OUTAGE REPORT

HGPI 2015 UCH Power Plant

16th Oct – 10th Nov 2015

Submitted by: Syed Fahim Abbas Kazmi

Trainee Engineer

Mechanical Maintenance Department

Contents HGPI Introduction ......................................................................................................................................... 3

Major jobs to be done by group Delta .......................................................................................................... 3

DETAILED OVERVIEW OF GROUP DELTA JOBS .............................................................................................. 4

HRSG Overview ......................................................................................................................................... 4

Hot and cold survey of hangars ................................................................................................................ 6

All drums internal inspection followed by NDT ........................................................................................ 8

Rectification of all the passing vavles according to thermography report ............................................. 14

Rectification of HRSG B HP MOV 014 ..................................................................................................... 17

Rectification of HRSG B HP MOV 013 ..................................................................................................... 21

Ice Blasting of HRSG Economizer tubes .................................................................................................. 24

Repair/Replacement of insulation in GT exhaust plenum and IFGD according to thermography report

................................................................................................................................................................ 28

HRSG C HP drum cyclones overhauling................................................................................................... 30

Lesson Learnt .............................................................................................................................................. 32

References .................................................................................................................................................. 33

HGPI Introduction

HGPI stands for Hot Gas Path Inspection which is the type of overall

preventive plant maintenance done on the gas turbines every

alternative year. It is the second biggest outage after the Major

Inspection (MI) in which the steam turbine is dismantled for

complete maintenance as well.

In HGPI, Gas turbines are dismantles and the pathways of hot gases

are inspected visually or through non destructive testing and is

consequently maintained or replaced according to the

manufacturer’s recommendation.

Major jobs to be done by group Delta

- Dry ice blasting of HRSG economizer tubes

- Bench testing of all the safety relief valves of the plant

- Repairing of piping insulation

- All drums internal inspection followed by NDT

- Rectification of all passing valves according to thermography

report.

- Rectification of vibration observed at ducts and its mountings.

- Rectification of HRSG A water leakage from HP hydrastep.

- Rectification of all hard to open valves.

- Repair/Replacement of insulation in GT exhaust plenum and IFGD

according to thermography report.

- Hot and cold survey of hangars of HRSG

- HRSG C HP drum cyclones overhauling.

DETAILED OVERVIEW OF GROUP DELTA JOBS

HRSG Overview:

Safety hazards

Injury to personnel by accidentally touching hot equipment or steam leaks: 1- Hot pipework should be insulated for heat retention and

personnel protection, where insulation is damaged, arrangements should be made to replace insulation and make the work area safe.

2- Protective clothing must be worn to operate or inspect equipment, where it is not practical to fit heat shields for personnel protection.

3- Where steam leaks are hazardous, for example Super-heated

steam leaks can be invisible, barriers should be erected to protect

personnel in the work area and arrangements made to stop the

steam leak as soon as possible.

Fire when oil impregnated insulation reaches its auto ignition

temperature:

1- Oil impregnated insulation will auto ignite at temperatures above

400°F.

2- Where leakage of oil can fall on hot pipework or contaminate

Insulation the oil should be diverted from the pipework and the

leak repaired as soon as possible. Oil contaminated lagging should

be removed from the work area.

Injury from chemicals used for treating feed water to the HRSG: 1- Hydrazine (Injected into the condensate)

2- Ammonia

3- Phosphate

Purpose of HRSG system:

1- The generation of electricity by supplying steam to the steam

turbine.

2- The Deaeration of feed water and pressure control of the

Deaerator.

The steam system consists of three Heat Recovery Steam Generators

and one Steam Turbine Generator.

A Heat Recovery Steam Generator (HRSG) is an unfired boiler,

generating steam in three separate pressure systems, from the heat in

the exhaust gas emitted by the Gas Turbine.

The three pressure systems are termed, High, Intermediate and Low

pressures steam.

1- High Pressure steam is utilised in the Steam Turbine Generator

2- Intermediate Pressure steam is used in the Steam Turbine

Generator and Deaerator.

