for at least 1 hour. This is a destructive test. If the vessel cannot withstand this pressure, it will break. But because water does not reduce its volume appreciably under pressure, it does not expand much too when it leaks out. The failed vessel does not burst out with a destructive force like a balloon.

If the vessel can withstand the hydrostatic test, it is considered to have fulfilled the final manufacturing requirement. The water is then drained off, the vessel dried up and several protective paint coatings can be applied to its surfaces both internally and externally.

The pressure vessel is then shipped out to the buyer who can rest assured that the pressure vessel will not burst like a balloon provided it is used according to the designed conditions.