Pumps PDF

Pumps and working principle Dr. Anoop Kumar Gupta Assistant Professor, IIT Patna FM (CB203) Pumps Pump is a mechanical device that is used to provide energy to a fluid to move it from one place to another. A device that raises, transfers, delivers, or compresses flui...

Pumps and working principle Dr. Anoop Kumar Gupta Assistant Professor, IIT Patna FM (CB203) Pumps Pump is a mechanical device that is used to provide energy to a fluid to move it from one place to another. A device that raises, transfers, delivers, or compresses fluids or that attenuates gases especially by suction or pressure or both. It assists to increase the pressure energy / kinetic energy / potential energy, or all three of the fluid by converting mechanical energy. 2 FM (CB203) Classification of Pumps Centrifugal pump 3 FM (CB203) Classification of Pumps Classification based on method of displacement: 1. Positive displacement pumps – Makes a fluid move by trapping a fixed amount and displacing the trapped volume into discharge pipe. Applies direct pressure to the fluid. Volumetric flow rate fixed by displacement per cycle of the moving component (either reciprocating or rotating) and the cycles per unit time. 2. Rotodynamic pumps – Produces head and flow by increasing the velocity of the liquid with the help of a rotating vane impeller. Uses torque to generate rotation. Kinetic energy imparted by the angular momentum of the impeller. 4 FM (CB203) FM (CB203) Positive displacement pumps Operates by drawing fluid into an expanding cavity on the suction side and then discharging the fluid as the cavity collapses into the discharge side. The discharge volume will be a constant for each cycle of operation. Can be divided into 2 classes: ‒ Reciprocating positive displacement pumps (piston, plunger, diaphragm pumps) ‒ Rotary positive displacement pumps (gear, lobe, screw, vane, peristaltic pumps) 6 FM (CB203) Reciprocating positive displacement pumps Reciprocating pumps move the fluid using one or more oscillating pistons, plungers, or membranes (diaphragms), while valves restrict fluid motion to the desired direction. Classified as Single acting and Double acting pumps. Single-acting pumps discharge on either the forward or return stroke of the piston or plunger; every cycle of the pump displaces only one volume of liquid. In double-acting pumps, liquid is discharged on both the forward and return stroke of the piston. 7 FM (CB203) In Diaphragm pumps, Reciprocating member is a flexible diaphragm made of polymers (plastic), rubber or metal. Can handle only small amount of liquids, and moderate pressure. Commonly used to handle hazardous or toxic fluids. 8 FM (CB203) Gear pumps, lobe pumps, screw pumps Also called rotary pumps that displaces fluid by the rotating action of the moving element. Do not require check valves (fluid movement only in one direction). Close tolerance between the moving and stationary parts minimize the leakage and reverse suction. Best for viscous fluids. External gear/Spur Internal Gear Pump for gear Pump high viscosity fluids 9 FM (CB203) Lobe pump Peristaltic (roller) pump Progressive cavity or screw pump 10 FM (CB203) Comparison Centrifugal Pumps Reciprocating Pumps Runs at high speeds and result in high Speed is less; low discharge discharge Less wear and tear because of less moving More wear and tear due to larger number parts of moving parts Can handle grit and slurry Can handle clean liquids only Occupy less space Need larger space Less maintenance cost and high initial Less capital cost. expense Uniform flow Non uniform flow unless modified Requires priming Priming not required 11 FM (CB203) Centrifugal pumps Blades/vanes 12 FM (CB203) The foot valve is a one-way valve that opens in the upward direction. The strainer is used to filter the unwanted particle. FM (CB203) Components of a Centrifugal pump A centrifugal pump consists of (1) impeller, (2) casing, (3) suction pipe, and (4) delivery pipe. Impeller – is the rotor with a series of backward curved vanes / blades, mounted on to a shaft connected to a motor. Casing – an airtight chamber surrounding the pump impeller. Inside the casing, area of flow (termed as Volute) gradually increases from the impeller outlet to the delivery pipe. Suction pipe – pipe that connects the center / eye of the impeller to the sump from which the fluid is to be lifted. The suction pump is provided with a strainer and a non- return valve Delivery pipe – connects the outlet of the pump to the required heights Since the area in the volute increases, the velocity decreases. The velocity head is thus converted to pressure head on the fluid. 