3- Low Pressure steam is used in the Deaerator

Hot and cold survey of hangars

All the HRSG pipes are hanged with the help of stainless steel

hangars which are installed with a spring to compensate the expansion

of the pipes due to various loadings. It is very important to check the

limits of the springs as to how much they are expanding during the

plant running and how much they are contracting after the HRSG has

been cooled.

For this technique, before the shutdown of the plant, cameras were

installed to specific locations to take the reading of the expansion of all

the hangars. Then after the shutdown when all the pipes have been

cooled and got to ambient temperature, again the reading was taken

with the help of a camera.

All the readings taken were then compared to the range given in the

maintenance manuals and were within the range of the manufacturer’s

recommendation.

All drums internal inspection followed by NDT

All the HRSGs HP, IP and LP drums including the blowdown tanks

together with the Deaerator and the Feedwater tank were to be

thoroughly inspected.

The procedure started off with opening up of all the boiler drums

doors. It took almost 2 days for all the boilers inside temperature to get

normal relative to the ambient temperature since the HP temperature

is above 600 degree Celcius during normal operating conditions.

Exhaust fans were installed after opening the doors to hasten up the

cooling process.

HRSG LP drum yokes before opening

Descon workers opening Feedwater drum door after shutdown

After all the drums were cooled down to ambient temperature, they

were handed over to the 3rd party (Descon) for thorough internal visual

inspection. During this inspection, the surface inside of the boiler was

checked to ensure its quality with accordance of the manufacturer’s

recommendation. Furthermore the boilers inner parts condition was

checked for example if the baffle plates were not lose or broken.

The next step was to perform Non-destructive testing at the welds to

ensure there are no cracks or pinholes that were not visible through the

naked eye. There were two NDT techniques used.

1. Dye Penetrant Testing

DPT is based upon capillary action, where low surface tension

fluid penetrates into clean and dry surface-breaking

discontinuities. Penetrant may be applied to the test component

by dipping, spraying, or brushing. After adequate penetration

time has been allowed, the excess penetrant is removed and a

developer is applied. The developer helps to draw penetrant out

of the flaw so that an invisible indication becomes visible to the

inspector. Inspection is performed under ultraviolet or white light,

depending on the type of dye used - fluorescent or

nonfluorescent (visible).

Below are the main steps of Liquid Penetrant Inspection:

1. Pre-cleaning:

The test surface is cleaned to remove any dirt, paint, oil, grease or

any loose scale that could either keep penetrant out of a defect,

or cause irrelevant or false indications. Cleaning methods may

include solvents, alkaline cleaning steps, vapor degreasing, or

media blasting. The end goal of this step is a clean surface where

any defects present are open to the surface, dry, and free of

contamination. Note that if media blasting is used, it may "work

over" small discontinuities in the part, and an etching bath is

recommended as a post-blasting treatment.

2. Application of Penetrant:

The penetrant is then applied to the surface of the item being

tested. The penetrant is allowed "dwell time" to soak into any

flaws (generally 5 to 30 minutes). The dwell time mainly depends

upon the penetrant being used, material being tested and the size

of flaws sought. As expected, smaller flaws require a longer

penetration time. Due to their incompatible nature one must be

careful not to apply solvent-based penetrant to a surface which is

to be inspected with a water-washable penetrant.

3. Excess Penetrant Removal:

The excess penetrant is then removed from the surface. The

removal method is controlled by the type of penetrant used.

Water-washable, solvent-removable, lipophilic post-emulsifiable,

or hydrophilic post-emulsifiable are the common choices.

Emulsifiers represent the highest sensitivity level, and chemically

interact with the oily penetrant to make it removable with a

water spray. When using solvent remover and lint-free cloth it is

important to not spray the solvent on the test surface directly,

because this can remove the penetrant from the flaws. If excess

penetrant is not properly removed, once the developer is applied,

it may leave a background in the developed area that can mask

indications or defects. In addition, this may also produce false

indications severely hindering your ability to do a proper

inspection. Also, the removal of excessive penetrant is done

towards one direction either vertically or horizontally as the case

may be.

