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hammer solutions - International Water Power

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Waterhammer solutions

October 2006

all hydro plant designers often choose long penstockstead of the traditional 'open channel' option, as they can be

eaper, quickly installed, and easier to maintain. However,

se longer penstocks are usually affected by waterhammer

enomena, which must be faced with great care and suitable

culation models to verify in a reliable way that the stresses

main in a correct range as was the case with one small

dro scheme in Italy

done V.T. is a small hydro plant, the result of the rehabilitation and

rade of a plant once owned by a military factory, where the head of

en other very small upstream plants, which were all decommissioned

ng the last 50 years, was concentrated. The water for Gardone V.T.

inates from the river Mella, by means of a weir located in the north

e of Gardone Val Trompia, a small industrial town in the province of

scia, northern Italy. The drainage area is 241km2 with an averagefall of 1.2mm per year.

ore the construction of the new plant, the water ran in a long canal,

med Acqualunga, created specifically for it, crossing the whole town

m north to south. Later, a secondary canal, called Gramineto, was built

he middle part. The existing plants were located on both the main

al and on the secondary ones, so some exploited the whole diverted

er amount and some only a part of it. Nowadays, this partition has

n removed and the whole flow rate runs in the new penstock.

e present maximum flow is 4.5m3/sec, corresponding to an average

w rate of about 3m3/sec (the maximum flow being increased slightly

m 4m3/sec in order to guarantee the 3m 3/sec average), but under a

erved flow obligation which is expected to increase dramatically in the

re.

exploit the whole head potential without any altimetric constraint, thesting open channels were substituted with a pressured pipe a long

stock that carries the water from the first plant upstream to the last

downstream, saving the available head as pressure.

optimum penstock diameter has been calculated as 1800mm, which

ws an increase in the head exploitation of 6,64% thanks to the

over of the unexploited heads existing between every couple of

tiguous plants. The main characteristics of the new plant in the final

figuration are:

aximum flow rate: 4.5m3/sec.

erage flow rate: 3m3/sec.

oss head: 27.3m.

Waterhammer 18

Waterhammer 17

Waterhammer 4

Waterhammer 5

5

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minal capacity: 803kW.

talled capacity: 1.3kVA.

nual production: 4MkWh.

e power plant and the tailrace, 360m long, will remain about the same

he last downstream plant.

the length (>1.5km) of the depleted reach and the storage constant

significant, a relief discharge device was installed close to the plant to

er flow rate variations (emptying out and filling up) in the river caused

he unit start up and shut-down. Essentially, it is a Howell Bunger

ve, installed under water, automatically driven by the control panel to

ntain the flow rate in the penstock when the turbine is out of service.

nstock characteristics

e first 202.8 m of the existing canal will remain unchanged; only a new

hrack and an automatic cleaner will be installed. In the following

.1m, an underground reinforced concrete tunnel is now working in

ressurised (approximately 1m of water) conditions to solve a mistake

de during the construction: the bottom of the channel was rebuilt about

m higher than the old one, so only 20m3/sec can run in the open air

nnel. As the tunnel cannot be reshaped, nor can another one be

avated in the heart of the town, the study decided to create a

ression column of about 1m, after having checked that the existing

ctures could tolerate the new external loads and having put a plastic

ting to make it as air-proof as possible.

e water then begins to run underground in the 1.38m long penstock,

ch was made with three different technologies. The first 254.5m of

stock, under the national road of Val Trompia, consists of cast iron

es, the following 768.4 m, which follow the borders of a new

dential area, are glass reinforced pipes (GRP) and the last 357.1m,

ng into the existing canals, is steel pipe coated with a reinforced

crete structure. Table 1 sums up the situation.

e first tests on the plant confirmed the good quality of the theoretical

culation, as the total reflection time was measured as 3.75 secs,

responding to an average wave speed of 736m/sec, slightly lower than

calculation, possibly because of the lower actual value of the modulus

lasticity of GRP pipes and because of the smaller contribution of the

erage soil along this part of the penstock.

terhammer calculation

e waterhammer effects on the penstock have been investigated in two

ges. First, the hydraulic functioning of the penstock and how the

bine and dissipation valve actions affect the pipes was calculated (by

ans of a simplified mathematical model); the calculations detected the

re dangerous operations for the system and suggested the operating

tation of the unit to avoid overstress in the penstock. In a second

ge, three sophisticated models were implemented to investigate the

t solution to the waterhammer problems, (which turned out to be a

ge tank) and to define its design parameters. Before the second model

ge, a set of field tests was carried out to verify the hydraulic

ameters of the penstock and the results of the theoretical calculations.

he end, another field tests campaign was carried out to set the

rating parameters of the turbine and the dissipation valve, and to test

plant in the hardest working conditions.

undary conditions

nstock relevant data

tic water level: 329.1m asl.

pressurised length: 321.1m.

nstock length: 1.380m.

nstock total length (depressurised + pressurised length): 1,701.1m.

draulic area of the pressurised penstock: 2.596m2.

eoretical average wave speed: 895m/sec.

eoretical reflection time: 3.8sec.

