leakage and seals

8/13/2019 Leakage and Seals

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LEAKAGE AND SEALS ITS RESTORATION IN HYDRO POWER STATION.

Hydro Power Station, as the name implies use water at high potential for driving of

turbine-generator combine set to generate electricity. Besides running of turbine, water is also used for

cooling of component of hydro-generating unit and other work. Hydraulic oil is being used for servo

mechanism control, lubrication and heat transfer of the hydrogenating set.

Generator ransformer, Potential ransformer, !urrent ransformer, !apacities "oltage,

Station Service ransformer used transformer for insulation and also for heat transfer from the core and

winding. #ll these components work very fine when they are controlled and not allow to go out from

there designate position. $hen they come out of the enclosed position, we called it leakage. !ontrolled of

this leakage is one of the main components in %anagement of Hydro Power Station.

#ir in Power Station is used for pneumatic braking of rotor for maintain &P' at high

pressure and also used for servicing work. (n olden days, air at high pressure is used for !ircuit Breakingin the form of #ir Blast !ircuit Breaker. However, this has become obsolete to space and slow in

operation. he recent turned is the used of S)* gas in !ircuit Breaker.

#nother leakage, which is not including in the above but of utmost importance, is theelectricity current leakage. %any of the damage of electricity e+uipment are due to the uncontrolled of

this item. Here we will discuss only the mechanical leakage as mention above and electrical leakage will

be dealt with another time.

et us know how hydropower work, before we proceed to eplain the leakage, its source and how

to tackle with it. Basically hydropower station gets its energy from the storage water at a higher level.

his water is taken through conduct, which may be turned, concrete liner or steel liner or both, to the

power station. &n reading the power station, the water is taken to the spiral casing and thereafter to the

turbine. he inflow to the spiral casing is contracting by %ain (nlet "alve %("/ and to the turbine by the

wicket gates or Guide "anes. #fter doing its work, the water is discharge to the tailrace through the draft

tube. #ll through, the water is sub0ected to various pressure. he turbine is turn device the shaft which iscoupled to the generator and power is generated. (n order to enable the shaft to rotate in particular ais, it

is guided by difficult bearing, like turbine guide bearing, upper or lower guide bearing or both. he whole

weight of the rotating component are support by thrust bearing. 1uring rotation, heat is developed in thisbearing and is being cooled by oil lubricant which are immense in a sump or force from outside by a

pump. his oil take away the heat form the bearing beside lubricating it and it is being cooled by &il

cooler. his oil cooler used water at around 2kgkm2 to cool the oil, through cooling water pump.

he water is confined in turbine and not allows to leak along the shaft by using shaft seal

arrangement. 1raft tube is used to guide the water after runner.

he turbine on rotating will have to be controlled so that it will remain at constant speed under

different load condition. his is achieved by control the opening and closing of water inlet to the runneri.e. by controlled the guide vane opening. his is achieved by governor action through guide vane

servomotor and gate mechanism, which used hydraulic oil at high pressure as medium for transfer of

mechanical power. Similarly, opening and closing of %(", Pressure 3elief "alve and achieved in similar

way. he oil is kept at high pressure in oil pressure tank and this high pressure is achieved by compressed

air or nitrogen. &n stopping the unit, the speed come doom slowly and it ahs to be brought to stand still to

avoid damage to the hrust Bearing. (n most power station, pneumatic brake are used, while in other

hydraulic oil is used for braking. eakage can be classified in two categories

a/ 4ternal leakage 5 eakage from inside the conduct, machine to atmosphere.

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b/ (nternal leakage 5 eakage inside without coming out to atmosphere.

Basically there are two types of fluid leakage namely (nternal eakage and 4ternal eakage.

(nternal eakage are those leakage which leak out inside the enclosure without coming out to theatmosphere whereas 4ternal eakage of fluid are those type of leakage where the fluid leaks out from

inside the enclosure at high pressure to the atmosphere. Both type of leakage are coming loss to the power

house. he internal leakage will not hamper the working personnel but it is etremely dangerous for thegenerating unit.

he eternal leakage may cause flood, damage to the personnel and also to the hydro-generating

unit. (t is most difficult and challenging task to conceal both type of leakage. (t is for this reason that all

the 0oints are sealed by special type of sealing material depending in the location where the seal is fitted.

