Conference of The Electric Power Supply Industry 2016

EXPERIENCE WITH D11 STEAM TURBINE BOWED ROTOR OPERATION Mr.Sarun Sahanavin, Engineer Level 7, Operation Shift 4, Ratchaburi Combined Cycle Power Plant,

Electricity Generating Authority of Thailand, +66814414764, sarun.s@egat.co.th

Mr.Peerapong Wanichthanarak, Engineer Level 5, Mechanical Maintenance Section, Ratchaburi Combined

Cycle Power Plant, Electricity Generating Authority of Thailand, +66851503515, peerapong.w@egat.co.th

Abstract- During a shutdown process for major overhaul at ratchaburi combined cycle power plant block

3 in 2009, the D11 steam turbine, which manufactured by GE, had experienced a bowed rotor problem. It had

caused a severe rub between turbine rotor and stationary part which resulted in HP/IP turbine casing crack at N2

packing male main fit. The casing was temporarily repaired by stitching method during major overhaul in 2009,

then needed a further welding repair during minor inspection in 2011, and finally a casing replacement during

planned outage in 2014.

Since major overhaul in 2009, a vibration high trip at a critical speed during a startup process, which

caused by a bowed rotor, had frequently occurred. It was solved by a field balancing but the problems still

remain at warm and hot condition. A startup process is improved by increase a reheat steam temperature at least

50 degF higher than a reheat upper metal temperature, control reheat pressure to roll steam turbine between 60-

70 psi, and startup manually by fast ramp rate. The results are 97.7% successful startups but vibration measuring

is rather high at warm and hot condition. It was solved by both low speed balancing and field balancing once

again during major overhaul in 2015 to ensure a responsive operation according to NCC’s command.

Keyword- D11 Steam Turbine, Bowed Rotor, HP/IP Turbine Casing, N2 Packing Male Main Fit, Vibration

High Trip

I. Introduction

During a shutdown process for major overhaul at ratchaburi combined cycle power plant block 3 in 2009,

the D11 steam turbine, which manufactured by GE, had vibration high trip at a critical speed. The maximum

casing vibration is 18 mils (Alarm 6.0 mils / Trip 9.0 mils)

It had caused a severe rub between turbine rotor and stationary part which resulted in HP/IP turbine

casing crack at both side of N2 packing head as shown in Fig. 1

Figure 1: HP/IP Turbine Casing Crack

The casing was temporarily repaired by stitching method, then welded repair later, and finally replaced

with a new casing.

Table 1 : Advantages and Disadvantages of Stitching, Welding and Replacement Casing

Method Advantages Disadvantages

Stitching - No casing distortion

- Short time to repair

- Temporarily repaired

Low alloy welding - Permanent repairs

- It takes a long time to repair

- There is risk of casing distortion

- Weaker than high alloy welding

High alloy welding - Permanent repairs

- It takes a long time to repair

- There is risk of casing distortion

- More difficult than low alloy

welding

Replace new casing - The root cause solution - It takes a long time to replace

casing

- Must be similar in dimensions to

the old casing

The casing was temporarily repaired by stitching method during major overhaul in 2009. Because it takes

short time to repair and has no effect on major overhaul schedule as shown in Fig. 2

Figure 2: Stitching Repair

During minor inspection in 2011, a casing crack was found at the same location and some stitching pin

disappear as shown in Fig. 3

Figure 3: Before-After Repaired by Stitching method

Casing Damage on 2009 Casing Damage on 2011

Therefore repair method was changed to low alloy welding as shown in Fig. 4

Figure 4 : Welding Repair HP/IP turbine casing RGC-C30

Finally a casing replacement during planned outage in 2014, a new HP/IP turbine casing had an

improvement design of N2 Shell Male Main Fit and N2 Packing Head to have more strength as shown in Fig. 5.

A casing Installation as shown in Fig. 6.

Figure 5 : New HP/IP Turbine Casing

Remove Crack and Grinding Groove Install Jig & Support Preheat @ 200 degC

Welding Post heat @ 350 degC Hardness Check

Figure 6 : HP/IP Turbine Casing Replacement

II. Main Content

Bowed rotor problem - RGC-C30 HP/IP turbine casing crack had caused a severe rub during shutdown

process for major overhaul 2009. It resulted in rotor bow as shown in Fig. 7. Maximum rotor run out have found

at middle of HP/IP Rotor with a measured value equals to 0.12 mm as shown in Fig. 8

Figure 7 : Bowed Rotor Problem

Pipe cutting Remove old casing Install new casing

Pipe welding Laser alignment Final assembly

Figure 8 : RGC-C30 Rotor Runout Measurement

The rotor eccentricity monitoring at control room is 3.6 mils as shown in Fig. 9, while the operating

permissive to auto start up is below 3.0 mils.

