Chapter 6: PUMPS*

The Goals Describe how centrifugal and positive-displacement pumps operate and common applications.Calculate system head requirements.Determine head, pump efficiency, and pump. horsepower from a typical centrifugal pump curve.Define net positive suction head (NPSH) and understand how it relates to cavitation.Compute NPSH required by a pump.Determine an appropriate pump (impeller diameter, efficiency, etc.) for a given required head.Describe how to modify system to operate on the appropriate pump curve.

BackgroundFluid Moving EquipmentFluids are moved through flow systems using pumps, fans, blowers, and compressors. Such devices increase the mechanical energy of the fluid. The additional energy can be used to increaseVelocity (flow rate)PressureElevation

Backgroundpumps move liquids while compressors add energy to gasses.Pump is a device which converts mechanical power into fluid power, while turbine converts fluid power into mechanical power. Mechanical power is usually obtained by shaft rotation

Pumps and fans do not appreciably affect the density of the fluids that they move and thus incompressible flow theory is applicable.

There are two basic types of pumps: positive-displacement and dynamic or momentum changepumps. There are several billion of each type in use in the world today.

1- Positive-displacement pumps (PDPs) force the fluid along by volume changes. A cavity opens, and the fluid is admitted through an inlet. The cavity then closes, and the fluid is squeezed through an outlet.

Classification of Pumps

A. Reciprocating 1. Piston or plunger 2. DiaphragmB. Rotary 1. Single rotor a. Sliding vane b. Flexible tube or lining c. Screw d. Peristaltic (wave contraction) 2. Multiple rotors a. Gear b. Lobe c. Screw d. Circumferential piston

A brief classification of PDP designs is as follows:

(a) reciprocating piston or plunger, (b) external gear pump,(c) double-screw pump, (d) sliding vane, (e) three-lobe pump, (f) double circumferential piston, (g) flexible-tube squeegee.

A Schematic design of positive-displacement pumps:

2- Dynamic pumps simply add momentum to the fluid by means of fast-moving bladesor vanes or certain special designs. There is no closed volume: The fluid increases momentum while moving through open passages and then converts its high velocity to a pressure increase by exiting into a diffuser section.

Dynamic pumps can be classified as follows:

A. Rotary 1. Centrifugal or radial exit flow 2. Axial flow 3. Mixed flow (between radial and axial)B. Special designs 1. Jet pump or ejector 2. Electromagnetic pumps for liquid metals 3. Fluid-actuated: gas-lift or hydraulic-ram

Dynamic pumps generally provide a higher flow rate than PDPs and a much steadier discharge but are ineffective in handling high- viscosity liquids. Dynamic pumps, also generally need priming; i.e., if they are filled with gas, they cannot suck up a liquid from below into their inlet. The PDP, on the other hand, is self-priming for most applications. A dynamic pump can provide very high flow rates (up to 300,000 gal/min) but usually with moderate pressure rises (a few atmospheres). In contrast, a PDP can operate up to very high pressures (300 atm) but typically produces low flow rates (100gal/min).The relative performance (p versus Q) is quite different for the two types At constant shaft rotation speed, the PDP produces nearly constant flow rate and virtually unlimited pressure rise, with little effect of viscosity. The flow rate of a PDP cannot be varied except by changing the displacement or the speedThe dynamic pump, provides a continuous constant-speed variation of performance, from near-maximum p at zero flow (shutoff conditions) to zero p at maximum flow rate. High-viscosity fluids sharply degrade the performance of a dynamic pump.Comparisons Between the Two types

Comparison of performance curves of typical dynamicand positive-displacement pumps at constant speed.

Positive Displacement PumpsTo move fluids positive displacement pumps admit a fixed volume of liquid from the inlet into a chamber and eject it into the discharge. Positive displacement pumps are used when higher head increases are required. Generally they do not increase velocity.

Positive Displacement PumpWorks on the principle of letting fluid flow into a cavity from a low-pressure source, trapping the fluid, and forcing it out to a high-pressure receiver by decreasing the volume of the cavitySimplest pump that can be found anywhere from liquid soap dispensers, to automobile fuel injectors, to the human heart.

Centrifugal PumpsMost common type of pumping machinery. There are many types, sizes, and designs from various manufacturers who also publish operating characteristics of each pump in the form of performance (pump) curves.

Pump curves describe head delivered, pump efficiency, and net positive suction head (NPSH) for a properly operating specific model pump.

Centrifugal pumps are generally used where high flow rates and moderate head increases are required.

