Recirculation Lines Research Articles

Условно динамический анализ работы центробежного компрессора в агрегатную линию рециркуляции

The procedure for conditionally dynamic (quasi-dynamic) hydraulic and thermal calculation of recirculation lines of compressor stations has been developed. As an example, the solution to the problem of reconstruction of the linear compressor station of the main gas pipeline is considered. Using the developed methodology, options for upgrading the existing recirculation line in order to increase its throughput are proposed. The gas temperature change in the recirculation line is analyzed due to various factors (heat exchange with soil, throttling) at the starting modes of constant and variable rotations of the compressor rotor. The analysis of the change in the recirculation cycle time and the rate of temperature growth at the compressor inlet per unit time is performed.

Pressurized SOFC System Fuelled by Biogas: Control Approaches and Degradation Impact

Abstract This paper shows control approaches for managing a pressurized solid oxide fuel cell (SOFC) system fuelled by biogas. This is an advanced solution to integrate the high efficiency benefits of a pressurized SOFC with a renewable source. The operative conditions of these analyses are based on the matching with an emulator rig including a T100 machine for tests in cyber-physical mode. So, this paper presents a real-time model including the fuel cell, the off-gas burner (OFB), and the recirculation lines. Although the microturbine components are planned to be evaluated with the hardware devices, the model includes also the T100 expander for machine control reasons. The simulations shown in this paper regard the assessment of an innovative control tool based on the model predictive control (MPC) technology. This controller and an additional tool based on the coupling of MPC and proportional integral derivative (PID) approaches were assessed against the application of PID controllers. The control targets consider both steady-state and dynamic aspects. Moreover, different control solutions are presented to operate the system during fuel cell degradation. The results include the system response to load variations, and SOFC voltage decrease. Considering the simulations including SOFC degradation, the MPC was able to decrease the thermal stress, but it was not able to compensate the degradation. On the other hand, the tool based on the coupling of the MPC and the PID approaches produced the best results in terms of set-point matching, and SOFC thermal stress containment.

Troubleshooting in a full-scale wastewater treatment plant: what can be learnt from tracer tests

The experimental assessment of the actual reactor hydrodynamic behavior is aimed at detecting possible deviations from the design configuration, as well as suggesting corrective/improving modifications, efficacy of which can be also assessed in the aftermath. The hydrodynamic scheme can be defined by means of tracer tests (stimulus–response method) used for detecting the residence time distribution curve. This study shows the results of three hydrodynamic tests performed on a wastewater treatment plant half-line, in which a biological process (denitrification and oxidation–nitrification) is carried out. Hydrodynamic faults (in terms of dead space and/or flow bypass) were identified, and consequent actions suggested (i.e., change in submerged mixers orientation in the reactors; sinking of the final section of the sludge recirculation pipe below the free surface to avoid foaming phenomena; introduction of new mixed liquor recirculation lines). These actions, together with a careful selection of wastes to be treated, led to an improvement of the overall process performance.

Assessment of passive safety system of a Small Modular Reactor (SMR)

Innovative SMRs are designed with enhanced safety features based on lessons learnt from past experience of plant operation. Reliance on natural circulation and addition of passive safety systems made them inherently safe and simple in design. It is required to study reliability assessment of passive safety systems during postulated transients prior to their deployment on commercial scale. Test facilities and best estimate system codes are playing significant role in assessment of passive safety systems as well as in design, certification and evaluation of these innovative types of reactors. RELAP5 code is widely used for thermal-hydraulic analysis of nuclear reactors. In this work, the passive safety systems of Multi-Application Small Light Water (MASLWR) have been assessed. The complete loop of the MASLWR test facility has been modeled in RELAP5-SCDAP Mod 4.0. The RELAP5 model is validated by comparing the simulation results with the experimental data. Results obtained for various transients show that high pressure vent and sump recirculation lines provide natural circulation flow path for long term cooling of core to avoid core heat up.Some of the components of passive safety system of MASLWR still rely on active power. Therefore, it was necessary to investigate their performance under failure of these components. New scenarios are considered in which the effect of vent and sump recirculation valves failure has been investigated in the present work. It is found from the results that continuous loss of inventory occurs due to lack of recirculation. It is thus concluded from this study that the high pressure vent valves in the MASLWR safety system require more redundancy.

Convective Mixing in Distal Pipes Exacerbates Legionella pneumophila Growth in Hot Water Plumbing.

