Biermann Ch. Handbook of Pulping and Papermaking

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644 30. HYDRAULICS

Fig. 30-5. Volute type of centrifugal pump.

After McGiU, 1980.

/-"•^^r^ — Rotating shaft

Axial flow

Propeller

Fig. 30-6. Impeller pump.

Metering pumps

Positive displacement pumps are often used

as metering pumps because they accurately control

flow rates by changing the speed at which the

pump is driven without regard to the back pres-

sure.

They are also used to pump relatively small

volumes of liquids against high pressures.

Possible metering pumps include lealq)roof

diaphragm pump, piston pump for precision,

rotary designs for simplicity and low maintenance,

or solenoid for proportional impulse control. The

corrosive nature of the fluid to be pumped (which

is usually a function of temperature) is a factor in

the selection of a pump. A fluid that is listed as

compatible with a pump material at room tempera-

ture may not be compatible at high temperatures.

30.4 CENTRIFUGAL (DYNAMIC) PUMPS

Introduction

The impeller is the piece that rotates inside

the casing to effect pumping (Fig. 30-5). The

input of the pump is at the eye and the liquid

enters perpendicular to the plane of

the

hnpeller in

the volute or spiral design. Since the impeller is

never in contact with the case, the input and

output are connected hydraulically. One important

characteristic of centrifugal pumps is that they

must be primed (filled with water) before they can

generate suction to overcome any lift required for

the liquid (i.e, if the source of water is below the

pump).

Also, buildup of air in the system may

cause the pump to stop pumping if lift is required.

Another type of dynamic pump, the impeller

pump (Fig. 30-6), uses an inline input and output

inside a tube—shaped housing. Fluid is conduct-

ed axially instead of radially.

Because centriftigal pumps do not enclose the

fluid to be pumped, the flow rate (Q), for a given

pump speed, will be a ftinction of the back pres-

sure,

the pressure at which the pump discharges

the liquid {H). H denotes the equivalent head

(height) of water or other liquid required to

achieve that pressure; a column of water 2.3067

feet high exerts a pressure of

psi at the bottom.

The efficiency of a centrifugal pump depends on

the pumping head and flow rate; the level of

highest efficiency is called the best efficiency point

(BEP),

which is also the ideal condition to run

pumps for low maintenance.

Centrifugal pump action is characterized by

the pump performance curve, as demonstrated in

Fig. 30-7 (upper curve of the graph on the right).

The maximum pressure that can be achieved is

usually sufficiently low such that a pressure relief

valve is not required.

The system response versus flow rate is

shown in Fig. 30-7 (lower curve in the graph on

the right). The operating flow rate and pressure

are the balance of system performance (flow and

pressure) and pump (flow and pressure) perfor-

mance. The system response is a ftinction of the

CENTRIFUGAL (DYNAMIC) PUMPS 645

Head (pressure)

Q, Flow rate Q, Flow rate

Fig. 30-7. Centrifugal pump efficiency curve (left) and pump performance curve (right).

height to which the liquid is pumped (the static

head,

which is constant with flow rate); the

frictional resistance of the pipes, valves, and

bends;

and the velocity head, the energy required

to accelerate the fluid (the kinetic energy is equal

to 0.5

mv^).

The velocity head is often small and

usually ignored.

Cavitation in

centrifugal

pumps

Problems of centrifugal pumps related to

cavitation include entrained air, suction recircu-

lation, and discharge recirculation (Table 30-1).

These three conditions cause pitting of

the

impeller

due to the formation and collapse of gas bubbles.

Air

entrainment,

where air is already in the liquid

or is drawn into the system, causes pitting at the

eye of the impeller, as does cavitation. Suction

recirculation, due to unstable flow patterns be-

tween the impeller vanes at low flow rates (and

high pressures), causes pitting halfway along the

vanes.

Many manufacturers specify a minimum

pump flow rate to avoid pitting, vibration, and

noise associated with suction recirculation. Dis-

charge recirculation, which also usually occurs at

low flow rates, causes pitting at the tips of the

vanes and is often caused by a hydraulic design

flaw known as vane passing frequency.

