List the two classifications of pumps.

kinetic (dynamic) and positive displacement

Capacity

Cavitation

Cavitation is the formation of vapor bubbles in eye of the pump and the subsequent collapse of the vapor bubbles in the impeller or in the volute.

When the pressure at the eye of the impeller is less than saturation pressure, some of the liquid water changes phase from a liquid to a gas, and vapor bubbles form. The bubbles collapse when the bubbles enter an area where the pressure is greater than the saturation pressure. This is usually in the vanes of the impeller

Cavitation can be detected by its distinctive sound. The pump sounds like it is pumping rocks. This noise is produced when the vapor bubbles collapse.

Indications of cavitation are:

·           Fluctuating pump discharge pressure,

·           Fluctuating pump motor current, and

·           Excessive pump noise (pump sounds like it is pumping rocks).

·           Fluctuating pump flow rate

If cavitation occurs, follow plant procedures and manufacturer and vendor guidelines. These steps may include the following, not necessarily in this order.

1.      Verify the suction valve is open.

2.      Reduce the speed of the pump.

3.      Reduce the temperature of the fluid entering the pump.

4.      Increase pressure on the suction of the pump. Pressurize the head (expansion) tank or raise the height/volume of liquid in the head tank.

5.      Throttle closed the discharge valve of a centrifugal pump.

Pump efficiency (h_p)

Ratio of work required by ideal pump to work required by real pump

[image]

Where:

Velocity Head

Suction Head

Static Suction Head

Static suction head describes energy available due to actual vertical distance between fluid level and eye of pump, as well as pressure exerted on fluid surface.

[image]

Total Suction Head

Total suction head corresponds to total suction lift. However, it is sum of static suction head, velocity head, and frictional losses in suction piping and fittings

[image]

Discharge Head

Discharge head is total energy of fluid at discharge of pump. It is design characteristic equal to number of feet pump moves liquid up pipe.

 [image]

Total Discharge Head

Includes static discharge head (L_D), friction losses (h_loss), and velocity head (h_velocity).

[image]

Pump Head

Total Static Head

Total Head

Total head (also referred to as total dynamic head or TDH) is difference between total discharge head and total suction head

[image]

 [image]

Suction Lift

Static Suction Lift

Static suction lift is actual work required to lift fluid to eye of pump for given set of conditions.

 [image]

Total Suction Lift

Total suction lift is sum of static suction lift, velocity head, and all frictional losses in suction piping and fittings

[image]

Net positive suction head (NPSH)

The difference between pump inlet, or suction, pressure and saturation pressure of fluid being pumped is called available net positive suction head.  The required NPSH is the amount of available NPSH that must be provided to prevent cavitation.

[image]

General trends for parameter adjustments

Centrifugal Pump

Centrifugal pumps create flow by moving the fluids faster and faster in a circular motion, and then changing the increased speed of the fluid into pressure. The kinetic energy (speed) is transformed into flow energy (pressure).

Casing

Gaskets

 Impeller

The impeller is the rotating component of the pump that converts mechanical energy of prime mover (the pump motor) into kinetic energy (speed) in fluid.

Volute

The volute is gradually expanding spiral that is integral to casing.  It reduces fluid velocity and increases fluid pressure.  The interaction of fluid flowing through impeller and volute is what makes centrifugal pump work.

Shaft

The shaft connects prime mover to impeller.  This may be done either directly or through flexible coupling.  The impeller is mounted on pump shaft.  The impeller is held on shaft with key and keyway and is pressed on/off shaft.

Bearings

The shaft is supported by bearings.  Bearings provide two types of support, radial support for side to side motion and axial support for movement along axis.  The radial support is provided by anti-friction journal bearings.  The bearings that provide axial support are called thrust bearings.

Seal Rings

The casing rings and wearing rings provide the seal needed between the impeller and the casing. 

Shaft Seals

There must be seal where shaft penetrates casing to prevent pumped fluid from leaking out around rotating shaft.  This is accomplished with shaft seal.  A shaft seal can be stuffing box or mechanical seal.

Centrifugal Pump Image

Volute Pump

In volute pumps, casing forms chamber called volute, which widens at discharge point of pump.  Fluid entering volute at high rate of speed is forced to fill widening chamber.  As fluid fills this larger area, it slows down, and its energy of motion (kinetic energy) is converted into pressure (flow energy).

Diffuser Pump

In diffuser pumps, volute is enhanced with diffuser and series of stationary vanes arranged around impeller.  The diffuser vanes direct fluid outward as it leaves impeller.  Since vanes are farther apart at their outermost points than at edge of impeller, they create series of widening chambers that convert kinetic energy into pressure in same way that volute does. The diffuser vane helps balance radial thrust loads on impeller, shaft, and journal bearings for varying flow conditions.

Volute and Diffuser Pump Image

Radial Flow Pump

In radial-flow pumps, for example, impeller is designed to direct fluid out from shaft at 90° angle.  By radically changing direction of flow and directing fluid through volute, radial-flow pumps are able to create high discharge pressure.