4. Application of Developer:

After excess penetrant has been removed, a white developer is

applied to the sample. Several developer types are available,

including: non-aqueous wet developer, dry powder, water-

suspendable, and water-soluble. Choice of developer is

governed by penetrant compatibility (one can't use water-

soluble or -suspendable developer with water-washable

penetrant), and by inspection conditions. When using non-

aqueous wet developer (NAWD) or dry powder, the sample

must be dried prior to application, while soluble and

suspendable developers are applied with the part still wet from

the previous step. NAWD is commercially available in aerosol

spray cans, and may employ acetone, isopropyl alcohol, or a

propellant that is a combination of the two. Developer should

form a semi-transparent, even coating on the surface.

The developer draws penetrant from defects out onto the

surface to form a visible indication, commonly known as bleed-

out. Any areas that bleed out can indicate the location,

orientation and possible types of defects on the surface.

Interpreting the results and characterizing defects from the

indications found may require some training and/or experience

[the indication size is not the actual size of the defect].

5. Inspection:

The inspector will use visible light with adequate intensity (100

foot-candles or 1100 lux is typical) for visible dye penetrant.

Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-

watts per centimeter squared is common), along with low

ambient light levels (less than 2 foot-candles) for fluorescent

penetrant examinations. Inspection of the test surface should

take place after 10- to 30-minute development time, depends

of product kind. This time delay allows the blotting action to

occur. The inspector may observe the sample for indication

formation when using visible dye. It is also good practice to

observe indications as they form because the characteristics of

the bleed out are a significant part of interpretation

characterization of flaws.

6. Post Cleaning:

The test surface is often cleaned after inspection and recording

of defects, especially if post-inspection coating processes are

scheduled.

2. Magnetic Particle Inspection

Magnetic particle Inspection (MPI) is a non-destructive testing

(NDT) process for detecting surface and slightly subsurface

discontinuities in ferromagnetic materials such as iron, nickel,

cobalt, and some of their alloys. The process puts a magnetic field

into the part.

Descon worker doing DPT inside the HP drum of the HRSG

After all the NDT work, any cracks or pinholes detected were repaired

by the welding team with the help of welding and grinding. Before

closing the boilers, they were also checked for quality by a boiler

inspector.

Rectification of all the passing vavles according to

thermography report

Before the outage, the technical service department used

thermography cameras to detect which of the valves were passing.

Thermal images, or thermograms, are actually visual displays of the

amount of infrared energy emitted, transmitted, and reflected by an

object. Because there are multiple sources of the infrared energy, it is

difficult to get an accurate temperature of an object using this method.

A thermal imaging camera is capable of performing algorithms to

interpret that data and build an image. Although the image shows the

viewer an approximation of the temperature at which the object is

operating, the camera is actually using multiple sources of data based

on the areas surrounding the object to determine that value rather

than detecting the actual temperature.

During the normal operation of the plant, it was informed by the

operations department that the blowdown of the water overall was

well above 35000 tonnes per day. This indicated that the blowdown

drain valves were heavily passing. The thermography report showed

numerous valves which had to be replaced.

The blowdown drain valves consisted of manually operated 1 inch

globe valves, manually operated 2 inch globe valves and pneumatically

operated 2 inch valves. A Globe valves is a linear motion valve and are

primarily designed to stop, start and regulate flow. The disk of a Globe

valve can be totally removed from the flowpath or it can completely

close the flowpath.

The following table shows the arrangement of these valves in an HRSG:

Blowdown drain valves diagram

The procedure of changing the valve was to first cut the valve from its

pipes at both the ends. This was followed by installation of new valve

and then welding it from both the sides. In the end these new welds

integrity was checked with the help of Dye penetrant testing.

In total 8 blowdown valves of HRSG A, 9 blowdown valves of HRSG B

and 2 blowdown valves of HRSG C were replaced with new ones.

1 inch manual isolation globe valves before replacement

MD AOV 034 B1 new valve being installed

Rectification of HRSG B HP MOV 014

HP MOV 013 is the high pressure steam bypass motor operated gate

valve. This gate valve has wedge type gates. The valve was declared

passing after the thermography survey by the technical service

department. This valve passing issue was very critical since its failure to

shut off steam would then make the job dependent on the manual

isolation valve.