NPSH requirements can be reduced by using

an inducer, a low head, axial flow impeller used

in front of the impeller of a centrifugal pump.

Pump size

Selection of the proper size pump is very

important. In pulp and paper, flow rates are

traditionally controlled by use of

pump operating

at ftiU power and a throttling valve to impede the

flow as necessary. This has been likened to

driving a car with the accelerator pressed all the

way down at all times and using the brake to

control the speed!

In this type of operation it is very important

to choose the proper size pump. Often pumps are

oversized for lack of proper information, to allow

for increased production in the future, or to coun-

teract the effect of scale that may build up in

pipes.

A control valve should account for 30% or

less of the pressure drop in piping at the maximum

flow

rate.

The wasted power absorbed by a valve

(which also causes the valve to wear out more

quickly) is:

P = q /Sp/iUUrj)

where

P = power, in horsepower

q = flow rate, gal/min

Ap = pressure drop of the valve, psi

rj = efficiency of the pump

At $0.10/kWh, each one horsepower loss over the

course of a year corresponds to $653 (for a mill

that operates continuously). So wasting 100 hp

costs $65,350 each year.

646 30. HYDRAULICS

Table 30-1. Check chart for centrifugal pump problems. Reprinted from Karassik (1989) by

courtesy of Marcel Dekker, Inc.

Symptoms Possible cause of trouble

Pump does not deliver liquid 1-3, 5, 10, 12-14, 16, 21, 22, 25, 30, 32, 38, 40

Insufficient capacity delivered 2-7, 10-18, 21-25, 31, 32, 40, 44, 63, 64

Insufficient pressure developed 4, 6, 7, 10-16, 18, 21-25, 34,

39-41,

44, 63, 64

Pump loses prime after starting 2, 4, 6-11

Pump requires excessive power 20, 22-24, 26, 32-35,

39-41,

44, 45, 61, 69-71

6. Pump vibrates or is noisy at all flows 2, 16, 37,

43-61,

67,

78-81,

83-85

Pump vibrates or is noisy at low flows 2, 3, 17, 19, 27-29, 35, 38, 77

8. Pump vibrates or is noisy at high flows 2, 3, 10-18, 33, 34, 41

9. Shaft oscillates axially 17-19, 27, 29, 35, 38

10.

Impeller vanes are eroded on visible side 3, 12-15, 17, 41

11.

Impeller vanes are eroded on invisible side 12, 17, 19, 29

12.

Impeller vanes are eroded at discharge near center 37

13.

Impeller vanes are eroded at discharge near 27, 29

shroud or vane fillets

14.

Impeller shrouds bowed out or fractured 27, 29

15.

Pump overheats and seizes 1, 3, 12, 28, 29, 38, 42, 43, 45, 50-55, 57-62,

77,

78, 82

16.

Internal parts are corroded prematurely 66

17.

Internal clearances wear too rapidly 3, 28, 29, 45, 50-55, 57, 59, 61, 62, 66, 67

18.

Axially-split casing is cut through wire-drawing 63-65

19.

Internal stationary joints are cut through wire- 53, 63-65

drawing

20.

Packed box leaks excessively or packing has 8, 9, 45, 54, 55, 57, 68-74

short Hfe

21.

Packed box: sleeve scored 8, 9

22.

Mechanical seal leaks excessively 45, 54, 55, 57, 58, 62, 75, 76

23.

Mechanical seal: a) faces damaged; 45, 54, 55, 57, 58, 62, 75, 76

b) sleeve damaged; c) metal bellows fails

24.

Bearings have short life 3, 29, 41, 42, 45, 50, 51, 54, 55, 58, 77-85

25.

Coupling fails 45, 50, 51, 54, 67

Table 30-1. Check chart for centrifugal pump problems,

courtesy of Marcel Dekker, Inc., Contmued.

