Factors affecting Service Features

Piping in operation fails by cracking, corrosion or sometimes by combination of the two due to the following reasons:

a) Non-Flexibility: While designing, provision of insufficient flexibility leads to cracking failure of steam line or any hotline. While in shut down, if gets cooled and contract and during operation it gets heated up and expands. This thermal contraction and expansion in the line lead to service failure if sufficient flexibility in all direction is not provided.

b) Notches: When heavy wall with higher thickness is designed and welded with a pipe of light wall thickness, a sharp corner or sudden change in section occurs in the line. Also in socket weld design a sudden change in section occurs in the line. In case of design of reinforcement pads or rings where the weld does not blend gradually into the piping wall, a sudden change in section occurs. The sudden change in sections or thickness work as a notch at that location and cracking take place due to thermal or mechanical figure.

c) Weld Defect: The location of shop weld joint and field weld joints with respect to accessibility for NDT inspection to find out the defects and space to attend the repair, if any. If the defects exist, it affects service features. Sometimes the wrong design of type of weld such as butt weld with groove angle or socket weld or slip-on weld joint is also the cause of system failure by crack.

d) Material: The selection of material based on their use with upper temperature, lower temperature and transition temperature is also reason for cracking in the heat-affected zone near the weld due to graphitization.

e) Weld Metals:Improper selection of the weld filter metal or electrode, specially, when temperature exceeds 800 F, has caused the crack across the interface zone of the weld metal and base metal.

f) Base-Metal Defects: Mechanical defects such as laminations. Laps, scabs & tears, if it is perpendicular to the pipe surface or diagonal to the pipe surface, acts as a very critical notches and cause cracking failure near the weld joints.

g) Hardness (Metallurgical notches): The hardness of steel varies with its chemical composition variation & heat treatment of the steel. When the difference in hardness value exceeds 70 to 100 points Brinnel in thermal & mechanical fatigue condition then the junction point or line between two different hardness materials behave like a notch and a crack takes place. This is known as metallurgical notches. For example:

Area Brinnel hardness value

Base metal 180

Heat affected Zone 232 Heat affected zone near the weld

280

Weld deposit 179

i) Carbonization during Hot Forming: Hot forming into plate, pipe and fitting, during manufacturing, by conventional method, is done by heating by means of gas burner to a temperature of 150⁰ to 185⁰ F with commercial gas (not a natural gas). Then the steel surface, most likely, gets carbonized. This carbonized surface, after welding, fails in service due to severe stresses caused due to pipe movement.

Such type of failure takes place in service after fabrication and all blame goes to the fabricator &

inspector but not to the manufacturer who has carbonized the pipe or elbow surface while making elbow by gas heating.

However, such carbonization can be detected only by weld ability test, particularly by bend test because it will develop crack in the parent metal. It can be detected by photomicrographs of the surface. It is very costly affair.

j) Incorrect Material: Generally, painting technique is applied during storing the different material in fabrication shop. But in long time, the paint goes away and it is very difficult to identify carbon steel & alloy steel piping components. Vary often, by mistake, Alloy Steel pipe is welded with carbon steel pipe or fittings or vice versa. Hence it becomes a case of metallurgical notch and fail in service in severe condition of thermal mechanical fatigue. A number of service failures take place in steam power plant due to material identification mistake before welding together.

k) Fabrication Mistake: Fabrication mistake such as deep cut during gas cutting or

machining for end preparation and fit up, or welding defects at root pass such as lack of penetration, slag inclusion etc. work as a notch and hence joint fails in service during severe thermal & mechanical fatigue condition. That is the reason; the root pass is done always by inert gas tungsten arc welding in a high temperature high pressure piping system.

l) Heat Treatment: Some carbon-steel pipe is furnished in the hot finished condition. Hot finishing is generally performed between 160⁰ and 220⁰ F and is followed by air-cooling. Under these conditions these steels can be compared to normalized steels, although it should be recognized that the temperature of finishing is an important factor. When piping materials are cold worked, their strength and hardness are increased and ductility is decreased. That is why cold expansion while bending of pipe or cold working is done intentionally to obtain the higher strength value in A106 or some API grade material. However, the effects of cold work can be removed by heat treatment.

m) Multi-axial stress: Multi-axial tensile stresses raise the transition temperature. This is particularly true at the base of a notch or crack where multi-axial tensile stresses of considerable magnitude may develop.

n) Section Size: If the section size is increased without other changes in geometry, the transition temperature will also be increased.

o) Design: Design based upon conventional tensile-test data gives no assurance that piping will not fail in a brittle manner. Nor can such assurance be obtained by simply increasing the section size with the intent of increasing the factor of safety. In the presence of notches, increase in section size will most likely increase restraint and may even lead to failure at lower applied loads.’

Table: Limitation of temperature & pressure on Materials

Material Max. Pressure

& temperature

Application

Cast grey

Irons (A 278)

250 psi, 250⁰ F 250 psi, 450⁰ F

Pipe, valves &

fitting

-DO-Malleable Cast

Irons

300 psi, 500 F Pipe, valves &

fitting

Carbon steels 775 F Pipe, valves &

fitting 1 ¼ Cr-½ Mo

steels

950 to

1000 F

Boiler piping & steam piping.

¼ Cr-1 Mo steels

1060 F Steam & power plant piping

5Cr-½ Mo Steel 1500 F Refinery Piping 7Cr-½ Mo Steel 1500 F Refinery Piping 9Cr-1 Mo Steel 1500 F Refinery Piping