[image]

Axial Flow Pump

Axial-flow pumps can move tremendous volumes of fluid but at relatively low discharge pressures.  Axial flow impellers provide flow where main flow through pump is along axis of shaft.  The impeller of axial-flow pump is basically propeller, moving fluid along path parallel to shaft (axis) of pump.  Because pump does not use centrifugal force to add energy to fluid, or volute to convert speed of fluid to pressure, it can generate only lower discharge pressures. Therefore, axial-flow pumps are used where high volume and relatively low discharge pressure is required (e.g., Circulating Water System).

  [image]

Mixed Flow Pump

Mixed-flow pumps combine, to some degree, functions of radial-flow pumps and axial-flow pumps.  Mixed-flow impellers are designed to discharge fluid at angle greater than 0° (that of axial-flow pumps) and less than 90° (that of radial-flow pumps) from shaft. 

[image]

Regenerative Pump

The term “regenerative” actually describes action of pump during its operation and method it uses to build up required discharge pressure.  In this type of pump, fluid is introduced on both sides of impeller.  This helps balance axial thrust on impeller.  As impeller rotates, water is circulated both in direction of rotation and in circular motion on face of impeller and discharge passage in pump casing.  This inner rotation of fluid within discharge passage and its interaction with impeller gives pump its name, “regenerative”.  The impeller in regenerative pump resembles turbine wheel of steam turbine more closely than centrifugal or propeller type impeller.  Blades on impeller give pump its ability to operate and pump water.

[image]

Centrifugal Pump NPSH

To prevent cavitation, pump must have some minimum positive pressure at pump suction.

 [image]

Starting and venting a centrifugal pump

• Before operating centrifugal pump, driver should be tested for its direction of rotation.
• Cooling water should be introduced carefully to pump bearings and lubricating oil.
• A centrifugal pump should not be operated until it is filled with liquid.
• Centrifugal pumps are started with discharge valve closed, because pump operates at only 35-50% of full load when discharge valve is closed.

Operation with a centrifugal pump dead headed

• Overheating will occur if pump is operated against closed valve ("dead headed") for more than few minutes.

Minimum flow

Bypass or minimum flow line prevents pump from operating at shutoff head.  The smallest amount of flow that prevents overheating is called pump minimum flow requirement.

Prevention of runout

Proper venting and filling of systems before startup helps prevent runout.

Shutoff Head

Shutoff head - maximum value of head that pump can produce with pump filled and operated at normal speed with discharge valve shut

At shutoff head, pump cannot overcome downstream pressure.  The resulting friction increases pump and fluid temperature.  Overheating of pump may occur at this point.  If pump discharges directly to atmosphere, pump capacity is at its maximum

Runout

Runout - abnormally high flow rate from centrifugal pump, can lead to mechanical stress on pump and excessive motor current.  Motor currents will be high and oscillating, not steady.

Runout is result of loss of downstream pressure (piping not filled and vented, leak or pipe break).  The drop in pressure speeds up pump and causes excessive motor current, which may result in damage to windings.  The low backpressure can also result in cavitation and excessive flow. 

Describe the operation of jet pumps

Jet pumps are static (that is, have no moving parts) devices that convert high pressure driving flow developed by external source into high velocity jet flow at low pressure.

The high velocity jet is surrounded by fluid to be moved.  The low pressure at outlet of jet nozzle draws surrounding fluid into throat or mixing section, where low pressure suction or driven flow mixes with driving flow.  The mixed fluid flows into divergent diffuser, where expanding area converts velocity back to high pressure.  This results in high pressure, high volumetric output pump.

[image]

Reciprocating Pump

Reciprocating pumps feature piston, plunger, or diaphragm, with movement back and forth (reciprocating) within cylinder or casing.  A reciprocating pump may be defined as pump that operates using back and forth, straight-line motion.

[image]

Rotary Pump

Rotary pumps, on other hand, have gears, vanes, rotating plungers, or screw-type rotors that rotate to move fluids.

 [image]

 Positive Displacement Pump NPSH

An increase in positive displacement pump speed causes available NPSH for the pump to decrease due to the decrease in the suction pressure.

Starting a positive displacement pump

Open the suction and discharge valves.

Operation with a positive displacement pump deadheaded

Need to have an outlet, i.e. a relief valve, otherwise things will break.

A positive displacement pump should never be started with the discharge blocked or the discharge valve closed. Additionally, the discharge valve should never be closed with a positive displacement pump running.

The relief valve at the discharge of most positive displacement pumps protects against the potential damage from operating the pump dead-headed. For this reason, proper operation of the relief valve should be checked periodically.

3 Centrifugal Pump Laws

Characteristic curves for centrifugal pumps

Characteristic curves for positive displacement pumps

Centrifugal pumps in parallel arrangements

Parallel operation of centrifugal pumps is obtained by having two pumps discharge into same header.  This type of configuration is useful when demand for volume fluctuates.  One pump can be operated when demand is low, and second pump can be started and used when fluid flow requirement increases beyond capacity of first pump.  By having two pumps discharge into common system, capacity is increased while same pressure is maintained. Capacity is just slightly less then doubled, if the pumps are similar.

 [image]

Centrifugal pumps in series arrangements

Series operation, as name suggests, combines two or more pumps in line with each other.  In this case, first pump discharges into suction of second, this raises pump head at given capacity.

 [image]