api - centifugal compressors

When the total energy stays the same, and energy in the form of velocity increases, energy in the form of pressure must increase / decrease.. EXHIBIT 2 STARTUP PROCEDURE — TURBINE DRIVE

CENTRIFUGAL COMPRESSORS

In the petroleum industry, gas is com- pressed for transportation to consuming markets and for refining processes This program is about the construction and operation of compressors

Unit 3 of Compressors teaches the oper- ating principles of centrifugal and axial compressors and the construction and operation of centrifugal compressors

INSTRUCTIONS

This is a programed learning course

Programed learning gives information in a series of steps

called frames Each frame gives some information and asks

you to make use of it

Here is how it works First, cover the response column at the

right with a mask

Read this frame and use the information it gives to fill in the

Move the mask down to uncover the word at the right of the

frame If you have filled the blank with that word or a word

that means the same, you are ready to go ahead to the next

frame

The drawing of a micrometer provides information that will

help you fill in the next blanks

RATCHET CAP

FRAME

Seven major parts are shown in the drawing, but only

the _and the contact the object

to be measured

small

anvil, spindle

The next frame calls for a choice Circle or underline the ap-

propriate word

Of the two parts that contact the object, only the (anvil/

spindle) moves

A program is a series of frames that work like the ones you

have just done

Read the frame

Use the information to fill in the blanks or make a

choice

Move the mask down and check the response column

Go on to the next frame

Remember to cover the response column with a mask before

you begin each page

Notice that the left-hand pages from here on are printed upside

down The program is designed so that you will go through all

the right-hand pages first, and then turn the book upside down

spindle

CENTRIFUGAL COMPRESSORS

Section One

POTENTIAL AND KINETIC ENERGY

Exhibits for this program are placed in the center of the book so

that they may be removed easily for reference Please remove

them now so that you will have them available when needed

10

11

12

To do work, some form of energy is needed An electric

motor needs _ energy

Under certain conditions, matter can do work A wound-up

clock spring ( can do work / cannot do work )

A moving hammer, due to its motion, ( can / cannot )

do work

Both the moving hammer and the wound-up spring possess

some kind of that enables them to do work

The hammer is moving and the spring is not

The energy of a moving hammer and the energy of a

wound-up spring is ( the same/ a different ) kind of energy

A moving hammer and flowing water possess kinetic

energy

Kinetic energy is the energy that a body possesses due

to its (motion / molecular arrangement )

A wound spring and still water behind a dam have potential

energy

Potential energy is the energy that a body possesses due

to its position or arrangement

When a spring is wound,_———————— is done on it

Because of the work that was done on it, the wound-up

spring possesses potential

Suppose a ball of iron is lifted 20 feet off the ground

Work was done in lifting the iron ball The ball, due to its

position, possesses _ energy

The ball is allowed to drop As the ball drops, it acquires

energy, due to its motion

When the ball hits the ground, it does

energy

potential

kinetic

work

Something possessing kinetic energy can do —

when it is slowed down or stopped

Work is done in lifting the ball Due to the work done on it,

the ball acquires energy

The potential energy of the ball is turned into

energy as the ball drops

The kinetic energy in turn can be converted into

—————— 8s the ball is stopped

Energy cannot be created or destroyed, but it can be

converted from one_————_———— to another

Potential energy can be converted into

When an amount of gas is compressed into a smaller

volume, the pressure of the gas ( increases / stays

the same )

To compress the gas, some — has to be

done to it

Compressed gas in a static state exerts its energy in the

form of pressure in all directions

When a gas is flowing, some of its energy in the form of

pressure is converted to energy in the form of motion in

(a single direction / all directions )

work

potential kinetic

work

form, or type kinetic

24 A compressed gas possesses potential energy due to the

27 Velocity is the speed of flow The higher the speed of flow,

28 If the gas is allowed to flow, some of its pressure is

converted into

29 The total energy of a flowing gas is a function of its

velocity, plus its 2

30 If no work is done and no energy is lost, the total energy

of a gas during flow ( changes / remains constant )

31 When the total energy stays the same, and energy in the

form of velocity increases, energy in the form of pressure

must ( increase / decrease )

