Flow Characteristics of Valves - Essay Example

? Introduction: Flow characteristics of Valves Flow characteristics of valves refers to therelationship between the volumetric rate of flow (F) plotted in the Y axis through the valve and the valve travel or opening position denoted by m(X- axis),as the valve is seen through its various degree of opening right from opening position to the closing position. The symbols, m and Z are essentially meant to indicate or stand for the valve travel or the opening part in percentage (%). The flow characteristic of an incompressible fluid that flows under constant pressure differential is known as the inherent flow characteristic. This flow is usually expressed by plotting the valve travel on the (y-axis) against the flow percentage under constant pressure differential on the (X-axis). There are two main types of the inherent flow characteristics; these are linear and equal percentage. A control valve is said to be linear if its inherent flow characteristics can be represented on a graph by a straight line. The flow is usually directly proportional to the valve travel. It is a straight line when represented on a rectangular coordinate of flow rate F or % F or Cv or % Cv versus or against the percentage travel at the optimum condition of constant drop in valve pressure (DPv). It can be deduced that an equal increase in travel rate or opening position, m, results in an equal increase in flow F (or Cv) at constant valve pressure drop (DPv). If m represents travel and F represents the rate of flow, then, dm= k dF where K is a proportional constant. Therefore the valve gain or sensitivity is independent of flow rate and valve opening. Shown by; Kv = Df/dm = constant The valve gain (Kv) is the slope or tangent of the rectangular coordinate or plot drawn. The slope of the graph remains constant and independent of the flow rate. A control valve is said to be equal percentage if equal increase in valve travel or opening m (or Z) produces or results in equal percentage increase in flow F. If the situation emerges at the optimum condition of constant drop in valve pressure (DPv), the valve is said to be inherently equal percentage. The valve gain is not constant in this case but increase with an increase in flow rate. Therefore, when the control valve is just beginning to open at low flow rate, the valve gain is quite low and the change in flow is quite insensitive. At high flow during the upper portion of the travel, the valve gain is high which translates to rapid flow or sensitively for the same increase in valve travel. When plotting the flow rate against valve travel or position, the slope is flat near the initial travel position, but the slope increases drastically with the flow as the valve opens on the upper section of the travel. It can be shown by a straight line plotted on a semi-logarithmic paper for log F against m. The Quick open control valve characteristic is flattened in shape as opposed to the contoured valve plug. The flow of this valve increases rapidly up to the highest flow level with the least initial opening of the valve. In the lower section of travel position, the valve gain is so high for application and utilization in modulating control. The slope is then flat where the flow rate hardly increases with valve openings. Therefore, the Quick open control valve is restricted to a service of ON and OFF and in a specified application that needs discharge of flow. Applications of Control Valves Linear plugs are mostly used on the systems where the valve pressure drop in a major section of the total system pressure drop , where the valve is the major pressure loss mechanism. Linear control valves are used in about 10-15 % of all applications. Linear control valves are also used in gas process. In the gas process, there is a large volume, decreasing DP with an increasing load DP at maximum load. The maximum load DP is greater than 20% of minimum load DP. It is also applied in the gas compressor recycle control valve. Also, the linear valves are used in water systems for controlling the systems. Equal percentage valves are applied generally in the steam systems. The best choice of the applications is the linear valve given that the installed and inherent characteristics are always linear and similar, resulting in limited gain in the control loop.The equal percentage valves are mainly applied in areas or places where it is fully in control of the flow generated from a pump. When the valve opens, the inwards pressure falls. The resulting percentage characteristic consequently increases the port area as the valve finally opens. This ultimately result into a linear flow. Calculating the Right Cv Value The flow co-efficient (Cv) is determined generally through an experiment. The flow co-efficient expresses the flow capacity in imperial units. The text will commence by looking at the flow co-efficient in liquids. For the liquids, the Cv expresses the capacity