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CIIARLKS IIKXRV .lACOiiHENROY IvENNKTll MURDUCKl

THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE

liV MECHANICAL ENGINEERING

IIV THE

COLLEGE OF ENGINEERING

OF THE

UNIVERSITY OF ILLINOIS

JUNE, 1910

UNIVERSITY OF ILLINOIS

May. 3.1 i9ao

THIS IS TO CERTIFY TH.AT THE THESIS PREPARED UNDER MY SUPERVISION BY

Charl.es Henry Jacobsen and Roy Kann.e.tli....Mur.duck

ENTITLED Tlie Erosion of. Steam T.ur.bine. Blades.

IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE

DEGREE OF Bachelor of Science in....Iy;;..ecM.n^^ Engin.e.e.r.lng

Instructor in Charge.

APPROVED:

HEAD OF DEPARTMENT OF Mechanical En^.i.n.e..e.r..i..n4..

167534

Digitized by the Internet Archive

in 2013

http://archive.org/details/ei;psionofsteamtuOOjaco

TABLE OP COTTTl'iNTS

Page

.

INTRODUCTION 1

THEORY OF THE FLOW OF STEAM. .

Determination of the steam velocity 2

Design of the nozzle 4

DESCRIPTION OF APPARATUS.

Description of the blades. 7

Description of the testing apparatus. ... 7

METHOD

.

Method of conducting tests 12

DISCUSSION.

Discussion of results 14

RESULTS

.

Data sheets. 15 to 30

Result sheet. , 31

Ciirves. 32 and 53

LIST OP ILLUSTRATIONS.

Fig, 1 Diverging^ nozzle. , 6

" 2 Converging nozzle 6

" 3 Cross-secion of the apparatus. . . 8

" 4 Photograph of the apparatus. ... 9

" 5 Diaphragm at the exit side 10

" 6 Westinghouse Parsons turhine "blades. 11

" 7 Cross-section of a blade 12

1.

THi^ OP riTEAM TUKBIN.. BLAD^sS.

Tntroduc tior

.

Unlike the steam engine the steam turbine is not subject

to much wear. In the steam turbine, practically the only we:-r is

caused by the cutting action of the steam upon the blades. In most

cases Lhis wear is not serious, but it may cause an increase in

the steam consumption.

In 1904 Mr. Francis Hodgkinson presented a paper before

The American Society of Mechanical Engineers, in which he gave the

results of some experiments concerning the erosion of steam turbine

blades. In these experiments hard drawn Delta metal blades were

exposed to the action of steam for 128 hours. Two nozzles were used

with velocities of approximately 2900 feet per second and 600 feet

per second, the wear being much greater in the case of the high

velocity than with the low velocity.

In 1909 Messers Ekblaw and Wolf performed a series of ex-

periments on Keir Turbine buckets comparing the action of wet and

dry steam at different velocities. It was found that the wear was

greater in case of the wet steam than with the dry steam. These

tests also confirmed Mr. Hodgkinson' s statement concerning the

effect of different velocities.

Aside from these there is an absence of data concerning

the erosive action of steam upon turbine blades.

To determine, then, the action of steam at practically

commercial quality, at different velocities upon different blades

is the object of this thesis.

Theory of the Flow of Steam

Steam in dropping from a region of pressure to a

region of lower pressure {^through a channel or no^izle, and, if the

expansion is adiahatic , the drop in heat contents is used in in-

creasing the kinetic energy of the jet. This heat drop is repie-

sented "by (H;^ - H^), ( where = the total heat in one pound of

steam at pressure p^^, and Ho = the heat in one pound of steam at

pressure p^.). It may be assumed that the velocity of the steam at

entrance is so small as to he neglected.

If Cq represents the theoretical velocity with which the

steam leaves the nozzle, then the kinetic energy of one pound of

steam is Gq/^& foot pounds. This ir the work equivalent of the

heat given up during expansion. One B. T. U. is equivalent to

778 foot pounds of work , or

Co/-g = 778(H3_ - Hg)

Letting g represent 32.16 this equation reduces to

Go = V 2g X 778(Hi -

or Go = 223.7 V(Hl - K^) — — — — (l)

The heat drop for a certain adiahatic expansion may he readily

obtained from a Heat Chart, or by getting the total heat in one

pound of steam at pressure p^, correcting for quality;

^1 " ^1*

Then subtract from this the total heat in one pound of steam at

pressure p,^, correcting for quality;

The quality of the steam at the initial pressure was obtained by

use of a throttling calorimeter. The formula used was

XI = (H^ - q^4. 0.48(Tq - Tg^))/r^— — (2)

6.