32 If the flowing gas is slowed down so that its velocity is

decreased, then its pressure must

33 Anything possessing potential energy must have had some

—————————— done on it at some previous time

34 A gas with pressure has had done on it

35 By doing work on something, one ( can / cannot ) increase

its total energy

36 A compressor does work on a gas and thus adds or in-

creases the total of the gas

LAWS OF MOTION

37 When a car accelerates quickly from rest, the driver is

thrown ( forward / backward )

38 When the brakes are applied as the car is moving, the

driver is thrown ( forward / backward )

39 If the wheels of a moving car are turned, but there is no

traction, as on ice, the car tends to ( turn / continue in a

straight line )

40 A body at rest tends to remain at

41 A body in motion tends to continue in

42 When a body is in motion and there is no outside force

acting on it, it tends to continue in a ( straight line /

curved line )

43 When a driver turns his car, the car ( opposes / does not

oppose ) the change of direction

44 A body at rest remains at rest unless it is acted upon by

some outside

45 If a gas in a pipe is not flowing, the gas tends to ( remain

static / flow )

DYNAMIC COMPRESSORS

46 A dynamic compressor adds energy to gas in the same

manner that an electric fan does

The rotating blades of the fan force air to

47 Air that is at rest tends to remain at

48 As the fan blades start turning, they push on the air

resists

As the air resists the blades, the molecules of the air are

brought ( closer together / further apart )

When the air molecules are compressed, the volume of

the air ( decreases / increases )

As the volume of the air decreases, its pressure

The blades of the fan overcome the resistance of the air

and thrust the air forward

The faster the blades turn, the ( faster / slower ) the air

is pushed

The fan, by doing work on the air, actually increases the

and velocity of the air

When velocity and pressure are added to a gas, its total

A dynamic compressor increases total gas energy by

adding and to the gas

The total energy of a gas leaving a compressor is ( less

than / greater than ) the total energy of the gas entering

the compressor

The energy that a gas gains in a compressor is due to

the _ done on it

Centrifugal Compressors

58

59

Any body set in motion tends to continue in motion

If there is no gravity pull, nor any obstacle to deflect it,

any body in motion travels in a ( straight / curved ) line

Suppose a ball attached to a string is set in motion Assume

that there is no gravity and that the string has no effect

60 Suppose the string is fastened to a fixed pivot point and

then the ball is set in motion

FIXED PIVOT POINT

At first, the ball moves ( in the direction of motion /

toward the pivot )

61 When the string becomes taut, it deflects the ball

At each instant of its travel, the physical tendency of the

ball is to travelina_—————_—— line

But instead, the ball travels in a circle because it is held

or deflected by the

The string actually applies centripetal ( pulling-in-

toward-the-center ) force, causing the path of the ball

—————

If the string breaks, the ball flies out in a ( larger circle /

straight line )

Any object traveling in a circle is kept in that path of

travel by ( centripetal / centrifugal ) force

If the centripetal force is eliminated, the object then

moves ina———————— line

The force pulling an object in a circular path toward the

center is _ force

The centrifugal tendency of the object is its tendency to

pull away from the of rotation, or to pull

against the centripetal force

The centrifugal tendency acts in ( the same direction as /

the direction opposite to ) the centripetal force

71 The centrifugal tendency is actually not a force but is

the result of the tendency of the object to move in a

— _ line while being pulled toward a center

of rotation by force

72 A ball bearing is placed close to the center of a disc

that has blades

BALL BEARING BLADES

74 The drawing shows the actual path of the ball bearing

as the disc rotates

Centripetal ( pulling-in-toward-the-center ) force ( is /

is not ) acting on the bearing

75 Because of the lack of centripetal force, the bearing is

forced ( toward / away from ) the center of the disc

straight centripetal

move

straight

is not

away from

As the disc rotates, the ball bearing ( is / is not ) in

contact with the vane

This disc is rotating

For each rotation, point ( A/ B ) has the largest distance

to cover

When the disc is rotating, point ( A/ B ) moves faster

Anything that is being carried along by the rotation of the

disc has a greater velocity when it is near ( the center /

the outer rim ) of the disc

If anything being carried along by the rotation of the

disc also travels outward from the center to the outer

rim, it gains

This is a compressor impeller

PLATES

An impeller is made of two plates separated by

the outer rim

velocity

blades

Air molecules tend to travelina —————— — line

Because there is no centripetal force, the rotation forces

the air molecules outward from the_—_———_— — of

the impeller

As the air molecules move outward, they gain

The air also tends to oppose the push of the blades, so

the pressure of the air is

The impeller adds both _ and

to the air

The tendency of air or gas to move outward from the center

of a rotating impeller is the centrifugal tendency

A compressor that uses centrifugal tendency to impart

pressure and velocity to a gas is a _—_—_—

As the impeller rotates, it moves the gas toward the

rim of the impeller

pressure, velocity

centrifugal

outer

This increase in velocity away from the eye creates a low-

pressure area at the ( eye/ outer rim )