(n the case of water the eternal leakage may occur at

i/ runnion of %(" due to damage of cup-seal

ii/ Spiral casing manhole. 1ue to damage of seal.

iii/ &utput pipe of by pass valve due to damage of the pipe by cavitations after prolong use.

iv/ )rom the flange of at different part- of %(", by-pass valve and others due to damage of

gasket or looseness of bolts.v/ urbine top cover from the guide vane stem due to damage of cup-seal.

vi/ Prominently from the shaft due to failure of shaft seal.

vii/ )rom man-hole of draft tube gate due to faculty seal or damage of the cone.viii/ )rom the cooling water of piping due to damage of seal.

(nternal leakage of water normally occur at the following7-

i/ #t the %(" during closed passing due to damage of seal, this leakage is very dangerous not

only that precious water is lost but also it will damage the guide vane in the long run.

ii/ eakage at the by-pass valve or sealing valve due to failure of seal.

iii/ #t the guide vane, this leakage is also very dangerous as it will not only cause loss of water

of water out reduction in the efficiency of the machine and damage to the lower part of the

turbine.

iv/ eakage form the P3". his leakage will cause loss of water and also turbulence to the water

entering the turbine further it may damage the PP3" discharge iner.

Beside the above leakage of water there is another type of internal leakage where the water leaks

in to the oil cooler to the bearing sump due to cooler failure. his is very ha8ardous to the hydrogenerating unit as the water may enter the bearing. Parts and subse+uently damage the bearing namely

GB, GB, 'GB 9 B. (n such cases if its happen the immediate solution for the maintenance engineer

of a hydro Power Station, repair:replace the leaked cooler to put the unit under shutdown thoroughly

before a fresh oil or filtered out is put back.

Similarly, leakage of water in to the oil in ;-inner will cause a low insulation to the ;-inner and

if not control will damage the out and subse+uently the ;-inner. his is more prominent in case of those

type of ;-inner where forced water is used for cooling the oil. $ater may also ingress in to the ;-inner

oil thru the damaged gasket or thru the improper use of air breather.

eakage in the case of oil may occur etremely at the following points7

i/ G" servomotor shaft seal. (n most cases the leakage through the servomotor shaft occurs due

to damaged seal. his is because due to pressure of impurities in oil. (n source cases it may be

happen due to loose fitting of adapter ring.

ii/ )rom the P3" shaft. his is also from the servomotor.

iii/ )rom the (nlet "alve servomotor piston.

iv/ )rom servomotor by-pass valve:sealing valve.

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v/ )rom the control valve and piping.

vi/ &il also may leak from the sealing end where &-ring is used in the end cover of the

servomotor.

vii/ &il may also leak in the 0acking system.

viii/ ;-inner oil leakage may also occur in !:!" due to damaged seal.i/

his type of leakage should be control or repair at the earliest so as to avoid loss of oil and alsodamaged to the environment. (nternally the oil may leak in the servomotor of %(": By-pass:Scaling

"alve:P3":G" Servomotor due to failure of piston seal. (f the re is a leakage of such cases there is a loss

of pressure oil to the sump thru leakage unit. his type of leakage can be observed by the fre+uency of

pump loading.

(n etreme cases it may not be possible to operate the servomotor due to ecessive leakage. (n

such cases the unit has to be put immediately under emergency Sub-1ivision.

#ir leakage is usually out in nature. he use of air in hydro power station we mostly limited to

brake system and &P' system. So the possible leakages occur also in those areas. he leakage can easily

be detected because it will make a lot of noise when leaked. Some yearow a days the used of leather is decreasing to cost factor

and also difficult replacement. >itrite rubber is of many types depending o the percent of carbon added to

the rubber. Proper selection of nitrite is very important otherwise the seal may not be able to sustain the

pressure for e.g. the nitrite rubber seal is to sustain the movement of piston at high pressure where as thestationery seal may not be sub0ected to same pressure. he following are the type of seal used in hydro

power station.

i/ !up seal 5 his seal are so made in the form of " shape. his is to enable it to increase the

surface area and when pressure is applied it will cause the sealing. hey are used in cup

sealing of %(" trunnion, cup-seal of G" stem where the rotary movement is small and at

slow speed.ii/ ?-Packing.

