FINAL PART OF CRITICAL PIPING -ABC OF THERMAL POWER PLANT

For critical Piping and Large LP piping are very important areas of thermal Power Station .

Erection of the piping starts around 16 -18 months from Zero date

( Project Zero date) and continues up to 32 to 35 months for a super thermal power project.

It has been observed that all critical materials and their accessories were imported from 

developed countries because of their material composition. New material are introduced

to reduce the overall weight of the package in turn it will reduce the structure weight

(sizing) ,hanger sizes etc.

Because of that, restriction on welding are imposed . One has to follow Welding

Procedure Sheet given by OEM in totality . No compromise should be made.

Otherwise bigger problem will be crop up in coming days. Now for this welding you

require different consumables starting from filler wire, welding electrode

( 2.5 ,3.15,4 mm) ,  gases specially argon , and many other items required during

welding. Most of the items are available in India but electrode and filler wire may

not be available at the time of crisis. It is advisable  to calculate the requirement

before hand and keep checking the stock available . This is

true for critical piping area only.

All the critical piping drawings to be taken and mark the joint number as per code followed

at site. Calculate the requirement of filler rod ( including wastage ) and welding electrode

different sizes. This will give fair amount of idea about the requirement.

Always check the straight and associated fitting specially internal Diameter. Piping

vendor may source it from different countries . There are developed countries

who may not be Following ASME or IBR code . Error happens may be very less but it

comes out at later date and there will be no time for fresh procurement . The project

will suffer for that.

Please also verify the availability of Test material ( welder test) at your disposal.

You need few materials to qualify your HP welder in P91 /P22 etc both pipe and tube.

A concept of orbital welding is now getting popular in India. This type of welding is being followed in developed countries at site but in India the same is limited to manufacturing unit only.

A short description of Orbital welding is given below .

Orbital welding uses the gas tungsten arc welding (GTAW) process as the source of the electric arc that melts the base material and forms the weld. In the GTAW process, an electric arc in established between a tungsten electrode and the part to be welded. To start the arc, a high-voltage signal is used to break down (ionize) the insulating properties of the shield gas and make it electrically conductive to pass through a tiny amount of current. A capacitor dumps current into this electrical path, which reduces the arc voltage to a level at which the power supply can then supply current for the arc. The power supply responds to the demand and provides weld current to keep the arc established. The metal to be welded is melted by the intense heat of the arc and fuses together.

 Following sizes of pipes are generally used in a super thermal power station.

Sl No.	Base Metal	Dia.(mm)	Thickness(mm)
1.	SA182F22CL3	356	45
2.	SA335P12	356	45
3.	SA234WP12CL1	300	45
4.	SA182F12CL2	559	67.3
5.	SA106GC.C	559	67.3
6.	SA105	356	46.3
7.	SA 335P22	508	63.1
8.	SA182F12CL2	219	34
9.	SA335P12	219	34
10.	SA234WP12CL1	219	34
11.	SA234WP91	450	63.5
12.	SA335P91	457	63.5
13.	SA324WPC	500	63.1
14.	SA234P91	470	60
15.	SA234P91	89	15.5
16.	SA234P91	630	85

There are many reasons for using orbital welding equipment. The ability to make high quality, consistent welds repeatedly, at a speed close to the maximum weld speed, offer many benefits to the user:

1. Productivity. An orbital welding system will drastically outperform manual welders, many times paying for the cost of the orbital equipment in a single job.

2. Quality. The quality of a weld created by an orbital welding system (with the correct weld program) will be superior to that of manual welding. In applications such as semiconductor or pharmaceutical tube welding, orbital welding is the only means to reach the weld quality requirements.

3. Consistency. Once a weld program has been established, an orbital welding system can repeatedly perform the same weld hundreds of times, eliminating the normal variability, inconsistencies, errors, and defects of manual welding.

4. Skill level. Certified welders are increasingly hard to find. With orbital welding equipment, you don't need a certified welding operator. All it takes is a skilled mechanic with some weld training.

5. Versatility. Orbital welding may be used in applications where a tube or pipe to be welded cannot be rotated or where rotation of the part is not practical. In addition, orbital welding may be used in applications where access space restrictions limit the physical size of the welding device. Weld heads may be used in rows of boiler tubing, where it would be difficult for a manual welder to use a welding torch or view the weld joint.

Few pictures of weld held is given below for information.

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TIPS ON CRITICAL PIPING ERECTION- ABC THERMAL POWER STATION

A few information on critical piping is written below for knowledge:-

Life cycle designed for critical piping is 30 years approximately.

All the piping  design ,erection and testing will be done according to ASME code 31.1 and Indian Boiler Regulation.

Inside diameters of piping shall first be calculated for the flow requirement of various systems. For that velocity of fluid flow to be known ( e.g for 660MW Main Steam Piping is 76M/sec , BFD (4.0-6.0 M/sec).

Piping system shall be of carbon steel for design temperature up to 400 deg. C and alloy steel for design temperature above 400 deg. C.

To account losses /drops 1.00/0.75 mm thickness are added on calculation based on ASME B31.1.

All high points in piping system shall be provided with vents. All low points shall be provided with drains. Provisions of drains on steam piping shall be as per ASME code TDP-1.

All piping shall be sloped towards the system low point such that slope is maintained in both hot and cold condition.

All drain and vent lines in piping system with design pressure 40 Kg/cm2 (g) and above or with vacuum service shall be double valved.

A clear head room of not less than 2.5 metres above the walkways/working area is available .

Cold pulling is not permitted.

All hangers and supports shall be erected such that they are vertical when the piping is in hot condition (rated parameters).- Important

Maximum cladding temperature after insulation should be approximately 60 degree C.

All stubs welded to the pipe including welded thermowells and instrument source tappings shall be installed on the pipe prior to stress relieving.

For all globe and check valves, the direction of flow to be clearly stamped on the body of the valve.

Where control valves, flow nozzles, orifices and other piping items are to be installed, they shall be installed only after steam blowing and chemical cleaning operation. After the completion of the steam blowing/chemical cleaning spool pieces will be removed and original components installed.

Field run piping shall be erected only after completion of erection of all other piping system, structures and equipment.

The hydrostatic testing of the piping system shall be done after proper installation of all permanent hangers/supports. Spring hangers shall be locked during hydrostatic test. Prior to steam blowing all hangers which had been locked for the hydrostatic testing shall be unlocked.

 All piping systems shall be hydro tested at 1.5 times the design pressure subject to regulation of 374 IBR. However, for such systems where it is practically not possible to do hydro tests, the tests as called for in ANSI B31.1& IBR in lieu of hydro test shall also be acceptable.

Two or three major problem arises during erection of critical piping of super thermal power. One is mismatching of Internal diameter of pipe and fittings differences ranges between 10 -15 mm . The other problem belongs oversize pipe which was cut to size and fouling ,shifting of hanger support.

A few pictures are attached to elaborate piping.

Area is Boiler furnace roof top and i.e starting point of critical Piping.

For 500 MW /600 MW or lesser sub critical unit boiler down comers are erected

first . Here the unit is 660 MW super critical therefore with downcomer seperator piping is also can be seen.

heating and stress releiving equipment for Pressure Parts and critical Piping.

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