Before discussing overhauling and problem diagnosis, let us see in

detail the working and components of a gate valve:

A gate valve functions by lifting a rectangular or circular gate out of the

path of the fluid. When the valve is fully open, gate valves are full bore,

meaning there is nothing to obstruct the flow because the gate and

pipeline diameter have the same opening. This bore diameter also

determines the valve size. An advantage of this full-bore design is very

low friction loss, which saves energy and reduces total cost of

ownership.

There are four primary designs for gate valves – a slab gate, an

expanding gate, a wedge valve and a knife gate valve.

Slab gate valves are comprised of a single gate unit which raises and

lowers between two seat rings and are primarily used for transporting

crude oil and NGLs.

Unlike a slab gate valve that only has one unit, an expanding gate valve

includes two units – a gate and segment. The gate and segment units

collapse against each other for travel, and separate when the valve is

fully opened or fully closed, to affect a mechanical seal.

Wedge gate valves are comprised of a tapered gate that is metal-to-

metal sealing. In contrast to a slab gate valve or an expanding gate

valve, wedge gate valves are not piggable because of the void that is

left in the bottom of the valve body when the valve is open. These

valves do not have a bore through the gate itself – instead, the gate

retracts into the valve body when open – which saves height space that

is necessary for slab and expanding gate valves.

A knife gate valve is used to cut through extremely thick fluids and dry

bulk solids. The design of this valve makes it inherently self-cleaning, as

the knife is cleared of abrasives with each stroke as it passes the seat

rings and skirts. The gate unit of this type of valve is thin compared to

other gate valve types and is guided in place by the water-type body

that sandwiches the gate.

Stem

Gate valves can have either a rising or non-rising stem design. Rising

stems are attached directly to the gate and provide a visual indicator of

the valve position. Non-rising stems are generally threaded into the

upper part of the gate and have a pointer threaded onto the top to

indicate position. Non-rising stem designs are ideally suited for

applications where vertical space is limited, in well applications and

where scraping/pigging is not required.

Bonnets

Gate valves generally have one of four types of bonnets, which provide

closure from leaks for the body of the valve. Screw-in bonnets are

simple, durable sealing units that use pressure to affect a seal. Union

bonnets provide easy access to the valve body for applications that may

require frequent maintenance or inspection. Bolted bonnets are

generally used for larger valves in higher pressure applications. Finally,

pressure seal bonnets are designed for services with high pressure in

excess of 15MPa (2250 psi).

Diagnosis of problem

After dismantling of the valve, thorough visual inspection was done of

the seat and the gate. There was a minor cut found on the seat which

was causing the passing. So lapping was done with the help of lapping

machine on the face of the seat. As an extra precaution, lapping was

also done on the face of both the gates.

After this DPT was performed on the seat and no cut was found. We

further used BLUE on the seat to check if the surface of the gate is

completely in contact with the seat which was successful. Finally the

valve was reassembled and ready to work again.

Rectification of HRSG B HP MOV 013

This is a major valve which is used to stop main HP steam. This valve is a

wedge type gate valve which is motor operated. The technical service

department, after doing thermography on this valve declared it as

passing.

Diagnosis of the problem

The dismantling of the valve was a major problem due to its weight and

its location. Rigging equipment was used to firstly remove the actuator

part and hang it nearby the valve to be reassembled later. Then the

valve was marked at every point for assembly reference.

Lapping in process of the seat of HP MOV o14

The valve was then fully closed and unbolted to first remove the gland and

the seals inside. Then the stem was removed and ultimately the two gates.

The disassembly was followed by detailed visual inspection of the

components. The gates faces were found damaged at multiple locations and

were beyond the state of repair. There was also a deep cut found on the

seat of the valve, hence being the main reason for the passing of the valve.

This valve was overall serviced with new parts installed. The new parts

included new gates, new stem and new gland seals. The seat of the valve

was lapped for two days to remove the cut. Finally DPT was done on the

seat and it was found to be clear. The valve was assembled again and the

job was complete.

