Answer / subash krishnan

Instruments required:
Multimeter, potentiometer, point position trnasmitter (PPT), Valve Positioner, allen keys 3mm and 4 mm , double-end spanner (12-13), mini screw driver.

1.Initially check the motor-operated valve for open/close manually since the torque set should be accurate inorder for accurate calibration process.

2. Choose a potentiometer a desired based on the number of rounds in which the valve opens or closes. For example if the rounds are maximum choose a potentiometer which has the lowest resitance (1 kilo ohm).

3. Also choose the matching teeth wheel for the chosen potentiometer.

4. Take a point position transmitter (PPT) which is used to give the present position of the motor-operated valve. The PPT is of two different ranges based on resistance (0 to 230 ohms) & (0 to 500 ohms)

5. For every motor-operated valve the close position resistance is set as 100 ohms and the open position is set as 230 (to 500 ohms).

6. The point position transmitter is now given the power supply of 24V DC .

7. Now the resitance for the close position i.e. 100 ohms is given as input and the current should be tuned as 4mA.

8. Now the resitance for the open position i.e. 230 to 500 ohms is given as input and the current should be tuned as 20mA.

9. The steps 7 & 8 are repeated until the PPT gets set to 4 to 20mA.

10.Now the teeth wheel is matched with the matching wheel in the motor-operated valve setup so that the resistance could change in accordance with the valve's open/close operation.

11. The valve is now checked for open/close so that the current for open/close should be in the range of 4-20mA.

12. Now the positoner unit should checked whether it displays 0-100% for the open/close position. Otherwise the positioner display should be tuned for 0-100%

13. There is an actuator tuning adjustment in the positioner unit inorder to do fine calibration for zero and span if there are any minor errors in open/close position.

In control valve there are two type of valve seat we can use metal to metal seat & soft seat.On this valve seat i have following doubt. 1)I have never seen practically the internal assembly of valve so my doubt is that basically weather the seat is soft seat or metal its basic MOC is same as valve body(SS or CS) only in soft seat we are using PTFE ring & place on valve seat & in metal this PTFE ring is not threr am i correct??????? 2)what are the common general rule for using metal to metal seat & soft seat,means its depend on process parameter like liquid,gas, steam & on valve leakage class.

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