9. Seal Systems · 2018-01-13 · Heat & Fluid Techniques in Power 9. Seal Systems 11 / 116 Areas Addressed in Steam Path Audit • nozzles • buckets • shaft end packings where

9. Seal Systems 1 / 116Heat & Fluid Techniques in Power

9. Seal Systems

Vacuum

Pressure

High

Pressure


Introduction to Seal Systems 163

Brush Seal 635

Other Seals 766

Throttling Process 21

Steam Path Audit 92

Labyrinth Seal 444

Hydrogen Seal for Generator 877


Throttling Process [1/6]

유체가 노즐이나 오리피스와 같이 갑자기 유로가 좁아지는 곳을 통과하면 외부와 열량이나 일의 교환 없

이도 압력이 감소하는 교축과정(throttling process) 발생.

교축과정이 발생하면 와류가 생성되어 에너지가 손실되면서 압력손실 발생.

작동유체가 액체인 경우 교축과정이 일어나서 압력이 액체의 포화압력보다 낮아지면 액체의 일부가 증발

하며, 증발에 필요한 열을 액체 자신으로부터 흡수하기 때문에 액체의 온도는 감소.

Pre

ssu

re P

1 2


열역학 제1법칙:

벽면에서의 열전달이 없으며, 이루어진 일이나 공급된 일도 없으며, 위치에너지 변화량도 무시할

수 있으므로,

속도가 40m/s 이하인 경우 운동에너지는 엔탈피 크기에 비해 매우 작다.

교축과정은 발전설비에서 자주 일어나는 과정인데, 특히 증기가 밸브를 통과할 때 압력강하가 발

생하며, 이를 교축과정이라 한다.

12 hh (교축과정 = 등엔탈피 과정)

1212

2

1

2

212122

1wzzgcchhq

02

1 2

1

2

212 cchh



작동유체가 이상기체인 경우 교축과정이 발생한 후에 엔탈피는 일정하게 유지됨.

엔탈피는 온도만의 함수이므로 교축과정 발생 후에 온도변화 없음.

그러나 작동유체가 증기인 경우에는 교축과정이 발생하면 압력과 온도가 떨어져서 에너지 수준이 낮아

짐. 주울-톰슨 효과(Joule-Thomson effect).

증기터빈 버켓커버 상부에는 증기누설을 방지하기 위해서 seal을 설치하여 증기누설 방지.

Seal을 통해서 누설되는 증기는 seal strips을 통과하면서 교축과정이 발생하기 때문에 실을 빠져나온

증기는 온도와 압력이 떨어져서 엔탈피가 낮아짐.

따라서 누설증기가 다음 단에서 주유동과 합류하더라도 주유동의 에너지 수준을 높이지 못하기 때문에

손실 발생 누설손실

즉 누설증기가 실을 빠져나오면서 에너지를 잃지 않았다면 다음 단에서 사용할 수 있지만 이미 잃어버

렸기 때문에 손실이 됨.

증기 특성



[Exercise 1] Compare the velocity at 2

그림에서 A와 B는동일한규격의도관이다. 도관 B에오리피스를설치하였다. 그리고도관 B 입구압력은도관 A와동일하게유지시킨상태에서질량유량을절반으로줄였다. 그리고이때도관 B의하류 2에서압력을측정하였더니입구압력의절반이었다. 이때오리피스하류 2에서유속을비교하시오.

1 2

A

1 2

B

Exercise



[Solution]

문제에서주어진조건은다음과같다.

(1)

그리고

교축과정이일어나면온도는변하지않는다. 따라서이상기체라고가정하면다음관계식이성립한다.

그러므로다음관계식이성립한다.

and , therefore, (2)

유동단면적이일정하기때문에식 (1)은다음과같이된다.

(3)

식 (2)와식 (3)을결합하면다음과같은식을얻는다.

따라서질량유량이달라지더라도압력을조절하여하류에서일정한속도를얻을수있다.

2,2,2,2,

2

1

2

1ABAB VAVAmm

1,1,2,2

1

2

1ABB ppp

2,2,1,1, BBBB pp

2,1,2 BB

2,2,2,2,2

1AABB VV

2,1,1, AAB 2,2,2 BA

2,2, AB VV

Exercise



The steam has an initial pressure P1 at the entry to the seal

assembly.

After expanding past the first constriction, the pressure will

have been reduced to condition Xo, with pressure P2.

In the chamber formed between the first and second seal

strips, the kinetic energy of the steam is destroyed and

reconverted at constant pressure P2 to condition X.

From point X, there is then a further expansion of the steam

past the second constriction, with the pressure falling to P3 at

condition Yo.

The kinetic energy is again reconverted in the chamber

between the second and third seal strips, raising the thermal

energy level from Yo to Y at constant pressure P3.

This process of expansion and kinetic energy reconversion is

continued throughout the series of seal strips until the final

expansion takes the steam to condition Qo at pressure P5.

The locus of the points Xo….Qo is called the Fanno curve.

h

s

T1

P1 P2P3 P4

P5

Xo YoZo

Qo

X Y Z

Leakage

Flow

P1 P2 P3 P4 P5

X Y Z

Rotation Side

Principle of Labyrinth Seal




Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



Definition and Purpose

Steam path audit is to assess the condition of the steam path to identify performance

degradation of the unit and to point out the causes and locations of power and efficiency

losses.

The results of the audit identify specific problem areas and quantify the impact of the

problems in order to assist the owner in making decisions whether to repair or replace

steam path components.

Steam Path Audit

Benefits of Steam Path Audit

Provides detailed inspection of steam path.

Quantify heat rate, power and efficiency impact on component-by-component basis.

