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Power Control
Published in Stephen W. Fardo, Dale R. Patrick, Electrical Power Systems Technology, 2020
Stephen W. Fardo, Dale R. Patrick
Another type of electrical control device is called a pressure switch. A pressure switch has a set of electrical contacts that change states as the result of a variation in the pressure of air, hydraulic fluid, water, or some other medium. Some pressure switches are diaphragm operated. They rely upon the intake or expelling of a medium, such as air, which takes place in a diaphragm assembly within the pressure-switch enclosure. Another type of pressure switch uses a piston mechanism to initiate the action of opening or closing the switch contacts. In this type of switch, the movement of a piston is controlled by the pressure of the medium (air, water, et cetera).
Hardware Components for Automation and Process Control
Published in Stamatios Manesis, George Nikolakopoulos, Introduction to Industrial Automation, 2018
Stamatios Manesis, George Nikolakopoulos
Pressure and temperature are the two most commonly measured or simply detected quantities in industrial processes. As happens for temperature, there are many ways of sensing fluid pressure. Pressure sensors and switches consist of a mechanical part sensitive to pressure and an electrical part producing the output signal, analog (measurement) sensors, or digital (switching contact) switches. In general, there are different sensing elements, each of which prescribes the design of the mechanical part. The goal of all the pressure sensing elements is to produce a movement as a result of the presence of fluid pressure, while the most common of them base their operation on a diaphragm and piston configuration, as presented in Figure 2.52. The air or liquid pressure at the inlet port acts on a movable surface (e.g., a flexible membrane or piston base surface). The force applied to the movable surface depends on the area of the membrane or piston base and the pressure of the compressed air or liquid. Since the area is constant, the produced force is directly proportional to the pressure. Therefore, the resulting movement is proportional to the pressure, and is converted either to an analog voltage, current signal, or to a contact switching signal, when movement is greater than a predefined limit. In the second case, which concerns a pressure switch, the pressure set-point at which the switch is activated may be constant or adjustable. The above-described relation between the movement and the pressure is valid when the pressure on the other side of the membrane is the atmospheric pressure. In the case that the pressure switch accepts two different pressures, it is called a “differential-pressure switch” and the movement is proportional to the difference between the two pressures. The term “differential” should not be confused with the differential behavior of the pressure switch output during rising and falling of the pressure, as shown in Figure 2.53a. The difference between the switch operating point on rising pressure (Pmax) and the switch operating point on falling pressure (Pmin) is called the “dead band”, and may be adjustable either independently in the setting points or as a fixed range.
Exploring the relationships between safety and maintenance in the cold generation process: insights from the functional resonance analysis method
Published in International Journal of Occupational Safety and Ergonomics, 2023
Marcelo Fabiano Costella, Graciela Aparecida Pelegrini, Heleia Bortolosso, Paulo Vicari, Francieli Dalcanton
The system consists of five distinct stages. The first stage is the storage of liquid ammonia (container). This step is controlled by safety and blocking valves that can be operated manually in any failure. Pressure gauges and level controllers generate data collected manually for analysis and interpretation by the people responsible for the maintenance. Second is the evaporation of liquid ammonia (evaporator), where the control is done through solenoid valves to control temperature, which operates in automatic mode. Safety and blocking valves are operated manually when demanded. The third stage is the separation of the liquid ammonia and vapor (separator), where the control is done through a level controller, which works without human intervention. Safety and blocking valves are operated manually when necessary, and the pressure gauges show the pressure values for analysis by those responsible for the maintenance. Fourth is the compression of the ammonia vapor (compressor), where the control is through pressure switch monitoring and automatically responding to the compressor’s pressure conditions. Safety and blocking valves are activated manually when necessary. The flow switch to monitor water flow also operates automatically. Pressure gauges measure the pressure and generate results that can be gathered and analyzed by the operators. The final stage is the condensation of the ammonia vapor (condenser), where the control is an automatically operated vent point and manually operated safety and blocking valves.
Valorisation of sodium lignosulfonate by ultrafiltration of spent sulphite liquor using commercial polyethersulfone membrane
Published in Indian Chemical Engineer, 2022
Kaushik Nath, Vinay B. Patel, Haresh K. Dave, Suresh C. Panchani
The schematic of the UF experimental set-up for the present study is outlined in Figure 1. The set-up has a feed tank (B) having a capacity of 40 l, high-pressure plunger pump (C) with a variable frequency drive (VFD) (D), test cell (A) housing the flat sheet membrane, rotameter (G), inlet (E) and outlet (F) pressure gauges and permeate collector (H). The UF pilot plant also consists of a high-pressure switch ensuring pressure not to exceed the set point value. The flat sheet (A4 size) UF membrane is placed between two thick stainless steel plates. The diluted feed solution of SSL of different concentrations was pumped to the test cell (dimension: 225 × 150 × 50 mm). The desired pressure in the system was controlled by means of a control valve placed in the reject line. At a given pressure the fluid cross-flow velocity (CFV) could vary using VFD. The effective surface area of the membrane available for permeation was 160 cm2. Due to pressure differences in the upstream and downstream side (across) of the membrane, the permeate was passed through the membrane and subsequently collected in the permeate collector. The volumetric permeate flux was measured at a regular period. The reject stream was passed through a rotameter and recycled to the feed tank to maintain the constant feed concentration. Concentrations of the permeate and reject stream were analysed by the UV–VIS spectrophotometer. Prior to its use the membrane was compacted as described by Bhattacharjee et al. [2] for about an hour with distilled water at a pressure of 690 kPa.
Integrated model for comparison of one- and two-pipe ground-coupled heat pump network configurations
Published in Science and Technology for the Built Environment, 2018
Laurent Gagné-Boisvert, Michel Bernier
For two-pipe networks, the main flow is a function of the number of heat pumps in operation, as shown in Figure 3a. The main flow decreases linearly as a function of the required flow to the heat pumps up to a certain minimum (30% in this case) after which the main flow rate remains constant. The VFD regulates the main pump speed based on the signal generated by a differential pressure switch measuring the pressure difference between the inlet and outlet of the farthest heat pump branch (between points 4 and 8 in Figure 1a). The differential pressure switch set-point is generally set to the pressure drop in a heat pump branch at nominal flow. Each heat pump is equipped with a motorized two-way control valve which closes when the heat pump is off. When a heat pump is turned off, more flow will be supplied to other units, increasing momentarily the differential pressure in each operating heat pump branch. In turn, this induces a reduction of the VFD speed to supply each heat pump with its required flow. This common two-pipe control strategy requires a similar pressure drop in each parallel branch, which is achieved by adding balancing valves in direct-return networks. The balancing valve, which is frequently an automatic flow limiting valve (Mescher 2009), allows a specific flow to a heat pump. If too much flow is supplied to a balancing valve after another unit shut-off, its pressure drop increases to limit the flow, increasing the differential pressure and reducing the VFD speed. Balancing valves are useful devices but present higher pressure drop compared to other valves, even when fully opened. More details on hydronic balancing and balancing valves are given by Taylor and Stein (2002).