This low-pressure area at the eye causes a suction which

( allows / does not allow ) more gas to enter

The impeller does work on the gas The work is converted

into the _ that the gas gains

The energy that the gas gains is in the form of both

8ïfÏd—————:

When the gas is at the tips of the impeller blades, it is at

( maximum / minimum ) velocity

As the gas leaves the impeller, it is thrust into a passage-

DIFFUSER PASSAGE

When the gas enters the diffuser, the impeller (is / is not )

acting directly on the gas

The radius of the diffuser is ( larger / smaller ) than the

radius of the impeller

Due to the larger radius, the flow path of the gas through

the diffuser is in a ( larger / smaller ) spiral

Since the flow path is longer and there is no direct

action by the impeller blades, the velocity of the gas

In the volute, the conversion from velocity to pressure

( continues / does not continue )

A centrifugal compressor, by doing work on a gas, imparts

both _ and _ to the gas

Then, the velocity of the gas is converted into pressure

( within / outside of ) the compressor

lthas ——————_—— separate impellers

Each impeller and diffuser make a stage

This is a -stage centrifugal compressor

As the gas leaves the first impeller it gains some

and

The increased velocity is partially converted into pressure

As the gas leaves the diffuser, it enters the return

passage, which guides it into the —————— of the

next impeller

When the gas enters the eye of the second impeller, it has

( greater pressure than / the same pressure as ) when it

entered the eye of the first impeller

Each impeller adds to the total _————————— of the gas

Greater increased pressure can be obtained from a

( single-stage / multi-stage ) centrifugal compressor

Although velocity is added to gas by the impeller, the

velocity is converted into — _ within the

diffuser

13

†our

four velocity, pressure

diffuser

eye

greater pressure than

energy multi-stage

pressure

114 When the gas leaves the compressor, its pressure is

( higher / lower ) than when it entered higher

115 The work done by a compressor is the total — _ energy added to a gas by a compressor

116 A gas leaving a compressor contains added energy,

usually in the form of increased ( pressure / velocity ) pressure and temperature

Axial motion is ( circular / straight-line ) motion straight-line

118 A compressor that moves gas parallel to the axis of its

shaft is an compressor axial

119 An axial compressor has stator and rotor blades

STATOR BLADES

CASING

The rotor blades are attached to the and shaft

rotate with it

14

The stator blades are attached to the

Look at this drawing

STATOR BLADES

CASING

The arrangement of blades is such that there is a set

of stator blades between each two sets of

blades

The rotor blades act in the same manner as the blades

of a fan

As they rotate, they force the gas to

The rotor blades impart both pressure and

to the gas

The rotor blades force the gas into the

_ blades

As the gas is thrust into the stator blades, the openings

between the blades act as diffusers, thus decreasing the

of the gas

With the decrease in velocity, the pressure of the gas

The stator blades also guide the gas into the next set

t.——._— +-_——_ blades:

Thus, the gas entering the second set of rotor blades

has a slightly ( higher / lower ) pressure

Each set of stator and rotor blades the

stator, rotor

131 The blades in this compressor are not the same size

The blades get gradually smaller toward the

end of the compressor

132 As the gas flows through an axial compressor, it is forced

to occupy successively ( more / less ) volume

133 As an amount of gas is forced to occupy less volume,

its pressure 1

134 The parts of the axial compressor that do work on the gas

are the _\ _. _ blades

135 Partial conversion from velocity to pressure is achieved

by the _ blades

136 Further pressure increase is caused by forcing the gas

into a smaller

137 The flow of gas through an axial compressor is in a

(somewhat straight / spiral ) line of flow

REVIEW

138 Two forms of gas energy are

139 Energy cannot be created or destroyed but it can be

from one form to another

140 By doing work on a gas, a compressor can add

142 When the velocity of a gas decreases, its pressure

143 When a flowing gas is slowed down, the velocity loss is

converted into

144 During flow, the total energy of a gas, less energy loss

due to friction or heat ( changes / remains constant )

