NET 112:Nuclear Power Plant Components

NET 112:Nuclear Power Plant Components

Module 1: Pumps

Introduction

•A nuclear power plant consists of numerous pieces of equipment which are interconnected in such a way that they can perform their function as part of the complete system.

•Pumps: components that provide the motive force to move various fluids through the systems

•Valves: devices to control this flow (later)

Pumps

•One of the most common pieces of mechanical equipment found in a power plant

•Every operator is involved to some extent with the proper operation of a pump.

•Many different types of pumps available

•Utilize different principles in their operation

•PURPOSE:

–add energy to a fluid (liquid or gas)

–Energy increase causes pressure increase

–Added pressure permits flow of the liquid through a piping system to some area of lower pressure

•Application examples:

–Supply lubricant to moving parts

–Deliver coolant where it is needed

–Transport fluid (substances that can be pumped) from one location to another.

•Too important to just “set and forget”

–Must be carefully maintained and monitored to insure efficiency and reliability

•General purpose of all pumps is the same: to move fluids.

–Different things in different situations.

–Examples:

•Easy: Transporting water from A to B on the same horizontal level

•Less Easy: Transport water from an open reservoir 150ft up into a high-pressure cooling tower. Much more sophisticated pump required.

•Different pumps designed to fulfill different functions

•Some: large volumes of fluid at low pressure

•Others: small volumes of fluid at very high pressures

•Some: move viscous (thick and heavy) fluids like fuel oil or slurry

•Others: move clear, light fluids (like water) or gases (like air)

•Nonetheless, the basic purpose of a pump never changes: All pumps move fluids by adding energy and increasing pressure.

•Two most common types of pumps

–Centrifugal (kinetic energy) pumps

–Positive displacement pumps

Centrifugal Pumps: Basic Idea

•Spinning impeller vanes pick up fluid and “sling” it back out

•Most common pumps in use today

–Versatile

–Industrial workhorse

Positive Displacement Pumps: Basic Idea

•Apply direct force to fluid instead of just accelerating it

•Works according to a basic law of nature:

–No two objects can occupy same space at same time.

•Displaces (moves) the same volume of liquid with each pumping cycle

•Two classes:

–Reciprocating (back and forth) (e.g., piston)

–Rotary (e.g., gear pump)

Pump Theory: Fluid Mechanics

•The fundamental laws of fluid flow apply equally well to all fluids, but are DRAMATICALLY simplified when limited to a discussion of incompressible fluids.

•Most liquids are considered to be incompressible.

•Most gases (including steam) are compressible.

•To understand incompressibility, consider water flowing a pipe

–If no leaks and no water is added to or removed from the system, the amount of water flowing past any one point is equal to that flowing past any other point.

–Holds true regardless of any changes in pipe size or shape.

Pump Theory: Continuity Eqn.

•The Continuity Equation is a mathematical description of incompressible flow.

•For a simple piping system such as that shown below, the flow at point 1 must be the same as the flow at point 2 and point 3 as long as no water is added or removed.

Pump Theory: Continuity Eqn.

•The flow rate at any point in a pipe depends on the area available for flow and the velocity of the flow

•Since the flow is incompressible, and thus Vdot cannot change from point to point:

Energy

•Three energy types:

–Potential energy (due to gravity)

–Kinetic energy (due to velocity)

–Flow energy (due to fluid pressure)

Pump Theory: Energy Equation

•Law of Conservation of Energy

–Energy can be neither created nor destroyed.

–Can only be transformed from one form to another.

•Total energy at one point in a system must be the same at any other point in the system, if no energy is added or removed between the two points.

Pump Theory: Energy Equation

•Energy equation:

PE1 + KE1 + FE1 = PE2 + KE2 + FE2 = PE3 + KE3 + FE3

•If no elevation change (pipe is perfectly horizontal), then PE1 = PE2 = PE3 and these terms drop out to leave just kinetic and flow energy:

KE1 + FE1 = KE2 + FE2 = KE3 + FE3

•KE can be written in terms of velocity, and FE in terms of pressure:

v12 + P1 = v22 + P2 = v32 + P3

Pump Theory: Losses

•So far, we have only covered the ideal situation.