Reprinted from Karassik (1989) by

Legend for possible cause of trouble

Suction

troubles

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

Pump not primed

Pump suction

pipe

not completely

filled with liquid

Insufficient available

NPSH

Excessive amount of air or gas in

liquid

Air pocket

suction line

Air leaks into suction line

Air leaks into pump through

stuffing boxes or through me-

chanical seal; Source of sealing

liquid has air in it

Water seal pipe plugged

Seal cage improperly mounted in

stuffing box

Inlet of suction

pipe

insufficiently

submerged

Vortex formation at suction

Pump operated with closed or

partially close suction valve

Clogged suction strainer

Obstruction in suction line

Excessive friction losses

suction

line

Clogged impeller

Suction elbow in plane parallel to

the shaft (for double-suction

pumps)

Two elbows in suction piping at

90° to each other, creating swirl

and prerotation

Selection of

pump

with too high a

suction specific speed

Other hydraulic problems

20.

Speed of

pump

too high

21.

Speed of

pump

too low

22.

Wrong direction of rotation

23.

Reverse mounting of double-suc-

tion impeller

24.

Uncalibrated instruments

25.

Impeller diameter smaller than

specified

26.

Impeller diameter larger than

specified

27.

Impeller selection

with

abnormally

high head coefficient

28.

Running the pump against a

closed discharge valve without

opening a

bypass

29.

Operating pump below recom-

mended minimum flow

30.

Static head higher than shut off

head

31.

Friction

losses

in discharge higher

than calculated

32.

Total head of system higher than

design of pump

33.

Total head of system lower than

design of pump

34.

Running

pump

at too high a flow

(for low specific speed pumps)

35.

Running pump at too low a flow

(for high specific speed pumps)

36.

Leaky or stuck check valve

37.

Too close a gap between impeller

vanes and volute tongue or

difftiser vanes

38.

Parallel operation of pumps un-

suitable for this purpose

39.

Specific gravity of liquid differs

from design conditions

40.

Viscosity of liquid differs from

design condition

41.

Excessive wear at internal running

clearances

42.

Obstruction in balancing device

leak-offline

43.

Transients at suction source (im-

balance between pressure at sur-

face of liquid and vapor pressure

at suction flange)

Mechanical troubles:

a. General

44.

Foreign matter in impellers

45.

Misalignment

46.

Foundations insufficiently rigid

47.

Loose foundation bolts

48.

Loose pump or motor bolts

49.

Inadequate grouting of baseplate

50.

Excessive forces and moments

from piping on pump nozzles

51.

Improperly mounted expansion

joints

52.

Starting the pump without proper

warm-up

53.

Mounting surfaces of internal fits

(at wearing

rings,

impellers, shaft

sleeves, shaft nuts, bearing hous-

ings,

(and so on) not 100% per-

pendicular to shaft axis

54.

Bent shaft

55.

Rotor out of balance

56.

Parts loose on the shaft

57.

Shaft running off center because

of worn bearings

58.

Pump running at or near critical

speed

59.

Use of too long a shaft span or

too small a shaft diameter

60.

Resonance between operating

speed and natural frequency of

foundation,

baseplate,

or piping

61.

Rotating part rubbing on station-

ary part

62.

Incursion of hard solid particles

into running clearances

63.

Use of improper casing gasket

material

64.

Inadequate installation of gasket

65.

Inadequate tightening of casing

bolts

66.

Pump materials not suitable for

liquid handled

67.

Lack of lubrication of certain

couplings

C Mechanical troubles

Sealing area

68.

Shaft or shaft sleeves worn or

scored at packing

69.

Incorrect type of packing for

operating conditions

70.

Packing improperly installed

71.

Gland too tight, resulting in no

flow of liquid to lubricate packing

72.

Excessive clearance at bottom of

stuffing box, causing packing to

be forced into pump interior

73.

Dirt or grit in sealing liquid

74.

Failure to provide adequate cool-

ing liquid to water-cooled stuffing

boxes

75.

Incorrect type of mechanical seal

for prevailing conditions

76.

Mechanical seal improperly in-

stalled

Mechanical troubles

c. At bearings

77.

Excessive radial thrust in single-

volute pumps

78.

Excessive axial thrust caused by

excessive wear at internal clear-

ances or by failure or excessive

wear of balancing

device

(if such

is used)

79.

Wrong grade of grease or oil

80.

Excessive grease or oil in

antifriction bearing housings

81.

Lack of lubrication

82.

