pumps

• 1. Topic: PUMPS-PIPINGS CENTRIFUGAL PUMPS Pumps are machines that are used to transfer liquid from a location of low elevation to a higher elevation. Classifications: A centrifugal pump consists of a stationary casing and an (1) Dynamic or kinetic impeller connected in rotating shaft. Types of pumps in which energy is continuously added to the fluid to increase its velocity. Centrifugal Jet Turbine pumps (2) Positive displacement Types of pump in which energy is continuously added by application of force to an enclosed volume of fluid and resulting to a direct increase in its pressure. Reciprocating Rotary Diaphragm DISADVANTAGES Poor suction power. Needs multistage to increase discharge pressure. Usually needs priming. Cannot handle very viscous fluid. Cavitation may develop during operation. Check valve is required to avoid back flow. ADVANTAGES Simple and compact. Little vibrations. Easy to maintain. Flow can be controlled from full to non-discharge Adaptability to motor with high rpm. without shutting the pump. PUMP FUNDAMENTALS Water or Hydraulic Power, WP Water or hydraulic power is the energy added by the impeller to the water or fluid as it moves through the pump. WP = Qγ W TDH Where: WP=water or hydraulic power, kW or hp . 1 hp = 0.746 kW ⎡ g ⎤ mW ⎡ go ⎤ . γ W = ρW ⎢ o ⎥ = . ⎢ ⎥ Q= V =pump capacity or volume flow rate, m3/s ⎣ gC ⎦ VW ⎣ gC ⎦ γ W = specific weight of water or fluid For water at standard condition @ 4oC: . ⎡g ⎤ SI units: 9.81 kN/m3 WP = m W ⎢ o ⎥ TDH Eng units: 62.4 lb/ft3 ⎣ gC ⎦ . mW = mass flow rate of water, kg/s or lbm/sec TDH=total dynamic head, m or ft The Total Dynamic Head, TDH Total dynamic head or also known as “pump head” is the total amount of work needed by pump, (usually measured in meters or feet) per specific weight of water flowing through the pump. It is the sum of the elevation head, velocity head, pressure head and head losses. Assumption: Suction Lift Conservation of Energy: [E IN = E OUT ] −PE1 + KE1 + Wf1 + U1 + WP = PE 2 + KE2 + Wf 2 + U2 WP = (PE 2 + PE1 ) + (KE2 − KE1 ) + (Wf 2 − Wf1 ) + (U2 − U1 ) If we assume that the temperature from the suction and discharge of the pump are almost equal, t1 ≈ t 2 , then we can say that the . . change in internal energy is negligible, ΔU ≈ 0 . We can also assume a steady state pumping process for the pump, V 1 ≈ V 2 . DAY 10 Copyright 2010 www.e-reviewonline.com
• 2. Topic: PUMPS-PIPINGS . WP = mW . go gC 2 gC ( (z 2 + z1 ) + 1 mW v 2 2 − v12 + V W (P2 − P1 ) . ) gC Multiplying both sides of the equations by . . mW go gC WP = (z 2 + z1 ) + ( 1 v 2 − v1 2 2 )+ (P 2 − P1 ) . 2 go . mW go m W ⎡ go ⎤ ⎢ ⎥ . g VW ⎣ C ⎦ TDH = gC WP = (z 2 + z1 ) + ( 1 v 2 − v1 2 2 )+ (P 2 − P1 ) . 2 go . mW go mW ⎡ go ⎤ ⎢ ⎥ . g VW ⎣ C ⎦ Therefore: TDH = (z 2 + z1 ) + ( 2 1 v 2 − v1 2 ) + (P 2 − P1 ) 2 go γW If we take into considerations all the head losses, hL due to fluid friction in the pipes, elbows, valves and fittings we will have: TDH = (z 2 + z1 ) + ( 2 1 v 2 − v1 2 ) + (P 2 − P1 ) + hL 2 go γW TDH = (total static head) + (velocity head) + (pressure head) + (friction head) TDH = hS + hV + hP + hL Total Static Head, hs Static head is the vertical distance where the pump must lift the water. It measured from the water level surface to the discharge point. Static Suction Lift Suction lift is present when the water source is below the pump centerline. Static Suction Head Suction lift is present when the water source is above the pump centerline. Velocity Head, hV Velocity head is the energy possessed by the fluid because of its velocity. Velocity head is usually very small that it can be neglected in the calculation. Pressure Head, hP Pressure head is the pressure required for the pump to distribute water properly. A 60 psi water pressure is equivalent to 138.5 feet of water. Friction Head, HL Friction head