145 Centrifugal and axial compressors are ( dynamic / positive

displacement ) compressors

146 In dynamic compressors, added velocity is changed into

within the compressor

147 The function of both diffusers and volutes is to convert

gas into

148 The stator blades of an axial compressor act as ( diffusers

/ impellers )

149 The total energy gain of a gas leaving a compressor is

due to the done by the compressor

RATIO OF COMPRESSION

150 A compressor is a machine that by doing work on a gas

151 A gas normally enters a compressor at one pressure and

leaves it at a ( higher / lower ) pressure

152 The difference between the suction pressure and the

discharge pressure represents the done

on the gas by the compressor less losses due to heat

and friction

153, The ratio of compression, R, is the relationship between

the absolute discharge pressure and the absolute suction

pressure, P2/P;, where P2 is absolute

pressure

154 P,is absolute pressure

155 R is how many times the suction pressure goes into the

pressure

156 In determining R, ( absolute / gage ) pressure is used

157 Gages are usually calibrated to read zero pressure at

do not

EXHIBIT 2

STARTUP PROCEDURE — TURBINE DRIVEN

COMPRESSOR

Pressure In System

® open suction valve

* open discharge valve (discharge check valve

closed)

© open bypass or vent

¢ start and bring up to speed (operating path

passes through points 1, 2, and 3)

® close bypass or vent

@ place bypass or vent on automatic control

No Pressure In System

® open suction and discharge valves

place bypass or vent (if used) on automatic

control

¢ start and bring up to speed (operating path

passes through points 1, 2, and 3)

STARTUP PROCEDURE — MOTOR DRIVEN

COMPRESSOR

Pressure In System

® throttle suction valve

© open discharge valve (discharge check valve is

closed, bypass or vent is normally open)

e start and bring up to speed (operating path

passes through points 1, 2, and 3)

© open suction valve slowly

place bypass or vent (if used) on automatic

control

No Pressure In System

® throttle suction valve

© open discharge valve

© open bypass or vent

¢ start unit (operating path passes through points

1, 2, and 3)

® slowly open suction valve

© put bypass or vent on automatic control

90% SPEED

HEAD (R)

