Power Consuming Fluid Machines BY Dr. P M V Subbarao Mechanical Engineering Department I I T Delhi Small Things Make Huge Events possible!!!

Power Consumption for Human Needs Few basic amenities responsible Industrialization & Urbanization. One of the important need responsible for Motive Power. Pumps, Fans, Blower, Compressors etc. Fluid Machines which need mechanical Power. Mechanical power of a shaft is converted into Fluid Power. Significant feature : Rise in Pressure or Position of the fluid. Volume changes are very small to very high. Main Goals: Maximization of Efficiency and Maximization of Specific fluid handling.

Power Consuming Machines First Laws for SG in SSSF Mode: Q cv W CV m i m e Always a negative value of Power. Heat transfer is discouraged… Ideal Machines are Reversible Isothermal. Relatively Better are Isentropic machines. Practically better are Polytropic Low frictional Machines.

Polytropic Process in Fluid Machines Shaft power Disc Power Fluid Power. Flow Machines & Non Flow Machines. Compressible fluids & Incompressible Fluids. Rotary Machines & Reciprocating Machines.

Pump Rotate a cylinder containing fluid at constant speed. Supply continuously fluid from bottom. See What happens? Flow in Any More Ideas?

Definition of A Pump 1.Physiology. A molecular mechanism for the active transport of ions or molecules across a cell membrane. 2.Physics. Electromagnetic radiation used to raise atoms or molecules to a higher energy level. 3.Informal. The heart.

Definition of A Pump 1.A pump stands essentially as the earliest form of machine for substituting natural energy for human physical effort. 2.A machine or device for raising, compressing, or transferring incompressible fluids.

Centrifugal Pump

Axial Flow Pump

The role of A Pump in a Fluid Handling System Elevation or Static Head Suction Head Delivery Head

Basic Pump Terminology Suction Head or Lift: A pump is expected to suck fluid from a lower elevation. Delivery Head: Pump should transport the fluid to an higher elevation. The entire fluid will be enclosed in a piping system. A finite mass flow rate in a finite diameter pipe is possible only if fluid attains a velocity. Fluid requires kinetic power to possess velocity. Suction Velocity: V s : Velocity of flow in suction pipe. Delivery Velocity: V d Any fluid flow is associated with frictional energy loss. A pump which can supply an excess of friction loss can only satisfy the pumping requirements.

Relation of Pressure to Elevation In a static liquid (a body of liquid at rest), the pressure difference between any two points is in direct proportion only to the vertical distance between the points. Calculate this pressure difference by multiplying the vertical distance by the density (or vertical distance x density of water x specific gravity of the fluid). Static Head The hydraulic pressure at a point in a fluid when the liquid is at rest. Friction Head The loss in pressure or energy due to frictional losses in flow. Discharge Head The outlet pressure of a pump in operation. Total Head The total pressure difference between the inlet and outlet of a pump in operation. Suction Head The inlet pressure of a pump when above atmospheric pressure. Suction Lift The inlet pressure of a pump when below atmospheric pressure.

Actual Burden on A Pump Kinetic power mv s 2 /2 Frictional loss in suction piping Kinetic power mv d 2 /2 Frictional loss in de;overy piping Pump power or System power:

Pump Head or System Head:

Classification of Pumps

Centrifugal Pumps Centrifugal pumps: the fluid enters the centre of a rotating impeller, where it is accelerated along the impeller blades. The fluid gains kinetic energy which is converted in pressure energy in the diffuser. The performance of centrifugal pumps depends on a big number of parameters and a satisfactory solution to face with the problem is dimensional analysis. Two explicit variables are of our interest in the characteristics of pumps: the pressure rise and the power consumption. It is expected the pressure rise, ΔP (N/m 2 ), depends on: Impeller size, D (m); rotation rate, N (s -1 ); volumetric flow rate, Q (m 3 /s); fluid properties: density ρ (kg/m 3 ) and viscosity µ (kg/ms);

