Nozzles are a vital part of pressure vessels, it is pivotal in connecting the rest of the vessel to the process. The secret formula that ensures the connection of nozzles and pipelines is to use standard flanges. An ideal pressure vessel must be equipped with nozzles and other connections. Nozzles can only be added to the vessel by making holes, this may create stress paths tangentially. In order to counter the removed area, reinforcements should be added.
Attachment of nozzle to a pressure vessel involves localised increase in stress or geometric discontinuity that should be taken note during calculations. We can employ so many practices to achieve this. The most common is by removing stress concentrators by rounding out the sharp edges.
Through this article we can discuss about flanged nozzles that are more prone to fluid leakage than welded nozzles. A flanged nozzle constitutes three elements: flange, reinforcement and neck. The figure given below gives different nozzle configurations.
Mostly nozzle configuration will be having a reinforcing plate or pad. But in cases like high pressure, high temperature or high thermal gradient services it is obliged to avoid unnecessary parts of nozzle as per ASME standards. Nowadays reinforcing pad is replaced by a self-reinforced-nozzle. These kind of self-reinforced nozzle can be designed with varying thickness hub which is more expensive. While another kind is known as straight hub which uses long welding neck. Moreover, if there are vibrations affecting the nozzle, a one-piece configuration should be considered if the equipment in under cyclic service and the amount of cycles is high or, even (built-up nozzle would not be allowable).
Apart from using a reinforcement plate or pad, there are two ways through which we can attach nozzle with the shell.
1. Set-on: The inner diameter of the neck and the diameter opening the shell compliments to each other. Here the nozzle is supported by the shell.
2. Set-in: The outer diameter of the neck and the diameter of the opening in the shell compliments to each other. The nozzle gets through shell.
In Set-on, the bevel is performed in the nozzle neck, in Set-in, the bevel is performed in the shell or head. Set on is mostly welcomed by manufacturers as it only requires very less bevels to prepare and less thickness of weld.Set-on nozzles are probably preferred by customers, but largely it is taken when vessel thickness is high regarding the neck of the nozzle. Especially this is done when there is no special services are considered.
Aforementioned, when designing a nozzle in an optimal way more care should be given on maximum allowable loads. These kinds of nozzles hardly fail under external loads. If this is the case the thickness needs to be increased, the problem comes with the shell.
Types of flanges used in industry of pressure vessel are:
Slip-on: This kind of flange is only desirable for less critical services. Here the nozzle neck and the flange are joined using fillet weld.
Welding Neck: Here Butt weld joint is used. The nozzle neck is welded to the flange using a butt weld joint
Socket Welding: The nozzle neck is inserted up to flange limit.
Lapped: Nozzle neck projects the flange and spreads over it.
Threaded: Neck and flange are assembled by means of thread.
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