Shuttle Pageantry - Term Paper Example

Reciprocating Pump The hydraulic machines that convert the mechanical energy into hydraulic energy are called as Pumps. The hydraulic energy is in the form of pressure energy. When the mechanical energy is converted into hydraulic energy by means of centrifugal force acting on the liquid, it is called as a Centrifugal pump. When the mechanical energy is converted to hydraulic energy by sucking the liquid into the cylinder in which a piston is reciprocating, which exerts the thrust on the liquid and increases its hydraulic energy, the pump is called as Reciprocating pump (Bansal 2009). (Image Source: www.catpumps.com) Reciprocating pumps are generally used where high-pressure service is required. In most applications exceeding 1,000 psig, reciprocating pumps are more suitable, particularly in low to medium capacity service. Reciprocating pumps operate at a constant flow rate regardless of the discharge pressure. There are specific applications that require either constant flow or variable flow. Metering pumps rely on a constant flow at varying pressures, which makes reciprocating pumps suitable for this application. Reciprocating pump are self priming pumps and hence best suited for applications where prime cannot be maintained on the pump. Reciprocating pumps are widely used in the petrochemical and water treatment industries where a high degree of accuracy and reliability is required. They are well suited for transferring clear, non-abrasive fluids, as well as abrasive slurries. Reciprocating pumps have relatively low velocities of moving parts this make it particularly resistant to erosion in abrasive-slurry applications. Reciprocating pumps maintain high efficiencies when pumping highly viscous fluids and can easily handle 50% and higher volumes of entrained gas. Classification of Reciprocating pumps: 1. As per Axis: a) Horizontal pump: The axial centerline of the cylinder is horizontal. b) Vertical pump: The axial centerline of the cylinder is vertical. 2. As per component used in the pump: a) Piston pump: A piston is used for suction and discharge. b) Plunger pump: A plunger is used for suction and discharge. 3. As per Discharge per stroke: a) Single-acting pump: Liquid is discharged only during the forward stroke of the plunger or piston, that is. The discharge is achieved only during half the revolution. b) Double-acting pump: Liquid is discharged during both the forward and return strokes of the piston or pair of opposed plungers. Discharge is achieved during the entire revolution. 4. Number of Pistons/Plungers: a) Simplex pump: Consists of one piston or one plunger or a pair of opposed plungers driven by one connecting rod. b) Duplex pump: Consists of two pistons or two plungers or two pair of opposed plungers driven by two connecting rods. c) Multiplex pump: Consists of more than two pistons or two single-acting or opposed plungers. (Credit: The Hydraulic Institute, visit www.pumps.org for more information.) Working of Reciprocating pumps: A specific volume of liquid is sucked through the inlet inside the cylinder, and discharged at a higher pressure through the discharge. A piston or plunger is used to suck and discharge the fluid. The piston may be motor driven through reducing gears or a steam cylinder may be used to drive the piston rod directly. The maximum discharge pressure for piston pumps is about 50 atm. For higher pressure services plunger pumps are used. A reciprocating plunger is merely an extension of the piston rod. At the limit of its stroke, the plunger fills nearly all the space in the cylinder. Plunger pumps are usually are motor driven. They can discharge a pressure of 1500 atm. or more. Diaphragm pumps are be used to handle toxic or corrosive liquids. The reciprocating member is a flexible diaphragm of metal, plastic or rubber. They handle small to moderate amounts of liquid, and can develop pressures in excess of 100 atm. Discharge through a single acting Reciprocating Pump: Let D = Diameter of the cylinder A = Cross-section area of the cylinder = π/4 * D² r = Radius of Crank N = r. p. m. of the Crank L = Length of the stroke = 2 * r hs = Height of the axis of the cylinder from water surface in sump hd = Height of delivery outlet above the cylinder axis Volume of water delivered in one revolution = Area * Length of the stroke = A * L Number of Revolutions per second = N/60 Therefore Discharge of the pump per second, Q = Discharge in one revolution * No. of revolution per second = A * L * N/10 = ALN/60 Weight of water delivered per second, W = ρ * g * Q = ρgALN/60 Work done by Reciprocating Pump: Work done by the reciprocating pump per second is given by the relation as Work done per second = Weight of water lifted per second * Total height through which water is lifted = W * (hs + hd) Where (hs + hd) = Total height through which water is lifted. From equation, Weight, W, is given by W = ρgALN/60 Substituting the value of W, we get Work done per second = ρgALN/60 * (hs + hd) Therefore Power required to drive the pump, in kw P = Work done per second / 1000 = ρg * ALN * (hs + hd)/60 * 100 = ρg * ALN * (hs + hd)/60 000 Slip of Reciprocating Pump: Slip of Reciprocating Pump is defined as the difference between the theoretical discharge and the actual discharge of the pump. The actual discharge of the pump is less than the theoretical discharge due to leakage. Slip = Qth – Qact Percentage Slip = (Qth – Qact)/Qth * 100 = (1 – Qact/Qth) * 100 = (1 - Cd) * 100 Where Cd = Coefficient of discharge. Air Vessels: Air vessel is a closed cylindrical Cast Iron chamber connected to the delivery or suction pipe as close to the pump as possible. It contains water upto a certain level above which there is air. It acts as a temporary reservoir and damps down the fluctuations of flow. An air vessel fitted on the delivery side accumulates excess discharge when the discharge from the pump exceeds the average discharge and supplies it to the delivery pipe beyond the air vessel when the discharge from the pump is less than the mean discharge. Thus the rate of discharge and hence, the velocity is averaged out in the delivery pipe beyond the air vessel. Advantages of Reciprocating Pumps: At any given speed, reciprocating operate at a constant flow rate regardless of the discharge pressure. Reciprocating pumps are self priming Reciprocating pumps maintain high efficiencies when pumping highly viscous fluids and can easily handle 50% and higher volumes of entrained gas. Reciprocating pumps produce the highest pressure Disadvantages of Reciprocating pump: Reciprocating pumps require considerable maintenance The discharge id discontinuous Reciprocating pumps have high initial costs Practical example for overcoming discontinuous discharge: Pulse-Free Pump from NIKKISO (Image source: http://www.nikkiso.com/rd/main/005.html) A pulse-free pump, employs the characteristics of a reciprocating controlled-volume type pump, and enables the flow capacity to be accurately changed, and allows an extremely small volume of flow to be transported under high pressure. Since pulsation dampers, including an accumulator, are not required, a change in flow capacity does not result in flow delays. The pump responds immediately to changes in flow capacity and provides constant flow. References: Bansal, R.K., 2008. Fluid Mechanics And Hydraulic Machines., Laxmi Publication. About Pumps [Online] Available at: http://www.pumps.org/content_detail.aspx?id=1776 [Accessed 1st April 2010] Development of Pulse-Free Pump [Online] Available at: http://www.nikkiso.com/rd/main/005.html [Accessed 1st April 2010] Read More