\Vhero H^^ ~ Total heat in one pound of steam at atmospheric pressan

q-j^- Heat of the liquid at pressure p^.

r^ = Heat of vaporization at pressure p^.

- Temperature of the steam in the calorimeter in

degrees Pahr.

= Temperature of the steam at atmospheric pressure in

degrees Pahr.

0»48 = Specific heat of the steam.

The quality of the steam after expansion was obtained by using the

relation;

e *{rjX^)/Ti = e +(rixi)/Ti - (3)

or , with adiabatic expansion there is no change of entropy,

where 6 = Entropy of the liquid .

(x-j^ ^l)Al = Entropyof vaporization, corrected for quality.

All terms are known except X2 which can be obtained as above

explained from equation (3) .

Having the total heat drop due to the adiabatic expansion

the theoretical velocity can be obtained as explained above, from

equation (1). But there a certain amount of the heat drop lost

due to the friction of the steam in the nozzle*

Let G-j^ = The actual velocity.

ra = The fraction of the heat drop lost due to the friction in

the nozzle.

Then from the energy formula -

G^/2g = (l-m)(Cy2g)

or = C^V 1 - in

Taking the friction loss m. as 0.1 , which is an average value

( from the I. C. S. pamphlet "Steam Turbines" ), then

4.

\/ I - m = 0.95 approximately, oi a lofes of velocity

of 5^ , or

= 0.95 Co

Ol = 0.95 . 223.7 " )

= 212.5 V(H]L " ^^2^"

- — — — —Design of the Nozzle.- The factors entering into the

design of the proper cross-section of a nozzle at any point of

its length, are , the pressure, the velocity of the jet, the

weight of the steamflowing per second, and the specific volume of

the steam.

Let G = The numher of pounds of steam flowing per second.

G]L = The velocity of thesteam in feet per second.

V = Thevolume of one pound of steam in cuhic feet.

F = The area of cross-section in sq. ft,

f = The area of cross-section in sq.in.

Then F = G v

or p = Gv/C;j^

or f =r l44Gv/ C^^ — — — — — — (f)

It is found that the cross-section is smallest where the

pressure is 0.57 of the initial pressure. Thus if the initial

pressure is 100 pounds^ the pressure will be 57 pounds at the

smallest cross-section or the throat. In the preceding statement

it is assumed that the final pressure is less than ^7f of the

initial pressure. If this were not the case— that is, if the drop

in pressure is small, or if the final pressure is greater than

57^1 of the initial pressure the walls of the nozzle should

converge

.

In "brief, if the final pressure pg is less than 0.57 p^^

5.

the nozzle should have a throat, then diverge, ns shov/r^ ir fig. (1)^

but if Lhu final prcissuro po is greater than 0,57 uho nozzle

should convergu, as shown in fig. (2).

The nozzle '.vas horijd out with a uniform taper for the

portion in which expansion takes place. The taper was approximately

1 in 10 as shown in fig.(l). The area of cross-section of the

throat was determined from formula (5), "by using pressure p' equal

to 0.57 p;j^, and the area of the outlet end by using the pressure

P2« If the nozzle is too short the taper will be too great and

the jet will spread. If the nozzle is too long the loss due to

friction will be increased. The usual taper is about a 10 degree

angle

.

Let = The diameter in inches at the large end .

b = The diameter in inches at the throat,

c = The angle between the sides of the nozzle .

L = The length of the nozzle in inches.

L = (0 - b)/2 tan(c/2).

And if c = 10 degrees,

L = 5.715(0 -b).

6.

Fig. 2.

Co/vy£/r6//v^A/OZZLE.

Description of Apparatus.

Apparatus Tested,- The blades wure of the type used in the

Westinghouse Parsons pressure turbine. A sketch showing a cross

section is shown by Fig. 7, pagefE. There were four blades,

each of different composition, hnov.'u oaly to the manufacturers,

the Westinghouse Machine Go.

Apparatus for Testing,- Two pressure chambers of 6" pipe were

bolted together with a blank flange between them. The steam nozzl

extended through this flange and was held in place by a flange

and strap over it bolted to the blank flange. One chamber was

connected through a reduction to a 1" steam pipe from the steam

main. The other chamber connected with a Worthington surface

condenser through a 4" pipe.

