Benefits of Monolithic vessels
1) Cost is typically lower
2) They are capable of more thorough inspection at any point in their life
3) Thermocouple gland and electrode replacement is much easier.
4) The cooling jacket is much easier to remove
5) Lower seals are more reliable and much longer lasting as no movement occurs
6) Very high heat capacity and virtually unaffected by power loss
7) Does not require venting in the event of power loss which would cause furnace damage or part cracking.
8) No failures of ASME code HIP systems
There are various styles of vessels and I will outline what their points are. Some common things to all vessels are:
1) All vessel designs can be made safe. Likewise all vessel designs can fail.
2) All vessel designs can be made cheaply or with quality. This can be by intention or with error.
3) All vessel designs can incorporate low cyclic design or high cyclic design. One can pay more for a longer cyclic life.
4) All vessels are subject to various levels of maintenance, which plays a part in safety and longevity.
5) Each vessel design has merits, which may be desirable from a users standpoint. This may be cost, weight, operation, cyclic life, ease of NDT testing, replacement options, and possibly delivery time.
Taking these into consideration a customer must choose not only a vessel design, but a credible manufacturer to supply this design.
1) Fully threaded monolithic forged vessel.
This is the most economical vessel. It is also the most widely used.
There is a long history of design and understanding involving these vessels.
It is covered by the ASME code (and foreign codes) and can be stamped.
It can be ultrasonically checked for defects at any point in it's life.
The analysis of design is straightforward and universally understood.
It is simple to operate and maintain. It is available from many high quality sources including AIP.
It is a medium weight vessel and is not as susceptible to overheating damage as some other designs are.
If damaged during it's life there have been no failures of an ASME code stamped Hot Isostatic Press vessel and no AIP, or vessels supplied by AIP personnel since our involvement in isostatic presses beginning in 1975. Note there has been at least one non ASME failure. Typical cyclic life is chosen at around 10,000 cycles, however designs can offer higher cyclic life at added cost, but often replacement is a more effective savings. It can be offered in both small and large vessels.
2) Breach threaded monolithic forged vessel.
This design is similar to the above model with the exceptions that it is more costly, heavier, and typically has higher stresses in the design. It is widely used in the cold isostatic press due to it's quick opening capability, but not in the hot isostatic press as there is usually no need to open quickly and the added cost is not justified. AIP has supplied this design to nuclear installations that need fully remote operation. There has not been any ASME HIP failures, but there has been non ASME failures. Cyclic life is similar to above but often designs are used to gain very high cyclic life as cold pressing is usually a 5 minute cycle versus a 6 to 8 hour HIP cycle. Some cold presses have been operated to 2 million cycles.
3) Non-threaded forged vessel and frame.
This includes both multiple plate and single piece frames (yokes).
This design is very large and heavy weight, and thus quite expensive. It can be ASME and can offer high cyclic life. It can be NDT'd well. It has the same capability to handle heating as the two designs above. It is suited best for medium to large vessel sizes where production becomes demanding and building size can be designed to handle the large size and weight. The benefit to this design is that the stress of the threaded closure are moved into the frame and cyclic life can typically be higher for a given design provided a conservative design approach is used. The vessel can be automated quite easily, however on a small system the cost to add automation is not justified and the lack of automation can make operation more difficult. AIP supplies these type of vessel in ASME code designs. Non-threaded forged vessels have also been made with wire wound frames by companies such as Kobe Steel but only on extremely large cold isostatic systems.
4) Non-threaded wire wound vessel with either forged frame or wire wound frame.
This vessel type can be designed in a few different arrangements. It can be either of pre-stressed or non-stressed design (no companies are currently building non-stressed designs, but they are available). The wire wound vessel has an excellent history and large installed user base. It is the most expensive pressure vessel requiring sophisticated equipment to wind layers of wire at prescribed tensions around a thin monolithic forging. The design is marketed as a lightweight vessel, however the weight advantage is not always inherent due to the massive frame required. It is typically somewhat lighter than a threaded design and much lighter than a solid frame design. The benefit to a wire wound has always been portrayed as a vessel that would not fail catastrophically but rather leak prior to breaking. This advantage was only theorized until 3 years ago when a wire wound vessel failed resulting in the death of one person. This is one of the only deaths ever attributed to a HIP system vessel failure. The cause of failure has yet to be determined and may never be found, however the leak before break theory is now in question and many engineers are analyzing this situation. Another problem with the wire wound vessel is the lack of capability to easily qualify the NDT over the life. Normal NDT such as dye penetrant, magnetic particle, and ultrasonic testing can not be applied to the vessel and only measurement of the dimensions can be done. This leaves much to be desired. The pre-stressed vessel is also very susceptible to heat damage and must be vented of pressure in the event of cooling loss. There have been instances of vessels overheating requiring pressure de-rating. Until 1997 wire wound vessel were not covered under ASME codes. Now there is ASME Division 3 which allows code stamping. AIP can provide wire wound vessels and would recommend these for very large installations where the vessel cost is not as important as the weight savings. Wire wound vessels can be made larger than monolithic due to the manufacturing capability available in the world.
The uniqueness of AIP is the conservatism and quality applied to our designs while also offering cost benefits due to our company structure and overhead.