0 —————————VAPACITY————————>

NORMAL CAPACITY LIMIT

158 Absolute pressure is total presSure

To convert from gage pressure ( PSIG ), to absolute

pressure ( PSIA ), the pressure ofthe_———_—_———— must

be added

159 Atmospheric pressure at sea level is about 14.7 PSI

When a gage reads 20 PSIG at sea level, the absolute

pressureis_———_—_——— PSIA

160 A compressor takes in gas at atmospheric pressure, 14.7

PSIA, and discharges it at 58.8 PSIG

Suction pressure is PSIA

161 The absolute discharge pressure is 58.8 + _ =

164 Since compression always increases gas pressure, the

discharge pressure during compression is always ( higher

/ lower ) than the suction pressure

165 Since discharge pressure is always higher than suction

pressure during compression, R is always ( smaller /

greater ) than 1

166 R is an abbreviation for _——————————— of compression

167 A compressor with an R of 2 takes in gas at 20 PSIA

The discharge pressure is ( 20/2/20X2 ) PSIA

168 R is an indicator of the amount of that the

compressor adds to the gas

169 The greater the R, the greater the increase

in the gas

CAPACITY OF COMPRESSORS

170 The capacity of a compressor is the volume of gas it

moves in a given period of time

Cubic feet per minute (CFM) indicates the ( capacity /

171 The flow rate of a gas in CFM depends on the velocity of

the gas and the diameter of the pipe or flow path

At the same velocity, the rate in CFM is higher if the gas is

flowing through a ( larger / smaller ) diameter passage

172 With the same size passage, the flow rate is higher when

the gas flows at a higher ————————

173 When the velocity of a gas flowing through a com-

pressor increases, then the capacity of the compressor

174 If the gas velocity at discharge is greater, then the pres-

sure at discharge is —-— —

175 Since a compressor compresses the gas that it handles,

the volume of gas entering the compressor is ( greater

than / less than ) the volume leaving the compressor

176 The capacity of a compressor is the volume of gas that

it moves in a given period of time

The actual CFM that a compressor moves represents the

volume of gas ( before / after ) compression

177 The actual CFM, must be measured at the ( suction /

discharge ) end

REVIEW

178 Rstands forthe_“ _———— of compression

179 R indicates the amount of _ increase that

occurs to the gas due to the compressor

180 In determining R, ( gage / absolute ) pressure is used

181 The capacity of a compressor is the _————————— of

gas that a compressor moves in a given period of time

182 The actual CFM represents the volume of the gas moved

in a given period of time ( before / after ) compression

183 The capacity limit of a compressor represents the

( maximum / minimum ) rate of flow of gas through it

184 When a dynamic compressor nears its capacity limit, its

efficiency ( increases / falls off )

185 For maximum efficiency, a dynamic compressor should be

operated ( at/ at less than ) its capacity limit

absolute amount, or volume

before

maximum

falls off

at less than

FOOT-POUNDS AND HORSEPOWER

186 - A unitfor measuring work is the foot-pound

{

1 FOOT

One foot-pound is the amount of work needed to raise a

weight of one a distance of one

187 When a weight of one pound is raised a distance of 100

feet, —\ _ —— foot-pounds of work is done

188 Horsepower is a unit for measuring power

One horsepower is equal to 550 foot-pounds per second

Horsepower is a unit that measures the ( amount of work

done / rate at which work is done )

189 A machine rated at one horsepower, if allowed to operate

for one minute is capable of doing 60 x _ =

foot-pounds of work

190 To raise a weight of one pound a distance of 33,000 feet,

foot-pounds of work are required

191 To raise a weight of one pound a distance of 33,000 feet in

one minute, one _ _is needed

192 A foot-pound represents ( amount/ rate ) of work

HEAD OF COMPRESSION

193 To compress any amount of gas, a compressor must do a

certain amount of on the gas

194 The amount of work that is done by the compressor can

20

pound foot

100

rate at which work is done

550 33,000

195 The work of compression can be thought of as straight

lifting of a given weight of gas

For every pound of gas that the compressor lifts a dis-

tance of one foot, one - of work is done

196 This centrifugal compressor is lifting a gas

To lift the gas, the compressor ( increases / decreases )

the velocity of the gas

In a compressor, the velocity of a gas is increased by an

When the speed of the impeller is increased, the velocity

When the velocity of the gas increases, the head devel-

oped by the compressor _1

The head that the compressor develops represents the

the height to which a column of gas is _

21

foot-pound

head feet

204 When the unit of weight is in pounds, for each foot of

distance this liquid is raised, one ———-

of work is done

205 When compression is thought of as straight lifting ofa

column of gas, the head in feet represents the output of

the compressor in foot _ per of gas handled

206 For each pound of gas that the compressor raises to the

top of the head column, a corresponding amount of

“—————— of work is needed

207 If the head increases, the number of foot-pounds of work

per pound of gas must

RPM AND HORSEPOWER

208 RPM is an abbreviation for _ per minute

209 The impeller of a centrifugal compressor has to

to move the gas

210 As the RPM of the impeller increases, the velocity of the

211 The work done by the impeller is reflected in the

: imparted to the gas

212 The faster the RPM of the impeller, the ( more / less ) work

is done on the gas

213 For any given RPM, a set amount of work in units of

iss done per unit weight

of gas

214 For any given RPM, the head developed by the compressor

is fairly ( constant / variable )

215 The density of various gases, or the weight per given

volume differs

A gas with a higher density weighs ( more / less ) for

the same volume as a less dense gas

216 However, for any given RPM of the compressor, the work

done per pound of gas handled is ( the same / different )

217 When a compressor at a given RPM is handling a heavier

gas, the work it does per pound of gas handled is ( the

same as / different from ) the work done on a pound of

218 Head represents the amount of foot-pounds of work done

per unit weight

For the same RPM when a compressor is handling a

heavier gas, the head it develops ( remains the same /

changes )

219 A compressor at a given RPM handles two different kinds

of gas

The gas that requires the larger volume per given weight

is (denser / less dense )

220 The amount of work done per each unit of weight is ( the

same / different ) for both gases

221 The gas that has the most units of weight per given

volume is

222 The gas that results in the highest discharge pressure

for the same head developed is ( the denser gas / the less

dense gas )

