PWHT HOW TO READ A PWHT CHART Heat treatment is done because of of two reasons 1 ) Process requirement requirement 2) Material Requirement
Heat treatment can be done in many ways. wa ys. Furnace heating, Oil fired heating, Electric resistance heating etc. There are different type of heat treatments like normalizing, annealing etc. Here we are going to discuss about the graph which is generated in electric resistance resis tance heating for PWHT (Post Weld Heat Treatment).
In this chart, X-axis represents temperature and Y-axis represents Time. In X-axis each line to line distance represents 10Deg C (See it is written as X 100°C, that means in X axis, 2 means 2 X 100°C= 200°C, 3 means 3 X 100°C= 300°C etc.) In Y-axis each line to line distance represents half an hour. That means the time between 2 and 3 represents one hour. These are the basics. How to read the graph?
The graph contains several lines, AB, BC, CD, AE 1) AB is RATE OF HEATING curve 2) BC is SOAKING TEMPERATURE 3) CD is RATE OF COOLING curve 4) AE is ROOM TEMPERATURE curve when you check the WPS you can see the required values for each of these. 1) AB: Rate of heating: In WPS it is mentioned as a maximum value. In the graph the rate of
heating is the slope of the curve AB. So the value of the slope should not be more than the value in WPS. How to calculate the slope: Slope = kl/lm Now in the graph given above, for AB line, kl=80DegC and lm=1/2 hr slope = 80 / (1/2) = 160DegC/hr ; So in this graph the rate of heating is 160DegC/hr. 2) BC: Soaking Temperature: In WPS it is mentioned as a single value with some
plus/minus allowance along with time it is to be steady(mentioned as minimum) Here in this graph it is 720Deg (Reading in X-axis) and for approximate 2hr and 10minute(this is an approximation-Reading in Y-axis ) 3) CD: Rate of cooling: In WPS it is mentioned as a maximum value. In the graph the rate of
heating is the slope of the curve CD. So the value of the slope should not be more than the value in WPS. How to calculate the slope: Slope = qr/pq Now in the graph given above, for CD line, qr=130DegC and pq=45 Min(this is taken as an approximation from the graph.) Slope = 130 / 45 = 2.89DegC/Min; so in this graph the rate of heating is 173DegC/hr. 4) AE: Room Temperature: This is the room temperature, here it is 40 deg C.
Loading Temperature: Loading temperature is the temperature at which controlled heating
(maintaining the rate of heating in the WPS) starts, up to which there is no need for controlled heating.
Unloading Temperature: Unloading temperature is the temperature up to which controlled
cooling (maintaining the rate of cooling in the WPS) occurs, below which there is no need for controlled cooling.Normally the power connection is disconnected below the temperature, keeping the insulation. After getting cooled to the atmospheric temperature the insulation is removed. NB: Please check the speed of the chart movement in the PWHT machine, Normally it is 25mm/hr. Thermocouples
Thermocouples are used to detect the temperatures in the process of PWHT. If the ends of the dissimilar wires are in contact with each other and also with a hot and a cold surface, a voltage difference between the two junctions is created. These voltage differences are proportional to the differences in temperature. Therefore a properly calibrated instrument to measure the voltage difference can translate voltages in to temperature readings at the hot junction. Each combination of wires required a separate cal ibration and an instrument designated for that combination. The three commonly used thermocouple combinations are (Data from AWS D10.10)
The letter designation applies only to the temperature voltage relationship and not to the material. For ease of understanding it is to be noted that the negative side in all cases is of red colour. Thermocouples are available in various sizes. Faster response is obtained from thinner wires but there are chances that the thinner wires are easily get damaged.
