1. Introduction
There are many types of a tube to #tubesheet joints, however, in this article, we focus on the challenges associated with a tube to tubesheet full penetration joints of a superheaters and recovery units in the chemical and petrochemical industry.
The employment of full penetration joint contrary to filled weld or expanded is led by the need for a robust and gapless joint. The purpose of gapless joint it to avoid any gaps where crevice corrosion could take place.
Heat exchangers operating in the chemical and petrochemical industry are subjected to high temperature and pressure and often operate at very high hydrogen partial pressure, therefore, Cr-Mo steel grades 1.25Cr0.5Mo and 2.25Cr1Mo are often specified.
The number of holes in the tubesheet, thereby, of joints can range from hundreds up to few thousands. It should be emphasized that even if the only single joint fails it can interrupt the operation of an entire plant. Therefore, these type of welds has to be made with care and attention to details.
Comments regarding the #PWHT and hardness problems listed in the article are common for all welded types of joints that are subjected to hydrogen service.
2. Tube to tubesheet gapless full penetration joint configuration
Full penetration joints are located at the back of the tubesheet and has to be welded with a fully automated welding machines.
Figure 1. Tube-to-tubesheet joint configuration for internal bore welding behind the tubesheet.
Figure 2. Tube to tubesheet weld - seen from the back - root pass
3. Challenges with fabrication
3.1 Restricted accessibility to the weld area
- Non Destructive Testing (NDT)
One of the main hindrances are limited capabilities of performing thorough NDT examination, thus a quality of the joints are relying on thorough preparation, close tolerances, plenty of mockups and welding with fully automatic processes.
Although to some extend radiography examination is possible, but the execution is very challenging, mainly because examination cannot be performed after entire tubesheet is complete.
The examination has to follow the production sequence each joint has to be inspected before next row of tubes is welded. Moreover, Cr-Mo grades require preheating that can reach 200 deg C this can be another issue.
3.2. Welding execution
Due to the mentioned examination limitation welding approach aims to transfer the adequate result from the mockup to the production joint in the tubesheet. Welding parameters, required quality and reproducibility have to be well established prior to welding activities on the tubesheet. Futheremore, welding at the back of the tubesheet and thus demand to the reproducibility requires much higher geometrical tolerances.
3.3. Heat treatment - Essential for hydrogen service
Cr-Mo steels are air hardened grades, means that after welding weldments have to be heat treated (PWHT) after completion.
PWHT is considered as one of the essential steps to guarantee safe operation in the hydrogen service. Too high hardness in the weldment sooner or later will lead to a fracture and leakages.
Hardness test is a very simple examination to verify whether the PWHT was correctly executed, but it should be emphasized that it is not possible to measure hardness in the tube to tubesheet joint proximity. Therefore, even though the weld can be perfectly executed with the state of the art welding machines, inadequate PWHT can ruin all efforts.
During standard in furnace PWHT, heat is transferred from the outside of the vessel to center of the heat exchanger. Further considering the tube to tubesheet joint particularly at the center of the tubesheet can reach the required soaking time as last.
Means that if the heat treatment process is not controlled adequately, e.g thermocouples located far from the tube to tubesheet joint can result in a situation were central are of the tubesheet and thus joints are not sufficiently heat treated.
Too short time or too low soaking temperature might not be sufficient to decrease the hardness of the weldment to the required levels for the hydrogen service. This scenario will result in cracking in the weldment during the operation.
To avoid this problem PWHT controlling thermocouples must be located in the close vicinity of the tube to tubesheet joints.
4. In service repair
Repair of the tube to tubesheet joint is extremely difficult.
First of all, because of the limited accessibility to the weld area consequently often closing a hole with a plug approach is an only option.
Secondly, after a repair, a PWHT has to be performed to operate to decrease hardness to the limits required for hydrogen service.
It should be noted that PWHT cannot be performed only on the small part of the tubesheet, that will result in a significant residual stresses in the nearby region and thus cracking during the operation. Rather a complete tube to tubesheet has to be subjected to the PWHT cycle.
That brings another challenge, the material can only take few PWHT cycles, means that reappearing problems can be repaired only a few times after that material will lose its strength.
A replacement of unrepairable heat exchanger would be the most reasonable approach, but if a spare one is available, the fabrication of a new one can take more than a year.
5. Summary:
Taking all of the above facts into consideration welding of the gapless tube to tubesheet joints can be challenging, however, with the appropriate preparation, care and attention to detail it is possible to achieve good quality and satisfactory results.
We want to emphasize that PWHT is crucial particularly for the hydrogen service. And since there is lack of access to perform a hardness test, PWHT must be under a special control and thermocouples must be located with careful consideration, as close as possible to the tube to tube sheet joint.