Improper installation of

antifriction bearings (damage

during installation, incorrect

assembly of stacked

bearings,

use

of unmatched bearings as a pair,

(and so on)

83.

Dirt getting into the bearings

84.

Moisture contamination of lubri-

cant

85.

Excessive cooling

of water-cooled

bearings

648 30. HYDRAULICS

Controlling

the process flow

The process flow can be controlled by three

methods. 1) With flow splitting a portion of the

flow can be recirculated to the supply vessel or

suction side of the pump, as in the stuff box of a

paper machine. 2) The pump head may be

changed by varying the speed of the pump (either

by using a variable speed motor or some type of

transmission). This method offers the potential to

be energy efficient where flow rate differences are

high. 3) The system curve can be altered, for

example, by using a throttling valve to increase

the system flow losses.

Variable

speed motors

Variable speed motors (especially NEMA B

squirrel cage AC induction motors) may be operat-

ed by alternating current (AC) variable frequency

drive (VFD) or by varying the voltage and current

to DC motors (Brink and Van Lieshout, 1993).

With VFD normal alternating current is converted

to direct current (DC) and from DC, using elec-

tronic circuits, to variable frequency and voltage

AC.

The speed of AC motors is dependent upon

the frequency of the power source, if an adequate

voltage is supplied. Variable torque motors are

often used with centrifugal pumps. While the

initial cost of these systems is higher than using a

pump with a throttling valve, the energy savings

can be substantial in many applications.

Impeller

design

and centrifugal pump performance

Over relatively small ranges for a given

centrifugal pump (because of the effect on the

volute, pumps losses, and other factors), the pump

head is proportional to the peripheral speed of the

impeller (and, therefore, proportional to the

impeller diameter for a given rotational speed).

The pump capacity is proportional to the square of

the peripheral speed or propeller diameter for a

given rotational speed (angular velocity), and the

power consumption varies as the cube of the

peripheral speed or propeller diameter for a given

rotational speed. These relationships can be

applied to a pump with a fixed propeller diameter

operating at varying rotational speeds; i.e., pump

head is proportional to rotational speed, and pump

capacity is proportional to rotation speed squared.

Impellers are sometimes cut or trimmed in

size to reduce output or motor overloading. When

impellers are cut, both the vanes and the shroud

should be cut to decreases losses from disk losses,

which are proportional to the fifth power of the

impeller diameter.

The capacity of a pump is proportional to the

propeller width over a narrow range, but the static

head is not usually affected to a large degree.

Vibrational analysis

and troubleshooting

Vibrational analysis of pumps indicates some

possible sources of trouble. The ratio of the

frequency of the major vibrations to the motor

speed gives important information. A ratio below

0.5 indicates suction recirculation; a ratio near

0.5 indicates oil whirl or whip in the bearings; a

ratio of 1 indicates rotor unbalance or coupling

problems; a ratio of 2 indicates axial misalign-

ment of couplings; a ratio equal to the number of

impeller vanes indicates vane passing or, less

likely, suction recirculation; a ratio above 2 may

indicate problems with antifrictional bearings.

Table 30-1 is an extensive guide to the

troubleshooting of centrifugal pump by a leading

authority in the field.

Sealless

pumps

Diaphragm and peristaltic pumps are by their

nature sealless. Centrifugal pumps can be made

sealless by using magnetic drives or canned mo-

tors.

Magnetic drives are more suited to pumping

hot liquids (on the order of 750°F or 400°C)

unless canned motor pumps are water cooled.

Canned motor pumps can tolerate high pressures.

30.5 VALVES

Flow in valves (Anderson, 1980, page 670)

Valves are used as control elements for

numerous processes with the pulp and paper

industry. They may be used in an on or off mode

only, but more often they are used to regulate the

flow in conjunction with instrumental process

control. In this mode they become a variable

orifice to regulate the flow where an actuator is

used to control the position of the valve.

This section is applicable to fluids of relative-

ly low viscosity. Bernoulli's theorem predicts the

flow through an orifice to be:

VALVES 649

but small enough to provide adequate control.