Figure 9 : Electricity Monitoring

Vibration high trip problem - Since major overhaul steam turbine in 2009, a vibration high trip at a

critical speed during a startup process happened frequently. A detailed steam turbine start up from 7-26 May

2009 as shown in Table 2

Table 2 : Start Up History After Major Overhaul RGC-C30

Start Up Condition Number of Start Number of Success

Cold Start 5 4

Warm Start 3 2

Hot Start 7 1

Total 15 7

The repetitive steam turbine rotor bow – First method to help a vibration high trip problem is using

steam to soak turbine at speed of 400 rpm for 30 min. and then at 1000 rpm for 60 min. repeatedly 3 times until

all shaft vibration have no change of phase angle. But this process cannot guaranteed a successful startup every

time and uses more fuel cost, so next method used is a field balance by putting weight 300 g. at turbine bearing

no.1

Since a field balance in 2009, the startup data during 2009 to 2010 still shown some problems when

startup with hot mode or warm mode. By doing further field balancing it can decrease a vibration at critical

speed, but likely cause problem at the normal operation at higher load and may cause a vibration high trip.

III. Results

Startup procedure improvement - Before D11 steam turbine auto start up, the requirement

parameters will be as following:

a. Steam Pressure and Temperature, b. Metal Temperature, c. Lube Oil Temperature, d. Eccentricity

When these values met the requirement, the operator will begin to roll turbine from turning gear to full

speed no load, synchronize to electrical transmission system, and then steam turbine flow change mode from

reverse flow to forward flow, inlet pressure control in-service and finally LP admission steam in-service.

The turbine acceleration rate will vary depends on calculated stress value. A vibration high trip may

occurs at critical speed about 1500 rpm as shown in Fig. 10

Figure 10 : D11 Steam Turbine Start Up Diagram

In Fig.11 during a start up steam turbine process, when a first admit steam to the turbine. It warms the

turbine and then start to rotate by RH steam. Then it changes to forward flow with an additional HP steam flow

through the HP turbine.

When a turbine speed reach 75%, the reverse flow valve mounted in CRH line will open to cool down

at HP Turbine (only hot start or metal temperature above 700 degF)

Figure 11 : Steam Flow Diagram

HP turbine bowed rotor has frequently cause a vibration high trip during start up process, although it

has been improved by a field balancing but the problem still remains.

To reduce the risk of being fined by contract of power plant purchase agreement, therefore the steam

turbine block 3 is prioritize as the last one to reserve shutdown. But on 14th

March 2010, National Control

Center (NCC) needed to reserve shutdown block 3 and then startup on 15th

March 2010. The results are RGC-

C30 cannot startup steam turbine according to NCC’s command, the operator attempted restart a steam turbine

until a successful 4th

startup on 17th

March 2010 as shown in Fig. 12

Figure 12 : Startup Steam Turbine Event from 15

th – 17

th March 2010

From analysis, the causes of vibration high trip is the rubbing between HP/IP turbine rotor and N2

packing. The only possibility collects from many startup is hot turbine starts with low steam temperature as

shown in Fig. 13

Figure 13 : Root Cause Analysis of Vibration High Trip

The data collecting of the difference between RH steam temperature and RH metal temperature during

start up steam turbine from 15th

– 17th

March 2010 shown that every time before turbine rolls a reheat

temperature difference (RHT_DIF) is less than 500 degF and met the requirement for auto start up permissive as

shown in Table 3

But the vibration high trip still remain which means the condition might not be appropriate for a bowed

rotor and needs to be adjusted to ensure a successful startup.

