壓力容器焊接需要考慮的因素

In many industries, liquids and gases must be stored or transported in a container. However, simply filling the containers up to a normal atmospheric pressure would be highly inefficient. Also, certain substances that are gases at normal pressures become more transportable liquids at higher pressures, or where delivery of the container contents needs to occur at high velocities. As such, these containers need to be able to store significant potential energy.

Because of these potential efficiency gains and the need for stored energy, many containers storing liquids and gases are pressurized. When a container has a liquid or gas stored under pressure inside of it, it is known as a pressure vessel.

While pressure vessels are extremely useful, they also come with potential hazards. Overpressurization can result in a pressure vessel completely rupturing. A leak, even a small one, can cause the pressurized contents to spill out at a rapid rate. A quick change in the pressure of a volatile substance caused by a ruptured or leaky pressure vessel can result in fire or an explosion. For these reasons, pressure vessels must be constructed with the utmost precision, quality and craftsmanship. Welding is a key process in the construction and maintenance of many pressure vessels and knowing the special requirements for welding a pressure vessel is paramount to ensuring safe pressure vessel operation.

Pressure vessel design

Pressure vessels come in many different shapes and sizes. Some pressure vessels are static tanks meant only for storage, while others are meant to be picked up and transported. Other pressure vessels are pressurized systems such as pipelines.

Common characteristics of pressure vessels include a main body, which is most commonly cylindrical, and a variety of inlets and outlets attached to it. Many times, these inlets and outlets must be welded to the main pressure body because they cannot be attached to it in the initial forming process. Depending on how the pressure vessel was formed, the main body itself may need to be welded.

One common type of pressure vessel weld is a seam weld, which is needed after the pressure vessel body material has been rolled to create a tube shape. Another common set of welds are end cap welds. These are the welds that plug on each side of the tube to create a cylinder capable of containing liquids or gases. Finally, welds for the various inlets, outlets and safety valves are commonly required as well.

Welding considerations

As a result of the safety risks inherent in storing a gas or liquid under pressure, pressure vessel construction must generally adhere to strict construction codes to ensure that quality is preserved and danger to people and the environment is mitigated. While many codes exist, the code that must be followed is dependent on the country, industry and end user that the pressure vessel will be used for.

Figure 1: A typical pressure vessel.In the United States, a code that is frequently considered mandatory is the American Society of Mechanical Engineers’ (ASME’s) Boiler and Pressure Vessel Code (BPVC). The code has many sections, and the section that specifies qualified welding procedures is ASME BPVC Section IX. In general, this is perhaps the most important pressure vessel welding code in the U.S. Welding materials, joints, processes and operators must all be qualified to the pressure vessel welding code. Each pressure vessel application is different, and it is important to understand how to use the code to ensure that all aspects of the welding process are qualified.

Prior to welding, it is critical that the correct joint preparation is used. Materials should be cleaned beforehand so that no contaminants make their way into the weld pool. For thicker sections, weld joint preparation such as beveling should be used to ensure that proper weld penetration can be achieved. Depending on the material and the thickness of the material, preheating may be required to prevent hydrogen cracking. The specific inputs for preheating, joint preparation, fit-up and welding filler material selection should follow the pressure vessel welding code and be recorded in a procedure qualification record (PQR) that will be later referenced in a qualified welding procedure specification (WPS).

Welding process is another key consideration when determining how a pressure vessel will be welded. Common processes include gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), submerged arc welding (SAW), flux cored arc welding (FCAW) and laser beam welding (LBW), among others. Many more can be used so long as they can be qualified to ASME standards. Each process comes with its own sets of advantages and disadvantages that must be weighed from a quality and economic standpoint. In addition to having a qualified welding procedure specification, a qualified welder is required to carry out the welding. The process for qualifying a pressure vessel welder can also be found in ASME Section IX.

Following the welding procedure, examination of the weld must be performed. This typically includes a visual inspection by a qualified welding inspector as well as other types of non-destructive evaluation. Other common forms of non-destructive testing for pressure vessel welding include X-ray (RT), dye penetrant testing (PT), ultrasonic testing (UT) and magnetic particle testing (MT). In some instances, especially for initial welding procedure or welder qualification, destructive samples for tensile testing, toughness testing, hardness testing and others must be used to ensure that a qualified welding procedure specification is being used.

Safety considerations

Welding a pressure vessel can be extremely dangerous work. Some pressure vessels are welded from the inside. This is typically considered a confined space, so proper ventilation and other safety considerations must be followed when welding inside a pressure vessel.

Pressure vessels made from materials that emit carcinogens during the welding process also require special ventilation equipment. Many pressure vessels are extremely heavy and may be quite tall. Because of this, proper harnessing for both the vessel and the welder must be provided when setting up a pressure vessel welding procedure.