ASCE NATIONAL CONVENTION

2542

M November 3-7, 1975 Denver, Colorado

50t

HYDRAULICS BRANCH OFFICIAL FILE COPY

CAVITATION IN SUBMERGED

JET FLOW GATES

Thomas J. Isbester

Meeting Preprint 2542

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CAVITATION IN SUBMERGED JET FLOW GATES

By

Thomas J. Isbester*

INTRODUCTION

The jet flow gate was developed in 1944 by the Bureau of Reclamation for the

102-inch outlets at Shasta Dam. The gate is a relatively simple, economical

device for regulating high head flows. It was intended for use in discharging

flow to a highly aerated downstream conduit, or directly to the atmosphere.

Jet flow gates have been used at a number of facilities since the initial

development with only minor modifications (figure 1). The gate can vary in

size from a few inches to 10 feet in diameter or more. Use of the gate to

discharge to an unaerated expanded conduit with considerable back pressure is

unusual and is the topic of this paper. This study was made to investigate a

proposed control gate and sudden expansion energy dissipater for a canal outlet

works at Teton Dam.

The tests included obtaining head losses, discharge coefficients, cavitation

r characteristics, and back pressure requirements for the facility.

From the standpoint of energy dissipation, the most appropriate control upstream

from the sudden expansion would be a valve where all partial openings produce a

concentric release. Only a slight deviation in concentricity can increase the

cavitation potential considerably [1]**. A needle valve seems best suited to

* Hydraulic Engineer, Division of General Research, Bureau of Reclamation, Denver, Colorado. ** Items in brackets refer to the reference list.

Gate

PT, 45° Air vent 90,

0 0 0 0 0 o a o m —

0 50R

A. SHASTA DAM JET FLOW GATE

ant I — O.SOR

CR6 sq

B. TRINITY DAM JET FLOW GATE

Hood, 45°

0 0 0 0 ooh

o r" 01 -_

~ 9

0.328 ~ 1.75

C. EAST CANYON DAM JET FLOW GATE

75D

.08D

0 r

D. CRYSTAL DAM JET FLOW GATE

N

Figure 1. - Other jet-flow gate configurations.

this end; however, the jet flow gate is considerably cheaper. Some of the

major advantages in the use of the jet flow gate for free discharge are elimi-

nated when the gate is operated submerged. Hydraulic downpull, nonexistent

with free flow, becomes a sizable quantity. Also, flow into the gate slots

becomes a problem with submergence. Downpull and flow into the gate slot are

common problems encountered with conventional slide gates operating under

similar submerged conditions. The slot of the jet flow gate, which is con-

siderably wider than the slot in present-day high-pressure slide gates, could

be a source of low pressure and possible cavitation damage if high back pres-

sure were not maintained. A subsequent design (figure 1D), which was in part

due to findings from this study, utilizes a very narrow slot far removed from

the orifice flow area and should provide excellent circulation to the jet and

provide a minimum potential for cavitation damage.

THE MODEL

The structure (figures 2 and 3) was modeled on a scale of 1:5.66 to conform to

the orifice size of an existing model gate. The model components upstream

from the gate were made of metal as observation of flow in this area was

{ unnecessary. Downstream components, including the initial two-diameter expan-

sion section, subsequent three-diameter expansion, and rectangular well were

•made of transparent acrylic plastic. A 10-inch (25.4-centimeter) gate valve

was placed downstream from the well to adjust back pressure and an elbow down-

stream from the valve was turned vertically upward to maintain the pipe full

at all times. Flow was supplied to the model from permanent laboratory pumps

9 River outlet works r I 1 E1. 5024.50

A% o ~E1511CII

.

14,~00

E1. 5014.00

60"I .D. Steel pipe tD - 1 A

a. 26'-6".m M

12'_0.

0

ELEVATION

36° I. D. Steel pip

A

SECTION A - A

Figure 2. - Canal outlet works jet-flow gate-sudden

expansion energy dissipator. ,

9.56° No. 4

4.414° ~i No. 2 8i No. 3

0 5.00 Q

No. I 0 J

TOP VIEW

0.525'

Gate leaf

Piezometers

m

FLT 40% D

C) No.4~

0.018"

u SIDE VIEW

I

Y

Figure 3. - Model jet-flow gate.

5

THE INVESTIGATION

Tests were performed first to determine head loss through the fully opened

gate discharging into the two-diameter sudden expansion energy dissipator

(figure 4). Subsequently, mild cavitation was observed while testing partial

gate openings. With cavitation occurring in the model, prototype cavitation

could be expected to be more severe. This expectation is based on past experi-

ence of other observers and on the fact that in scaling by Froude relationships,

atmospheric pressure and vapor pressure are not scaled Only one instance can

be recalled in which cavitation appeared to be greater in a scaled model than

in the prototype which it represented. Mention of this is found in reference

[1], page 1633, where it states that cavitation formed at an index of less

than about 0.7 in the prototype, compared to 1.5 in the model.

