interference testing case histories

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Dyn am ic in ter ference or con t inuous ly vary ing in ma gni tude/d i rect ion) w i ll no t be

addressed in th is paper . However , the cor ros ion cont ro l personnel shou ld be

aw are that i t i s out there and is enc oun tered in the v ic in i ty o f t rans i t systems, in

the v i c in i ty o f m in ing , t e l lu r ic o r geo ma gne t i c sou rces and we ld ing opera t ions .

S T A T I C I N T E R F E R E N C E T E S T I N G

Stat ic s t ray cur rent in ter ference test ing cons is ts o f severa l log ica l s teps that need

to be fo l lowed to ident i fy the source s) , def ine the magni tude o f the prob lem and

mi t i ga te the p rob lem. T he s teps a re as fo l l ow s

Per form a p ipe to so i l survey annual , c lose- in terva l , e tc . ) and determ ine

f rom that survey that an in ter ference cond i t ion ex is ts .

From the survey resu l ts determine the in i t ia l magni tude o f s t ray cur rent

and the po in t o f max im um e xposure d i scharge ) .

De te rm ine the sou rce o f t he s t ray cu r re n t - no rma l l y a p ipe l ine c ross ing a t

the po in t o f max imum d ischarge .

• Per form jo in t tes t ing w i th the com pan y ow ning the s tray cur rent source.

Determine f rom the jo in t tes t resu l ts the best method o f mi t iga t ion for the

prob lem.

• Instal l the m it igat ion system .

• Retest .

Pipe to Soi l Survev

This is necessary to determine i f indeed an in ter ference cond i t ion ex is ts . The

survey w i l l ind icate by vo l tage sh i f ts f rom prev ious surveys or a la rge var ia t ion on

a c lose in terva l survey CIS) . Norma l ly , you wo uld have lowe r nega t ive or

poss ib ly pos i t ive va lues present or h igh negat ive va lues w i th no loca l

c thodic

protect ion CP) system.

Survey Results

Look ing ov e r the da ta can he lp p inpo in t t he p rob lem a rea and / o r sou rce . Us ing

a CIS, the cur rent d ischarge locat ion is normal ly the area o f low negat ive

potent ia l and the cur rent p ick-up locat ion the area o f h igh negat ive potent ia ls . On

an annu a l o r spo t po ten tia l su rvey the a reas o f p i ck -up an d d i scharge may n o t

show up .

Sour c e o f S t r a y C ur r e n t

I f the c ur rent d isch arge locat ion is a t a p ipe l ine cross ing, i t w i l l most l i ke ly be that

p ipe l ine which is the source o f in f luenc ing cur rent . Contact the operator and

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determ ine i f a new CP source has been added to the i r sys tem. The y m ay a lso

know of o ther CP sources in the area. I f the current d ischarge o r p ick up locat ion

is near or inside a faci l i ty ( tank farm, compressor stat ion, pump stat ion, storage

fie ld, etc.) then the source may be more di f f icul t to pinpoint. Again, work wi th the

operator or operators of that faci l i ty .

J o i n t T e s t in a

After the source or sources have b een ident if ied , jo in t or mutua l in ter ference

test ing needs to be per formed. Th is invo lves the current source be ing in ter rupted

on a t imed (on/o f f cycle) bas is and the a f fec ted co mp anies d etermin ing the

ma gni tude o f the e f fec t. The a f fec ted p ipe line s potent ia l w i th the current source

of f a t the po in t o f max imum expos ure is the base l ine potentia l. W i th the current

sourc e on the affected p ipel ine s poten t ia l is the interference potent ia l. Attem pt to

change the interference potent ia l back to the basel ine potent ia l by insert ing a

var iab le res is tor between the two s t ruc tures and a d just the res is tor from low

resistance to a higher resistance unt i l the basel ine potent ia l is achieved. Record

the potent ia ls, the current f low through the resistor and the f inal resistance

sett ing of the variable resistor.

Note : A de qu ate tes t fac i li ties w i l l be n ece ssa ry on bo th s truc tures to prope r ly

perform joint interference test ing.

M i t i a a t i o n S y s t e m

Depen ding on the current requ i rements ,

resist nce

level, etc. of the circuit

severa l m i tigat ion me thods are ava i lab le . Typ ica l l y the fo l lowing method s (but not

l imi ted to) may be used:

• Recoa t ing the p ipe l ines a t the cross ing

• Ins ta l ling ga lvan ic anode s a t the cross ing

• Instal ling a di rect bond betwee n the structures at the cross ing

• Instal ling a CP system on the interfered wi th structure

App ly the mi t igat ion system that i s agreed upon. Mo st operators pre fer to recoat

or instal l galvanic anodes. A di rect bond should be avoided i f possible, as wel l as

a new CP system.