The dismantled components of HP MOV 013

Seat of HP MOV 013. Cut can be seen on the left side

Dismantling of HP MOV 013 using rigging equipment

Ice Blasting of HRSG Economizer tubes

Due to the low temperature of the flue gases at the very end of the

HRSG, the economizer tubes are subjected to high sulfur deposits on

the fins which decrease the heat exchanging tendency of the tubes.

Method

Dry-ice blasting involves propelling pellets at extremely high speeds.

The actual dry-ice pellets are quite soft, and much less dense than

Damaged gate of HP MOV 013

other media used in blast-cleaning (i.e. sand or plastic pellets). Upon

impact, the pellet sublimates almost immediately, transferring minimal

kinetic energy to the surface on impact and producing minimal

abrasion. The sublimation process absorbs a large volume of heat from

the surface, producing shear stresses due to thermal shock.[2] This is

assumed to improve cleaning as the top layer of dirt or contaminant is

expected to transfer more heat than the underlying substrate and flake

off more easily. The efficiency and effectiveness of this process

depends on the thermal conductivity of the substrate and contaminant.

The rapid change in state from solid to gas also causes microscopic

shock waves, which are also thought to assist in removing the

contaminant.

Equipment

The ice used can be in solid pellet form or shaved from a larger block of

ice. The shaved ice block produces a less dense ice medium and is more

delicate than the solid pellet system.

Dry-ice blasting technology can trace its roots to conventional abrasive

blasting. The differences between an abrasive-blasting machine and a

dry-ice blasting machine are in how they handle the blast media. Unlike

sand or other media, dry ice is generally used at its sublimation

temperature. This means that a pressurized hopper system could

potentially build up dangerous amounts of pressure (see dry ice bomb).

Other differences include systems for preventing the ice from forming

snowball-like jams, and different materials to allow operation at very

low temperatures.

Single-hose dry-ice blasters share many of the advantages of single-

hose abrasive-blast systems. To avoid the potential dangers of a

pressurized hopper, single-hose dry-ice blasters make use of a quickly

cycling airlock. The single-hose system can use a longer hose than its

double-hose counterpart without a significant drop in pressure when

the ice leaves the hose. The additional power comes at the cost of

increased complexity. Single-hose systems are used where more

aggressive cleaning is an advantage. This allows heavier build-up to be

cleaned and allows moderate buildup to be cleaned faster.

Safety Hazards

Carbon dioxide is increasingly toxic starting at concentrations above

1%, and can also displace oxygen resulting in asphyxia if equipment is

not used in a ventilated area. In addition, because carbon dioxide is

heavier than air, exhaust vents are required to be at or near ground

level to efficiently remove the gas. At normal pressure dry ice is −78 °C

(−108 °F) and must be handled with insulated gloves. Eye and ear

protection are required to safely use dry ice cleaning equipment.

Compared to other blasting-cleaning methods, dry ice blasting

produces fewer waste products and does not require clean-up of a

blasting medium. The waste products can be swept up, vacuumed or

washed away depending on the containment.

Result

The total deposits of sulfur removed from the HRSG’s were as follows:

HRSG A: 820 kg

HRSG B: 460 kg

HRSG C: 480 kg

HRSG A inspection after Ice blasting. It could be seen that only the sulfur deposits from the front side were removed. The back side remains unattended.