is the head associated with the decrease in pressure due to friction when the fluid is flowing through the pipes, valves and fittings. Pump Efficiency The pump efficiency is the ratio of the water or hydraulic power output of the pump over the brake power input to the pump. WP ηP = * 100% BP PUMP SPECIFIC SPEED Pump specific speed is a dimensionless parameter Where: Ns = specific speed of the pump, rpm used to describe hydraulic features of centrifugal pumps. It is Q = capacity of the pump, gpm defined as the revolution per minute at which a given 1 gallon = 3.78 Liters geometrically similar impeller of a pump would operate if H = pump head per stage, ft reduced proportionally in size so as to deliver a rated capacity 1 meter = 3.28 feet of 1 gallon per minute against a head of 1-foot. Note: For double suction pumps Q is divided by 2 and for multi-stage pumps, H is divided by the number of stages. N=impeller speed, in revolutions per minute N Q NS = H3 / 4 DAY 10 Copyright 2010 www.e-reviewonline.com
• 3. Topic: PUMPS-PIPINGS NET POSITIVE SUCTION HEAD, NPSH Net Positive Suction Head, NPSH, is an index where the pump may operate without cavitation. Cavitation Cavitation occurs when the pressure at any point inside the pump drops below the vapor pressure corresponding to the temperature of the liquid. The liquid vaporize and forms vapor bubbles as it enters the inlet of the pump. This vapor bubbles then collapse or implodes at the surface of the impeller creating tremendous physical shock to the edges of the impeller. NPSH Required NPSHr for a particular pump is experimentally determined and provided by the manufacturer and is a function of the pump design. NPSH Available NPSHa is determined by plant designer during the design and proposed installation of the pump and is a function of the system where the pump will operate. To avoid cavitation: NPSHa ≥ NPSHr NET POSITIVE SUCTION HEAD AVAILABLE, NPSHa NPSHa = hP ± hSL − hV − HL Where: hP = absolute pressure head on the surface of the liquid source, in meters. This will be the atmospheric pressure corresponding to its altitude when the liquid surface is open. hSL = the height of the liquid surface from the pump center line, designated as positive when suction head and negative when suction lift, in meters hV = head corresponding to the vapor pressure of the liquid at liquid temperature, can be determined using steam tables, in meters HL = head loss due to friction and turbulence, in meters If NPSHa ≤ NPSHr , there are two possible options available: (1) Decrease the suction lift by changing the plant layout and raising the source on which the pump draws water. (2) Reduce the suction head by using pumps with larger capacity but operating it in partial loads or speeds. PUMP AFFINITY LAWS Pump affinity laws are rules that express the relationship of pump capacity, head and BHP when the speed or impeller diameter is changed. In applying the following equations, we consider that the efficiency is the same for both conditions. Impeller Speed, N, held diameter, D, constant Where: Q = Capacity, in gpm held constant N1 = N2 H = Total head, in feet D1 = D2 BHP = Brake horsepower, in hp Q1 N Q1 D N = Pump speed, rpm Capacity = 1 = 1 Q2 N2 Q2 D2 Subscript 1 refers to initial condition and subscript 2 refers to the new condition. 2 2 H1 ⎛ N1 ⎞ H1 ⎛D ⎞ Head =⎜ ⎟ =⎜ 1 ⎟ H2 ⎜ N2 ⎝ ⎟ ⎠ H2 ⎜D ⎝ 2 ⎟ ⎠ 3 3 BHP1 ⎛N ⎞ BHP1 ⎛D ⎞ BHP =⎜ 1 ⎜N ⎟ ⎟ =⎜ 1 ⎜D ⎟ ⎟ BHP2 ⎝ 2 ⎠ BHP2 ⎝ 2 ⎠ Characteristics of Reciprocating Pumps b. Considering Piston Rod VD = π 2 4 π ( D LN + D2 − d2 LN 4 ) 2. Slip = VD - Q 3. Percent slip Q % slip = 1 − 1. Piston displacement VD a. Neglecting Piston Rod ⎡ D2 ⎤ ⎡ c a (n / 60) 2 ⎤ 4. Volumetric Efficiency, η V From: VD = ⎢π ⎥L⎢ ⎥ ⎣ 4⎦ ⎣ s ⎦ s=2 for pumps and compressors η V = 1 − slip c=1 and a=2 Q ⎛ V1 ⎞ ⎡π ⎤ ηV = = 1+c −c ⎜ ⎟ VD = 2 ⎢ D2LN⎥ VD ⎜V ⎟ ⎝ 2 ⎠ ⎣ 4 ⎦ DAY 10 Copyright 2010 www.e-reviewonline.com