The blade holder was bolted to the exhaust side of the

blank flange, to one side of the nozzle. The blade fitted into a

split cylinder, which, in turn, fitted into a hole in the holder,

where it was secured in the proper position by three set screws.

The nozzle for the first set of tests- at the lo7/est vel-

ocity of steam was a straight nozzle. The nozzlo used in the re-

mainder of the tests was tapered one in ten, being a diverging

nozzle.

Two Crosby steam gages were used to obtain the steam

pressure. They were connected to the two chambers, the one on the

exhaust chamber reading' both vacuum and pressure.

Each chamber was provided with a drip cock.

/7G. 4

Photogrrph of the Apparatu6

6.

\/\/E5 J //V6 H0U5E f^/^ff50A'5 Tu/fB//VF Slf.O£S.

The throttling calorliaQtor was connected to the high

pressui'e chamber through a l/r" sampling nipple.

Method Ox Conducting Test.

Four ten hour tests were run on each of the four blades

The steam velocity was varied, while all other conditions were

maintained as near constant as possible. The velocities used,

were approximately 1400, 2100, £800 and 3400 feet per second,

the latter being the maximum attainable under the conditions.

The pressures, initial and exhaust, used to obtain the desired

steam velocities were taken from Prof. 0. G. Thomas' "Steam

Turbines".

The quality of the steam was maintained as high and as

constant as possible. Steam was taken from the top of the main

and the piping was lagged.

The blade on which the test was to be conducted, was

thoroughly cleaned, dried in a desiccator and weighed on a sensi

tive balance. It was then fastened on the blank flange close to

the end of the nozzle, in such a position, that the steam flowin

through the nozzle would strike it on a tangent to the inside

short curve, as shown in figure 7.

13.

After connecting up the apparatus, ctearn war; turned on and

the valves on the high and low pressure sides adjusted to give the

proper initial and exhaust pressures to give the desired steam

velocity

.

A condenser vvas used to condense the steam and to obtain

a vacuUiTi on the exhaust side when needed. A throttling calorimeter

was used to obtain the quality of the steam.

The following readings were taken every thirty minutes;

Initial steam pressure.

Exhaust steam pressure.

Temperature of steam in calorimeter.

Temperature of steam in exhaust chamber.

Three barometer readings were taken during each test.

All instruments used were calibrated and readings corrected

At the end of the test, the blade was removed, allowed

to cool, cleaned, dried in a desiccator and weighed. The amount

of erosion was obtained by taking the difference between the

initial and final weights.

14,

Discassion of Results.

The results of the series of experiments given on pages

15, to 31, show rather clearly the action of steam at different

velocities upon turbine blades of different materials. Enough data

was obtained to make a fair comparison between the action of the

steam at high and low velocities. In all cases there was an increase

of the wear with an increase of velocity. In the two higher vel-

ocities, in all cases, the quality of the steam ran up slightly,

which would cause the wear to be less than if the quality had been

more constant

.

With blade No.(l) subject to the action of steam at a vel-

ocity of 1400 feet per second for 10 hours the wear was 0.002 grams,

and at a velocity of 3350 feet per second the wear was 0.0041 grams,

or an increase of 0.0021 grams , or 105/^ with an increase of 1950

feet per second in the velocity.

Taking the highest velocity with blade No. (4), the wear

was 0.0035 grams, but in these tests the velocity of the steam is

absolute, while in the turbine the effective velocity is the veloc-

ity relative to the blades. The blades tested belong to the Parsons

type of pressure turbine. The relative velocity in a turbine of

this type is much less than that used in these tests, being less

than 1000 feet per second. The curves on pages 32 and 33 show the

relative hardness of the different blades.

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31.

Result Shuet

Kxp. No. Blade No. Avge

.

mFt • per

1 1 1400

2 2 1387

3 3 1405

4 4 1375

5 1 1975

6 2 1990

7 3

8 4 \j \j \j

9 1 2760

10 2 2745

11 5 2760

12 4 2750

13 1 3350

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15 3 3340

16 4 3360

1. Avge, Qual. Wear.of i n

steam Gram

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98.1 0.001

98.6 0,0017

98.5 0.0013

98.9 0. 0022

99.0 0.0012

99.1 0.0022

98.8 0.0018

99.5 0.0027

99.5 0.0024

99.6 0.0031

99.4 0.0022

99.4 0.0041

99.4 0.0035

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