Legionella pneumophila is known to proliferate in hot water plumbing systems, but little is known about the specific physicochemical factors that contribute to its regrowth. Here, L. pneumophila trends were examined in controlled, replicated pilot-scale hot water systems with continuous recirculation lines subject to two water heater settings (40 °C and 58 °C) and three distal tap water use frequencies (high, medium, and low) with two pipe configurations (oriented upward to promote convective mixing with the recirculating line and downward to prevent it). Water heater temperature setting determined where L. pneumophila regrowth occurred in each system, with an increase of up to 4.4 log gene copies/mL in the 40 °C system tank and recirculating line relative to influent water compared to only 2.5 log gene copies/mL regrowth in the 58 °C system. Distal pipes without convective mixing cooled to room temperature (23–24 °C) during periods of no water use, but pipes with convective mixing equilibrated to 30.5 °C in the 40 °C system and 38.8 °C in the 58 °C system. Corresponding with known temperature effects on L. pneumophila growth and enhanced delivery of nutrients, distal pipes with convective mixing had on average 0.2 log more gene copies/mL in the 40 °C system and 0.8 log more gene copies/mL in the 58 °C system. Importantly, this work demonstrated the potential for thermal control strategies to be undermined by distal taps in general, and convective mixing in particular.

Efficiency of recirculation membrane sorption systems for producing oxygen-enriched air

Hybrid membrane–sorption systems for producing oxygen-enriched air have been considered in terms of the general theory of multimodular separation systems with recirculation lines. The performance of the hybrid systems has been examined as a function of the product stream pressure. The hybrid and the membrane air separation systems have been compared. It has been shown that an optimal scheme is the membrane–sorption system in which a recycle is formed by returning the retentate stream from the membrane module to the feed stream using an ejector, with the feed gas mixture being sent to adsorbers from the ejector outlet.

Possible design improvements and a high power density fuel design for integral type small modular pressurized water reactors

The study evaluates potential weaknesses and possible improvements for integral type small modular pressurized water reactor designs. By taking International Reactor Innovative and Secure (IRIS) as the reference design and keeping the power output as the same, a new fuel and reactor design were proposed. The proposed design relocates the primary coolant pumps and the pressurizer outside the reactor pressure vessel (RPV). Three recirculation lines and jet pumps/centrifugal pumps are introduced to provide the coolant circulation similar to Boiling Water Reactor designs. The pressurizer component is expected to be similar to the AP600 design. It is located at one of the recirculation lines. The new fuel assembly adopts 264 solid cylindrical fuel pins with 10 mm diameter and 2.3 m height, arranged at a hexagonal tight lattice configuration. Large water rods are introduced to preserve the moderating power and to accommodate finger type control rods. The resulting fuel can operate with 104.5 kW/l power density while having substantially higher margin for boiling crisis compared to typical large PWRs. Full core neutronic analysis shows that 24-month cycle length and 50 MWd/kg burnup is achievable with a two-batch refueling scheme. Furthermore, the fuel behavior study shows that the new fuel with M5 type Zircaloy cladding show fairly acceptable steady state performance. A preliminary Loss of Coolant analysis shows that the new design could be advantageous over IRIS due to its low ratio of the water inventory below the top of the active fuel to total RPV water inventory. The proposed reactor pressure vessel height and the containment volume are 30% lower than the reference IRIS design.

Analysis and control of a hybrid fuel delivery system for a polymer electrolyte membrane fuel cell

A polymer electrolyte membrane fuel cell (PEM FC) system as a power source used in mobile applications should be able to produce electric power continuously and dynamically to meet the demand of the driver by consuming the fuel, hydrogen. The hydrogen stored in the tank is supplied to the anode of the stack by a fuel delivery system (FDS) that is comprised of supply and recirculation lines controlled by different actuators. Design of such a system and its operation should take into account several aspects, particularly efficient fuel usage and safe operation of the stack. The exiting unconsumed hydrogen is circulated and reused to increase the efficiency and at the same time maintain the humidity in the anode side of the stack, thereby preventing drying and flooding in the channel which can affect the stack performance. A high pressure difference across a cell between the anode and cathode could cause damage on thin layers of the cell components and water imbalance in the membranes. In this paper, we analyze a hybrid fuel delivery system that consists of two supply and two recirculation lines. The major components were a pressure regulator, a flow control valve, an ejector, and a blower. These models were developed and connected in order to analyze dynamic behavior of the fuel delivery system. Based on the models, two control strategies, a decentralized classic proportional and integral control and a state feed-back control were designed and optimized to keep a constant pressure in the anode flow channel and a constant ratio of mass flow rates from recirculation to supply lines. The integrated system with the two different controllers was simulated to evaluate its tracking and rejection performance at different references and disturbances.

