Compressors control devices

The purpose of pneumatics is to do work in a controlled manner. The control of pneumatic power is accomplished using valves and other control devices that are connected together in an organized circuit. The starting point in this organized circuit is the air compressor, where the air is pressurized. 

Most packaged water chillers are large enough to have capacity control devices in the compressor. The main control thermostat will unload the compressor as the water temperature approaches a lower safe limit, so as to keep the water as cold as possible without risk of freeze damage. 

In order to achieve a complete transportation system a host of other subsystems support the transportation system operations. Loading facilities, pumping and compressor stations, tank farms and metering and control devices are necessary for a complete transportation system of liquid or gases hydrocarbon commodities. 

Control Devices In many installations the use of gas is intermittent, and some means of controlling the output of the compressor is therefore necessary. In other cases constant output is required despite variations in discharge pressure, and the control device must operate to maintain a constant compressor speed. Compressor capacity, speed, or pressure may be varied in accordance with requirements. The nature of the control device will depend on the function to be regulated. Regulation of pressure, volume, temperature, or some other factor determines the type of regulation required and the type of the compressor driver. 

The key to smooth operation of a CFB system is the effective control of the solids recirculation rate to the riser. The solids flow control device serves two major functions, namely, sealing riser gas flow to the downcomer and controlling solids circulation rate. Both mechanical valves or feeders (see Figs. 10.1(a) and (d)) and nonmechanical valves (see Figs. 10.1(b) and (c)) are used to perform these functions. Typical mechanical valves are rotary, screw, butterfly, and sliding valves. Nonmechanical valves include L-valves, J-valves (see Chapter 8), V-valves, seal pots, and their variations. Blowers and compressors are commonly used as the gas suppliers. Operating characteristics of these gas suppliers which are directly associated with the dynamics and instability of the riser operation must be considered (see 10.3.3.2). 

Air supply subsystem, including blower or compressor, air humidification, heat exchanger and pressure control devices to deliver process air at the required mass flow, pressure temperature and humidity to the fuel cell stack. Separation of water from the used air stream and, in some cases, an expander add more complexity to the air supply subsystem ... 

Separation processes typically consist of hrmdreds of elements (pipes, vessels, heat exchangers, columns, pumps, compressors, engines, measuring and control devices, etc.) that are too complex for exact graphical depiction. Therefore, the process structures are depicted in a very abstract and geometrically nonsimilar manner in the form of flow sheets. Flow sheets show the process structure with symbols for the essential process elements according to international standards (e.g., DIN, ISO). 

Many instrumentation systems and control devices require power sources, electrical or pneumatic, which have to be reticulated around the plant. The cost of instrument air is routinely underestimated, mainly because the associated piping costs are not appreciated when the P I diagrams are drawn up, but also because of a disinclination on the part of the instrument system designers to make any compromise on air-quality requirements. Even a few parts-per-million of oil may be regarded as unacceptable, with consequent cost increase to the air compressor. Both instrument power supplies and instrument air have to be provided in an adequately secure fashion in relation to the consequences of their failure. 

To ensure a more reliable temperature control and the formation of a cylindrical layer of cold gas in each section of the reactor, several collectors for its introduction can be mounted, with thermocouple 7 and control device 8 ahead of each to adjust the volume of supplied cold gas. As the cold gas, the discharge gas or initial natural gas or any combination thereof can be used. Furfhermore, the oxidation by-products can be pumped from the vat of column 11 by pump 12 into mixer 14, from which, along with the circulation gas (boosted by compressor 13) or initial gas, they move in the cooling gas flow to final oxidation. Making use of the method for temperature control of the process by supplying additional amount of cold gas, the AMTEK-engineering company has developed a technical project of a plant with a capacity of 5000 tons of methanol per year. 

Nuclear Radiation Effects. Components of a nuclear reactor system that require lubrication include control-rod drives, coolant circulating pumps or compressors, motor-operated valves, and fuel handling devices, and, of course, are exposed to varying amounts of ionising (14). 

A compressor is typically a specially designed device, and comes with far less surplus capacity than other process components. As a result compressors merit great care in specification of flow, inlet pressure, and discharge pressure. Similarly, the control system and equipment need to be carefully matched to provide turndown with maximum efficiency. 

When no capacity control or unloading device is provided, it is nec-essaiy to provide bypasses between the inlet and discharge in order that the compressor can be started against no load (see Fig. 10-93). 

For protection and temperature control of the compressor the follow ing safety devices may be provided ... 

Gas compressor anti-surge (GM-OFF) control circuit, comprising transmitters, computers and pneumatic control valve Reverse flow protection (on axnal compressors only) as supplementary protection device against surging, working independently of the control circuit Expander emergency stop valve with pneumatic actuator and solenoid valve... 

This standard could be adapted to the fuel compressor for the natural gas to be brought up to the injection pressure required for the gas turbine. Covers the minimum requirements for reciprocating compressors and their drivers used in petroleum, chemical, and gas industry services for handling process air or gas with either lubricated or nonlubricated cylinders. Compressors covered by this standard are of moderate-to-low speed and in critical services. The nonlubricated cylinder types of compressors are used for injecting fuel in gas turbines at the high pressure needed. Also covered are related lubricating systems, controls, instrumentation, intercoolers, after-coolers, pulsation suppression devices, and other auxiliary equipment. 

In all static surge-detection devices, the actual plienonemon of flow reversal (surge) is not directly monitored. What is monitored are other conditions related to surge. Control limits are set from past experience and a study of compressor characteristics. 

The pulsation control elements can have several forms, such as plain volume bottles, volume bottles with baffles, bottles and orifices, and proprietary acoustical filters. See Figure 3-26 for an example of a compressor with a set of attached volume bottles. Regardless of which device or element is selected, a pressure loss evaluation must be made before the selection is finalized because each of these devices causes a pressure drop. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

The discharge pressure developed hy the compressor must he equal to the process gas s total system resistance, of control valves, hand valves, orifices, heat exchangers, and any other process-related devices through which the discharge gas from the compressor must flow. As this resistance changes, the gas flow through the compressor will automatically adjust itself to equal the new resistance. ... 

Usually a constant speed, motor-driven compressor will be equipped with inlet guide vanes to the first wheel to allow suction volume control. If the guide vanes are not used, some other throtding device should be available. Figure 12-62 presents the constant speed performance of a centrifugal machine with inlet guide vanes. 

A wide range of controls is available to match compressor output to demand. Consultation with the equipment supplier is essential. The output of a compressor can be controlled by several methods as outlined below. Pneumatic, hydro-pneumatic and electronic devices can perform the following functions ... 

The basic advantages of this process are (a) elimination of a mechanical device (recycle gas compressor) for controlling the adiabatic temperature rise, (b) combination of CO shift with methanation, (c) significant increase in byproduct steam recovery, and (d) significant capital advantages. 

To minimize damage due to power outage, the Celanese Corporation in their plant at Newark, N.J., instituted a policy of always generating half its own power. Merck Company installed additional auxiliary steam power to insure constant refrigeration for its biochemicals at its West Point, Pa., plant. At Allied Chemical s phenol plant in Frankfort, Pa., electric devices on air compressors and pumps were replaced by steam-operated controls, and diesel generators were installed to maintain cooling water circulation. 19... 

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