A series of tests was performed to define the cavitation characteristics of

the model gate with the two-diameter expansion attached downstream. A hydro-

phone and a sound level meter were initially used in an attempt to detect the

presence of cavitation in the model. Construction work adjacent to the model

produced an extremely high level of background noise and made the electronic

equipment readings unreliable. As the tests were performed in a clear plastic

pipe, visual and aural observations were finally used as the method to deter-

mine when cavitation began (i.e., incipient cavitation). For these tests,

incipient cavitation was arbitrarily defined as that magnitude which:

1. Produced a crackling sound which could just be heard when the ear was_

placed on the wall of the expanded pipe near the gate. The sound had to be a

present between 50 and 75 percent of the time.

6

19C

IBC

17C

V

I6C

15C

140

W I-- 13C

9 120

0

w 110

0 100

a W x 90

J

f 60 O F

70 Z

N 60

O J

50

40

30

20

10

0 0

100% GATE OPENING

0 Head loss from 2.632 feet upstream 0f gate to 3.776 feet upstream of well

a He .

ad loss from 2.832 feet upstream of gate to 0.944 feet upstream of the downstream end of well.

o Head loss from 2.832 feet upstream of gate to 19.661 feet downstream of well.

(Note recovery of head downstream of well.)

DI D2

02 - 201

10 20 30 40 50 60 70 90 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250

DISCHARGE THROUGH ONE GATE (FT3/S1

Figure 4. - Head losses for two-diameter expansion

(100 percent gate opening).

2. Produced a faintly visible vortex emanating from the gate-orifice

intersection. The vortex had to be visible between 50 and 75 percent of

the time. Intensity with a fully opened gate was based on sound alone, as

the origin of the vortex varied, making its presence difficult to detect.

If on a particular rum the cavitation intensity was too low, a reduction in

back pressure was made. Conversely, if the intensity was too high, ..n increase -

in back pressure was made. In both cases, slight adjustments in discharge

were required so as to maintain a constant total upstream head for all gate

openings.

To better define the cavitation characteristics of the gate, an index K was

used as defined by Ball [2]:

K = h2 - by

ht - h2

where: h2 = Pressure head downstream of the gate in an area where

flow is uniform

by = Vapor pressure head of water relative to atmospheric

pressure (i.e., approximately 27 feet [8.23 meters]

of water below atmospheric at approximately 5,280 feet

[1,610 meters] of elevation)

8

ht = Upstream total head (pressure head plus velocity head

in the pipe upstream from the jet flow gate)

The heads ht and h2 were obtained while operating the model at the conditions

defined as incipient cavitation for a range of gate openings. A peak cavita-

tion index value of 4.2 at a gate opening of 75 percent was determined for the

_initial model configuration. The curve of model incipient cavitation versus

gate openings is shown on figure S.

Within limits, a larger diamater expansion was expected to improve the circu-

lation to the jet and reduce the cavitation potential.

A length of three-diameter expansion was installed downstream from the gate as

shown in figure 6. To simplify the model modifications, only three diameters

of length were included. After the system was tested for losses (figure 7),

the cavitation index was investigated as before. A sizable improvement to the

cavitation characteristics resulted from this modification, as can be seen by

comparing the test results shown in figure 8 to those in figure S. While

possessing the same general shape, the peak index value was lowered from 4.2

to 2.7. As before, the peak value occurred at a 75 percent gate opening. This

improvement was considered adequate for the Teton installation. Similar

improvements resulting from the use of larger expansions to improve circulation

are contained in [3] by Tullis and Marschner. Rouse [4] also discussed the

advantage of larger expansions on the index of incipient damage for a sub-

merged jet.

9

6.5

6.0

5.5

5.0

_ 4.5 Y

z 0 4.0

a ~ 3.5

Q v H 3.0 z W_

CL 2.5 U z

We

1.5

1.0

0.5

K = hB-hV

hT -h B

Where h e = Pressure head

in pipe 19.66 ft.

downstream of

well.

by = Vapor pressure

head of water.

h T = Total head

(pressure plus

velocity) in pipe

upstream of

jet-flow gate.

d

/ NO CAVITATION

\ CAVITATION

we, 90 80 70 60 50 40 30 20 10 0

GATE OPENING (PERCENT)

Figure S. - Model incipient cavitation curve

(two-diameter expansion).