Aid economic decisions during the repair outage.

Prioritize maintenance decisions on a benefit-to cost ratio.

Quantify the quality of turbine repairs by performing a closing steam path audit.

Provides excellent record/history of equipment conditions for future reference.


Areas Addressed in Steam Path Audit

• nozzles

• buckets

• shaft end packings where rotors emerge

from casing

• poorly fitting joints

• other miscellaneous leakages

1) Leakage

• deposits

• corrosion

• solid particle erosion

• mechanical damage

2) Surface Roughness

• deposits

• foreign objects


3) Flow Blockage

• solid particle erosion

• water droplet erosion


4) Flow Path Modification


Spill Strip

Surface RoughnessSolid Particle Erosion

Clearance Measurement

Inspection


Sample –200 MW, 3000 rpm, 14710 kPa/535C/535C, 2 Flow LP

Power Loss Distribution

Opening Audit Closing Audit




Power Loss Distribution – HP Turbine




Heat Rate Loss Distribution



Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



TO ATMOSPHEREGLAND EXHAUSTERS

A Typical Steam Seal System


Seal

설치 위치 및 목적

• 로터 양 끝 케이싱 관통 부위

- HP/IP터빈: 터빈 내부 증기 외부(대기중) 누설 방지

- LP터빈: 외부 공기 LP터빈 누입 방지

• 버켓 팁: 버켓 열 압력차에 의한 증기누설 최소화

• 다이아프램 축 관통 부위: 노즐 열 압력차에 의한 증기누설 최소화

https://www.google.co.kr/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=-2ASYEg78gRSPM&tbnid=FQflhhJWGA372M:&ved=0CAUQjRw&url=https://www.sulzer.com/en/Products-and-Services/Coating-Materials/Thermal-Spray/Abradables-and-Polymer-Fillers&ei=CwdCU97OHoagigfxxYC4BA&bvm=bv.64125504,d.dGI&psig=AFQjCNFelGkLH9lwkw287xQvw17NB1QgUQ&ust=1396922195478401


Turbine Stage Sealing System

Flow Coefficient

0.58

0.45

0.46

0.38

0.30

Nozzle Profile

15%

Bucket

Profile

15%

Nozzle

Secondary

15%Bucket Secondary

15%

Tip

Leakage

22%

Shaft Packing

Leakage 7%

Root Leakage 4%Rotation 3%

Carryover 4%


Balance Piston

Welding Balance Piston

The reaction design has a significant pressure drop

across the buckets and high thrust. Therefore, a

balance piston, which is normally built into the rotor,

is installed in high-pressure zones of single-flow

turbines to offset the thrust. Otherwise, the turbine is

designed with double-flow. Some designers also use

a balance piston on impulse turbines that have a high

thrust. Balance Piston

(Siemens)


① Valve and admission

② Profile

③ Shaft & inter-stage seal

④ Extraction & exhaust

⑤ Reheat pressure

⑥ X-over pressure

⑦ Moisture

⑧ Mechanical

Com

po

nen

t L

osse

s R

ela

ted

to T

urb

ine O

utp

ut

HP Turbine IP Turbine LP Turbine

Steam Turbine Efficiency

1970

1990

87%

90%

%

2.0

1.0

Siemens

Losses of Turbine Components

600 MW Reheat Steam Turbine


The installation of steam seals is a cost-effective

means for improving steam turbine efficiency and

power output.

Steam leakage losses are a major component of

controllable losses in large steam turbines.

A demonstrated improvement in turbine section

efficiencies of 5% can be achieved for full section

seal upgrades.

The leakage loss can be increased up to 1% of

available energy of steam turbine.

The amount of steam leakage is strongly related to

the operating clearance between the rotating and

stationary elements that are separated by the seal.

A second major influence on steam leakage is the

pressure difference that separates the inlet and exit

of the seal assembly. Seals located in regions of high

pressure drop experience greater leakage flow.

Generals


Tip Leakage [1/7]

Tip leakage flow go through the tip clearance driven by the pressure difference between the up- and

downstream at tip. This flow is function of clearance area and sealing arrangement.

The leakage flow jet mixes out firstly in the clearance space and this mixing process is irreversible because

of throttling loss.

The output of the steam turbine reduces as the leakage flow increases. This is because the amount and

velocity of the main flow reduce as the leakage flow increases.

Front stages, where the blade height is low and pressure difference across the seal is high, are more

affected by leakage loss than rear ones.

In order to decrease tip losses, for any given stage, the radial distribution of the row pressure drop (degree of

reaction) has to be optimized. This means that the degree of reaction at the tip should be decreased, but the

degree of reaction at the hub should be increased to minimize the leakage losses.

The modification of the degree of reaction is done by means of twist and lean.

The seal design is dependent on the pressure drop across the blades. Reaction turbines that high pressure

drops across the blades require the labyrinth type seal, whereas impulse turbines with small pressure drops

across the blades require only spill strips.


This loss is considered in different ways

depending on whether the blade is covered or

uncovered.

The major parameters affecting leakage flow

are tip clearance, degree of reaction, blade

turning, and blade loading (local pressure

difference between the pressure and suction

side).

The tip flow goes through the tip clearance from

the pressure side to the suction side of the

blade.

Tip leakage flow rolls up into a vortex and

interacts with the secondary flow.

The secondary flow and tip leakage flow

interaction produces a distinct interface.