•Total energies at two points in a system are never exactly the same. Why??

•LOSSES: The inner walls of a pipe exert a friction force on the fluid and slow it down.

•Kinetic energy converted to heat

•The flow must be strong

enough to overcome this

friction.

•System factors that increase frictional losses:

–Increasing the velocity

–Decreasing the flow area (area so small that fluid can’t “get away from” walls)

–Increasing the length of piping

–Increasing the roughness of the inside of the pipe

•One more general rule dealing with fluid flow:

–Fluid always flows from an area of high total energy to an area of lower total energy.

–Simpler: fluid flows from a high pressure area to a low pressure (ONLY when pressure high and velocity low)

Pumps (In-Depth)

•PDPs can exert great amounts of pressure on a fluid and “self-prime” (do not have to be full of fluid when started)

•However, PDPs cannot move high volumes of fluid and are typically high maintenance relative to other types of pumps.

•This is where centrifugal pumps come in

Centrifugal Pumps (In-Depth)

•No pistons or cylinders

•Outer casing contains a circular impeller with a series of curved vanes extending out from the center.

•Impeller attached to a rotating shaft

•CP must be filled with fluid at all

times during operation.

•Fluid enters the pump through the suction eye at the center of the impeller

•Impeller vanes stir the water, causing the fluid to rotate with the impeller.

•Rapid circular motion creates a centrifugal force that pushes the water away from the center of the impeller

•Outward motion has two effects:

–Creates a suction or vacuum in the eye that draws in more water from the suction piping

–Moves the water to the outside edge of the impeller and against the casing wall

•The outside edge of a circular object travels faster than its center. Therefore, to keep up with the impeller, the water must continue to gain speed as it moves outward. This further increases the kinetic energy in the water.

•At the discharge edges of the impeller, the casing forms a volute.

•The shape of the volute changes the water’s KE into flow (pressure) energy.

•Volute is wider at its discharge point than at the point where water enters from the impeller… water has room to expand.

•Expansion causes water to slow down and to give up KE.

•Because energy cannot simply disappear, it is transformed into pressure, which forces the fluid out of the pump and through the discharge piping.

CPs: Casing

•Contains the fluid being pumped and directs the flow path of the fluid as it moves from the impeller to the volute and out through the discharge.

•Forms the volute and supplies connection points for suction and discharge piping and necessary fixtures.

•Houses the internal parts, holding them in their proper place and protecting them from dirt and damage.

•Usually consists of two halves held in place by casing bolts or studs.

•Seam between mount flanges of each half is sealed by a gasket

•Inside surface of casing shaped to accommodate internal parts, direct the flow path, and form the volute.

CPs: External Parts

•Parts that can be seen when the casing is intact. Include:

–Casing and volute

–Stuffing box

–Slop drain

–Flinger

–Gland follower

–Bearing housing

–Gland sealing line

•The stuffing boxes hold the packing gland, which regulates leakage along the shaft.

•Lower casing half often has small open wells for catching leakage. At the bottom of this well is the slop drain through which leakoff can be directed to a drainage system.

•Gland followers are bushings that hold the packing gland assembly and compress the packing in the stuffing box.

–Held in place by position-adjusting bolts

•Flingers are attached to the shaft with set screws.

–“Fling” leakoff from the packing gland away from the shaft, preventing the fluid from leaking into the bearings.

•Bearing housings support and protect the shaft bearings on either end of the pump.

–Held in place by housing bolts

•Some pumps have lines that supply sealing water to the packing gland.

–Sealing line delivers water to the stuffing box from impeller discharge or from an external source.