10

Figure 6. - Model equipped with length of three-diameter expansion.

11

210

20C

19C

I80

170

16C

15C

w

~ 14C a 3

13C

W 12C w w

a w IOC S

Q 9C

0 O ~ BC

2

N 7C W O J 6C

5C

4C

30

2C

IC

o Head loss from 2.832 feet upstream of gate to 3.776 feet upstrea m of well.

o Head loss from 2.832 feet upstream of gate to 0.944 feet upstream of the downstream end of well.

a Head loss from 2.832 feet upstream of gate to 19.661 feet downstream of well.

DI D2

D2 = 3D1

ol

Figure 7. - Head losses for three-diameter expansion

0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 II

DISCHARGE THROUGH ONE GATE (FT3/S)

12

O.V

5.5

5.0

4.5

Y 4.0

Z O

3.5 Q

- H

> 3.0 Q U

f- Z 2.5 w

a

v 2.0 z

1.5

1.0

0.5

0 100 90 80 70 60 50 40 30 20 10 0

GATE OPENING (PERCENT)

Figure 8. - Model incipient cavitation curve

(three-diameter expansion).

13

The scaling of cavitation from model to prototype (see reference [5],

page 2S4, summary item 2) does not follow the Froude relationships. An

attempt was made to establish some significant values through a series of

graphs. The graphs were all similar and were made for a range of gate openings

from 100 percent to 25 percent in 5-percent increments. The graphs show the

expected head conditions for prototype incipient cavitation. Figure 9 con-

tains the proposed scaling curve for 75 percent gate opening. The graphs are

based on the assumption that incipient cavitation in the model is a valid

starting point. Working with water and using Froude relationships, no

similarity exists between model and prototype when operating below a model

head which scales to prototype vapor pressure. However, no error is expected

when merely increasing pressure from the observable (not measurable) point of

model vapor pressure to a value in the model which corresponds to vapor pres-

sure in the prototype.

Beginning with a discharge coefficient curve for partial openings (figure 10)

and the incipient cavitation coefficient curve (figure 8), the graphs were

developed in the following manner. Plotting total head upstream against total

head downstream, a no-flow line was established on a 1:1 slope. Above and

parallel to the no-flow line a series of lines of constant discharge was laid

out. In addition, beginning at a vapor pressure head of minus 27 feet

(8.24 meters) of water, a model incipient cavitation line was drawn which was

determined from the back pressure requirements to barely maintain incipient

cavitation in the model. Based on tests in the model, which were limited by

model structural strength, the incipient cavitation coefficient was found to

be a constant for a fixed-gate opening. This is supported by reference [6],

page 423, where pressure scale effects on orifices are discussed. By dividing

14

J W

0 E

100

90

80

70 0

N

60

W H 50 a

0 40

LLJ 30 H

a 20

0 a W = 10 J a

0 0

-10

-20

w a r F-O r INCIPIENT

--

CAVITATION (MODEL) 0 m CL

1.50 c.f.s. (114) 350

Max Reservoir El. (Teton) 300 `row

Max. Discharge for 4\ 250 1 Gate (110c.f.s.) 00`0

INCIPIENT CAVITATION ky.

200 ( Prototype)

150 C, QP 5

100

- ~O

- 50

TetOn Back Presure

VAPOR PRESS(Proto)

VAPOR PRESS(Model) 75% Gate Opening

-50 0 50 IOb 15b (PROTOTY

Figure 9. - Proposed scaling curve.

is

PE) -30

LL_ -30 -20 -10 0 10 20 30 40 50 (MODEL)

TOTAL HEAD DOWNSTREAM OF GATE (Ft. H 20)

Cd = Q , Where, A 2g VWIT -H2T

HIT = The total head 2.83 feet upstream of the gate,

H2T = The total head 19.66 feet downstream of the well,

Note: Total head equals the sum of pressure

head and velocity head at the some elevation for the two stations.

A =The area of the 24—inch-diameter pipe,

g =gravitational acceleration, and

Q =discharge In cubic feet per second (FT3/S).

CdI = Q , Where, A 2g h l -h2

h l and h2 are the pressure heads at the

two stations.

Cdl

Cd

10 20 30 40 50 60 70 80 90 100

GATE OPENING (PERCENT)

0 0

.1 .

0.9 U)

z w 0.8

U

LL LL 0.7 W O U

w O.E

Q:

= 0.° U N

0 0.4

0.2

0.2

0.1

Figure 10. - Discharge coefficient curves for gate with

three-diameter expansion.

