Pressure let-down

More details of operation in an actual study can be seen in Berty et al, (1982). In tliis work, a condenser and a liquid-gas separator were used in the product line before the pressure let-down. Keeping the products all in the vapor phase was difficult. Other improvements later included a continuous, four-component, feedgas make-up system with a compressor. 

It is hoped that this volume will provide some guidance to operators, plant safety engineers and designers, and that some of the design practices are adopted particularly in older plant operations that do not apply proper pressure let down procedures and flaring practices. Many unsafe operations and even catastrophic failures have been observed by the author in overseas assignments to such countries as Ukraine and other Newly Independent States (NIS) in older plant designs. These incidents have prompted the preparation of this reference. 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

A continuous-flow reactor with a fixed catalyst bed was employed under pressurized conditions. The reactor was made of stainless steel with an inner diameter of 6 mm. All products and unreacted feed materials were withdrawn in the gaseous state from the reactor through a heated pressure let-down valve, A quantitative analysis of the products was carried out by gas chromatography. The time factor, which corresponds to contact time, is expressed by W/F, where W is the weight of catalyst (g) and F is the total flow rate of feed (mol/hr). Chemisorption of H2, CO and methyl iodide (Mel) were measured by a conventional glass vacuum system. 

Recommendation 3-la. The Army should develop criteria and a schedule for resolving design and operational issues raised in the 1998 report, Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization, that have not yet been resolved for supercritical water oxidation operation at Newport. These issues include materials of construction, fabrication methods, system plugging, pressure let-down, and the duration of successful continuous pilot-scale operations. 

The selection of appropriate MOCs is a key requirement for the successful treatment of VX hydrolysate by SCWO (see Finding 2 and Recommendation 2 of the SCWO report [NRC, 1998]). SCWO treatment of VX hydrolysate involves highly corrosive and erosive environments that change drastically as the hydrolysate passes through the reactor and its peripheral components (inlet, outlet, and pressure let-down... 

Problems with salt/solids management, including the potential for plugging and difficulties in pressure let-down, was thoroughly described in the SCWO report (NRC, 1998) but have not been resolved in the ADP as currently modified. Further development work and testing will be necessary to address this important issue. The tests carried out by General Atomics to demonstrate high destruction efficiencies... 

This control configuration can only be used when the excess steam energy is utilized for the cogeneration of electricity, which can vary. Although energy conservation dictates that the flow through pressure let-down line be minimized, control dynamics require its existence. This is because the speed of response of a let-down valve is much faster than that of a turbine. Therefore, the sensitive control of the LP steam pressure is provided by the let-down pressure controller, while the bulk of the steam passes through the turbine and is used to make electricity. 

If the demand for LPS exceeds the amount of exhaust steam available, the PC opens the pressure let-down bypass. FC-1 serves to minimize the bypass flow. If the pressure controller (PC) opens the bypass and the let-down flow rate exceeds the set point of FC-1, the previously inactive (saturated) FC-3 becomes active and starts cutting back the LPS flow to the boiler feedwater preheater and thereby reduces the plant s demand for LPS. This is an energy-efficient response, because the energy recovered from the LPS supplied to the feedwater preheater is less than the energy content of the HP steam that is needed to produce that LPS. 

The second type of equipment used for volatile removal from polystyrene is the devolatilizing extruder. In these devices, an extruder is equipped with one or more pressure let-down sequences where vacuum is applied. In these devices, polymer surfaces are constantly being renewed, giving excellent mass transfer. Another advantage with the devolatilizing extruder is the ability to add and mix additives after devolatilization. This is especially useful if the additive has a... 

Catalyst beds were completely saturated with sulfur during poisoning tests. In each experiment the catalyst bed was exposed to the desired HjS concentration for 4 - 6 hours to st ilize the catalyst performance. After the experiments, the catalyst beds were cooled to room temperature under dry nitrogen flow before opening the reactor vessel. In pressurized tests the pressure let down was not started until 20-30 min after nitrogen flushing to be sure that all remaining reactive gas components had been removed. 

Physical solvent systems in which gas is absorbed by physical means in a solvent and released by heating and/or pressure let-down. 

Natural gas is often pressurized above 3000 psig, and to take a sample at atmospheric pressure it is necessary to design a suitable pressure let down system, which avoids liquefaction of the hydrocarbon gases. For coal fired gas sampling, it is important to avoid any condensation of moisture present in the stack gas, since with the matrix containing sulfior dioxide, nitrous oxide, and carbon dioxide, these may result in acidic solutions. 

Figure 2 shows a schematic diagram of a pressure let down system, which conforms to the European Standard ISO 6978 (ASTM 6350) for natural gas sampling. By careful choice of components and temperature of operation it is possible to provide reliable data on the mercury levels present in the natural gas samples. By injecting defined samples of mercury into the sample line prior to the sample collection, it is possible to check the validity of the measurements by testing the spike recovery. Recoveries in the region of 95-105 % will show that there is little, if any, loss with the system, and the analyst can be confident of the levels found in the gas streams. 

Dissolving vessel 2 Loop reactor 3 Catalyst vessel 4 Pressure-let-down vessel 5 Scrubber 6 Solvent recovery 7 Dewatering column 8 and 9 Fractionating columns... 

Oxidation reactor 2 Purification column 3 Extraction column 4 Post treatment 5 Diels-Alder reactor 6 Pressure let-down vessel 7 Extraction column 8 Oxidation reactor 9 Gas separator 10 Melting pot 11 and 12 Distillation columns 13 Crystallizer 14 Dehydration column 15 and 16 Distillation columns... 

P.B. Queneau, et al.. Control of Silica Deposition During Pressure Let-Down of Acidic Leach Slurries, Hydrometallurgy - Research, Development and Plant Practice, K. Osseo-Asare, J.D. Miller Eds., 112 AIME Annual Meeting, Atlanta, Georgia, USA, 1983, 121-137. 

High pressure let-down valves. If a control valve is used to reduce the pressure of a gas stream, considerable noise can be generated. 

Failure of the pressure containment system (piping, SCWO reactor, post-reactor air cooler, or pressure let-down system) could result in rapid depressurization and dispersal of hot fluids and debris at high velocities. Similarly, failure of the pressure let-down system could result in a large pressure surge that could rupture equipment downstream.. .. The pressure let-down system may be the weak Unk in the full-scale SCWO process chain (NRC, 1998b). 

Minimal number of processing steps Good. Uses a reactor, compressors, pressure let-down, evaporators, effluent air coxrlers, pumps, and gas/Uquid separator. 

Chapter 11 is an exercise involving SIL targeting for a pressure let-down system. The design is then compared with the target and improvements are evaluated and subjected to ALARP criteria. The answers are provided in Appendix 5. 

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