Waterhammer 6

Waterhammer 10

Waterhammer 8

Waterhammer 3

Waterhammer 11

Waterhammer 9
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tual reflection time: 3.75sec.

tual wave speed: 736m/sec.

draulic losses coefficient: 0.1068sec2/m5.

n turbine characteristics

e: Kaplan.

mber of blades: five.

nner diameter: 950mm.

cket gates axis level: 302.7m asl.

nner axis level: 302.33m asl.

odetic head: 27.1m.

t head: 25.8m.

aximum flow rate: 4.5m3/sec.

ted power: 980kW.

ted speed: 750rpm.

aximum runway speed: 2,030rpm.

it inertia (PD2): 5,258Nm2.

ater contribution (PD2): 432Nm2.

tal inertia (PD2): 5,690Nm2.

draulic relevant transients

a preliminary approach, the more dramatic situations for the

stock stress are assumed to be:

erspeed caused by electric shut-off without wicket gates

sing (with the runner at stationary runaway condition).

ectric shut off with wicket gates closing.

echanical shut-off without over speed.

preliminary simulations and the field tests are carried out under the

vementioned items.

terhammer stress minus surge tank

preliminary calculations showed that significant overpressures

sitive and negative) affect the penstock. So many field tests were

ried out on the penstock in its original configuration (without surge

k) to validate the theoretical results and to define exactly the actual

raulic working of the turbine/penstock system.

enomena connected with the negative pressures

hydraulic and mechanical transients depend on the peculiar

racteristics of the plant, which are:

ry high hydraulic inertia, due to the penstock length.

ry low mechanical inertia, due to the small value of the unit PD2.

characteristic time of the mechanical inertia is about 0.9 secs,responding, when an electric shut-down occurs, to the acceleration

e of the unit running speed.

the turbine is a Kaplan type, increasing the running speed

responds to a similar increase of the flow rate, which causes a strong

ression in the penstock. The maximum value is reached in case of

rspeed due to runaway condition, but it changes only a little during

normal shut-down because of the long closing time of the wicket

es needed to limit the positive waterhammer pressure.

the reflection time of the penstock is about 3.8 secs and the first

ease of the flow rate comes in less than 1 sec, a direct waterhammer

es place in the penstock, so the first depression wave runs practically

hanged along the pipeline until 450m downstream of the siphoned

nnel, decreasing only in the last part. Nearby, the whole penstock is


4/6



aged by a transient of negative pressure, as reference to the

ospheric value, with a first maximum close to the absolute vacuum at

progressive 790, while the main part of the siphoned channel would

affected by the absolute vacuum.

his situation, it would be necessary to install suitable devices to

rantee the air comes in, but the air bubbles in the penstock could

ve to the dissipation valve, causing a very strong waterhammer, with

itive pressure waves of about 250m as a water column.

the other side, without aeration valves, vacuum bubbles appear and

y could implode under the ensuing positive pressure wave, causing

y dangerous effects to the penstock. Referring to the positive

rpressure, it occurs at every electric shut-down with correct closingons of the wicket gates and runner blades. The calculation and the

d tests show that the maximum pressure waves can be limited to

ndard values using a long closing time and suitable law for the wicket

es (a bilinear closing law is enough).

main problem from the turbine operating transients is the negative

ssure wave, which cant be mitigated by external devices such as

dissipation valve, which is very efficient for the positive pressure.

reover, the dissipation valve must be operated under great care to

id making the penstock situation worse. The only technical solution,

ch is consistently reliable, is a surge tank, installed in the penstock

h between the power station and the progressive 790, where the

uum problem becomes dramatic.

ting for the erection of the surge tank and taking into account the

gerous effects of the negative pressure of the transient waves, plant

acity was limited to 250kW, so as to operate in a safe condition inry situation.

ge tank

owing the mathematical simulations and the morphology of the town

ere the penstock is located, the surge tank was erected at the

gressive 1524.7m. This is quite close to the power station (145m

tream) so that a long part of the penstock can enjoy the benefits of

work. The surge tank, circular shaped, was built quite completely over

ground level, so it resembles a simple cilindric tower made of steel.