(Likewise, one should pick the appropriate size

pump.) A large valve allowing only a small flow

will be almost shut during service, creating high

flows near the seat, which may lead to undue

wear. Flow rates can be given for liquids, gases,

and steam based on the published Q values:

liquids (30-1)

C =

1360 ^J

1FK

0 r "H I 1 1 1 1 1 r I I

0 10 20 30 40 50 60 70 80 90

100

Stroke action, %

Fig. 30-8. Valve flow rates for linear, A, equal

percentage, B, and quick opening, C types.

Q = CA(APy^

where

Q = flow rate

C = constant for flow conditions

A = open area of valve

AP = pressure drop across the valve

C =

The flow behavior of valves will certainly

influence the behavior of the overall process

control loop. Valve behavior (Fig. 30-8) is

usually reported based on a constant pressure drop

across the valve. It is described as linear (curve

A),

equal percentage (curve B), or quick opening

(curve C). In fact, the pressure drop of a valve

that is barely open is usually much higher than for

a fully open valve. Curve A' shows the flow

properties of a linear valve where the pressure

drop varies by a factor of 5 as the valve is

opened, and curve B' uses this pressure drop

behavior with an equal percentage valve.

The flow capacity of a valve is measured by

the hydraulic valve coefficient Q, the flow of

water through a wide—open valve in

U.S.

gal/min

for a pressure gradient of 1 psi. For a given

nominal valve size the highest Q is achieved with

ball valves followed by (in decreasing order) gate,

plug, and globe valves.

The proper size valve must be selected so

that it is large enough to allow the required flow.

63.3 \1

where

Q = flow rate gal/min (for liquids)

scfli (for gases)

W = Ib/h (for steam or vapor)

G = specific gravity

P2 = valve discharge pressure

V = downstream volume

gases (30-2)

steam (30-3)

If = temperature inRankine (°F + 491.67)

For gas and steam flow AP must be less than

ViPi, where P, is the pressure at the valve inlet.

If AP is greater than

ViP^,

set AP and Pj as equal

V2P1.

Pulp slurries may deviate from the

formula for a liquid.

The pressure within a valve is lower than

either P, or Pj because of the venturi effect (as a

fluid moves faster, its pressure decreases). The

lowest pressure (and cross sectional area) experi-

enced at the orifice is called the vena contracta.

If the vena contracta is less than the vapor pres-

sure of the liquid while the pressure at the output

of the valve is above the vapor pressure of the

liquid, vapor bubbles will contract as they exit the

valve, a condition known as

cavitation.

Cavitation

causes undue noise and valve wear and should be

avoided; it occurs when hot liquids are throttled.

Hashing occurs if the pressure at the input of the

valve is above the vapor pressure of the liquid

while output of the valve is below the vapor

pressure of the liquid.

Valve rangeability is the ratio of the maxi-

mum controlled flow to the minimum controlled

650 30. HYDRAULICS

gate valve

UT^T^

plug valve

Fig. 30-9. Diagrams of valves.

globe valve

End Piece with

integral Seat

Fig. 30-10. Exploded diagram of a ball valve. Courtesy of Keystone International, Inc.

flow. A rangeability of 100:1 is very high.

Valves are often selected so that the maximum

operating flow rate is 70% of the maximum

possible flow rate so that control can be main-

tained in unusual situations.

Valve types (Figs. 30-9 and 30-10)

Valves are used to control the flow of fluids.

Shutoff valves are used in an open or closed state.

Check valves (nonreturn valves) allow flow in one

direction but not the other direction. Control

valves are used to vary the flow as needed. We

will consider control valves in detail in this sec-

tion, although some of these designs are used for

shut—off valves.

Double—ported valves have balanced con-

struction allowing for the use of small actuators,

but these types are not suitable in applications

requiring tight

shutoff.

Cast-in Top Plate

is an integral part of the

body and is standardized to

allow direct mounting of all

Keystone actuators.

Upper Stem Bushing

absorbs actuator side thrust.

Bi-Directional Stem Seal

prevents external

contamination of the stem

area and is self adjusting.

RTFE Lined Stainless Steel

Upper & Lower Stem Bearings

are isolated from the flow media

and help reduce torque.

Patented

Torque Plug Connection

offers

rigid,

high strength

engagement of disc and stem

while allowing quick and

easy disassembly.

