Nace CP 1, Course Manual

Page: 471 / 535

RP0200-2000
14 NACE International
The calibration factor is calculated using a modified version of Equation (B3):
A/mV 8.70 =
mV 1.035
A 9.00
= Factor n Calibratio
(B3)
Note: This calibration method is similar to that used to calibrate a typical shunt used in a rectifier.
Check Step 1 using Table B1:
•
Find the Factor for a pipe with a 76.2-cm diameter and 7.92-cm wall thickness in the table.
•
Divide the Factor by the length to find the Calibration Factor. The equation is a modified version of Equation (B4):
A/mV 8.64 =
m 12.2
A/mV/m 105.41
= Factor n Calibratio
(B4)
Because these two calculations are within
±
5%, proceed to Step 2.
Step 2: Measure the voltage with current applied at the upstream end.
Calculate the distance (in percent) from contact to T4 using a modified version of Equation (B6):
Distance (in percent) to contact from T4 =
A 9.00
100% x A/mV 8.70 x mV 1.035
= 100%
(B6)
Check Step 2 Using Table B1:
•
Find the resistance for a pipe with a 76.2-cm diameter and 7.92-cm wall thickness in Table B1.
•
Determine the length from the positive, inside terminal to the short. The resulting equation is a modified version of
Equation (B5):
ohms/m
10
6 -
x 9.48 A x 9.00
10
-3
mV x 1.035
= Length
Length = 12.1 m
(B5)
Because the length from the positive, inside terminal to the short is equal to the entire length of the pipe, this test
indicates that the short is at the downstream end.
Step 3: Measure the voltage with current applied at the downstream end.
Calculate the distance (in percent) to contact from T3 using a modified version of Equation (B7):
Distance (in percent) to contact from 0% =
A 9.00
100% x A/mV 8.70 x mV 0
= T3
(B7)
A potential change due to the presence of current is not observed between T3 and T4, confirming that the metal-to-
metal contact is located at the downstream end of the casing.
Notes:
If a short is located near the middle of the casing and all of the currents are confirmed by the IR drop method, the
location of the short may be determined by finding the percentage of current at either end of the pipe and then
calculating the distance in A/m (A/ft) (as shown in Step 2).
If the currents do not sum algebraically, the results of the testing shall be considered inconclusive.