Considering the diameter and length ratio, AAC autoclaves are considered as long-length autoclaves in comparison with other industries in which autoclaves are used. In many AAC projects, the most standard autoclave diameter is 2.9 meters and the standard length is from 37 meters to 55 meters.

Due to the length of AAC autoclaves, special attention should be paid to the saddle design of the autoclaves. Wrongly designed saddles have a direct impact on the lifetime and safe use of AAC autoclaves. The manufacturing of the saddles and the placement on the foundation are utmost critical aspects of AAC autoclaves. As important as the in-factory manufacturing process, including the condensed water flow inclination, necessary checks should be done when placing the autoclave on the foundation at the AAC site. One of the most common issues AAC producers face is the lack of maintenance and observation for the discharge of condensed water. It is common to see the replacement of brand-new autoclaves after a few years of operation because of over-corrosion due to the failure to remove the condensed water.

For a typical AAC production process, a single cycle takes approximately 12 hours. As the autoclaves are high pressure equipment, the manufacturing must meet harmonized pressure vessel standards and regulations. While some countries have their own pressure equipment standards, or follow an accepted standard such as CE, the most well-known standards are 2014/68/EU (CE-Europe), ASME Sec VIII Div.1 - U stamp (USA) and EAC (Russia). 

The safety of AAC autoclaves starts with the design and manufacturing phase. The AAC plant investors and existing owners should consider purchasing autoclaves from reputable autoclave manufacturers with good AAC plant knowledge and experience. Due to the size and high investment costs, the mistakes made during the manufacturing stage are extremely difficult to fix at the site after the assembly and start of operations.

As periodic checks and maintenance of AAC autoclaves are crucial, it is a known fact that accidents, resulting in loss of lives, have occurred in autoclaves without proper maintenance. It is also worth noting that these accidents can easily be prevented by investing in autoclaves manufactured according to standards, and operated by trained and authorized personnel who are aware of the risks, as well as following more frequent maintenance cycles. The operators should be trained in running pressure vessels according to the pressure vessel regulations.

As a leading complete AAC autoclave and bayonet closure door manufacturer in Europe, Akarmak suggests the checks and controls for AAC autoclaves at the following stages:

1)     Manufacturing

2)     Assembly and Start Up

3)     During Operation

4)     After Repair and Modification

Inspection During Manufacturing

In case the bayonet door and final autoclave shell are manufactured by different companies, the integration of the door and autoclave vessel is very critical. The door must be properly installed at the vessel and the required checks must be done during manufacturing.

The AAC plant customers should require certifications from the bayonet door manufacturer for the design, and manufacturing certificate showing compliance with the working parameters (i.e.: pressure, temperature). The manufacturer should have its name-tag on the door. The doors without pressure vessel certifications and name-tag should not be accepted by the customer.

The bayonet doors must provide safe use. The second-hand doors used under different plant conditions and working parameters should be carefully examined before being installed in a new vessel. A door without recorded previous working conditions should not be used.

For the autoclave vessel, the connections of the metal sheet should follow the (Pmax= t/3 mm) ratio required by the standards. Failure to meet the ratio will cause unwanted aggressive stress resulting in welding cracks.

The alignment of the vessel segments during manufacturing and welding process is critical and should be within limits. Misalignment will cause distortion on the vessel resulting in the application of additional forces and extra stress on some areas of welded parts.

The autoclave vessel welding must be checked using Non-Destructive Tests (NDT) to avoid potential welding cracks during the welding process.

Akarmak also recommends that AAC autoclave investors assign technical experts in pressure vessels to be in close contact with the autoclave manufacturer to confirm the quality of the work being carried out. The expert party would also be a good bridge between the investor and autoclave manufacturer as many customers are not technical experts in autoclaves.

Checks During Assembly and Start Up

Onsite installation should ideally be done by the autoclave manufacturer. The foundation flatness, heights and alignment should strictly follow the design parameters. The condensed water flow and rail inclination should be carefully checked and reported after the placement of the autoclaves on the foundation.

Each saddle should be examined, making sure the saddles not sitting on foundation support points are identified. Due to thermal expansion, the concrete foundation should have sufficient space to allow the saddle rollers to move back and forth. Before placing the pads preventing the saddles from moving left or right, proper checks should be done for the straight alignment of the autoclave shell.

For the first steam pressurizing, the autoclave should be pressurized step by step (slower than normal rate) in the presence of the autoclave manufacturer. The saddle movements should also be carefully checked, including foundation marking and bayonet door final adjustments. The steam pressurizing test should be repeated at least three times before putting the autoclave in regular production cycles.  

Checks During Operation

Once the autoclave enters into operation, the personnel should follow the user manual and instructions from the manufacturer and AAC plant engineering supplier.

Trained operators should carefully follow the performance of the autoclaves and consider the following:

-        Autoclaves should not be used in the event of any steam leakage.

-        Observations should be made on autoclave saddles to make sure the expansion and movement of the saddles are within the marked areas. The expansion will usually be in standard length, and any significant reduction in expansion should be observed and reported. The reduction in expansion, which has a direct impact on the lifetime of the autoclave, may be due to potential forces preventing the expansion of the autoclave. A proper investigation should be done between autoclave cycles and the problem should be solved.

-        The autoclave’s bayonet door safety mechanism should be active at all times. An experienced operator should check the safety hardware to make sure it properly functions and is active.

-        The insulation of the autoclaves should also regularly be checked. In case of any observed moisture, water droplets or leakage, the observed area of the autoclave’s insulation should be removed and closely checked.

-        It is critical that there is no water accumulation inside the autoclave. This should be checked using proper lighting inside the autoclave. During this examination, the plant maintenance staff should schedule the next cleaning. In Akarmak’s many years of autoclave experience, the lack of regular cleaning is a main cause of over-corrosion usually in the bottom areas.

-        The autoclave’s condensed water flow and rail inclination should be checked annually as well as measuring the autoclave thickness in critical areas and applying NDT (Non-Destructive Tests). It is recommended that a hydrostatic test should also be run to forecast any potential problems with the autoclave vessel in the future.

Checks after After Repair and Modification

According to the measurement and test results, if repair or modification is required, this should be done by the autoclave manufacturer or under the manufacturer’s supervision. The manufacturer should be consulted and approval should be received in case welding work is necessary on the autoclave, in areas subject to pressured force.

Technical documentation for approval from local authorities in the customer country should be prepared. If necessary, re-certification should be done. The pressure vessel directives of the country where the autoclave is used should be strictly followed.

If repair and modification involving extensive welding are done, hydrostatic tests must be done and reported. The hydrostatic test is the most ideal test to identify problems in the autoclave vessel, not only during the present time, but also possible problems that may occur in the near future.

Once the repair or modification is completed, the autoclave should enter into operation following the same procedures as brand-new autoclaves, as described above.