VALVES

651

One-piece Thru-shaft Design

provides high strength and

positive disc control.

One-piece Body

allows adequate clearance for

flanges and insulation.

Disc Design

provides maximum

product flow.

Polished Disc Edge and Hubs

provide lower torques and

longer seat life.

Patented

Metal Reinforced Seat

incorporates a one-piece,

serpentine metal reinforcement

fully encapsulated within the

elastomer. This innovative seat

design reduces seating and

unseating torque and increases

sealing capabilities.

Fig.

30-11.

Diagram of a butterfly valve. Courtesy of Keystone International, Inc.

Pinch valves use a flexible tube which is

constricted against two members, at least one of

which is movable. Diaphragm valves use a

flexible diaphragm (disk) against a fixed seat.

Both of these allow the control mechanism to be

completely sealed from the fluid without the use of

seals or glands, although they are limited to

relatively low temperatures and pressures.

Conmion types of rigid valves include ball

plug (Fig. 30-10) [which include the segmented

(or V-notch) types], butterfly (Fig. 30-11), lubri-

cated plug or taper (eccentric) plug (like those

used on glass laboratory burettes), knife gate valve

(Fig. 30-12), and globe valves. Rotary valves

(rotating or quarter—turn), including ball, butter-

fly, and plug valves, are position—seated devices.

Gate (or slide) valves are linear—action valves

with a disk or wedge moving up and down to give

seating. Globe (or obturating) valves use a plug

(such as a sphere or needle) that is forced directly

into the seat; technically globe valves are one

particular type of obturating valve, based on the

shape of the stopper or plug, but the term is used

for almost all obturating valves. The properties of

valves are compared in Fig. 30-13 and Table 30-2.

Globe valves have high friction loss due to

the irregular flow path, but have high rangeability.

Globe valves have cage—guided, top—guided,

split—body, and top—and—bottom—guidedconfig-

urations of body type and plug guiding. Cage-

guided globe valves are fairly versatile and com-

mon since their introduction in the 1960s, but this

design is not suited for use with abrasive or highly

viscous liquids. The post—guided globe valve is

useful with harsh chemicals. If used with bellows

stem seals, fugitive emissions are decreased.

652 30. HYDRAULICS

Fig. 30-12. Gate valve.

International, Inc.

Courtesy of Keystone

Ball valves for use with abrasive fluids

should use a integral seat since there are no

behind—the—seat cavities to fill with particles.

Eccentric plug valves are suitable when solids

are present or crystallization may occur; they

offer stick resistance and erosion resistance due to

the camming action of the valve plug and the

geometry of the body. The flow path is stream-

lined, and the tapered seating surfaces are away

from the flow path. Erosion (abrasion) resistance

is enhanced by the selection of the proper material

for the trim materials. These include chrome

plated stainless steels, solid Alloy 6 parts, and

various ceramics. Reverse flow direction also

protects against erosion. Lubrication, in those

valves that use it, may be a source of difficulty.

Butterfly valves are fairly compact, but they

generally are not used with high fluid velocities

(above 30 ft/s) since they then require expensive,

high—torque actuators for their control. Butterfly

valves offer good protection against erosion only

in situations where shutoff is not required.

Gate valves give full port design and are

resistant to erosion at the seat. They are often

used for shutoff control. When large gate valves

are used with steam, they may jam when the valve

is closed and then cooled since thermal contraction

puts a high pressure on the gate; the valve may

have to be heated before it can be opened. Gate

12 3 4 6 8 10

Nominal valve size (inches)

Kg.

30-13. C, values of valves.

valves are subject to buildup of solids and have

high maintenance requirements.

Check valves come in several designs includ-

ing ball and seat (usually with a spring pushing the

ball against the seat as in

Fig.

30-16), wafer swing

(Fig. 30-14), or guided disk.

Selection of

valves

(Anon,, 1992)

Like most items, the choice of valves maxi-

mizes performance, cost, and service life; the

least expensive suitable valve is usually the best

choice. Corrosion is a major consideration for

One-Piece

Investment Cast

Disc/Arm

Fig. 30-14. Wafer check valve. Courtesy of

Keystone International, Inc.

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