Pressure vessels closure systems

When the pressure vessel is intended for batch service, particular attention often has to be paid on short opening / closing time of the closure system. 

In operation containers constructed of microwave-transparent materials, (e.g. quartz or fluoropolymers), are used to hold multiple samples inside the ultraCLAVE . The interior of the stainless steel vessel is protected by a titanium nitride or multi-layer PTFE plasma coating for complete acid and chemical resistance. Sample containers may be open or covered by a lid. After the samples are loaded (manually or robotically) the ultraCLAVE cover is lowered into place by an electric motor controlled from the system s PC. The vessel closure is engaged and secured in place to seal the ultraCLAVE for high pressure operation. 

The numerous possible high pressure applications and their associated requirements can be satisfied by a variety of different assemblies of reaction vessels, closures, tubing, valves, pumps or compressors. First, an overview of these different components is presented, after which several complete high-pressure systems found in the literature are discussed. At this point it should be emphasized that in showing equipment from one manufacturer or another, no preference for a particular product is being expressed. Further details on equipment employed for spectroscopy at high pressure are provided in Chapter 3. 

Nozzle attachments to high pressure vessels are almost always made through the heads, or end closures, rather than through the shell as is done in common pressure vessels. This is primarily due to the thickness of the shells and the fact that multilayered vessels are common in high pressure systems. Nozzles through multilayer vessels, while not impossible, are discouraged. 

The primary system, the reactor coolant pressure boundary, and important ancillary systems are enclosed in the primary contmnment, a cylindrical prestressed concrete structure that incorporates an embedded steel liner to ensure adequate leaktightness a steel dome is provided as a "removable" closure of the shaft above the reactor pressure vessel. 

In the case of pressure vessels which must be opened and closed frequently (such as batch extractors for example) a more convenient closure system is required and, in order to avoid long down times a quick closure system is then desirable. A distinction must be made between closures which provide an opening of diameter equal to the full vessel diameter (which are required when the material to be extracted is loaded in removable baskets) and those which do not. The latter are adequate if the vessel is to be charged with fluid or, in some cases, bulk solids (see chapter 5). Some of the available designs for quick closures are listed below. 

Figure 8.12 shows the segmented ring quick closure system [12]. This special development of the Bredtschneider closure makes it possible to open or close the vessel in a very short time (3-4 minutes) by a computer-controlled hydraulic system. The technique has been operated successfully with discontinuous extraction processes for several years. Figure 8.13, for example, shows this type of closure on a pressure vessel used for the decaffeination of tea. The inner diameter of the vessel is t/i = 1100 mm. Closure and vessel were designed for a fluctuating load. 

The importance of using mechanical or electronic safety systems in conjunction with quick closure systems should not be underestimated. These prevent the pressure vessel from being opened while it is still under pressure (see also chapter 5). 

The use of gas cooling, with the consequent high primary circuit pressure compared with a sodium-cooled reactor, increases the probability of a loss-of-coolant accident. With a prestressed concrete pressure vessel however, the only mechanism which could lead to a rapid depressurization is the failure of one of the penetration seals into the vessel. The maximum rate of depressurization can be limited by flow restrictions built into the penetration closures, and the reactivity worth of the helium is typically well below the delayed neutron fraction, so that reactivity transients are not a problem. A reliable emergency heat removal system, independent of the main cooling system, is essential. 

CATHARE is a system code developed by CEA, IPSN, EDF and FRAMATOME for PWR safety analysis. It can model light water reactors or test facilities using several available modules. Two-phase flows are described using a two-fluid six-equation model and the presence of non-condensable gases can be taken into account by one to four additive transport equations. The code allows a three-dimensional modelling of the pressure vessel. Successive sets of closure laws or "revisions " are developed in an iterative methodology of improvement. The Revision 6 of the closure laws is implemented in the Version VI.5. It includes models. 

The heart of the system is the oxidation vessel depicted in Figure 1. This vessel is a high pressure, 1000 cc, Hastelloy C-276, bolted closure reactor manufactured by Autoclave Engineers Inc. Hastelloy C-276 was chosen as the material of construction due to its excellent corrosion resistance to a wide variety of chemical process environments, which include processes utilizing strong oxidizers (5,6,7). The unit is fitted with 1/8" and 1/4" Hastelloy C-276 feed delivery and product outlet lines... 

Modifier Pump. The first feature in our adapted design is the introduction of a liquid pump via an instrument controlled VALCO (Model E04, Valeo Instruments, Houston, TX), four position selection valve. We have used an LKB Model 2150, dual piston pump for pumping modifier and entrainer fluids (LKB-Produkter AB, Bromma, Sweden). However, any suitable liquid pump could be substituted. Only pure fluids such as carbon dioxide have been introduced with the Suprex system syringe pump. With the addition of this second pump to deliver liquids, modifier is introduced directly into the extraction vessel. A wide range of alternative fluids and fluid mixtures can be rapidly selected with this dual pumping option. The criteria for selection of a modifier pump include the ability of the pump heads to withstand pressures in the range of 100 to 300 atm and interfacing capabilities, i.e. the ability to be turned on and off by the Suprex contact closure controls. 

Much information is available on the deformation and fatigue behavior of simple thick-walled cylinders [10-17], but it must be remembered that most process reactors will not be a simple hollow cylinder. Components such as connectors, threads and sleeves, windows, and removable closures make a complete analytical solution for a high-pressure system design problem quite involved. Useful design criteria for thick-walled vessels can be derived, however, under the assumption that the material of which the vessel is made is isotropic and that the cylinder is long (more than five diameters) and initially free from stress. The radial and tangential stresses in the walls are then only functions of the radius coordinate (r) and the internal pressure. Given the outer-to-inner wall radius ratio as o/i = w, and the yield point (To) of the material, the yield pressure (py) is... 

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