Repair/Replacement of insulation in GT exhaust plenum and

IFGD according to thermography report

Before the outage, the Technical Service Department carried out surveys of the HRSGs hotspots which showed areas of high thermal stresses. This was also visible by a visual survey which showed areas of the casing with burned or discolored paint. After the identification of the areas with very high thermal stresses, the apparent cause was found out to be burned insulation inside the baffle plates of the HRSG. The primary area in all three HRSG’s was the expansion joint A and the diffuser side. This is because the flue gases directly hit these areas after coming out of the exhaust diffuser. Casing temperatures above 350F are especially problematic if several square feet of casing are affected. Such high tem­peratures combined with the affected area cause restrained expansion which, in turn, results in cracking of casing steel-sometimes severe. Most paint will discolor above 350F and cracking is a possibility, espe­cially in areas where the casing is stiff-such as floors and roofs. After the shutdown, all the HRSG doors were opened and a selected person report was issued at first before anyone could enter the confided space. Due to safety hazard of confined space, it was necessary that at every entry point a designated person stood who would note down the name and time of every person entering and exiting. A detailed visual inspection was done by the field supervisor to identify the areas and diagnose the reason for the hotspots. This was followed by installation of scaffolding and removing of the baffle plates. Insulation was filled and inside the baffle plates and minor cleads that were broken were fixed.

Figure shows the installation of scaffolding inside the HRSG for insulation filling

Figure shows the insulation filled inside the baffle plates of HRSG before retightening of bolts

HRSG C HP drum cyclones overhauling

In small, low pressure boilers, steam-water separation can be

established with the use of a large steam drum approximately half full

of water. Natural gravity steam-water separation can be sufficient for

this case.

On the other hand, for high capacity, high pressure (HP) boilers, steam-

water separation at the steam drum is accomplished with the use of

mechanical steam-water separators. With the installation of these

devices in the steam drum, the drum's diameter and cost can be

significantly decreased.

Primary steam-water separation

Primary steam-water separation removes nearly all the steam from the

water so that very little steam is recirculated from the drum bottom

towards the heated tubes through the outlet connection (downcomer).

Primary separation equipment generally takes one of the following

three forms:

Gravity-driven separation: it is generally considered uneconomical and

its use nowadays is very limited,

Baffle-assisted separation: simple screens and baffle arrangements are

used for improving the steam-water separation process. Baffles provide

among others changes in the direction, more even distribution of the

steam-water mixture as well as additional flow resistance. Their use is

mostly limited to smaller, low capacity boilers.

Mechanical primary separators: they make use of centrifugal force or

radial acceleration. They are nowadays in use almost worldwide for

state of the art steam-water separators. Conical cyclone, horizontal

cyclone, vertical cyclone separators are some of the technologies used.

In the HP drum used in HRSG of Uch Power Plant, there are 20 cyclone

separators located on the left side and 22 located on the right side of

the boiler. They were all removed one by one and were taken to

workshop for washing and cleaning of their meshes. Those whose

meshes were of very deteriorated condition were replaced. These were

then installed back inside the drum.

Cyclone separators being overhauled at the workshop

Lesson Learnt

There were multiple lessons learnt during the outage. They are as

follows:

- One of the yokes required to shut down the door of LP drum of

HRSG C went missing. All the teams searched the nearby areas but

it was nowhere to be found. Ultimately by the decision of the

senior management a new yoke was fabricated for the job. For

the next outage it is very important to keep check on all the

dismantled equipment to avoid last minute hassle.

- The hose used for the ice blasting should be used of a different

design. The one used this time only removed sulfur content from

the front surface of the economizer tubes and not even from the

second batch behind it. The back surface of the front tubes also

remained unattended. It should be noted that using such high

cost technology yielded very unimpressive result.

- The valve HP MOV 013 that was repaired have a 12 inch diameter.

To cut and welt such a high diameter pipe, a post weld heat

treatment machine is required which the plant does not own.

Although the problem was solved without the installation of a

new valve but in case new valve had to be installed there were no

complete resources which could have caused delay in the plant

startup.

- After the startup of the plant, The HP drum of HRSG B started to

leak, resulting in tripping of the gas turbine. Later it was

diagnosed that the gasket inside the door was cut during the

closing of the door. Therefore for future reference it is necessary

that a senior supervisor attends to the process of closing of the

doors of the boilers and the vessels to ensure proper gasket

installation.

References

- UPS induction manuals - Wikipedia - http://www.answers.com/Q/When_boiler_use_cyclone_separato

r_and_why - http://www.enggcyclopedia.com/2012/01/drums-mechanical-

steam-water-separators/ - https://www.c-a-m.com/products-and-services/valves/valve-

academy/how-does-it-work-gate-valves