CPs: Internal Parts

•Visible only when the pump is drained and upper casing half is removed. Include:

–Impeller

–Volute

–Casing wearing rings

–Throat bushings

–Pump packing

–Lantern rings

•Impeller:

–Circular device with curved vanes that stir the fluid as the shaft rotates

–Sometimes vanes are enclosed between metal faces called shrouds.

–Intake water enters through the suction eye at the center of the impeller

–Water forced to the outside edge of the impeller by centrifugal force

–Impeller held in place on shaft in one of several ways, most often by a key and keyway.

•Impeller clearance and slip

–Space at all points between the impeller and the casing

–Impeller should never touch the casing at any point

–Clearance prevents unnecessary impeller wear

–Problem: water can leak from the discharge back into the suction eye through the gap between the impeller and casing. This is called slip.

–Slip results in the same fluid being pumped twice, lowering the efficiency of the pump.

•Casing wearing rings are installed between the suction eye and the casing.

–Minimize leakage and increase pump efficiency by sealing the space between the discharge and the suction eye.

–Do not rotate with the shaft and impeller

•If the wearing rings are fitted to the impeller, they are called impeller wearing rings.

–Rotate with the impeller.

–Not unusual for pumps to be fitted with both casing wearing rings and impeller wearing rings.

•Packing prevents leakage at the point where the shaft penetrates the casing.

•In multi-stage pumps (pumps with two or more impellers), water can leak between the different stages (impellers).

•To prevent such leakage, interstage diaphragms are installed between stages.

–Work similar to wearing rings

–Maintain close tolerance between rotating and stationary parts

Types of CPs

•All CPs operate on the same principle, but can differ in several ways

–Size

–Shape

–Differences in design

•Variations enable CPs to meet wide range of industrial requirements

–Low-pressure vs. high-pressure pumping

–Moving thin fluids vs. viscous fluids

–Pumping vertically vs. horizontally

•Centrifugal pumps are named according to certain design features

–Vertical or horizontal

–Single-suction or double-suction

–Single-stage, double-stage, or multi-stage

–Volute or diffuser

–Radial-flow, axial-flow, or mixed flow

•Name might also include information about the impeller

–Closed

–Open

–Semi-open

•Examples:

–Vertical, double-suction, single-stage, radial-flow, volute pump with an open impeller

–Horizontal, single-suction, multi-stage diffuser pump with closed impellers.

Vertical and Horizontal CPs

•Vertical and horizontal refer to position of shaft during normal operation

•Example: Vertical wet-pit pump

–Vertical shaft

–Motor positioned above the pump itself

–This arrangement allows the pump to be submerged in the fluid without causing water damage to the motor.

•Vertical pumps may be used where floor space is limited. Specific examples:

–Sump pumps

–Circulating water pumps

•Horizontal pumps

–Horizontal shaft

–Motor beside the pump

Single and Double-Suction CPs

•Single or double suction refers to number of suction eyes of impeller

•Single-suction: fluid enters through one side only

•Problem: low-pressure suction is on one side and the high-pressure discharge is on the other.

•Because of its higher pressure, the discharge exerts more thrust (force) on its side of the impeller than the suction exerts on its side.

•Impeller is constantly forced toward the suction.

•The constant thrust puts strain on the thrust bearing and can cause damage

•Most pumps with high discharge pressures either do not have single-suction impellers or are somehow counterbalanced.

Single and Double-Suction CPs

•Double-suction impellers are designed to eliminate this imbalance.

•Suction on each side

•Fluid enters the pump through a single intake port

•Fluid directed toward both sides of the impeller.