Intermittent use of copper-silver ionization for Legionella control in water distribution systems: a potential option in buildings housing individuals at low risk of infection.

One copper-silver ionization system was sequentially installed onto the hot-water recirculation lines of two hospital buildings colonized with Legionella pneumophila, serogroup 1. A third building with the same water supply and also colonized with Legionella served as a control. Four weeks after activation of the system, distal site positivity for Legionella in the first test building dropped to zero. After operating for 16 weeks, the system was disconnected and installed onto the second test building. Twelve weeks of disinfection reduced the distal site positivity for Legionella in the second test building to zero. Legionella recolonization did not occur in the first test building for 6-12 weeks and in the second test building for 8-12 weeks after inactivation of the system. The control building remained Legionella-positive throughout the experimental period. A significantly higher copper concentration was found in the biofilm taken from a sampling device than in that from water. This is likely to be the reason that the copper-silver ionization system had the residual effect of preventing early recolonization. Our study raises the possibility that one copper-silver unit could be rotated among several buildings to maintain a Legionella-free environment. Such an approach may be cost-effective for buildings housing individuals at low risk for contracting legionnaires' disease.

Experience with the dismantling of three secondary steam generators in unit A in Gundremmingen by the ‘ice-sawing’ technique

The unit A in Gundremmingen (KRB A), was the first commercial nuclear power plant in Germany. It had an electrical power of 250 MWe and was in operation from 1966–1977. The plant was equipped with a dual cycle boiling water reactor of a former General Electric design and includes three recirculation lines each with a big recirculation pump and a steam generator comparable with those of pressurized water reactors. Therefore dismantling experience is gained for systems and components of boiling water reactors as well as pressurized water reactors.In early 1980, it was decided to decommission the plant. Actual decommissioning work started in 1983 with the removal of the components and systems in the turbine house. Since 1990 the decommissioning activities have been expanded to all primary water systems inside the reactor building. In 1992 , KRB A obtained a licence for dismantling the remaining activated components like the reactor pressure vessel and the biological shield.Meanwhile more than 5200 tons of contaminated components have been dismantled. Special cutting and handling tools were tested, developed and optimized for the purpose of working in radiation fields and under water. The dismantling work of the contaminated systems and components ends up in about 6000 tons of material with a rather low amount of waste, especially due to optimized decontamination techniques Eickelpasch et al. (1992). For the dismantling of the three secondary steam generators in the reactor building the ‘ice-sawing’ technique was developed and patented.

Hydrogen injection in BWR and related radiation chemistry

Hydrogen injection to feed water systems in boiling water reactors (BWR) has drawn wide attention as one of the possible countermeasures to the stress corrosion cracking (SCC) of 304 type stainless steel piping. To confirm the effectiveness of the hydrogen injection, a computer simulation of the complicated radiolysis reactions was carried out. The result of the simulation showed that the reactor water data monitored at the usual sampling points in actual plants reflect mainly the reactions in the downcomer portion but not in the reactor core in BWR. The calculation claimed approximately 300 ppb hydrogen in feed water to reduce the oxygen concentration in the recirculation lines to a negligible level, while one order of magnitude higher level of hydrogen is necessary to suppress oxygen in the reactor core. The computer simulation requires many radiation chemical data as in-put, among which are G values of initial products for water radiolysis at high temperature. An experimental approach was made to confirm the G values for high temperature radiolysis of water. The result does not seem to be consistent with the high temperature G values reported by Burns.

Deposition of Cobalt-60 on the Stainless-Steel Surface Used for the Primary Cooling System of a Boiling Water Reactor

The characterization of radioactive corrosion products deposited on the internal surfaces of recirculation pipes was carried out to determine the deposited amounts and configurations. The test specimens were taken from the auxiliary pipes of the recirculation lines of Shimane Nuclear Power Plant after operation for ∼20 000 effective full-power hours. As a result of characterizing them, it was demonstrated that 1. 60Co, which determined the contact dose rate, was difficult to remove by mechanical washing2. the amount of 60Co deposited was reduced by more than a factor of 10 by oxalic acid polishing and by a factor of ∼1000 by electro-polishing3. both the particle size and 60Co specific activity of the deposits removed by the mechanical washing (designated as soft deposits) were the same as those of the crud suspended in the reactor water4. the 60Co specific activity of the deposits removed by the oxalic acid polishing (designated as hard deposits) was ten times greater than that of the suspended crud.It was inferred that the soft deposits were formed by direct deposition of the suspended crud, and the hard deposits were formed by oxidation of the stainless-steel base metal with 60Co being taken into the oxide layers.