hort steel pipe 800mm in diameter connects the penstock to the surge

k, constituting the surge tank diaphragm. It has an hydraulic smoothed

tion, conic shaped (in order to have an asymmetric losses coefficient)

he top side, towards the tower; the idea was to get higher hydraulic

ses for the water going into the surge tank and lower ones for the

er coming out.

anks to its steel structure, the tower, which works as a surge tank, was

cted in two weeks, during a scheduled out-of-service period of the

nt for routine maintenance.

e main characteristics of the surge tank are:

ernal diameter: 4m.

aterial: steel UNI EN 10025 S275 JR.

ckness: 11mm.

nstock axis level at the progressive 1524.7: 308.7m slm.

p of the tower: 336.6m slm.

wer net height from the ground: 23.6m.

al situation

anks to the preliminary simulation and the field tests on the plant at

ted capacity, a sophisticated mathematical model of the penstock was

lemented to investigate the situation of the plant with a surge tank

able to maintain the maximum stress in the penstock in the standard

ge. Because of the location of the plant in the town, the surge tank

the following two main constraints:

aximum diameter 4m, due to small available space.

duction of the positive oscillation of water in the surge tank to limit the

er height.

calculation scheme was resumed, then, where:


5/6



penstock length upstream of the surge tank: 1526.1m.

average penstock area: 2.596m2.

surge tank area: 12.566m2.

diaphragm area: 0.503m2.

u hydraulic losses coefficient for the water going into the tank (m=0.7):

49sec2/m5.

e hydraulic losses coefficient for the water coming out the tank

=0.9): 0.412sec2/m5.

S. static level of the water: 329.10m asl.

preliminary simulation by simplified software finds the situations

ch cause the hardest stresses to the penstock, which are:

ectric shut-down just at the end of the maximum positive load ramp:

is the worst condition for the negative pressure.

ectric shut-down at maximum load when the flow rate in the penstock

t its maximum transient value: this is the worst condition for the

itive pressure.

e simulation also supplies the law of the flow rate variation during the

sing transient, which was used as an input for more sophisticated

culations.

above conditions are used to simulate many different situations by

ans of three different software packages, to find out:

e best closing law of wicket gates and runner.

e diaphragm optimum size to fulfill the constraints.

erring to the turbine closing law, the simulations suggest as a

oretical approach:

cket gates: Quick closing time from 100% to 18% (corresponding to

braking start) 50 secs, braking time from 18% to 0% 30 secs.

nner blades: Linear closing time 120 secs.

calculations face the set of operating actions connected with the

bine opening till full load and the following shut-down as mentioned

ve, adopting a diaphragm with a diameter of 800mm, which the model

d would be the smallest one to satisfy the design constraints. The

culation results show the pressure variation in the penstock section

upstream of the surge tank and at the progressive 790 as a very

ortant issue also, corresponding to more critical vertex for the

ative pressure; the small perturbation waves demonstrates that the

ge tank creates a satisfying decoupling effect.

e maximum oscillation range of the water level in the surge tank has

n verified by means of three different software tools, which confirmed

following values, which became the input data for the surge tank

ign:

nimum water level in the tower (negative wave): 320.85m asl.

aximum water level in the tower (positive wave): 335.6m asl.

evation of the tower top: 336.6m asl.

eeboard over the maximum positive wave: 1m.

end of the erection of the surge tank, a field survey was carried out to

fy the actual situation of the penstock in the most dangerous

ations. The tests allow a better definition of the turbine parameters,

easing the runner blades and wicket gates closing time in order to

uce the effects of the related waterhammer phenomena.

cket gates: quick closing time from 100% to 12 % (corresponding to

braking start) 165 secs, braking time from 12% to 0% 140 secs.

nner blades: linear closing time - 250 secs.

e results of the tests show an excellent concordance with the

oretical calculations, so that every limitation to the plant which could

rt to work with the design flow rate could be removed.

nclusion

he particular situation at the Gardone V.T. plant, the calculations, and


6/6


adopted solutions are shown, where the most dangerous stresses

me from the negative waterhammer pressures caused by the very

ck increase of the flow rate (+30% on the rated flow in ~ 1 sec) when

Kaplan unit shuts down at full load. This event, if not completely

lysed and faced in the designing phase, would cause an absolute

uum in a part of the penstock, with possible severe damages to pipes

to nearby installations.

hor Info:

olo Cretti, Voith Siemens Hydro Power Generation, Fosse Ardeatine,

I-20092 Cinisello Balsamo (MI), Italy; and Nino Frosio, Studio Frosio,

F. Calvi, 9 I-25125 Brescia, Italy. Emails: [email protected]

[email protected]

s paper was orginally presented at the Hidroenergia 06 conference,

anised by the British Hydropower Association and the European Small

dropower Association

les

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