223 Atagiven RPM, the actual CFM of gas that the compressor

moves is constant, but with a denser gas there will be

( more / fewer ) pounds of gas moved

224 At a given RPM, as more weight of gas is handled in a

given time, the work done per pound ( remains the same /

increases )

225 Although the work per pound is the same, the number of

pounds of gas worked on in a given time ( increases /

stays unchanged ) with a denser gas

226 With an increase of weight handled for a given time, the

rate of work ( increases / decreases )

227 With an increase in the rate of work there is an increase

in required to compress the gas

228 At any given RPM, when a compressor handles a heavier

gas, the horsepower required

REVIEW

229 Work can be measured in units of — -

230 The rate of work done is usually expressed in units of

231 When the work of compression is thought of as straight

lifting of a weight of gas, then head represents the output

‘of aicompressor ini — = is I Bất

pound of gas handled

As the head developed increases, the amount of work done

on each unit of weight of gas handled

At any given RPM, the work done by the compressor on

each pound of gas handled is ( nearly constant / variable )

The density of a gas does not affect the

developed, but does affect the needed

When a denser gas is handled, the discharge pressure for

the same RPM _——————————

When RPM increases, there is ( more / less) work done

per unit weight

With an increase of RPM, there is ( an increase / no

change ) in the head developed

As the RPM increases, the capacity of the compressor

increases and the amount of weight of gas handled per

given time s

With an increase in RPM, the horsepower

Maximum head at any given RPM represents the ( maxi-

mum / minimum ) amount of work that a compressor can

do on each of gas handled

The ratio of compression, R, is the absolute

pressure divided by the absolute _ pressure

R is an indicator of the amount of _ that the

compressor adds to the gas

At a set RPM, a dynamic compressor adds a certain head

to the gas

The total head added depends on the design of the

compressor, the amount of flow, and the operating

As RPM increases, the total head of the compressor

At a fixed RPM and CFM, the dynamic compressor attains

approximately the same feet of head, regardless of the

weight of the gas handled

The head developed by a dynamic compressor ( depends /

does not depend ) on the density of the gas being handled

Feet of head ( can / cannot ) be converted into PSI

discharge

suction pressure

RPM, or speed

increases

does not depend

can

247 PSI ( can / cannot ) be converted into feet of head

248 Two compressors handling two different kinds of gas

develop the same head

The compressor generating the highest discharge pressure

is handling the ( denser / less dense ) gas

249 The density of a gas does not affect the head developed,

but does affect the _ pressure

250 The compressor with the higher R is handling the

——L-LL<C/———gas

251 TheR or pressure increase at the compressor ( depends /

does not depend ) on gas density

252 As the density of a gas increases, the R of the compressor

253 Changes in the density of the gas do not change the

( head/ R ), but they do change the ( head / R )

254 As suction pressure increases, the compressor causes a

higher discharge _ for the same head and R

255 As temperature increases at suction, the gas is lighter and

the same head causes a ( higher / lower ) R

256 BHP or brake horsepower, is the horsepower that is

required by the shaft of the compressor

As a gas becomes heavier the BHP required

257 Because of wide variations in gas density, the BHP re-

quired by a dynamic compressor tends to ( change / re-

main constant ) while the compressor is in operation

SURGING

258 Suppose a compressor is connected to a large system

having a high capacity or needing large amounts of air

Because there is a demand for air, when the compressor

is started there is ( little / great ) resistance to the

discharge of the air

259 With little resistance at the discharge end of the com-

pressor, the compressor capacity at first is ( high / low )

260 As more air is delivered and the system fills, the capacity

need of the system ( is reduced / stays the same )