Retest the structures wi th the mi t igat ion system in place and ensure i t is working

as des igned.

Note: The system should be re tested a t some f requency to be determined by the

operators or i f cond i t ions change.

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C A S E H I S TO R Y N U M B E R

Ope ra to r A has 2 coa ted p ipe l ines , one a 20 d iame te r and the second an 18

d iameter . The l ines are wel l p ro tected w i th an impressed cur rent system. These

pipe l ines are or iented in a genera l ly east -west d i rect ion.

Op erator B has one coated p ipe l ine, a 30 d iameter . The l ine is we l l p ro tected

wi th an impressed cur rent system. Th is p ipe l ine is or iented in a genera l ly nor th-

south d i rect ion.

The p ipe l ines are bu r ied in rocky/ loam so i l w i th a fa i rly h igh res is t iv i ty (25,000

ohm-cm) .

Where B 's p ipe l ine crosses A 's p ipe l ines both p ipe l ines are wel l above cr i te r ia

(850 m V.) and no a f fect is ind icated. In the v ic in i ty o f the cross ing, h ow eve r both

p ipe l ines (A & B ) have co a t ing da ma ge tha t has requ i red excava t ion and repa ir .

Good test fac i l i t ies ex is t a t the cross ing, and are adequate enough to per form

ini t ia l inter ference test ing.

Both p ipe l ines A & B para l le l and/or ingress/egress h igh vo l tage AC power l ine

cor r idors in th is area and there is a la rge AC in ter ference component on both

p ipe l ines.

Operator A & B schedule some mutua l in ter ference test ing a t the s i te to

determine what is the cause o f the coat ing fa i lu res. The test ing (AC & DC

potent ia ls) is conducted on the s i te and i t i s determined that the AC is caus ing

the coat ing fa i lu res. Fau l ts , l igh tn ing and o ther surges a long the power l ines and

p ipe l ines a re poss ib le causes fo r coa t ing fa i lu res wh en the AC cu r ren ts

d ischarge o f f the p ipe l ine.

The coat ing fa i lu re is random, the p ipe s tee l has burn or scorch marks on i t (no

dam age to the s tee l) and is t yp ica l o f AC cu r ren t d i scharge as p rev ious ly

observed by bo th p ipe l ine opera to rs .

Fu r the r tes t ing ( tak ing vo l tage mea sureme n ts us ing an osc i l loscope) and

d iscuss ions revea led the fo l low ing:

• P ipe l ine A is para l le l ing a pow er line d i f fe rent f rom the pow er line a long

Pipe l ine B.

• The induced AC vo l tage on Com pany A ' s p ipe l ines and Compa ny B 's

p ipe l ine were p rac t ica l ly 180 degrees o u t o f phase . Th e induced vo l tages

var ied be twe en 5 and 4 5 vo l ts AC .

• Both pow er lines have a h is tory o f fau l ts , l igh tn ing and o ther surges in th is

area.

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With the a bove know ledge of the induced vo ltages_being out o f phase, add i t iona l

tes t ing to determine the approx imate magni tude and leve ls o f the AC vo l tages

and currents was per formed on P ipe lines A & B and the fo l low ing so lu t ions were

recommended :

• Install a z inc grounding cel l at the c rossing of Pipel ine A (both lines) and

Pipel ine B.

• Conn ect the z inc ce ll to each p ipe l ine to a l low the AC current to f low

between the l ines via the cel l .

• Ins ta l l z inc grounding anode beds (10 - 6 0 # z inc anode s a t 10 ' spac ings

1' off the l ine) at the egress and ingress points of Pipel ine A & B wi th the

pow er l ine corr idors to lower the p ipe l ine res is tance and a l low for a be t ter

re turn path for the AC currents .

W hat happe ned in Case H is tory #1?

The z inc grounding ce l l s were ins ta l led, conn ected a nd a l lowed to operate . T he

resu l ts were that the AC cu rrent was ab le to flow through P ipe lines A & B v ia the

ce l l and the d ischarge po in ts were e l iminated. T he AC current was d ischarged

v ia the z inc anod e bed s bac k to the pow er l ine sources.

C A S E H I S T O R Y N U M B E R

Ope rator A has 2 p ipe lines, one a 24" d iam eter p ipe l ine w i th an averag e coat ing

and the second a 36" d iame ter p ipe l ine w i th a very good coat ing. The l ines are

wel l p ro tected w i th a 100 vo l t - 20 am pere im pressed current system.

Operator B has one pipel ine; a 6" diameter instal led bare (no coating). The l ine is

ma rg ina l ly pro tected w i th a d is tr ibuted ( impressed current anode s a t 125 '

spac ings on a common pos i t i ve header cab le) impressed current system.

The pipel ines are buried in rocky/ loam soi l wi th a fa i r ly high resist iv i ty (55,000

ohm-cm).