The commonly used thermocouple is having a diameter of 0.81mm (#20AWG). The thermocouple wires are to be in touch at the point where the heat is to be detected. Balance all portions of the thermocouples shall be protected from radiant heat and also from touching each other. Touching each other can be prevented by the sheath provided, usually by the manufacturer. Radiant energy can be prevented by proper protection by ceramic wool. Thermocouple extension wires: Usually the job will be farther from the instrument for recording and controlling the temperature. Thermocouple extension wires are used to connect the thermocouple at the job and the instrument. The thermocouple extension wires will be of same composition of thermocouples or designated to match the thermocouple system. For a J type thermocouple the thermocouple extension wire is designated as JX, for E it will be EX and for K it will be KX. The positive side of the thermocouple to be connected to the posit ive side of thermocouple extension wire and positive side of thermocouple extension wire to be connected to the positive lead of the temperature controlling instrument.Change in leads will cause error in the detected voltage and there by the recorded temperature. The thermocouples are usually connected to the job/material for PW HT by capacitor discharge method. PWHT EQUIPMENT CONNECTION
1. Control panel and Proportional Integral Derivative (PID) controller: Control panel receives the power input from the mains. ELCB/Fuse, over current protection system etc will be in the control panel. PID controller need power for working the same which will be taken f rom control panel. The PID controller contains PID's which can be programmed as per t he requirement of PWHT cycle. Then PID controller takes the input's as follows: Temperature up to soaking temperature. Time for reaching up to soaking temperature (rate of heating)
Soaking temperature Time for soaking. Temperature up to controlled cooling Time to reach that temperature (Rate of cooling) 2. Temperature recorder: Temperature recorder records the temperature against time. This will be recorded against each thermocouple and each graph corresponding to be each thermocouple will be of different colour. The temperature recorder is working on the principle of potentiometer 3. Thermocouples: Thermocouples detects the temperature as voltages based on the seebeck effect or thermoelectric effect. These voltages are used by PID controller to control the temperature and temperature recorder for recording it. 4. Thermocouple extension wires: Thermocouple extension wires are used to connect the thermocouple to the P ID controller and temperature recorder. 5. Coils: Coils heat the joints by resistance heating method. 6. Power cables: Power cables are used for connection between PID controller &Control Panel to the coils for power transmission. Refer the connection diagram. The coils are heated thereby temperature is developed through resistance heating. The current required to the coils are given by control panel through PID controller. The PID controller detects the temperature of the Job through the thermocouple connected to the PID controller and decides whether the current to be allowed to flow through the coils or not. This on and off of current flow through the coil by the feedback of thermocouple helps the process to generate a controlled temperature cycle. One coil can be used to connect more than one joint depends in the length of the coil (the length of the coil is decided by the current load.)
Suppose 2 joints are connected by a coil. One thermocouple from any one joint will be used to control the temperature and is called controlling thermocouple. This thermocouple will be taken from the joint and will be connected to the P ID controller and from PID controller it is connected to the temperature recorder. All other thermocouples from both the joints (on the assumption that two joints are connected using a coil) are directly connected to the temperature recorder. One temperature recorder can detect temperature from 6 points in a 6 point recorder (most common) and temperature 12 points in a 12 point recorder. A PID controller contains 3 PID's (most common design) which will control 3 different coils connected from the PID controller.
POINTS TO BE NOTED General points to be noted in the Job selection for PWHT (Examples based on PWHT of pipe joints.)
1. Using a coil, pipes of different schedules s hould not be connected to PWHT. 2. Using a coil, pipes of different materials should not be heated even though the PWHT cycle is same. 3. The controlling and detecting temperature thermocouple should be taken only from joints only. The thickness of pipe elbow flange etc will slightly different and tacking thermocouple on the parent material (adjacent to the joint) will cause error in temperature detection as more thickness requires more temperature for heating and in the meantime the less thickness portion will be overheated. 4. Thermocouple tacking shall be done by capacitance discharge method to the possible extend. 5. After the removal of thermocouple the thermocouple tacked area shall be visually inspected to see whether cracks or any defects are present.
PORTABLE HARDNESS TESTING
Hardness testing is considered as one of the best tools for judging the quality of heat treatment. For site inspections portable hardness testers are used. One of the popular method of the harness testing is using poldi hardness tester.
Poldi hardness tester is having a standard test bar and a test facility on which hammering is done for indentations. The diameter of indentations in the bar and the parent material are taken and the hardness value HB can be read from the standard chart available with poldi tester. Poldi uses impact method of indentation.
The procedure for using Poldi Hardness Tester is a s below 1. Preparation of the area for hardness testing - The area should be flat 2. Selection of standard Poldi bar - Bar factor nearly 1.00 3. Place the bar in the striker yoke of Poldi tester 4. Position the bar on the area and strike it with medium weight hammer (2kg) to get 3.0 to 4.0 mm indentation on the bar. 5. Measure the actual indentation on the bar and job by microscope of the Poldi set 6. Calculate BHN using the chart available with Poldi Set.
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ENLARGED VIEW OF THE TEST END OF THE POLDI TESTER