•Force is equal on both sides

Single and Multiple-Stage CPs

•Single-stage CP: one impeller

•Double-stage CP: two impellers

•Multi-stage CP: three or more impellers

•Purpose: produce higher discharge pressure than is possible with a single impeller

•Multi-stage pump

–Each of the impellers is a single-suction type

–Fluid enters through the suction eye of 1st stage, where it gains KE

–In the 1st volute, KE is converted into pressure

–Pressurized fluid is discharged into suction eye of 2nd stage

–2nd impeller gives fluid more KE, converted to additional pressure

–Fluid has thus been pressurized twice before it enters the 3rd impeller

–Pressure raised in each stage until pump discharge pressure is reached in the last stage

•Thrust on impellers: important consideration in designing multi-stage pumps

–Because impellers are usually single-suction

–Constantly unbalanced thrust against bearings

–Could result in excessive bearing wear

•2 common methods for minimizing thrust:

–Arrange impellers in opposition to one another

–Use a balancing device

•Figure below shows typical arrangement in which single-suction impellers oppose each other.

•Equal thrust is maintained in both directions along the shaft, thus preventing imbalance.

•Balancing devices

–Used to minimize thrust, usually hydraulic

–High pressure discharge of final stage is piped through the balance line

–Balance line connected to balance drum attached to the shaft.

–Thrust of discharge against drum opposes thrust of fluid against impellers, balancing the forces acting on the shaft.

Volute and Diffuser CPs

•Volute and diffuser describe the area in which CP converts KE of fluid into pressure.

•Volute pumps

–Casing forms a chamber (volute) that widens at discharge

–Fluid entering volute at high speed is forced to expand to fill the widening chamber

–As fluid expands, it slows down, and its KE is converted into pressure.

•Diffuser pumps

–Volute replaced by diffuser with series of stationary vanes arranged around impeller

–Diffuser vanes direct fluid outward as it leaves the impeller

–Since vanes are further apart at their outermost points than at the edge of the impeller, they create a series of widening chambers that convert kinetic energy into pressure in the same way that a volute does.

•Advantage of diffuser over volute is that it will reduce the radial distance required by a single volute.

Radial, Axial, and Mixed-Flow CPs

•Describe direction of fluid’s path through pump in relation to the shaft.

•Radial-flow pumps

–Impeller designed to direct fluid out from the shaft at a 90° angle

–High discharge pressure created by radically changing direction of flow and directing the fluid through volute

–Typically not large capacity

•Axial-flow pumps

–Move tremendous volumes of fluid

–Impeller is basically a propeller, moving the fluid along path parallel to shaft.

–Because pump does not use centrifugal force to add KE or a volute to convert KE to pressure, can only generate low discharge pressure.

–Thus not “true“ CPs, but classified as CPs because parts and maintenance requirements are similar to those of CPs

•Mixed-flow pumps: combine functions of radial and axial

•Impellers discharge fluid at angle greater than 0° (that of axial pumps) and less than 90° (that of radial pumps) from the shaft

•Angle of discharge

–Small enough to allow high-volume operation

–Large enough to impart some centrifugal force (higher discharge pressures)

•Used for high-volume, moderate-pressure applications

•Useful when a small amount of net positive suction head exists and cavitation is a potential problem (come back to this later)

Positive Displacement Pumps

•Constant, positive volume of liquid is displaced or moved during each pumping cycle.

•Volume equal to the volume displaced by the piston or other moving parts.

•PDPs can be further divided into two classes determined by the motion of the pumping element.

–Reciprocating (motion is back and forth

–Rotary (rotational motion)

•A reciprocating PDP consists of a cylinder, piston, piston rings, a suction valve, a discharge valve, and the casing.

•The piston moves back and forth within the cylinder, driven by the prime mover (motor)

•Each complete movement of the piston along the length of the cylinder is called a stroke.

•Starting from its position forward against the valve end of the cylinder, the piston moves back on the first stroke

•Suction created in valve end of cylinder

•This suction opens the suction valve, through which fluid enters the cylinder.

•On 2nd stroke, piston moves forward and presses against fluid, causing suction valve to close and discharge valve to open.

•Under pressure from piston, fluid escapes through discharge

•Valve

•Piston and fluid cannot occupy same space in cylinder at same time

•At end of 2nd stroke, piston is in its initial position, and cycle begins again.

Types of Positive Displacement Pumps