Flow in Free Tip Blades

Tip Leakage [2/7]


From Payne et. al. (2003)

CFD Result for Tip Leakage Flows – Free Tip Blade

Tip Leakage [3/7]


Tip Vortex – Free Tip Blade

Tip Leakage [4/7]


Leakage

clearance

Tip Leakage [5/7]


Tip Clearance, fraction of passage height

Tu

rbin

e E

ffic

ien

cy,

fra

ctio

n o

f e

ffic

iency w

ith

ou

t cle

ara

nce

.01 .02 .03 .040

100

99

98

97

96

95

94

93

92

91

90

010203040

50

60

70 De

gre

e o

f re

actio

n,

(%)

Tip-Clearance Correlation for Unshrouded Blades

Tip Leakage [6/7]


A slight movement of the rotor in the radial direction reduces tip leakage flow

on one side of the rotor, resulting in more flow through the rotor blades.

This results in a larger force on that side of the rotor.

On the opposite side the tip leakage flow is increased, resulting in less flow

through the rotating blades and this results in a smaller force acting on the

rotor.

The couple produced by these two forces is off center, which causes a

vibration pattern that is different than the vibration produced by oil whip or a

bowed rotor.

The vibrating rotor rubs radial spill strips, decreasing their effectiveness and

reducing the magnitude of the forces producing the couple.

Steam Whirl

Oil Whirl or Whip

Caused by the elliptical motion of the shaft within the bearing and may be as much as one cycle per 2

revolutions of the shaft.

Possibly caused by rotor imbalance.

Remedied by increasing the bearing load by reducing bearing length.

This increases side leakage and reduces oil film flexibility.

Tip Leakage [7/7]


Impulse Reaction

Bucket

Tip

Diaphragm

Root

cylindrical

drum type

rotor

disc wheels

shrunk on to

a rotor shaft

Comparison of Leakage


a b

HP IP

1

c d

LP

2

d

46

6

35

89

7

10

1- integrated HP-IP casing and its front (a), central (b), and rear (c) seals, 2-two double flow

LP casings and their end seals (d), 3&4-headers of the HP-IP and LP end seals, 5&6-gland

governors of steam pressure in these headers, 7-outside source of sealing steam, 8-gland

steam ejector, 9-gland steam condenser, 10-to a feedwater heater.

Shaft Packing Leakage [1/3]

Steam Flow for Gland Packing Seals


LP 터빈 끝 단에 형성되는 진공압력으로인한 외부 공기 유입 방지를 위한 seal steam을 LP터빈gland로공급

Steam이정체되어 응축수가 생성되는 것을 방지하기 위하여 seal steam과 air를 배출

배출된 seal steam과 air는 gland steam

condenser에서열교환 후 배출됨

Rotor Shaft

Air

Seal steam Seal steam

& Air

LP

Turbine

LP Turbine Gland





MHI

Root Leakage [1/3]

Retractable Packing


This figure shows the force diagram

around a labyrinth seal of a

retractable design during both

normal operation and at

startup/shutdown when the steam

pressures are significantly lower.

It can be seen that there is a

radially inward force, which is the

sum of the spring force and the

steam force.

In the HP and IP turbine, the steam

force is considerably larger than the

spring force.

Retractable Packing

Root Leakage [2/3]


Retractable packing was developed to avoid packing rubs that can occur during unit startup when the shaft

passes through critical speeds.

Retractable packing provides a very large shaft clearance during unit startup, when turbine steam flow and

local pressure are very low.

During normal operation when the flow and pressure are at design levels, the seal element close, or move

radially inward (closer to the shaft). In this case, leakage losses are reduced because of reduced clearance.

Unlike conventional spring-backed packing, the springs in retractable packing assemblies hold the packing

at a large radial clearance to the shaft (typically 0.15 in.).

As the turbine steam flow increases, the increasing steam pressure closes the packing, causing the springs

to compress to mechanical stops.

By operating in this manner, retractable packing eliminates startup problems caused by the shaft rubbing

and bowed rotors.

Over the life span of the equipment, this results in higher sustained efficiency and lower maintenance costs.

Retractable packing can be installed throughout the steam path with the exception of the three outer rings in

the shaft end packing boxes

Commercial suppliers of variable clearance packing include Brandon Engineering, TurboCare, and Dresser-

Rand.

Retractable Packing

Root Leakage [3/3]


Full-Annular

Rub Damage

Undamaged Seal Continuous Rub Damage on End-Glands

Damage and Degradation of Seals [1/4]


Seals are often damaged by shaft rubs that increase leakage flow and reduce efficiency. Therefore, seals

should be designed to withstand shaft vibration experienced during startups and shutdowns.

The stationary and rotating surfaces of turbine seal elements can be subjected to high-speed rubs, large

pressure differentials, and high temperatures of surrounding steam and metal.

If rubbing occurs with high-contact forces, the seal knives and shaft will heat up to excessive levels, raising

the temperature of the affected components beyond the austenitizing temperature.

The surrounding steam creates a quenching effect that can cause the seal material to become brittle and

susceptible to failure during next rubbing.

The knife deformation damage has two detrimental effects on sealing efficiency. First, it will increase leakage

area. Second, the rub will modify the shape of the knife profile, changing the sharp knife corner to a rounded

corner and increasing the flow coefficient.

Similarly, there is a potential for quenching damage to the shaft, resulting in areas of increased hardness and

possible crack initiation sites.

The resulting deformation of the shaft surface due to wear will also increase the seal clearance and leakage

flow.



Labyrinth seal knives have tapered cross-sections so that any accidental rub will limit frictional heating by

reducing the contact area.

A major part of the effort in reducing the steam leakage involves minimizing the incidence and severity of

these rubs during unit startup, shutdown, and system transients.

Shaft rubs can occur during startup when the rotor passes through critical speed and the resulting mid-span

lateral shaft deflections are high enough to exceed the clearance.