14 FM (CB203) Definition of various heads Suction head (hs): vertical height of the centerline of pump above the surface of tank from which fluid is to be lifted. Delivery head (hd): vertical distance between the centerline of the pump and liquid surface in the tank to which liquid is delivered. Static head (Hs): Hs = hs + hd Manometric head (Hm): Head against which a centrifugal pump has to work. pi/ρg = hs, po/ρg = hd , Zo = hs+hd, Zi = 0, Vo = Vd, Vi = 0 FM (CB203) Pump overall efficiency (η): ratio of power output of the pump to the power input to the pump Power output (W) = Weight of lifted water  H m  W  H m   gQ  H m (Water horsepower) Power input = power supplied by the electric motor Brake horsepower (bhp) = power input =  T   (  gQ  H m ) Power input  = angular speed (rad/s) T = shaft torque Problem: Find power required to drive a centrifugal pump which delivers 0.04 m3/s of water to a height of 20 m through a 15 cm diameter pipe and 100 m long. Overall efficiency of pump is 70%. Friction coefficient = 0.15. FM (CB203) Similarity analysis (model and prototype) N Q N Q Specific speed (N s )   3/4     3/4   H m m  H m  p N = impeller speed (rpm)  Hm   Hm  Head coefficient:     D = impeller diameter  DN   DN  P = Power requirement (W)  m  p Q = discharge (m3/s)  P   P  Power coefficient:  3   3    D 5 N m   D N  p 5 Capacity coefficient:  Q3    Q3   D N m  D N  p FM (CB203) Main characteristic curves FM (CB203) Centrifugal pumps – Characteristic curves Pump performance curves. Plot the following parameters against flow rate (Q): head (H), power input (P), pump efficiency (η) and NPSH.  gQH  Pin 19 FM (CB203) Operating characteristic curves (Speed N constant) Note: Even at zero discharge some power Shut off pressure is required to overcome mechanical losses, BEP = best efficiency point therefore input power curve does not pass through the origin. FM (CB203) Priming When centrifugal pump is filled with gas or vapour, the pump will not function – condition termed as “vapour lock”. Though the head developed by the pump will be the same, pressure is proportional to the fluid density. So for gas, the pressure will be several orders of magnitude lower than that for liquid. For the pump to function, the pump should be filled with liquid before it starts – termed as “priming”. Generally, the pump would be placed below the liquid level to enable self priming. Priming is a process in which impeller is kept fully submerged in liquid without any air trap when there is first start up. 21 FM (CB203) Priming of centrifugal pump FM (CB203) Cavitation in centrifugal pumps Centrifugal pumps increase fluid energy by imparting angular momentum, then converted to pressure head in the volute. Fluid velocity is highest around the impeller; pressure is the lowest where velocity is the highest. If anywhere the pressure decreases below the vapour pressure of the liquid, boiling will occur, forming vapour bubbles of liquid. This is called Cavitation. At higher temperature, the vapour pressure is higher  more likely to occur cavitation. The bubbles will then be transported to regions of higher pressure and collapses, creating local shock waves that damages pump. Considerable noise and vibrations are produced during cavitation. 23 FM (CB203) Cavitation in centrifugal pumps FM (CB203) Maximum suction lift (height) hs to avoid cavitation Apply Bernoulli’s equation between point ‘0’ and ‘1’ 1 Patm 0 Therefore,   FM (CB203) To avoid cavitation, the pressure at the inlet of pump (point 1) should not be less than vapour pressure of liquid, thus for the limiting case p1 = pv (vapour pressure of liquid)  where    Maximum allowable suction lift (height) to avoid cavitation. If suction height of pump is more, the vaporization of liquid at pump inlet will take place resulting into cavitation. FM (CB203) NPSH (Net positive suction head) To prevent cavitation, a minimum pressure exceeding the vapor pressure of the liquid by a certain value should be maintained inside the pump. This minimum excess pressure depends on the pump design, impeller size and speed, and flow rate. “Net positive suction head” is the minimum absolute head at the pump intake (difference b/w absolute stagnation pressure at pump suction and liquid vapour pressure) for which the pump will operate without cavitation.  p1 pv  vs2     NPSH where pv is the vapour pressure of the liquid.   g  g  2 g 1 Applying Bernoulli’s equation between points 0 & 1, p1 pa vs2 0   h h  g  g 2 g s fs where hs is the vertical distance between the center line of the pump and the free liquid surface in the sump. 27 FM (CB203) NPSH p1 pv vs2  pa vs2  pv vs2 This equation gives  NPSH       hs  h fs    available NPSH (NPSHA)  g  g 2g   g 2g   g 2g The larger is the NPSH, the  p  pv   h  h  a g  s fs  lesser are the chances of cavitation.  H a  H v  hs  h fs Generally, NPSH value is about 2 to 3 m for small centrifugal pumps, but increases with pump capacity, impeller speed, and discharge pressure. For large pumps, NPSH value might increase up to 15 m. Knowing the NPSH specified by the pump manufacture, a pump could be placed at the required height (hs) to avoid cavitation if the pressure on the free surface (point 0) and the frictional losses in the suction pipe is known. 28 FM (CB203) Available and required NPSH Net Positive Suction Head Available (NPSHA) is a measure of the pressure at the suction of a pump, while NPSH Required (NPSHR) is the minimum pressure required to prevent cavitation. Value of NPSHR (required) is given by pump manufacturer based on experiment testing to get maximum efficiency corresponding to minimum hs without any objectionable noise (cavitation free). To suppress cavitation: Available NPSH (NPSHA) > Required NPSH (NPSHR) NPSHR can be determined from performance curve. NPSHR increases with flow rate (Q) while NPSHA decreases with Q. At critical Q, NPSHA = NPSHR, this flow rate should not be exceeded to maintain the flow free from cavitation. NPSH Thoma's Cavitation factor,     c (critical, from empirical relation) Hm