INTERVIEW QUESTIONS ON PWHT - PART 1

POST WELD HEAT TREATMENT (PWHT)

INTERVIEW QUESTIONS

PART ONE

Post Weld Heat Treatment (PWHT) is one of the most used topics in mechanical construction industry. PWHT is done as material requirement or as a process requirement. PWHT is also known as stress relieving, though stress relieving is a boarder term as compared to PWHT. Pre heat and post heat is also associated with PWHT.

This series is to list out the questions that are commonly asked in interviews. 

The materials that require PWHT from the material point of view and based on the common usage in refinery are, SA 335 Gr.P11, SA 335 Gr.P22, SA 335 Gr.P5, SA 335 Gr.P9 and SA 335 Gr. P91 pipe form and their related flanges and fittings. 

 The materials that require PWHT from the process point of view (which is decided by the process designer) and based on the common usage in refinery are, SA 106 Gr.B pipe form and it's related flanges and fittings.

The output of a PWHT process, is the graph which is generated during the execution of PWHT.

The X axis of the graph is ?

Answer: Temperature 

The Y axis of the graph is ?

Answer: Time

The graph is generated with the help of a recorder. There are 6 point recorders and 12 point recorders.

How many thermocouples can be mapped with a 6 point recorder. 

Answer : 6 Thermocouples. 

How many thernocouples can be mapped with a 12 poiny recorder

Answer : 12 Thermocouples. 

What is the normal speed of the movement of the graph in a recorder? 

Answer : 25mm/hour.

What is the principle of working of a recorder?

Answer : Potentiometer

What are the colours that are normally used in a 6 point recorder?

Answer: Red, Blue, Black, Brown, Green, Violet

What is the input of a recorder to plot the graph?

Answer: voltage from thermocouple

The thermocouple brings the temperature as voltage to the recorder.

Which thermocouple is used normally for the detection of temperature?

Answer : 'K' type

What is the MOC of 'K' type thermocouple?

Answer: Chromel-Alumel

How the thermocouples are identified?

Answer: 'K' type is Yellow and Red combination thermocouple.

In 'K' type thermocouple Red represents which polarity?

Answer: Negative Polarity

What is the diameter/thickness of the 'K' type thermocouple?

Answer: 0.81mm (#20AWG)

What is the maximum temperature that can be measured by 'K' type thermocouple?

Answer: 1260 Deg.C

Can a recorder calibrated for 'E' type thermocouple be used for measuring the temperature detected by 'K' type thermocouple?

Answer: No

How the thermocouples are connected to the surface where the temperature is to be measured?

Answer: Capacitor Discharge Method

Is PQR required for the welding of thermocouples to the surface?

Answer : No as per ASME BPVC Sec IX

What is the principle of temperature measurement using the thermocouple?

Answer: Seebeck Effect

How the thermocouples are connected to the recorder?

Answer: Through thermocouple extension wire.

PLEASE FEEL FREE TO ASK QUESTIONS IF ANY, THROUGH THE 'COMMENTS' SECTION IN THE BOTTOM OF THIS BLOG

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MORE QUESTIONS CAN BE FOUND IN PART 2 OF THIS SERIES

PART 2 WILL BE PUBLISHED on June 21, 2020

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