Centrifugal PumpBased on the concept of raising the pressure of a liquid indirectly by increasing the kinetic energy via the centrifugal action of the impeller and converting this kinetic energy to fluid workUsed predominantly for high-flow applications, less expensive, and less complex thereby minimizing maintenanceMust be pre-charged with liquid or else it wont pump at start-up. Positive displacement pumps dont have this limitation.

Cavitation

A centrifugal pump increases the fluid pressure by first imparting angular momentum (or kinetic energy) to the fluid, which is converted to pressure in the diffuser section. Hence, the fluid velocity in and around the impeller is much higher than that either entering or leaving the pump, and the pressure is the lowest where the velocity is highest.

The minimum pressure at which a pump will operate properly must be above the vapor pressure of the fluid; otherwise the fluid will vaporize (or boil), a condition known as cavitation.

Obviously, the higher the temperature the higher the vapor pressure and the more likely that this condition will occur. When a centrifugal pump contains a gas or vapor it will still develop the same headCentrifugal Pump

Centrifugal Pumps

Impeller

Pump CurvesPumps from manufacturers are typically rated by how much fluid work that can be achieved as a function of fluid flowFluid work in the pump curves are typically expressed in head form

Potential work decreases with increasing flow due to increased losses incurred at higher flow velocitiesh is the pump head that equal to the change in pressure head flow between point 1, the eye, and point 2, the exit, as Z1=Z2=0 & V1=V2 around pump

Pump EfficiencyThe power delivered to the fluid simply equals the specific weight times the discharge times the net head change Pw=ghQ (hp) This is traditionally called the water horsepower. The power required to drive the pump is the brake horsepower bhp=T (hp) where is the shaft angular velocity and T the shaft torque. If there were no losses, Pw and brake horsepower would be equal, but of course Pw is actually less, and the efficiency of the pump is defined as

The chief aim of the pump designer is to make as high as possible over as broad a range of discharge Q as possible.The volumetric efficiency is

where QL is the loss of fluid due to leakage in the impeller-casing clearances. (1 hp 550 ft lbf/s = 746 W)

A pump delivers gasoline at 20C and 12 m3/h. At the inlet, p1 = 100 kPa, z1 = 1 m, and V1 = 2 m/s. At the exit p2 = 500 kPa, z2 = 4 m, and V2 = 3 m/s. How much power is required if the motor efficiency is 75%? Example

Centrifugal Pump Performance 1/2The operating characteristics of a pump are shown by plotting the total pump head ( h), pump power (P), pump efficiency () and the (NPSH)R versus the flow rate (Q), any rpm, impeller diameter and liquid viscosity.[Net Positive Suction Head ] Required to avoid cavitation BEP is the best effective point which led to design flow rate (Q)

In a particular system, a centrifugal pump can only operate at one point on the h against Q curve and that is the point where the pump h against Q curve intersects with the system h against Q curve as shown. The intersection point should be near to the best efficiency point of the pump. The point of intersection is called duty point. the system total head at a particular liquid flow rate.

Centrifugal Pump Performance 2/2duty pointWhere s for suction side & d for discharge side

Pump MapFor a given flow rate and pump head, the manufacturers pump map would indicate the operation of the pump at the physically available sizes

NPSH : is the difference between the absolute pressure head at the pump inlet and the absolute vapor pressure head of the liquid being pumped suction discharge side side Pa PvapNet Positive Suction Head(NPSH), which is the head required at the pump inlet to keep the liquid from cavitating or boiling. The pump inlet or suction side is the low-pressure point where cavitation will first occur.

NPSH Net Positive Suction HeadIn centrifugal pumps, the fluid must be brought up to the rotational speed of the impeller blades.Increasing the fluid velocity would result in a decrease in pressureThis can cause boiling of the fluid or cavitation around the eye of the impeller. To prevent this, there must be elevation of the fluid before the pump.This height is known as the net positive suction head (NPSH).

PaZahfsNet Positive Suction HeadIf the pump inlet is placed at a height Zi above a reservoir whose free surface is at pressure pa, we can use Bernoullis equation to rewrite NPSH aswhere h f i is the friction-head loss between the reservoir and the pump inlet. Knowing pa and h f i, we can set the pump at a height Zi which will keep the right-hand side greater than the required NPSH plotted in Figure of pump mapNPSH must be positive and larger than [NPSH]R

The 32-in pump of Figure below is to pump 24,000 gal/min of water at 1170 r/min from a reservoir whose surface is at 14.7 lbf/in2 absolute. If head loss from reservoir to pump inlet is 6 ft, where should the pump inlet be placed to avoid cavitation for water at (a) 60F, Pv = 0.26 lbf/in2 absolute, SG 1.0 and (b) 200F, Pv = 11.52 lbf/in2 absolute, SG 0.9635?Example