of flow in every litre of gallon at any given minute of water temperature of about 600 Fareheids Calculating and Expressing as Volume Cv = q (SG / dp)1/2 Where; q = water flow (US gallons per minute) SG = specific gravity (1 for water) dp = pressure drop (psi) Given a water flow of 20 units, the specific gravity to be equal to 1 and the pressure drop as well equal to 1. Calculate the flow co-efficient. On calculating we get 20. When calculating the flow in liquids; the flow can be given in terms of volume or alternatively in terms of S.I units. Flow co-efficient for Saturated steam. Given that steam and gases are incompressible the formula used must be altered to suit the needs and to accomodate density changes. For instance, at critical pressure drop, the oulet pressure(po)emminating from the valves is less than 58% of the inlet pressure(p1). The flow co-efficient can then be determined as Cv = m / 1.61 pi         (2) In which M refers to steam flow Pi caters for the absolute pressure of the steam And Po stands for absolute pressure for the outlet Steam flow (lb/h) Given that the steam flow is 1000 and the inlet steam absolute pressure is 100. Calculate the flow co-efficient. Therefore; Cv =1000/1.61(100) This equals; 6.21118 which is the flow of co-efficient. Types of Process heater There are various types of process heaters which can fall in the following categories: Combustion based process heating, electric process heating systems, heat recovery and heat exchange systems. The process heaters are to be discssed in detail and their characteristics potrayed. Combustion-based (Fuel-based) Process Heating In this type of process heating, heat is generated by combustion of solid, liquid, or gaseous fuels and then passed to the material directly or indirectly. The major types of fossil fuels used include oil, natural gas, and coal. The biomass fuel includes vegetable oil, wood chips, cellulose, ethanol and charcoal. In order to effectively enable combustion, the fuels both gaseous and liquids are mixed with oxidants such as oxygen, and air. The gases used for combustion in this process can be either in contact with the material, resulting in direct heating or can be confined and thus separated from the material. The indirect heating involves radiant burner tube, the radiant panel and the muffle. Some of the examples of this process heating equipment include; the kilns, ovens and melters. The combustion based processes heating systems are used in nearly every section or sector of the industry. This application is majorly seen in furnaces such as ovens, heaters, kilns and melters but also for surface treatments in ambient air. There are many types of combustion based process heaters. This could include; atmosphere generators, blast furnaces, crucible furnaces, dryer, flares, thermal oxidizers among others. Electric Process Heating In this type of process heating, electric currents or electromagnetic fields are used in heating the material. The direct heating methods generate heat within the given work piece. The heat is generated by either passing an electrical current through the material or by inducing an electrical current or eddy current into the material. The heating methods also generate heat which vibrates atoms within the substance with electromagnetic radiation for instance in a case of microwave. Indirect heating methods are employed in one of the three methods of heating element or susceptor and transfer the heat by either conduction, radiation, or a combination of the three to the work piece. Examples of these heating processors include; resistance heating, direct resistance heating, microwave heating and plasma and arc heating. Heat Recovery and Exchange Systems This kind of process heaters are end use specific. The applications employed often use heat exchangers to transmit energy from one part to another part. The other example is the chemical reaction vessels that depend on energy emitted by exothermic reactions to heat other successive processes. A common kind of heat exchange system is the thermal fluid systems. This system uses a medium that bases its heat transfer from oil or salt to carry heat from the source of generation to the heated product. This process is just like the process in which the steam is heated. The thermal fluid systems contain very lower vapor in comparison to pressure and temperature characteristics, which implies that thermal fluids can provide high temperature service without the high pressures that would be required with steam. DC Motor Control System Direct current motor (DC) is an electric motor that is mechanically commutated and driven by a direct current. It has a stationary stator in place same as the current. The angle between the stator and the rotor magnetic flux is maintained at 90 degrees. The relative angle between the two is achieved or reached upon switching the commutator to be in the stationary place. This therefore results into maximum