Holding pressure

Condensing Organic Va.por, The eutectic mixture of diphenyl and diphenyl oxide is an excellent vapor medium for precise temperature control at temperatures higher than those practical using steam. This mixture can achieve 315°C while holding pressure at 304 kPa (3 atm) absolute. In contrast, steam would require 10.6 MPa (105 atm) pressure. 

About half of all pumps in the shop today were pulled out of service because they were leaking or wouldn t hold pressure or pump. This is most likely a leaking o-ring. The o-ring is the rubber component of most mechanical seals. The o-ring controls the temperature, pressure, and chemical resistance of the mechanical seal (Figure 14-1). 

The injection and holding pressures were 100 MPa and 50 MPa, respectively. For thick (DIN 53455 Form 3) and thin (cut from the plates) dumbbell-shaped samples, an average shear rate of 230 s was kept constant. 

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. As an example, the injection mold cavity pressure profile is a parameter that is easily influenced by variations in the materials. Related to this parameter are four groups of variables that when put together influences the profile (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plastica-tor. TTius material variations may be directly related to the cavity pressure variation. Details on EQUIPMENT/PROCESSING VARIABLE are in Chapter 8. 

One of the major differences among the phases of water at the molecular level is the motions of the water molecules. Using the phase diagram (Figure 7), we can follow the effects of temperature and pressure on the molecular mobility of water. For example, if we hold pressure constant (say at 1 atm) and increase temperature, molecular mobility increases as we move from the solid to the liquid to the gas phase regions. Conversely, if we hold temperature constant (say at 100°Q and increase pressure, molecular mobility decreases as we move from the gas to the liquid phase region. 

A belt apparatus is capable of holding pressures of 7 GPa (70 kbar) and temperatures of up to 3300 K for periods of hours. The maximum steady-state temperatures are limited by melting of the refractory near the reaction zone (13). 

The tight lid of a pressure cooker holds pressurized vapor above the water surface, and this inhibits boiling. In this way, the boiling temperature of the water is increased. Any food placed in this hotter water cooks more quickly than food placed in water boiling at 100°C. 

We may treat this system two ways Hold temperature constant (isothermal conditions) and vary pressure and concentration, or hold pressure constant (isobaric conditions) and vary concentration and temperature. Both situations are shown in Figure 2.3. The plots in Figures 2.3a and 2.3b represent conditions for isothermal and isobaric treatments, respectively. 

We are now ready to expand our discussion of phase equilibria to consider composition as a variable. In doing so, we will usually hold temperature constant and see the effect of composition (usually mole fraction) on the equilibrium pressure (or pressure on the equilibrium mole fraction) or we will hold pressure constant and describe the relationship between temperature and mole fraction. 

Pilot-operated PRV A PRV in which the disc or piston is held closed by system pressure, and the holding pressure of that piston is controlled by a separate pilot valve actuated by system pressure (see Section 5.3). 

Set pressure must be demonstrated first and then pressure is again lowered. Then the inlet pressure is again raised to 90% of set pressure (hold pressure is 0.34 barg below set pressure if the set pressure is lower than 3.44 barg). The pressure needs to be held at 90% as follows ... 

The Arizona State University Chemistry Department, under contract to the BWIP, has designed a H2 diffusion membrane consisting of a 3-in.-long,l/4-in.-OD, closed-end Teflon tube with a wall thickness of 0.03 in. The mechanical strength of the membrane has been tested to 260°C and 1,500 bars and was able to hold pressure and pass H2 under these conditions. 

A typical injection pack/hold pressure during injection molding of polyamide 66 components is 1,000 bar and the injection temperature is 280°C. The gate freezes shut when the average temperature inside the mold reaches 225°C. 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

The point on the pvT diagram at which the final isobaric cooling begins (4), controls the total part shrinkage. This point is influenced by the two main processing conditions —the melt temperature and the holding pressure as depicted in Fig. 3.43. 

The plasticating and injection unit. A plasticating and injection unit is shown in Fig. 3.45. The major tasks of the plasticating unit are to melt the polymer, to accumulate the melt in the screw chamber, to inject the melt into the cavity, and to maintain the holding pressure during cooling. 

Pluronics F127 has been exploited to construct microfluidic valves. This material, which is an uncharged tri-block copolymer [(EO)106 (PO)70 (EO)106], is a commercially available surfactant. But within a certain concentration (18-30%), the material, which is a liquid of low viscosity (< 2 cP) at low temperature (0-5°C), will form a self-supporting gel (cubic liquid crystalline) at a higher temperature (e.g., room temperature) [937]. This material has been used as a one-shot, phase-change valve for PCR reaction. The Pluronics valve can hold up to 20 psi pressure, which is above the holding pressure (6.8 psi) required for PCR (up to 94°C) [937],... 

So as to fill the mould at the speed required, high pressure is applied to the melt to compensate for shrinkage, it usually is necessary to force in extra material in a second stage and to prevent over-filling at that time a lower second-stage or hold pressure may be used. 

Figure 6.14 Snap ring that holds pressure vessel end cap in place. Courtesy of Nako-Crossbow Water LLC.

At current development levels, the operator must implement the solution where it could be made directly by the machine controls. For example, where too low shrinkage can be corrected by a decrease in holding pressure. Because one is concerned with the risks of automatic implementation, and believes that, at least for a long time to come, there will always be problems requiring manual operations. An example is material clogging the throat of the hopper a problem which can be minimized by proper equipment design but never eliminated. Another is metal wear.163... 

During clamping and when applying pressure on the molds, the tie rods stretch. If everything is in balance, the platens and mold stretch evenly. The distance the rods stretch is directly proportional to the applied load. Sensors, such as electrical strain gauges, can be used to detect the stretch or load applied and if an unbalance situation occurs, an indicator can alert the operator or the process control system. Bar sensing can also be used as a means of signaling the switch from pack to hold pressures, a potential alternate or support to pressure transducer use. 

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