Table 3 : RH Steam/Metal Temperature Between Start Up in 15 – 17 March 2010

1st start 2

nd start 3

rd start 4

th start

TT_RHS1 : oF 550 688 489 581

TT_RHS2 : oF 541 681 480 575

TT_RHBUI1 : oF 767 707 488 464

TT_RHBLI1 : oF 526 466 274 267

TT_RHBLI2 : oF 506 448 259 257

RHT_DIF : oF 30 228 218 316

When comparing condition requirements between auto start up permissive and manufacture guide

(GEK 106752 & GEK 111301), there is one parameter that should be adjusted, it is a RHT_DIF. This parameter

is calculated from difference between an average of RH steam temperature and an average of RH lower metal

temperature. The steam turbine can auto startup when a RH temperature difference is lower than 500 degF, and

RH pressure (HRHP) must be over 1% as shown in Fig. 14

Figure 14 : D11 Steam Turbine Auto Start Up Permissive

The RH metal thermocouples were installed at 3 positions as following, one RH upper metal

temperature (TT_RHBUI1) and two RH lower metal temperature (TT_RHBLI1, TT_RHBLI2). The RH upper

metal thermocouple was installed near IP turbine rotor, and RH lower metal thermocouples were installed at RH

inlet pipe as shown in Fig. 15

Figure 15 : D11 Steam Turbine Reheat Metal Temperature Measurement

The position of RH upper metal thermocouple is closer to IP turbine than RH lower metal

thermocouples, so the measured value from RH upper metal thermocouple shows the true condition of IP

turbine.

From the trend of steam turbine during shutdown as shown in Fig. 16, the RH lower metal temperature

decrease faster than RH upper metal temperature. So when the turbine startup in hot mode or warm mode

chances are turbine will roll with low steam temperature, because the condition of auto start up permissive had

calculated RHT_DIF by average of RH lower metal temperature and excluded the RH upper metal temperature.

Figure 16 : RH Steam and Metal Temperture

The manufacture guide specify in GEK 111301 had defined the RH steam temperature must be 200-

250 degF higher than RH metal temperature as shown in Fig. 17. But in practice when the operator try to

increase RH steam temperature up to that point it will takes a lot of time. So an optimum adjustment for RH

steam temperature was changed to 50 degF higher than RH upper metal temperature.

Figure 17 : Allowable RH Steam Temperature for Roll Steam Turbine

The requirement of auto start up permissive defined a RH pressure more than 1% of design pressure.

The manufacture guide had specified RH pressure from 101-130 psi, and RH bypass pressure control valve set

point equal to 58 psi for cold start and 112 psi for warm / hot start.

To startup steam turbine by hot mode must have a very high RH steam temperature. Therefore, it needs

higher gas turbine load but doing so may cause a RH steam pressure too high which according to record, it had

caused vibration high trip in block 1 and block 2. Thus requiring to reduce flue gas flow by decrease HRSG

damper to 80%, and limit a RH steam pressure in the range of 60-70 psi.

The startup of steam turbine by auto mode, an acceleration rate is calculated by the control system based

on rotor stress as shown in Table 4. To avoid the risk of extensively operating at or near the critical speed, which

might cause rubbing and vibration high trip as shown in Fig. 18, operator chooses to do a manual start up by fast

ramp rate since on turning gear until full speed no load.

In conclusion, the startup procedures improvement to reduce a vibration high trip problem at critical

speed in steam turbine bowed rotor are as following:

a. Increase a reheat steam temperature at least 50 degF higher than a reheat upper metal temperature

b. Control a reheat pressure to roll steam turbine in the range of 60-70 psi

c. Start up manually by fast ramp rate

Table 4 : D11 Steam Turbine Start up Ramp Rate

Speed

%

Condition Acceleration

rate

TNHR_RS%

/ min

0-10 -

Fast 20

> 10 If stress > 50%

If 5 < stress < 50 %

If stress < 5 %

Slow

Medium

Fast

6.7

10

20

Figure 18 : Auto Start up Steam Turbine and Vibration High Trip Problem

IV. Conclusion

Since improvement a startup process in 2011 until a HP/IP turbine casing replacement in 2014, RGC-

C30 have a total startup of 20 times which divide into hot start 4 times, warm start 5 times and cold start 11

times. The result of startup are always successful. But even with a casing replacement the vibration

measuring is rather high at warm and hot condition, because steam turbine bowed rotor does not

replacement. To reduce vibration during normal operation, it was solved by both low speed balancing and

field balancing once again during major overhaul in 2015. Until now it had a 100% successful startup.

(Detail of startup as shown in Fig. 19)

Figure 19 : Detail Steam Turbine Start up after Replacement Casing Turbine

References :

GENERAL ELECTRIC COMPANY (1997). GEK 106752 : Basic Information for Starting and Loading

Reheat Combined Cycle Units.

GENERAL ELECTRIC COMPANY (2005). GEK111301: Allowable Steam to Metal Temperature Mismatch

Combined Cycle Units

GENERAL ELECTRIC COMPANY (2007). GEK110856d : Steam Seal System Requirements for Combined

Cycle Steam Turbines