Manifold thermal expansion

The Ba0-Ca0-Al203-Si02 (BCAS) glass ceramic seal is often used for joining dissimilar materials, i.e. ceramic cells, metallic manifolds and metallic interconnects [2]. The ceramic seal should possess a satisfactory matching of the thermal expansion coefficient with the cells and the chosen alloy and it should also exhibit a long-term stability under the operation conditions. 

The ceramic membrane typically must also be joined to a metal manifold to collect the product gas. The requirements of ceramic-metal joints are extremely stringent due to the large pressure differential across the membrane. Even very small leaks will create a drop in product purity. The joints are usually at the operating temperature, although cold seals are occasionally considered. The thermal expansion coefficient of the ceramic needs to match that of the metal manifold. 

Thermal expansion and fire cases are not required to be checked, if the existing equipment is re-used, with the same service and also the same level control setting. Overpressure relief requirements due to each utility failure, fire cases and any other combination scenarios need to be estimated. API 521 (2014) has a comprehensive list of effects for utilities failure. All the PRV manifolds shall be checked to estimate back pressures at the PRVs. PRD overpressure calculations for equipment shall be documented as shown in Table 3.4. Vacuum relief (if the vessel/s is/are not designed to withstand full vacuum) shall also be documented. All the flare scenarios and flare network shall be properly documented. An example of PRV sizing calculations for the system shown in Figure 3.5 is presented in Table 3.4. 

The pin closnre force is aroimd 160 Pa, while opening takes place at a melt pressure of about 8 MPa. At a temperature of over 300 C the spring force is liable to diminish. The body of the spring and pin guide are connected to the manifold, which renders the system independent of the effects of thermal expansion in the manifold. 

Manifold nozzle (b) has a large cylinder which may be driven either hy compressed air at a minimum 0.6 MPa (closure force around 1 kN) or hydraulically at a maximum of 6 MPa (closure force around 10 kN). The pin is guided in the nozzle, so thermal expansion of the manifold does not affect the pin position. This design, though, leads to a zigzag flow channel. The nozzle is closed by a cylindrical pin in a tapered gate aperture. 

This is possible because of the negligible thermal expansion of the system and the seal provided by the external layer of frozen melt in the channel. Mould assembly requires simultaneous insertion of manifold, nozzles and mould inserts. 

The manifold is pressed against the nozzles by ceramic pads in a steel body, which improves heat insulation. The manifold is held in place with the aid of hardened plates set in the clamping plate, which prevents damage (gashing) of the soft plate during thermal expansion of the manifold. 

The telescopic connector used between the bushings and the other components of the manifold enables thermal expansion to be compensated. 

Thermal expansion in manifold may cause bending of clamping plate if this is too thin (after opening of mould). 

The mould clamping plate is subject to large surface pressures originating from the manifold pressure discs, and there is also a risk of seizure of the plate surface by the discs during thermal expansion of the manifold. It is therefore recommended that the clamping plate be made of toughened steel, or that hardened inserts be located in it (see Figure 4.61). 

As has already been mentioned, the problem of thermal expansion does not arise in this type of manifold, but compensation for thermal expansion AL of torpedoes and heaters is required. 

One design novelty in the HR set described, is the way the nozzle seal is ensured by clamping it to the manifold using a set of disc springs (24). The leakproofing of the system is thus made independent of the operating temperature, i.e., the thermal expansion of the manifold (in other words, the set may be used for different plastics with a processing... 

The nozzle ends were adapted to the shape of the moulding. The manifold is equipped with 11 temperature regulation circuits. To compensate for the thermal expansion in a large manifold, nozzles set in intermediate blocks were used in this design. 

The flat sealing of the calottes area around the tips compensates the radial thermal expansion of the manifold and avoids stresses by hindering thermal expansion. A minimum injection pressure applied to the surface reduces buoyancy forces. The small material volume in the calottes area facilitates the rapid change of material. Tight tolerances for the tip position and length enable exact positioning of the tip... 

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