In many steam turbines backing springs are used to hold seal segments in place but also allow them to be

compliant in the radial direction in the event of a hard rub between the shaft and the seal.

Seal degradation and the need to periodically restore damaged elements to their original condition represent

a barrier to the current industry desire to extend scheduled maintenance intervals on large steam turbines.

Seal damages are also associated with water droplet erosion and solid particle erosion.



The most common source of seal damage is shaft rubs during unit startup and shutdown, which experience

high lateral vibration at the critical speeds.

Once a rub starts, local heating of the shaft due to friction can cause thermal distortion and worsen vibration

and the associated rub.

Plant operators need to be aware of the shaft critical speeds and avoid any extended operation during

startup/shutdown at these speeds.

Also, the degree of balance of the shaft system will affect the lateral vibration amplitude at critical speed, so

rotating unbalance should be maintained as low as possible in order to protect the seal elements.



Causes of Rubbing and Seal Damage

The unit will pass through critical speeds during startup.

The critical speed vibration amplitude is typically highest at the mid-span of the rotors, and it can damage the

shaft seals on the diaphragms if the vibration amplitude exceeds the clearances.

Conventional seals use stiff backing springs to hold the labyrinth seal rings radially inward.

These springs permit some level of protection by allowing the labyrinth assembly to move outward in the

event of hard rubbing.

Depending on the spring stiffness, the extent and consequences of startup rubbing remain serious.

Retractable seals allow the labyrinth seal assemblies to displace radially away from the rotor surface when

steam pressures are low. This kind of condition happens during unit startup and shutdown.

The rub-tolerant strip, which is designed slightly taller than the standard knives surrounding it, can be used

to minimize seal damage.

This strip is the first to come in contact with the shaft in rubbing conditions and thus protects the remaining

labyrinth knives.

1. Unit Startup [1/2]


This design is used in combination with a soft backing spring so that the protective strip pushes the seal

assembly away.

For those seals incorporating rub-tolerant strips, the clearances between the conventional knife-edged seal

strip and rotating surfaces are set at normal design levels.

The rub-tolerant strip has a marginally smaller clearances, but it is large enough so that during normal unit

startup, contact will not occur.

At startup, the pressures in the steam path are considerably lower, therefore, the steam force on the gland

rings are not large. In this case, relatively low contact pressures are developed at the rub surfaces, and the

contacting force is determined by the soft backing spring.

Distortion of turbine casings during thermal transients (e.g. in start-up or rapid load changes) can damage

radial seals between the rotor and stationary parts, especially if the fixed blades are mounted directly into the

casings.

1. Unit Startup [2/2]



When a unit is shutdown either under a controlled condition or emergency condition, the main and

intermediate stop and/or control valves close, and the steam pressure levels within the flow path decay as

the rotor slows.

For a normal shutdown, the steam pressure is controlled, but the rotor must pass through its critical speeds

with the potential to initiate rubs if procedures are not followed.

So, under controlled shutdown conditions, the radial inward force on the labyrinth seal assemblies when the

rotor passes through critical speed is governed by the spring device.

If the steam pressure is not reduced as required during shutdown, the steam forces are added to the backing

spring forces, hold the labyrinth assembly in place with tight clearances, and thus risk damaging the seals

during coast down.

3. Unit Operating Transients and Trips

Unit trip represent a risk of seal damage during an uncontrolled shutdown.

Under these conditions, the rotor can be accelerated momentarily to overspeed and vibrate either in going to

overspeed or coasting down through its critical speeds.

Immediately upon disconnection from the grid, the stop and control valves will assume a control status, but

there is a finite period when steam pressures are high, and heavy shaft-seal rubs can occur at critical

speeds.

2. Unit Shutdown




Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



Configuration

Labyrinth Seal


Stepped labyrinth seals (high-low packing)

with multiple knives are the most common

method used in order to reduce leakage

flow by providing a tortuous flow area.

The leakage jet in a stepped labyrinth seal

is bounded by counter-rotating vortices that

restrict the leakage flow.

Flow in a Stepped Labyrinth Seal

Labyrinth Seal


This type of seal is the most common one used in large steam turbines.

Backing springs push the seal radially inwards toward the turbine shaft. The compliance of these springs

permit some radial deflection during heavy rubbing.

Fluid pressure produces a radially inward force and makes a minimum clearance. This additional force is

normally proportional to turbine flow and is insignificant under no steam flow.

Rubbing between the shaft and the packing can cause nonuniform shaft heating that leads to a bowed rotor

condition and shaft vibration.

Turbine

Rotor

Conventional

Seal

(in a rub

condition)Steam Flow

Spring Tension

1. Spring-Backed Labyrinth Seal

Labyrinth Seal


Retractable Packing Arrangement Pressure Distribution

This is developed by Ronald E.

Brandon in the early 1980s and

is installed in many steam

turbines

2. Retractable Packing

Labyrinth Seal


MHI


Labyrinth Seal


This figure shows the force diagram

around a labyrinth seal of a

retractable design during both

normal operation and at

startup/shutdown when the steam

pressures are significantly lower.

It can be seen that there is a

radially inward force, which is the

sum of the spring force and the

steam force.

In the HP and IP turbine, the steam

force is considerably larger than the

spring force.


Labyrinth Seal


Retractable packing was developed to avoid packing rubs that can occur during unit startup when the shaft

passes through critical speeds.

Retractable packing provides a very large shaft clearance during unit startup, when turbine steam flow and

local pressure are very low.

During normal operation when the flow and pressure are at design levels, the seal element close, or move

radially inward (closer to the shaft). In this case, leakage losses are reduced because of reduced clearance.