261 If the system does not use the air as quickly as the

compressor delivers it, the pressure in the system

With an increase in pressure in the system, the resistance

to the discharge of the compressor

An increase in resistance to discharge causes the capacity

of the compressor to

As pressure in the system increases, the compressor has

to do ( more / less ) work per pound of air

The maximum head of a dynamic compressor represents

the maximum amount of ————————— it can do on the

gas to maintain flow

lf the system continues to use less air than is delivered

to it, the system’s pressure keeps increasing and the head

required to maintain flow keeps —_\

When the head needed to maintain flow increases above

the maximum head of the compressor, flow ——

When the flow stops, the pressure within the compressor

becomes less than the pressure in the system, and the air

may flow from the _———_——_—— into the

After some air flows back into the compressor or is used by

the system, the pressure of the system

As the pressure of the system drops below the maximum

head of the compressor, the compressor again delivers

gas into the

As the compressor begins to deliver gas again, it operates

at a lower and lower capacity and higher and higher

If the system still uses less air than is delivered to it, the

compressor reaches its maximum head and the flow

stoppage ( occurs / does not occur ) again

The rapid flow of gas back and forth in the compressor is

called surging

Surging occurs when the compressor is operated below

minimum ———

The rapid reversals of surging set up severe vibrations in

the compressor and piping which can cause

to the compressor

A compressor goes into surging because the flow of gas

( drops below / rises above ) the minimum stable level

Most dynamic compressors are equipped with protective

devices that guard the compressor against

Now refer to Exhibit 1

277 The left-hand edge of the graph shows percent of rated

278 A point higher on the graph indicates a ( higher / lower )

head

279 The bottom edge of the graph shows percent of rated

280 A point further to the right on the graph indicates ( higher /

lower ) capacity

281 The graph shows that as head increases, capacity

282 The compressor is discharging into a system that requires

100 percent of its rated head

According to the graph, the compressor is operating

at ——————— percent capacity

Suppose the discharge system does not use as much gas

as the compressor delivers

The pressure at the discharge end of the compressor

(increases / decreases )

The graph shows that as the required head (or pressure)

expressed in feet of gas increases, compressor capacity

Suppose the head at discharge necessary to maintain

flow reaches 102 percent of normal rated head

The head/capacity curve shows that at 102 percent head,

capacity is reduced to _ percent of normal

capacity

If pressure in the discharge system increases so that the

compressor must produce 104 percent of normal rated

head to maintain flow, capacity decreases to

percent of normal rated capacity

Locate the surge line on the head/capacity curve

As shown on the graph, the operating point of the com-

pressor at 104 per cent rated head is ( closer to / further

from ) the surge line than the operating point at 102 per-

cent rated head

As the head necessary to maintain flow increases, the

operating point of the compressor ( approaches / recedes

from ) the surge line

289 According to the graph, this compressor begins surging

at / _ percent rated head

290 Capacity at surge is - percent rated

capacity

Interpretation of Curves

291 Performance curves show the limits of the compressor,

which are the surge point on the ( right / left ) and the

normal capacity limit on the _ _ of the graph

292 Compression is controlled by making permissible changes

in pressure, tlow and temperature, to keep the compressor

from

293 The point where surge can occur is defined by the

end of the curve

294 Increasing the speed of a compressor will ( increase /

decrease ) the generated head

295 Increasing the speed will also increase the

required for compression

‘296 On a constant-speed compressor with a fixed suction

pressure, an increase in discharge pressure always causes

(a decrease / an increase ) in capacity and ( an increase /

a decrease ) inR

297 The reduction in capacity causes a in the

BHP required

298 If less volume of gas is required, the compressor should

be operated at a ( higher / lower ) speed

EFPECTS OF EXTERNAL SYSTEMS ON A COMPRESSOR

299 Some processes require a given weight of gas or air for

any given time of operation For example, a cat cracker

requires enough air by weight to maintain the effectiveness

of the catalyst

When a compressor is delivering into such a system, the

main control objective is ( volume / weight ) flow

300 For any given time, the requirement of such systems is

(constant / variable )

301 Therefore, a compressor in such a system must deliver a

-weight flow for any given operative time

28

105

50

left right

surging

left

increase BHP

In other systems, as with the production of light ends in a

fractionating tower or yard air, gas must be delivered

or taken out as the need arises

With such systems, the flow of gas is ( constant/ variable )

With a compressor working in such systems, the objective

is to move a _" quantity of gas only at the

rate it is produced or is z

Maintaining a flow equal to make means moving gas as

fast as it is ( produced / used ); and maintaining a flow

equal to demand means moving gas as fast as it is

( produced / used )

