Industrial Machinery Repair Part Episode 2 Part 7 pdf

Figure 21.1 is a simplified diagram of a typical centrifugal pump that showsthe relative locations of the pump suction, impeller, volute, and discharge.The pump casing guides the liquid f

Cut end of packing not staggered •

seal Internal

Seal not compatible with application •

Source: Integrated Systems Inc.

Seal Flushing

When installed in corrosive chemical applications, mechanical seals musthave a clear water flush system to prevent chemical attack The flushingsystem must provide a positive flow of clean liquid to the seal and alsoprovide an enclosed drain line that removes the flushing liquid The flowrate and pressure of the flushing liquid will vary depending on the specifictype of seal but must be enough to assure complete, continuous flushing

Packed Boxes

Packing is used to seal shafts in a variety of applications In equipmentwhere the shaft is not continuously rotating (e.g., valves), packed boxescan be used successfully without any leakage around the shaft In rotatingapplications, such as pump shafts, the application must be able to toleratesome leakage around the shaft

Nonrotating Applications

In nonrotating applications, packing can be installed tightly enough to vent leakage around the shaft As long as the packing is properly installedand the stuffing-box gland is properly tightened, there is very little probabil-ity that seal failure will occur This type of application does require periodicmaintenance to ensure that the stuffing-box gland is properly tightened orthat the packing is replaced when required

pre-Rotating Applications

In applications where a shaft continuously rotates, packing cannot be tightenough to prevent leakage In fact, some leakage is required to provideboth flushing and cooling of the packing Properly installed and main-tained packed boxes should not fail or contribute to equipment reliabilityproblems Proper installation is relatively easy, and routine maintenance islimited to periodic tightening of the stuffing-box gland

Precision measurement is an important part of any maintenance procedure.Without micrometers, telescopic gauges, dial calipers, edge finders, andother precision measuring tools, the job cannot be done correctly Theareas covered in this chapter are:

1 The proper use of an outside micrometer

2 The proper use of an inside micrometer

3 The proper use of telescopic gauges

4 The proper use of dial calipers

Micrometers

Precision measurement is an important part of the correct installation

of equipment One of the most important precision measurement toolsavailable to the technician is the micrometer

A difference of 0.001" may not seem important for most purposes, but someparts of equipment or tools must fit even more closely than that, even asclose as 0001"

The most common type of micrometer is operated by a screw that has

40 threads to the inch Each revolution of the screw moves the ing spindle 0.025" A scale revolving with the screw is divided into 25 partsand indicates, therefore, the fractions of a turn in units of 0.001"

5 3

1 0 2

22 24

Figure 20.1 Outside micrometer

Frame

ThimbleSpindle

Anvil

Lock

5 3 2 1 3

22

24 0

Sleeve

Ratchetstop

Figure 20.2 Defining parts of a micrometer

Standards

Standards are used to check the accuracy of the micrometers These areprecision blocks that are cut to an exact measurement The micrometer isthen used to measure the standard The measurement on the micrometermust match that of the standard If there is any variation then the micrometermust be adjusted

Let’s take a look at the names for the specific parts of the micrometer (seeFigure 20.2)

The scale on the sleeve is graduated in 025" The scale on the thimble isgraduated in 001" See Figures 20.3 and 20.4

Now let’s see if we can put the two parts together and come up with ameasurement Write down the measurement for the following drawing SeeFigure 20.5

22 23 24 3

.024"

.023"

.022"

Figure 20.4 Micrometer scale

To find the measurement we start at the sleeve and add 600"+ 075" +.000"= 675"

Vernier Scale

When using a micrometer, there may be a need to take measurements thatare closer than 001" When this is necessary, a micrometer with a Vernierscale is used

A Vernier scale will make measurements to within 0001 (one thousandth) of an inch The Vernier scale is located on top of the sleeveand is read by lining up the lines on the sleeve with those on the thimble

ten-In the Figure 20.6, we can see that the reading is 350", but we know that

in order to take the reading to within 0001" we must also line up the lines

0 1 2

200510

Figure 20.5 Micrometer

0 5

20

3 2 1 0

Figure 20.6 Micrometer readings

on the thimble with the lines on the sleeve So, our actual reading would

be 3501"

Inside Micrometer

In addition to outside micrometers, you must also become familiar withinside micrometers Inside micrometers work the same as outside micro-meters, except that they measure inside dimensions See Figure 20.7

2 3 4 5 6

1

12 14 16 18

Figure 20.7 Inside micrometer

5 3 1

1 22 24 3 0

Figure 20.8 Telescoping gauges measuring inside diameter

⫺0⫹

50

.001"