SEALS

Seals have been used since ancient times and have evolved into a wide variety of shapes and

materials. )or those who are not familiar with sealing technology, the number of options available can beconfusing.

Selecting the most suitable product for a given application can be difficult.

Sealing Theory

Static v! Dyna"ic Sealing

4very seal, whether static or dynamic, must seal against at least two contacting surfaces. (n static

applications, both surfaces are non-moving relative to one another. (n dynamic applications at least one

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4ngineered resins such as >ylons, sometimes called hard plastics. hese compounds ehibit high

tensile and compressive strength and are typically used in wear rings for bearing support and in auiliary

devices for etrusion resistance.

iv/ hermoset 4lastomers 3ubber'nlike thermoplastic elastomers, thermoset elastomers gain their strength from an irreversible

cross linking process that occurs when the compound is sub0ected to pressure and heat. 1uring thisprocess, or CcureD, special chemical agents within the compound react to the heat and pressure to

vulcani8e the molecules together. &nce cured, thermoset compounds obtain the necessary physical

properties needed to function in fluid sealing applications.

3eheating thermoset compounds will not cause them to melt as thermoplastics do.

v/ >itrile >B3/

>itrile rubber >B3/ is the general term for acrylonitrile butadiene copolymer. >itrile

compounds offer good resistance to abrasion, etrusion, and compression set. he acrylonitrile #!>/

content influences the physical properties of the compound. #s the #!> content increases, oil and

solvent resistance improve, tensile strength, hardness and abrasion resistance increase, while

permeability, low temperature fleibility, and resilience decrease.

Ty%e o& Seal 'e%en'ing on their ha%e an' location o& there (e'

a/ Gasket 5 gasket will be needed flanged oil and water pipe connections, oil tubes, generator air

coolers, the packing bo and other similar application.

b/ &-ring 5 o-ring material are seal which are formed in the form of letter E&


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areas to low pressure areas of the turbine. #s the clearance increases over time, the amount of

water flowing through the seal rings increases, decreasing the efficiency of the turbine. #s seal

ring clearances increase to 2 percent of the manufacturers design clearance, the efficiency can

decrease as much as 2 percent.

(deally, seal rings should last 2 years or more. his type of longevity is not alwayspossible, but to achieve the longest life possible, the seal rings are constructed of materials that

are resistant to wear and corrosion. $ear can occur as a result of cavitations or abrasive erosion.#brasive erosion caused by silt or other suspended particles in the water is usually not a concern

in turbines that receive their water from deep reservoirs, but it can be a problem in pump-turbines

or turbines that receive their water from a canal or other shallow waterway. !avitations erosion is

dependent on the amount of head of the unit, the seal ring clearance, and the hydraulic

characteristics of the unit. he occurrence of corrosion will depend on the seal ring material and

the +uality of the water.

he galling resistance between the rotating and stationary ring materials must also be

considered. 'nder normal operating conditions, the rings should never touch. However, in some

circumstances such as the failure of the turbine guide bearing, failure of a seal ring, or during unit

alignment, they can come in contact. (f contact is made between materials that are prone to

galling, the rings can be severely damaged, and in etreme cases, the rings may sei8e

Sha&t Seal

>ormally two types of shaft gland seals are in use in different power stations7

i/ !arbon or ferrobestos segment.

ii/ 3ubber flap type

6 $ar)on or *erro)eto Seg"ent Ty%e

he seal segments are housed in the stuffing bo. Stuffing bo being always in touch with the shaft is

sub0ected to ecessive wear and tear. he overhauling of the stuffing bo becomes necessary when it is

observed that consumption of cooling water has considerably increased or ecessive water in top cover

appears to be coming. (n general maintenance of the seal is re+uired to be done annually.

2 R())er &la% ty%e

%aintenance of rubber flap type gland seal is comparatively simpler and easier. &nly

precaution during assembly of rubber gland is 0ointing of the rubber seal in the proper way.

he +uality of rubber used plays a very important role for satisfactory performance of therubber gland. (n one of the recently commissioned power stations rubber gland seal used to fail very

fre+uently. he cause of fre+uent failure was found to be due to +uality of rubber.

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leakage and seals

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