There can be three basic control objectives with com-

pressors:

aconstant _/ flow of gas;

variable flow equal to _; or

variable flow equal to

Depending on the discharge systems, the change in dis-

charge pressure may be large or small depending on the

volume delivered to it

If the change is very small, it may be regarded as a

( constant / variable ) -pressure system

If the change in discharge pressure is large, then it must

be regarded as a _-pressure system

Drivers used with compressors either are constant-speed

or -speed drivers

Steam turbines are variable-speed drivers

Electric motors, on the other hand, are normally

-speed drivers

Most steam turbines have governors that control their

Neither the basic control objectives nor the system char-

acteristics change the method of control

With steam-turbine-driven compressors, a process signal

is used to activate the governor to raise or

the speed

As the speed of the compressor is increased, the mass or

weight flow through it

As the speed is lowered, the mass flow is also

29

variable

variable needed

produced used

314 Whenever mass flow increases, the horsepower required

senile s os So

315 When mass flow is decreased by a decrease in speed,

the actual CFM is always ( reduced / increased )

316 When the actual CFM through a compressor is reduced,

the compressor moves ( closer to / further away from )

surge

317 Constant-speed machines are usually equipped with either

variable guide vanes or a suction throttle When the guide

vanes are closed, they ( reduce / increase ) mass flow

318 When the suction valve is throttled, the suction pressure

is ( reduced / increased )

319 With a reduction in suction pressure, the density of the

gas is ——————————— and the total gas flow in pounds

is

320 If the suction is throttled and the discharge pressure is not

reduced, the actual CFM through the compressor usually

(increases / decreases )

321 Reducing mass flow by throttling may either reduce or

322 A flow meter is a control element that can be used for

constant-weight flow When the control objective is

constant-weight flow, the flow meter is placed at the

( discharge / suction ) end of the compressor

323 A pressure controller is a control element for meeting the

objective of flow equal to make or demand

For flow equal to demand, the pressure controller is

placed at the end of the compressor

324 For flow equal to make, the pressure controller is placed

at the _ end of the compressor

Review

325 There are three basic control objectives, constant-

———— flow, and variable flow equal to

, or variable flow equal to

326 The two basic system characteristics are ———

discharge pressure and discharge pres-

variable

speed, or RPM

328 A flow meter is used to control constant- ( volume / weight )

flow

329 A pressure controller is used to meet the delivery of

(variable / constant ) flow of gas as needed

330 Dynamic compressors operating into fixed pressure

systems are usually instrumented to protect against

FF

PERFORMANCE FEATURES

331 Dynamic compressors do not perform in the same way that

positive displacement compressors do

A positive displacement compressor first traps a volume

of gas and then _ the gas into a smaller

volume

332 The alternate trapping and displacement of gas is the

operating principle of the ( positive displacement /

dynamic ) compressor

333 Gas is not trapped, but flows continuously through the

compressor

334 Because of simple construction, the dynamic compressor

usually requires ( more / less ) maintenance

335 Dynamic compressors tend to take more horsepower for

compression than positive displacement compressors and

thus they have a ( higher / lower ) efficiency

336 The dynamic compressor is an economic choice where the

lower first costs and lower maintenance costs offset

the effects of their reduced

337 The dynamic compressor is normally more economic when

the volume of gas handled is ( large / small )

338 In general, axial compressors are used for relatively low

heads and relatively high

339 For large capacities and high heads, ( an axial / a

centrifugal ) compressor is used

340 The head through a positive displacement compressor

tends to vary

The head of a dynamic compressor at any operating point

tends to remain ( constant / variable )

341 For compressing large volumes of gas through relatively

constant heads and Rs, a _- compressor

HORIZONTALLY SPLIT CASING -

The horizontally split casing is in two halves that are

together to form a tight enclosure bolted

2 When the top half of the horizontally split casing is

removed, all the internal components ( are easily acces-

sible / must be removed from the case ) are easily accessible

32

This is a vertically split casing Some vertically split

casings have only one removable side, the other side being

part of the casing

VERTICALLY SPLIT CASING

It is sealed by two which are bolted to the covers

ends of the casing

To reach all the working components in the vertically split

casing, the cover has to be pulled and the internal com-

ponents must be from the case - removed, or pulled

For easier accessibility of the internal working components

‘of &: compressor, ‘the ——— horizontally split casing is preferred

When the compressor is operating, the pressure inside the

compressor is ( higher / lower ) than the outside pressure higher

The casing must be so constructed as to ~— prevent

gas from escaping through it

In the vertically split casing, joints through which gas can

escape are only at the _ of the casing end

In the horizontally split casing, the joint through which gas

can escape is of a ( larger / smaller ) area than in the larger

vertically split casing

33