10 20 30 40 60

5 3 1

1 22 24 3 0

Figure 20.10 Exercise 1

an outside micrometer Measure each drill bit and compare the reading thatyou get to the size on the drill index Write down the answers that you getand keep them, because you will need them in exercises that follow SeeFigure 20.10

Inside Micrometer

Now let’s try some inside measurements with the inside micrometer.Remember, this is similar to the outside micrometer, only you are measuringinside dimensions You will need an assortment of pillow-block bearings toperform these exercises

First note the dimension that is stamped on the outside of the bearing, thenconvert this from fractions to decimals To do this simply divide the topnumber (numerator) by the bottom number (denominator) The problemwill look like this:

Let’s say that the bearing size is34" Just divide the top number by the bottomnumber, which will give you the decimal equivalent

3

4 = 75Your measurement should be 75 on the micrometer scale Remember towrite down your answers, as you will need them in another exercise SeeFigure 20.11

counter-7 0 6

1 22 24 3 0

Figure 20.12 Exercise 3

50

.001"

10 20 30 40 60

in 100 of an inch, and the round dial is graduated in 001 of an inch SeeFigures 20.13 and 20.14

Wrap-Up Exercise

Using the same pillow-block bearings and drill bits that were used in theprevious exercises, measure the objects and compare the measurement thatyou get with the dial caliper to that of the micrometers If you did notwrite down your answers from the previous exercise, then repeat the otherexercises along with this one

Centrifugal Pumps

Centrifugal pumps basically consist of a stationary pump casing and animpeller mounted on a rotating shaft The pump casing provides a pres-sure boundary for the pump and contains channels to properly direct thesuction and discharge flow The pump casing has suction and dischargepenetrations for the main flow path of the pump and normally has a smalldrain and vent fittings to remove gases trapped in the pump casing or todrain the pump casing for maintenance

Figure 21.1 is a simplified diagram of a typical centrifugal pump that showsthe relative locations of the pump suction, impeller, volute, and discharge.The pump casing guides the liquid from the suction connection to the cen-ter, or eye, of the impeller The vanes of the rotatingimpeller impart a

radial and rotary motion to the liquid, forcing it to the outer periphery ofthe pump casing, where it is collected in the outer part of the pump casingcalled the volute

Thevolute is a region that expands in cross-sectional areas as it wraps around

the pump casing The purpose of the volute is to collect the liquid discharged

Impeller eye

Suction

Volute Impeller

Figure 21.1 Centrifugal pump

Single Double

Figure 21.2 Single and double volute

from the periphery of the impeller at high velocity and gradually cause areduction in fluid velocity by increasing the flow area This converts thevelocity head to static pressure The fluid is then discharged from the pumpthrough the discharge connection Figure 21.2 illustrates the two types ofvolutes

Centrifugal pumps can also be constructed in a manner that results intwo distinct volutes, each receiving the liquid that is discharged from a

180 degrees region of the impeller at any given time Pumps of this type arecalled double volute pumps In some applications the double volute mini-mizes radial forces imparted to the shaft and bearings due to imbalances inthe pressure around the impeller

Characteristics Curve

For a given centrifugal pump operating at a constant speed, the flow ratethrough the pump is dependent upon the differential pressure or headdeveloped by the pump The lower the pump head, the higher the flowrate A vendor manual for a specific pump usually contains a curve of pumpflow rate versus pump head called a pump characteristic curve After a pump

is installed in a system, it is usually tested to ensure that the flow rate andhead of the pump are within the required specifications A typical centrifugalpump characteristic curve is shown in Figure 21.3

There are several terms associated with the pump characteristic curve thatmust be defined.Shutoff head is the maximum head that can be developed

Shutoff head

PumprunoutFlow rate

Figure 21.3 Centrifugal pump characteristics curve

by a centrifugal pump operating at a set speed.Pump run-out is the point

where a centrifugal pump can develop the maximum flow without damagingthe pump Centrifugal pumps must be designed to be protected from theconditions of pump run-out or operating at shutoff head

Protection

A centrifugal pump is deadheaded when it is operated with a closed charge valve or against a seated check valve If the discharge valve is closedand there is no other flow path available to the pump, the impeller willchurn the same volume of water as it rotates in the pump casing This willincrease the temperature of the liquid in the pump casing to the point that

dis-it will flash to vapor If the pump is run in this conddis-ition for a significantamount of time, it will become damaged

When a centrifugal pump is installed in a system in such a way that it may

be subjected to periodic shutoff head conditions, it is necessary to providesome means of pump protection One method for protecting the pumpfrom running deadheaded is to provide a recirculation line from the pumpdischarge line upstream of the discharge valve, back to the pump’s supplysource The recirculation line should be sized to allow enough flow throughthe pump to prevent overheating and damage to the pump Protection mayalso be accomplished by use of an automatic flow control device

Centrifugal pumps must also be protected from runout One method forensuring that there is always adequate flow resistance at the pump discharge

to prevent excessive flow through the pump is to place an orifice or a throttlevalve immediately downstream of the pump discharge.