Axial Flow Pumps

Differential head of pump:

, C H = head coefficient, C Q = flow coefficient, Re = Reynolds number

A similar analysis may be carried out for the power consumption of the pump, P:

ElectricalMechanicalThermodynamical η elec η mech η therm P T = VI P = P T η elec η mech η therm = P T η = ΔP Q

Constant impeller diameter Constant impeller speed Capacity Head Power

Affinity Laws for Centrifugal Pumps

Head Vs Flow Rate & Selection of Operating Point

PUMPS Running Parallel

Operation of Pumps at Low Flows Just like there are many forms of cavitation, each demanding a unique solution, there are a number of unfavorable conditions which may occur separately or simultaneously when the pump is operated at reduced flows. Some include: Cases of heavy leakages from the casing, seal, and stuffing box Deflection and shearing of shafts Seizure of pump internals Close tolerances erosion Separation cavitation Product quality degradation Excessive hydraulic thrust Premature bearing failures Each condition may dictate a different minimum flow low requirement. The final decision on recommended minimum flow is taken after careful “techno-economical” analysis by both the pump user and the manufacturer.

Cavitation As the liquid flows onto the impeller of the pump it is accelerated and initially its pressure falls (Bernoulli). The pressure subsequently increases as the fluid leaves the impeller and as the kinetic energy is recovered in the volute chamber. If the pressure of the liquid falls below the vapour pressure, P v, the liquid boils, generating vapour bubbles or cavities-cavitation. The bubbles are swept into higher pressure regions by the liquid flow, where they collapse creating pressure waves and cause mechanical damage to solid surfaces. Moreover, pump discharge head is reduced at flow rates above the cavitation point. Operation under these conditions is not desirable and damages the equipment.

NPSH (Net Pressure Suction Head). Net Positive Suction Head Required, NPSHr NPSH is one of the most widely used and least understood terms associated with pumps. Understanding the significance of NPSH is very much essential during installation as well as operation of the pumps. Pumps can pump only liquids, not vapors Rise in temperature and fall in pressure induces vaporization NPSH as a measure to prevent liquid vaporization Net Positive Suction Head (NPSH) is the total head at the suction flange of the pump less the vapor pressure converted to fluid column height of the liquid.

NPSHr is a function of pump design NPSH required is a function of the pump design and is determined based on actual pump test by the vendor. As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH required is the positive head in feet absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. The NPSH is always positive since it is expressed in terms of absolute fluid column height. The term "Net" refers to the actual pressure head at the pump suction flange and not the static suction head.

NPSHr increases as capacity increases The NPSH required varies with speed and capacity within any particular pump. The NPSH required increase as the capacity is increasing because the velocity of the liquid is increasing, and as anytime the velocity of a liquid goes up, the pressure or head comes down. Pump manufacturer's curves normally provide this information. The NPSH is independent of the fluid density as are all head terms. Note: It is to be noted that the net positive suction head required (NPSHr) number shown on the pump curves is for fresh water at 20°C and not for the fluid or combinations of fluids being pumped.

Net Positive Suction Head available, NPSHa Net Positive Suction Head Available is a function of the system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction, to be sure that the pump selected does not cavitate. It is calculated based on system or process conditions

NPSHa in a nutshell NPSHa = Pressure head + Static head - Vapor pressure head of your product – Friction head loss in the piping, valves and fittings. “All terms in feet absolute” In an existing system, the NPSHa can also be approximated by a gauge on the pump suction using the formula: NPSHa = hp S - hvp S  hg S + hv S hp S = Barometric pressure in feet absolute. hvp S = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute. hg S = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline. hv S = Velocity head in the suction pipe at the gauge connection, expressed in feet. NPSHa should always be greater than NPSHr

Performance of A Damaged Impeller

Performance with Reduced Throat Area

Pump with Minor Wears

Pump with Excessive Wear

Pump with rough impeller & casing

Pump with lower NPSH