16

vapor pressure head by the model scale ratio (-27/5.66), the prototype

incipient cavitation point for no-flow was obtained (-4.77). As an approxi-

mation to the prototype incipient cavitation characteristics, a line was

drawn parallel to the model incipient cavitation line, but displaced to the

right and passing through vapor pressure condition in the prototype. The

slope of the constant discharge lines to the left of the incipient line was

not investigated to any great extent in the model; however, it was assumed

that an extremely high degree of cavitation would be required before the

discharge coefficient would be affected (choking).

While it was considered possible to extend the operation of the prototype

beyond the point of incipient cavitation without incurring damage, the point

at which damage would occur could not be determined from the model or the

series of graphs. While other investigators feel that incipient cavitation

is not a practical design limit [7], the only scalable cavitation condition

was thought to be that of inception. With incipient conditions defined for

a range of gate openings, releases may be made to avoid cavitation. For

example, gate openings of 75 percent should be restricted to low-head operation

only, as the cavitation index is a maximum for this setting. Full gate open-

ing with its reduced index may be used to release the maximum discharge per

gate (110 ft3/s [3.115 m/s]) without the occurence of cavitation. Figure 11

'relates the discharge for incipient cavitation for both single gate and

double gate operation. The symmetry of the full gate opening makes it less

conducive to cavitation than partial openings. With partial openings, a

vortex emanates from the points of intersection of the straight horizontal

gate bottom and circular orifice (figure 12).

17

220

200

F-- 180

z p 160

H a ~ 140

w a a v cD 1 2 0

z w w J — c~ a 1 0 c

Z U ~ z

8C

a

6C c~

a U 4C N

2C

o Balanced two gate operation

o Single gate operation

t

70 60 50 40 30 20 10 0

GATE OPENING (PERCENT)

Figure 11. - Discharge versus gate opening for incipient

cavitation.

18

Figure 12. - Vortex emanating from the junction

of the circular orifice and the hori-

zontal bottom of the gate.

Figure 13. - Erosion to parallel frame downstream

of gate slot resulting from an 8-hour

test (model) with a release of

of 207 ft3/s and 75 percent gate

opening.

19

Some additional cavitation tests were performed at exaggerated heads and dis-

charges to seek out areas of potential damage for the jet flow gate. These

included piezometric measurements with electronic transducers at selected

locations in the gate and cavitation erodible paint tests. Slight erosion was

observed to the paint on the downstream side of the parallel frame downstream

from the gate slots (figure 13). To improve circulation to the upstream corner

of the frame and minimize the potential for cavitation erosion, an away-from-

the-flow 45° divergence was recommended for the frame extending from a slight

flat on the upstream corner to the downstream side.

SUMMARY

A two-diameter expansion located downstream from the jet flow gate was not

large enough to provide good circulation to the jet and eliminate the

possibility of damage from cavitation occurring on the expansion walls. Use

of a three-diameter expansion greatly reduced the model cavitation index and

should eliminate the possibility of damage to the expansion walls. The most

severe cavitation characteristics occurred at a 7S percent gate opening.

Incipient cavitation will occur for some combinations of gate openings and

heads, but should not adversely affect the structure.

20

REFERENCES

[1] Tullis, J. Paul, and Albertson, Maurice L., 'Needle Valves as Pressure

Regulators," Journal of the Hydraulics Division, ASCE, Vol. 95,

No. HYS, September 1969

[2] Ball, J. W., "Cavitation Characteristics of Gate Valves and Globe Valves

Used as Flow Regulators under Heads up to 125 feet," Transactions,

ASME, August 1957

[3] Tullis, J. Paul and Marschner, Bernard W., "Review of Cavitation Research

on Valves," Journal of the Hydraulics Division, ASCE, Vol. 94,

No. HY1, January 1968

[4] Rouse, Hunter, "Cavitation and Energy Dissipation in Conduit Expansions,"

Proceedings of IAHR, 11th Congress, Leningrad, 1965

[S] Ripken, John F., and Hayakawa, Norio, "Cavitation in High-Head Conduit

Control Dissipators," Journal of the Hydraulics Divison, ASCE,

Vol. 98, No. HY1, January 1972

' [6] Tullis, J. Paul, and Govindarajan, Rangachari, "Cavitation and Size Scale

Effects for Orifices," Journal of the Hydraulics Division, ASCE,

Vol. 99, No. HY3, March 1973

[7] Tullis, J. Paul, "Cavitation Scale Effects for Valves," Journal of the

Hydraulics Division, ASCE, Vol 99, No. HY7, July 1973

21

[8] Isbester, T. J., "Hydraulic Model Studies of the Teton Canal Out

Energy Dissipator," USBR, REC-ERC-74-16, October 1974

22