torque (Kuo, 97). The different and varied connections of the field and armature windings provide or results into different torques. The speed of the motor can usually be checked through constant checks on the applied voltage or through varying of the current in the field. When variable resistance is introduced in the field circuit or armature circuit speed is thus controlled. Difference between Speed Control and Torque Control In any speed controller, the speed of the motor or the velocity of such a device is always controlled. There is always a gadget that detects speed then provides feedback to the speed controller to ensure that speed is either increased or decreased. The speed of the motor is increased or decreased by applying voltage to the motor. During the application of voltage, there is a resulting current that flows through the motor winding. The current consequently provides the spinning torque in the motor. The resulting current by any chance is never controlled directly but through applied voltage. There is a speed controller that can control speed in either clockwise or anticlockwise direction by applying either positive or negative voltage. This therefore is referred to as a two quadrant controller; clockwise controller; clockwise torque in a clockwise direction and counter clockwise torque in a counterclockwise direction. Hydraulic Control System Hydraulic control system use the concept of pressure applied at one given point in incompressible fluid is transmitted directly to other parts within the system. A hydraulic control system is made up of three major parts. These parts include the hydraulic pump driven or operated by an electric motor or a combustion engine; valves and filters and then the motor or the hydraulic cylinder. It majorly uses the Pascal’s principle; force applied at one point generates pressure that is transmitted equally and proportionately in the fluid. The major dependence is the area of contact (Burdock, 1). Hence a piston with a small surface area would result into a small force and a piston with a larger area results into a larger force. The hydraulic control system can actually use the hydraulic cylinders in which case a small torque can be transmitted to a large torque. Use of hydraulic position control in robotic applications Hydraulics has been used in different precision machinery. The applications of the hydraulic position control have expanded and changed over time. For instance it is used in fluid dynamics technology for hydraulic oils; in this case it regulates drops in pressure at each of the valve plate, the operating fluid is alternately pushed or displaced thus enhancing the hydraulic pump efficiency (Kuo, 581). It is also applied in the hybrid technology of electric and hydraulic power. The main reason behind this is to greatly reduce the energy consumption in the systems. There is also the application of hydraulic systems in the robotic picking bins. In this case, the robotic system that uses the proprietary 3D-vision system to automatically pick up the parts that have been stacked in a random manner and in the litter bins. Use of robotic hands applies the knowledge and technology of hydraulic control systems. They are being developed to aid in meeting demands of any targeted production line, including the general purpose use, high functionality, and even the durability and the compact size of the structures. Use of hydraulics has also been witnessed in the development of off-line programming tool. It is a software product whose main purpose is to reduce drastically time spent on teaching or tutoring on line. The software creates robot motion program automatically and then proceeds to evaluate the robot motion at off-line. Use of hydraulics has also been witnessed in the construction of work Instruction System for an Assembly Cell. The Instruction System aids a worker to grasp and fully learn the assembly processes. The monitor of this system gives a procedural display of the instructions in work so that the worker can easily design their product. Also the system checks important data, such as number the frequency of use of the torque wrench and the number of complete processes signals for automated devices used in the assembly cell. Works Cited Jerman, Hal. "Electrically-activated, normally-closed diaphragm valves." Solid-State Sensors and Actuators, 1991. Digest of Technical Papers, TRANSDUCERS'91., 1991 International Conference on. IEEE, 1991. Burdock, William. "Hydraulic control systems." U.S. Patent No. 6,206,383. 27 Mar. 2001. Kuo, Benjamin C., and M. Farid Golnaraghi. Automatic control systems. Vol. 4. New York: John Wiley & Sons, 2003. Masters, Keith. "Spray drying handbook." Spray drying handbook. Ed. 3 (1979). Alfayad, Samer, et al. "High performance integrated electro-hydraulic actuator for robotics–Part I: Principle, prototype design and first experiments." Sensors and Actuators A: Physical 169.1 (2011): 115-123. Read More