Unlike conventional spring-backed packing, the springs in retractable packing assemblies hold the packing at

a large radial clearance to the shaft (typically 0.15 in.).

As the turbine steam flow increases, the increasing steam pressure closes the packing, causing the springs

to compress to mechanical stops.

By operating in this manner, retractable packing eliminates startup problems caused by the shaft rubbing and

bowed rotors.

Over the life span of the equipment, this results in higher sustained efficiency and lower maintenance costs.

Retractable packing can be installed throughout the steam path with the exception of the three outer rings in

the shaft end packing boxes

Commercial suppliers of variable clearance packing include Brandon Engineering, TurboCare, and Dresser-

Rand.


Labyrinth Seal


3. Brandon Sensitized Packing

Labyrinth Seal


3. Brandon Sensitized Packing

Labyrinth Seal


The steam has an initial pressure P1 at the entry to the seal assembly.

After expanding past the first constriction, the pressure will have been reduced to condition Xo, with pressure

P2.

In the chamber formed between the first and second seal strips, the kinetic energy of the steam is destroyed

and reconverted at constant pressure P2 to condition X.

From point X, there is then a further expansion of the steam past the second constriction, with the pressure

falling to P3 at condition Yo.

The kinetic energy is again reconverted in the chamber between the second and third seal strips, raising the

thermal energy level from Yo to Y at constant pressure P3.

This process of expansion and kinetic energy reconversion is continued throughout the series of seal strips

until the final expansion takes the steam to condition Qo at pressure P5.

The locus of the points Xo….Qo is called the Fanno curve.

h

s

T1

P1 P2P3 P4

P5

Xo YoZo

Qo

X Y Z

Leakage

Flow

P1 P2 P3 P4 P5

X Y Z

Rotation Side

Labyrinth Seal


hmkW

xNVs

xpAekm

i

i

ln

/11 2

Ae = leakage area = Ds Cl

Ds = mean diameter of the leakage annulus (in.)

Cl = radial clearance (in.)

= flow coefficient

x = pressure ratio across the series seals (pi/pd)

pi = inlet pressure (psi)

pd = pressure at discharge (psi)

Vsi = specific volume corresponding to pi (ft3/lb)

N = the number of series constrictions

k = conversion constant (= 0.472 for the flow in lb/s)

(Martin’s equation)

m = leakage flow

h = enthalpy drop across the seals

Leakage Loss – Martins Equation

Labyrinth Seal


21

= flow coefficient for Martin’s equation

1 = basic flow factor

2 = knife sharpness factor

= 1.0 for 100% circumferential rub

= 0.9 for 50% circumferential rub

= 0.8 for sharp teeth

Seal ShapeFlow Coefficient

(Basic flow factor)

Flow Coefficient

Labyrinth Seal


The leakage flow will be at a maximum when the value of pressure ratio (x) reaches the critical value.

Because the flow cannot exceed that associated with the critical pressure ratio, if the value of ‘x’ exceeds

the critical value, shown in the above figure, should be used.

Critical Pressure Ratio - Martins Equation

Labyrinth Seal


The Guardian seal design concept (essentially an adaptation of

the spring-backed labyrinth seal) has been steadily increasing

application since it was introduced for commercial use in June of

1997.

The unique feature of this design is that two of the high knives in

each segment are replaced by a slightly higher sacrificial post

designed to contact the shaft in the event of a rub.

This post is essentially sacrificial and protects the remaining adjacent conventional knives.

In the event of contact between the shaft and the packing, this higher post pushes the entire spring-backed

assembly outward, preventing the shaft from contacting the adjacent knives and causing rub damage.

The coil spring supporting the Guardian packing rings are softer than those used in retractable packing

design in order to permit easier radial movement and reduced contact force in the event of rub.

The post is nominally 0.005 in. (o.127 mm) closer to the shaft than the adjacent high knives, which still

provides significant clearance under typical operating conditions when shaft vibration is normal.

Guradian Seal

Labyrinth Seal


The clearance between the shaft and the knives in the Guardian packing is typical of that used in

conventional labyrinth designs.

The main advantage of this design is the maintaining the desired clearance of all labyrinth knives for an

extended period of time between major section overhauls.

The Guardian strip is the main feature of this labyrinth design and is fabricated from Nitronic 60 stainless

steel alloy that is designed to prevent galling and exhibits low friction when it comes into contact with the

turbine shaft in the event of rub.

It has been reported from field experience that the Nitronic strip does not cause shaft damage as evidenced

by local areas of high hardness.

The body of the seal is made from 12-chrome stainless steel alloy in order to provide a longer service life in

the steam environment.

Guradian Seal

Labyrinth Seal


Guradian Seal

Labyrinth Seal


The seal design is dependent on the pressure drop

across the blades. Reaction turbines that high

pressure drops across the blades require the

labyrinth type seal, whereas impulse turbines with

small pressure drops across the blades require

only spill strips.

Labyrinth Seal


Labyrinth Seal



Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



Brush Seal

Brush seals are becoming standard features in advanced steam turbines,

particularly for the HP and IP modules of SC and USC steam turbines.

Brush seals provide a curtain of metal bristles between adjacent areas of different

pressures.

The bristles are canted at an angle relative to the radial direction of the shaft, and

sealing process starts as soon as differential pressure is created, even though

there is still a gap between the bristles and the rotor.

In this type of application, a 50% reduction of leakage flow is achieved compared with that of a conventional

seal.

The absence of any clearance between the brush and the surface of the parts reduces the leakage

considerably – 70% and more – and can improve turbine efficiency by 0.5%.

It is a modified labyrinth seal in which one of the high knives is replaced with an element consisting of fine

wire bristle material sandwiched between two plates.