Gas Binding

Gas binding of a centrifugal pump is a condition in which the pump casing

is filled with gases or vapors to the point where the impeller is no longerable to contact enough fluid to function correctly The impeller spins in thegas bubble but is unable to force liquid through the pump

Centrifugal pumps are designed so that their pump casings are completelyfilled with liquid during pump operation Most centrifugal pumps can stilloperate when a small amount of gas accumulates in the pump casing, butpumps in systems containing dissolved gases that are not designed to beself-venting should be periodically vented manually to ensure that gases donot build up in the pump casing

Priming

Most centrifugal pumps are not self-priming In other words, the pumpcasing must be filled with liquid before the pump is started, or the pumpwill not be able to function If the pump casing becomes filled with vapors

or gases, the pump impeller becomes gas-bound and incapable of pumping

To ensure that a centrifugal pump remains primed and does not becomegas-bound, most centrifugal pumps are located below the level of the sourcefrom which the pump is to take its suction The same effect can be gained bysupplying liquid to the pump suction under pressure supplied by anotherpump placed in the suction line

Classification by Flow

Centrifugal pumps can be classified based on the manner in which fluidflows through the pump The manner in which fluid flows through thepump is determined by the design of the pump casing and the impeller.The three types of flow through a centrifugal pump are radial flow, axialflow, and mixed flow

Radial Flow

In a radial flow pump, the liquid enters at the center of the impeller and

is directed out along the impeller blades in a direction at right angles to

VoluteImpeller

Figure 21.4 Radial flow centrifugal pump

Impeller

Figure 21.5 Typical axial flow centrifugal pump

the pump shaft The impeller of a typical radial flow pump and the flow isillustrated in Figure 21.4

Axial Flow

In an axial flow pump, the impeller pushes the liquid in a direction parallel

to the pump shaft Axial flow pumps are sometimes called propeller pumpsbecause they operate essentially the same as the propeller of a boat Theimpeller of a typical axial flow pump and the flow through a radial flowpump are shown in Figure 21.5

Impeller Volute casing

Multistage Pumps

A centrifugal pump with a single impeller that can develop a differentialpressure of more than 150 psid between the suction and the discharge isdifficult and costly to design and construct A more economical approach

to developing high pressures with a single centrifugal pump is to includemultiple impellers on a common shaft within the same pump casing.Internal channels in the pump casing route the discharge of one impeller tothe suction of another impeller Figure 21.7 shows a diagram of the arrange-ment of the impellers of a four-stage pump The water enters the pump fromthe top left and passes through each of the four impellers, going from left

to right The water goes from the volute surrounding the discharge of oneimpeller to the suction of the next impeller

Apump stage is defined as that portion of a centrifugal pump consisting

of one impeller and its associated components Most centrifugal pumps aresingle-stage pumps, containing only one impeller A pump containing sevenimpellers within a single casing would be referred to as a seven-stage pump,

or generally as a multistage pump

Figure 21.7 Multistage centrifugal pump

Components

Centrifugal pumps vary in design and construction from simple pumps withrelatively few parts to extremely complicated pumps with hundreds of indi-vidual parts Some of the most common components found in centrifugalpumps are wearing rings, stuffing boxes, packing, and lantern rings Thesecomponents are shown in Figure 21.8 and are described on the followingpages

Impellers

Impellers of pumps are classified based on the number of points at which theliquid can enter the impeller and also on the amount of webbing betweenthe impeller blades

Impellers can be either single-suction or double-suction A single-suctionimpeller allows liquid to enter the center of the blades from only onedirection A double-suction impeller allows liquid to enter the center ofthe impeller blades from both sides simultaneously Figure 21.9 showssimplified diagrams of single- and double-suction impellers

Impellers can be open, semi-open, or enclosed The open impeller consistsonly of blades attached to a hub The semi-open impeller is constructedwith a circular plate (the web) attached to one side of the blade Theenclosed impeller has circular plates attached to both sides of the blades