• 4. Topic: PUMPS-PIPINGS Pump Installation Pumps in Series Pumps in Parallel Pumps in series are done by staging two pumps as Pumps in parallel are the result of installing two shown in Figure. The total dynamic head is increased at a pumps as shown in Figure. The capacity is doubled while given capacity as shown in the performance curve. maintaining the total dynamic head. Pipe Friction Head Pipe is defined as a closed conduit in which fluids flow. Piping systems are very important in transporting liquid or gasses like in oil refineries, air conditioning, water works, and food processing industries. Pressure Losses The total pressure loss in a piping system is composed of major and minor losses. In this section, we will tackle the major and minor frictional losses caused by pipe surface friction, changes in fluid velocity and direction. Major Losses, hf Major losses or also known as pipe friction losses, hf, they are losses along the pipe surface and are assumed to be uniform as long as the size and the type of material of the pipe remain constant. Minor Losses, hL Minor losses, hL, consist of losses due to the following: Contraction of pipe cross section, hc Enlargement of pipe cross section, he Obstructions such as valves, gates and fittings, hg Bends or curves, hb The total losses, HL, in a pipe line are the sum of the major and minor losses; HL = hf + hL HL = hf + (hc + he + hg + hb) Darcy-Weisbach Equation Darcy-Weisbach equation expresses the loss of head Reynolds Number, Re in pipes in terms of the velocity head. Reynolds number is a dimensionless parameter discovered by Osborne Reynolds in 1883, which determines L v2 whether the flow in a closed conduit or a pipeline is laminar or hf = f turbulent. D 2go Dvρ Where: f = resistance coefficient or friction factor Re = Note: If tables are not available, use f = 0.02 μ L = Total pipe length including valves and fittings equivalent length, m Where: D = Inside diameter of the pipe, m D = Inside diameter of the pipe, m v = Liquid velocity, m/s v = Liquid velocity, m/s ρ = Density of the fluid, kg/m3 go = observed gravitational acceleration, m/2 μ = Dynamic viscosity, Pa-s or Poise dyne − s Note: 1 poise = 1 = 0.1 Pa-s m2 If the dynamic viscosity is divided by the density, kinematic viscosity, ν, can be obtained: μ ν = ρ Therefore; Dv Re = ν Where: ν = Kinematic viscosity, m2/s Note: 1 stroke = 1 cm2/sec DAY 10 Copyright 2010 www.e-reviewonline.com
• 5. Topic: PUMPS-PIPINGS Friction Factor, f, for Turbulent and Laminar Flow If R e ≥ 3000 the flow is turbulent and the friction If R e ≤ 2000 the flow is said to be laminar, and the factor, f, will be dependent on the Reynold’s number and the friction factor, f can be determined using the relationship: relative roughness: 64 f = ⎛ ε⎞ Re f = ∫ ⎜R e , ⎟ ⎝ D⎠ Relative Roughness Relative roughness is the ratio of the pipe inside surface irregularities (absolute roughness), ε , to the pipe diameter, D. ε Re lative Roughness = D Minor Losses Minor losses are determined using the relationship: Where: kL = loss coefficient for contraction, enlargement, gate, valves, fittings and v2 bends. hL = ∑ k L Note: The values of kL (if not given) can be 2go found in published tables Pipes of different diameters connected in series Figure shows a simplified diagram of flow in a series pipeline with different diameters connecting two Total head loss: reservoirs. HL = hf1 + hf2 + hf3 Volume flow rate: Q = Q1 = Q2 = Q3 Pipes connected in parallel Figure shows a simplified diagram of flow in a parallel pipeline connecting two reservoirs. The pipe Total head loss: line 1 from reservoir A divides the flow at C and then HL = hf1 + hf2 + hf4 joins again at D. Pipeline 4 leads to reservoir B. hf2 = hf3 Volume flow rate: Q1 = Q2 + Q3 Q1 = Q4 DAY