This assembly is inserted into an enlarged slot in the packing assembly.

The key design feature of the brush element is the bristle pack in which the brush elements are slanted in

the direction of the shaft rotation.

The brush is designed to be compliant when it contacts with the shaft, and this is why the slanted orientation

is used.


Since bristles are extruded at an angle relative to the radial direction

of the shaft, the seal closes as soon as pressure is applied and even

if the brush seal is built in with an initial gap between rotor and

bristles.

This “blow-down effect” account for the more than 50% reduction in

leakage flow compared to standard labyrinth seals.

The flexible nature of the bristle pack allows for sufficient relative

movement of rotating and stationary parts during transient operation,

while the clearance between hard parts can even be increased by the

use of brush seals.

While a single brush row withstands more than 10 bar pressure

difference, several adjacent rows to be used to seal higher pressure

drops.

Multiple stage brush seals are already being used in turbines since

several years.

Brush seal has also been applied to gland seal system for large

steam turbines.

Flow

Bristle

Pack

Backing

Plate

Fence

Height

Brush Seal


The function of the backing plate is to support and prevent

axial deflection of the relatively flexible bristles against the

pressure loading across the seal.

The backing plate does not extend to the tip of the bristle

pack, and its diameter is set to prevent contact with the

shaft during the maximum expected lateral excursions in

shaft vibration.

The bristle tips are not fully in contact with the shaft, but

maintain a small nominal clearance, that is significantly

less than a conventional knife element.

Brush seal has a better performance than conventional

labyrinth seal because of reduced clearance.

Brush Seal


Incorporating the brush seal element into a

spring-back packing provides additional

compliance in the event of a shaft rub.

This feature is expected to extend the life of the

bristle pack by reducing the likelihood of a hard

rub that can damage the bristles due to

excessive bristle deflection, overheating, or wear.

Retractable brush seals have been used in both

nuclear and fossil steam turbines.

Brush Seal


Gas Turbine Stage 2 Nozzle Inner Diaphragm

Brush Seal


The brush spill strip basically applies the brush pack

design from the retractable brush packing into the

existing knife grooves in the casing near the blade tips

of the turbine.

Similar to the shaft seals, the brush spill strip is

designed to run at a reduced nominal clearance as

compared to conventional spill strips, thus reducing

leakage flow.

Brush Spill Strip [TurboCare]

Brush Seal


Gland Seal Packing (Brush Seal)

Brush Seal


HPP

Applications in Gas Turbines

Brush Seal


HPP (High Pressure Packing)

In general, a rub of 20 mils on the labyrinth seal

teeth equates to at least 1.0% loss in unit

performance.

To increase unit performance and to reduce the

rate of performance degradation due to the wear

on labyrinth seal teeth, a new wire brush seal

design has been developed.

Since the wire brush seal is flexible and will bend

(not wear) on contact with the compressor aft shaft,

a closer clearance can be allowed for the initial

installation.

Since the wire brush seal will “bounce back” to its

original configuration after a “rub”, there will be

substantially less performance degradation for the

labyrinth seal.

Performance improvement by replacement of

labyrinth seal into brush seal is normally about 1%

output and 0.5% heat rate.

Brush Seal


1) Pressure capability

2) Frequency

3) Seal leakage

4) Seal blow-down

5) Seal stiffness

6) Bristle tip forces and pressure

stiffening effect

7) Seal heat generation

8) Bristle tip temperature

9) Rotor dynamics

10) Rotor thermal stability

11) Secondary flow and cavity

flow

12) Seal upstream protection

13) Seal HCF and LCF

analysis

14) Seal oxidation

15) Seal creep

16) Seal wear

17) Solid particle erosion

18) Seal performance

19) Oil seal

Parameters Evaluated during Seal Design

Brush Seal


HPP (High Pressure Packing)

Brush Seal


#2 Bearing Brush Seal

Brush Seal



Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



Leaf Seal [1/2]

Leaf seals were used to replace existing brush seal in both

gas turbines and steam turbines.

During rotation of shaft, hydrodynamic forces lift the leaf tips,

eliminating direct contact, reducing friction, heat generation

and vibration.

The tip of the leaf is lifted-up by a balance of the pushing

force due to pre-pressure of the setting, lifting force due to

hydrodynamic pressure generated during rotation of the

rotor, and lifting force due to the differential pressure of the

seal.


MHI

Application of Leaf Seal in Steam Turbines

Leaf Seal [2/2]


Vortex Shedder Spill Strip

In the spill strip, the knife profile is not constant in the circumferential direction, but instead includes a series

of dimples that protrude in the axial direction.

These dimples disturb the flow (that has a very strong circumferential component) and produce vortices in

the leakage jet.

In turn, these vortices increase the average pressure within the seal area and produce the pressure drop

across the seal.

This reduced pressure drop lowers the leakage flow proportionally.


An abradable coating is a coating made of an abradable

material – meaning if it rubs against a more abrasive

material in motion, the former will be worn whereas the

latter will face no wear.

Abradable coatings are used in aircraft jet engines in the

compressor and turbine sections where a minimal

clearance is needed between the blade tips and the casing.

Recently, it has been applied in USC steam turbines to

reduce leakage loss occurred clearance between bucket tip

and casing.

Abradable Coating

In typical turbo machinery, the clearance between blade tips and the casing must account for thermal

expansion as well as changes in concentricity due to shock loading events. To prevent catastrophic tip to

casing contact, conservatively large clearances must be employed.

