As we all know pressure vessels are used in industries like power generation, chemical, petroleum, food processing industries and soon, they are shell constructions that are fabricated from 1/2 '' or less steel plate material. During its course of period pressure vessels undergo various environmental, physical and chemical attacks. So many operators perform non destructive testing to know its durability.
Pressure vessels are humongous structures having more than one nozzle penetrating it through its head or shell. These nozzles are secured in place by fillet welding on both ID and OD surfaces of the unit. The desirable outcome driven vessel examination procedure includes testing circumferential welds, longitudinal welds and all the kinds of nozzle welds.
Apart from the visual examination of weld non destructive testing fluorescent magnetic particle technique is used by owners and operators. This examination enables us to identify surface and slightly subsurface indications in the material. Even though the viability of wet fluorescent magnetic particle is high it is not required by the original code construction. A more integrated result of examination is taken into account by evaluating the indications or cracks found during testing.
The indications found in the welds and plate material creates the confusion of what to do next. If we found repairing the cracks as relevant as the plate defects (such as laps, reduced minimum wall thickness), then for sure we can go for it. Frequent cracking in a weld is an obvious indicator of poor quality. Mostly as a solution for this, old weld is removed and a new weld is constituted instead of that.
Repairing the crack without knowing the actual cause acts like a short term solution that opens a pandoras box of problems. Let’s go through examples that shows how cracking pattern around the smaller nozzle in a pressure vessel can help to identify the root cause of the problems. These are not to be implied as the unilateral solution resulting from nozzle cracking.
Cracking due to external loads : Obnoxiously high nozzle loads that have crossed the foretold or designed nozzle loading is generally characterized by the appearance of stretch marks around the weld in the base metal. These cracks follow the periphery of the weld and mostly tear the base metal. The reason for this is higher tensile strength of weld filler metal than the base metal. The problem remains even weld repairs are performed in the same area.
The obvious reason for a high external load is the poor designing of support system. This is the resultant when loads are transferred from support system to the nozzles. These kind of loads can result from adding a piece of equipment to the nozzle or connecting piping without modifying the existing support arrangement. The high loads can result from a malfunctioning pipe support. The identification of the problem can be made by visual examination of the support system within the first 30 feet of the nozzle or pressure vessel.
In some cases many vessels faces thermal expansion because of the temperature increase. The thermal expansion increases the size of the vessels. Vessels that are supported and fixed at one end should have a sliding support on the other end that helps the vessel to slightly expand and contract. The fixed restraint results the piping being routed, (in a hi lo water level drain piping) through an undersized hole in the floor plate. Because the movement of the piping was cramped by the floor plate, eventually it caused the drain pipe to bend which caused recurrent cracking of the nozzle in the ID. In certain situations that resulted in akin crack pattern, restraints had been added to stand pipes, level transmitters, and chemical feed lines because of either vibration or long pipe connections to the vessel.
Cracking due to lack of penetration : If the weld filler metal deposit is not sufficient at the root of the joint it causes lack of penetration. The root of a nozzle joint is the interface between the nozzle wall and the shell or head. This type of cracking will generate through the weld in the same pattern as the root, but will break the surface of the weld in a radial direction around the nozzle if the crack come across a weld defect such as porosity, slag inclusions, or lack of fusion. This inclination change in the cracking can add confusion to the evaluation of the problem. The repair incorporates of removing all of the fillet weld and preparing the area for welding with a small diameter welding rod such as a 3/32” diameter rod.
Cracking due to Chemical Attack : Mostly Heat affected zones are prone to chemical attack of the weld usually affects the toe of the weld. The attack specifically happen to this are because it’s lightly a different form of micro structure. Generally it appears circumstantially around the toe of the weld or over the outer diameter of the weld. As it is chemical attack can be identified in the specific area like above or below the liquid level. However the pattern of cracking is different from that of fatigue (in fatigue the cracking pattern is in specific location and have recurring tensile load). The best solution for this is repair of the weld and adjustments to the chemical input.
The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.