The role of abradable coatings is not only to allow for closer clearances, but to automatically adjust

clearances, in-situ, to accept physical events and/or thermal scenarios that may be found in a devices

operational history


Carbon Packing

탄소는 회전축과 접촉하더라도 마찰이 적어 마찰열 발생이 적음

탄소는 고열에 견디는 성질을 가짐

이런 탄소의 특성을 이용하여 압축 성형한 탄소환을 원주상으로 등분하여 축 둘레에 감고 스프링으로 가볍게 접촉시켜 누설을 방지

현재 대용량 터빈에는 사용하지 않음


Combined seal with “Honeycomb” seal for balance piston

Honeycomb Seal


Honeycomb Shroud [1/3]


GE 6FA Stage 2 & 3 Diaphragm Seals



The clearance between the bucket tip and the stationary shroud blocks have

always been about 100 mils in order to avoid bucket tip rubbing.

This large clearance allows a significant amount of hot gas flow over the

bucket tip, resulting in significant performance loss.

This loss can be reduced by the insertion of honeycomb material in the

stage 2 and 3 shroud blocks.

The bucket tip shroud labyrinth seals are designed to cut a groove into the

honeycomb material.

The tight clearance between the bucket tip and the honeycomb shroud seal

provide a performance improvement up to 0.6% in both output and heat rate.



Diaphragm Seal

Stage 3 Nozzle of 6FA



Brush Seal 5

Other Seals 6

Throttling Process1

Steam Path Audit2

Labyrinth Seal 4



Hydrogen Cooled Generator


Seal Ring 구조


Seal Ring 사진


Turbine End Contact Face

Seal Ring 구조


1) Seal Oil 주입 압력 점진적 증가

2) 고온에 의한 시일 손상 발생

연구 배경


Seal Oil 주입 Seal Oil 주입

Rotor

Collar

회전

Oil flow out to atmosphere

Oil flow in to generator

Land는고정

작동 원리

Velocity Vector Distributions


CFD(Computational Fluid Dynamics)

- Code: FLUENT

- Turbulence model: standard k- model

- Scheme: second-order scheme

Grid generation

- Tool: Gambit

- Type: Hybrid

- Number of grid: 360,000

Viscous heating model was employed to consider temperature

rise

해석 방법


Temperature Measuring Points


Oil film thickness (0.04 mm)

Hybrid Grid

No. of Grid 360,000

Grid


Grid


Flat Seal Tapered Land Seal

Seal Type


0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

Oil film thickness [mm]

30000

32000

34000

36000

38000

40000

42000

44000

46000

48000

50000

Fo

rce

actin

go

nth

eco

llar

[N]

dP = 150kPa

dP = 83kPa

Film Thk. = 0.04 mm

Film Thk. = 0.10 mm

Film Thk. = 0.06 mm

Film Thk. = 0.08 mm

Collar에요구되는힘 57,000N 만족시키지못함 Flat seal 부적합

Flat Seal

Force acting on the Collar


0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11


310

315

320

325

330

335

340

345

350

355

360

365

370

375

380

385

390

395

400

Tem

pera

ture

[K]

dP (kPa)

150 Upper part of the seal

150 Lower part of the seal

83 Upper part of the seal

83 Lower part of the seal

Film Thk. = 0.04 mm

Film Thk. = 0.10 mm

Film Thk. = 0.06 mm

Film Thk. = 0.08 mm

하부에서과도한온도상승발생 Seal 하부고온손상발생원인

Seal Temperature

Flat Seal


0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11


-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Oil

flo

w[l/s

]

dP = 83kPa, Oil flow comming in

dP = 83kPa, Oil flow out to atmosphere

dP = 83kPa, Oil flow out to generator

dP = 150kPa, Oil flow comming in

dP = 150kPa, Oil flow out to atmosphere

dP = 150kPa, Oil flow out to generator

To Generator

To Atmosphere

Oil Flow

Flat Seal


0 50 100 150 200

Differential pressure between inlet and generator side [kPa]

30000

35000

40000

45000

50000

55000

60000

65000

70000

75000

80000

Fo

rce

actin

go

nth

eco

llar

[N]

Taper-Land ratio = 0.7Taper = 0.05mm Film thickness

0.04mm

0.06mm

0.08mm

dP = 35 kPa

dP = 83 kPa

dP = 150 kPa

Film Thk. = 0.04 mm

Film Thk. = 0.04 mm

Film Thk. = 0.04 mm

Film thickness가 0.05mm 이하인경우 Collar에요구되는힘(57,000N) 만족

Tapered Land Seal

Force acting on the Collar with the variation of dP


-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

The amount of taper [mm]

30000

35000

40000

45000

50000

55000

60000

65000

70000

75000

80000

85000

90000

Fo

rce

actin

go

nth

eco

llar

[N]

dP = 83kPa

dP = 150kPa

Oil film thickness = 0.04mmTaper-Land ratio = 0.7

Taper = 0.03 mm

dP = 83 kPa

Taper = 0.05 mm

dP = 83 kPa

Taper = 0.10 mm

dP = 83 kPa

Force acting on the Collar with the variation of the Taper

Tapered Land Seal


0.4 0.5 0.6 0.7 0.8 0.9 1

Taper-Land ratio

40000

42500

45000

47500

50000

52500

55000

57500

60000

62500

65000

67500

70000

72500

75000

77500

80000

Fo

rce

actin

go

nth

ela

nd

[N]

(150, 0.03)

(dP,Taper)

(150, 0.05)

(150, 0.10)

(dP,Taper)

(83, 0.10)

(83, 0.05)

(83, 0.03)

Film thickness = 0.04 mm

T-L ratio = 0.5 T-L ratio = 0.6 T-L ratio = 0.7 T-L ratio = 0.8 T-L ratio = 0.9

Force acting on the Collar with the variation of Taper-Land Ratio

Tapered Land Seal


0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11


35000

37500

40000

42500

45000

47500

50000

52500

55000

57500

60000

62500

65000

67500

70000

72500

75000

Fo

rce

actin

go

nth

eco

llar

[N]

Taper = 0.05mmdP = 83kPaTaper-Land ratio = 0.7

Film Thk. = 0.04 mm

Film Thk. = 0.06 mm

Film Thk. = 0.08 mm

Force acting on the Collar with the variation of Oil-Film Thickness

Tapered Land Seal


0 20 40 60 80 100 120 140 160 180 200


320

330

340

350

360

370

380

390

400

410

420

Tem

pera

ture

[K]

Taper-Land ratio = 0.7Taper = 0.05mmOil film thickness = 0.04mm

Lower part of the seal

Upper part of the seal

dP = 150 kPa

dP = 83 kPa

dP = 35 kPa

Seal Temperature with the variation of Oil Pressure

Tapered Land Seal


0.03 0.04 0.05 0.06 0.07 0.08 0.09


320

325

330

335

340

345

350

355

360

365

370

375

380

385

390

395

400

Tem

pera

ture

[K]



Taper = 0.05mmTaper-Land ratio = 0.7dP = 83kPa

F.T. = 0.04 mm

F.T. = 0.06 mm

F.T. = 0.08 mm

Seal Temperature with the variation of the Oil Film Thickness

Tapered Land Seal


-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11


320

325

330

335

340

345

350

355

360

365

370

375

380

385

390

395

400

Tem

pera

ture

[K]

Film thickness = 0.04mm






Taper-Land ratio = 0.7dP = 83kPa

Seal Temperature with the variation of the Amount of Taper

Tapered Land Seal


0.4 0.5 0.6 0.7 0.8 0.9 1.0

Taper-Land ratio

320

325

330

335

340

345

350

355

360

365

370

375

380

385

390

395

400T

em

pera

ture

[K]



Taper = 0.05mmdP = 83kPaOil film thickness = 0.04mm

Seal Temperature with the variation of the Taper-Land Ratio

Tapered Land Seal


0 50 100 150 200


-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0V

olu

me

flo

wra

teo

fth

eo

il[l/s

]

Oil comming in

Oil exiting through outlet facing with generator

Oil exiting through outlet facing with atmosphere

Taper-Land ratio = 0.7Taper = 0.05mmOil film thickness = 0.04mm

-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11


-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Vo

lum

eflo

wra

teo

fth

eo

il[l/s

]

Oil comming in



Oil film thickness = 0.04mmTaper-Land ratio = 0.7dP = 83kPa

0.4 0.5 0.6 0.7 0.8 0.9 1.0

Taper-Land ratio

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

Vo

lum

eflo

wra

teo

fth

eo

il[l/s

]

Taper = 0.05mmdP = 83kPaOil film thickness = 0.04mm

Oil comming in



0.03 0.04 0.05 0.06 0.07 0.08 0.09


-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Vo

lum

eflo

wra

teo

fth

eo

il[l/s

]

Oil comming in



Taper = 0.05mmTaper-Land ratio = 0.7dP = 83kPa

1) Oil

Pressure

2) Taper

3) T-L ratio 4) Oil Film

Thk.

Oil Flow

Tapered Land Seal


0.00 0.25 0.50 0.75 1.00

Throat depth of the groove [mm]

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Oil

flo

wra

te[l/s

]

Oil comming in

Oil exiting to atmosphere

Oil exiting to generator

Oil film thickness = 0.06mmTaper = 0.05mmTaper-Land ratio = 0.5dP = 83kPa

Throat depth = 0.75 mm



Oil Flow with the variation of the Throat Depth

Tapered Land Seal


0.00 0.25 0.50 0.75 1.00


20000

22500

25000

27500

30000

32500

35000

37500

40000

42500

45000

47500

50000

52500

55000

57500

60000

Fo

rce

actin

go

nth

eco

llar

[N]

dP = 150 kPa

dP = 83kPa

Oil film thickness = 0.06mmTaper = 0.05mmTaper-Land ratio = 0.5

T.D. = 0.25 mm

T.D. = 0.50 mm

T.D. = 0.75 mm

Force acting on the Collar with the variation of the Throat Depth

Tapered Land Seal


0.00 0.25 0.50 0.75 1.00


320

325

330

335

340

345

350

355

360

365

370

375

380

Tem

pera

ture

[K]



Oil film thickness = 0.06mmTaper = 0.05mmTaper-Land ratio = 0.5dP = 83kPa

T.D. = 0.25 mm

T.D. = 0.50 mm

T.D. = 0.75 mm

Seal Temperature with the variation of the Throat Depth

Tapered Land Seal


Slot

The Effect of a Slot on the Collar

Tapered Land Seal


50 100 150 200

Differential pressure between inlet and generator [kPa]

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

55000

60000

Fo

rce

actin

go

nth

eco

llar

[N]

Oil film thickness = 0.06mmTaper = 0.05mmTaper-Land ratio = 0.5

w/o

slo

t

with

slo

t

Force acting on the Collar with a Slot

Tapered Land Seal


질의 및 응답

작성자: 이 병 은 (공학박사)작성일: 2015.02.11 (Ver.0)연락처: [email protected]

Mobile: 010-3122-2262저서: 실무 발전설비 열역학/증기터빈 열유체기술

Documents

9. Seal Systems · 2018-01-13 · Heat & Fluid Techniques in Power 9. Seal Systems 11 / 116 Areas Addressed in Steam Path Audit • nozzles • buckets • shaft end packings where