W here Operator B 's p ipe l ine crosses Ope rator A 's p ipe l ines A is depressed

below thei r cr i ter ia (-850 mV.) and B is depressed, but above thei r cr i ter ia (-850

i n V . .

Adequate test faci l i t ies exist at the crossing.

Ope rator A has a Cathod ic Protect ion (CP) sou rce ( rec t if ie r & remote a node bed)

5-1/2 m i les awa y operat ing a t 95 vo lts and 4 amperes. Ope rator B has a CP

source ab out ½ m i le away operat ing a t 25 vo lts and 65 amperes. Ope rator B 's

CP so urce is in ter rupted and Ope rator A 's potent ia ls come back abo ve cr i te r ia . A

resistor is instal led between the two structures and after adjustment, a current

f low and bond res is tance is ca lcu la ted. See Table .

A d i rec t bond is ins ta lled and f lows 5 .9 am peres f rom A to B and A 's potent ia ls

are st i l l depressed below cr i ter ia at the crossing. B's potent ia ls come up a l i t t le

bit.

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Addi t iona l in ter ference tes t ing is per formed . A ga lvan ic ano de dra in i s

cons idered, but the am ount o f current requ i red (5 .90 ampe res) and the h igh so il

res is t iv i ty prec ludes the use o f ga lvan ic anodes.

The resul t is that a rect i f ier and local ized distr ibuted anode bed is instal led wi th

the rect i f ie r connected to Ope rator A 's p ipe l ine and the d is tr ibuted anod es

insta l led a long Operator B 's p ipe l ine. Th is forced dra in techn ique a l lows the

current be ing p icked up by Operator A f rom O perator B 's CP system to be

returned.

Severa l fac tors requ i red the use o f a forced dra inage bond inc lud ing:

• High dr iv ing vol tage

• Am oun t o f current to be dra ined

• L imi ta t ions o f d i rec t bond

• High soi l resist iv i ty

• Res is tance o f 6 p ipe

Table I

A = =

24

-1 .250

A =..=

36

B - 6 @

24

-0 .760

B - 6 @

36

On Potent ia l 1 .45 0 0 .81 0

Off Potent ia l -0 .810 -0 .920 -0 .910 -0 .990

Current

Bond

Solu t ion

5 .90 amps

Resistance .002 ohms

-1 .890

ra inage

bond on

Dra inage

-1 .650

-1 .650

-1.0101 .310

-1 .780

-1.200

bond of f

-1 .410

Recti f ier

So lu t ion

75 Vol ts

7 amps

Notes Rect i f ie r cyc led 10 secon ds on / 5 seconds o f f

Potent ia l is p ipe-to-soi l potent ia l vs. copper/copper sul fate reference

electrode in vol ts.

Wha t happened i n Case 2?

The d i rec t bond d id not a llow enough current to f low f rom the two coa ted

p ipe l ines to the s ing le ba re p ipe line. Wh y? Severa l fac tors d id not a l low the

current to dra in , a h igher driv ing vo l tage was need ed, the 6 p ipe res is tance was

l imi t ing the amount of current and the high soi l resist iv i ty was l imi t ing the return

path to the local ized crossing.

D I S C U S S I O N

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A go od def in i tion for Stray Current Co rros ion as taken f rom the Ap palach ian

Und erground Corros ion Shor t Course ~ Stray current corros ion is caused by an

ear th path d i rec t current from s om e source externa l to the underground meta l l ic

s t ruc ture which can be p icked up by the s t ruc ture a t one po in t (c reat ing a

cathod ic condi t ion) , f low a long the s t ruc ture for a d is tance, and then d ischarge

into the envi ronment (electrolyte) in order to complete the ci rcui t to the external

source. Wh ere the d ischarge occurs , an anodic condi t ion ex is ts w i th , in some

cases, a very severe corros ive e f fec t on the un derground s t ruc ture .

Stray current corros ion, in con junct ion w i th s ta t i c s t ray current can be qu i te

severe. More p ipe line & fac i li ty operators are add ing new o r add i tiona l cathod ic

protect ion as the pipel ines and faci l i t ies age and current requirements increase.

This is providing addi t ional sources of stray current. However, most pipel ine &

faci l i ty operators take responsibi l i ty for thei r current and are for the most part

concerned w i th s t ray current and where i t goes and how i t re turns. C aus ing a

leak, rup ture or system fai lure at or nea r a pipel ine cro ssing o r faci l i ty locat ion

causes severe prob lem s for a l l par t ies invo lved/a f fec ted and most operators t ry

to avo id caus ing a prob lem. The avo idance o f the prob lem can be done severa l

ways:

• Becom e act ive w i th the loca l Corros ion Coord inat ing Com mi t tee and / or

NAC E sect ion. Th is is a good locat ion to become awa re o f new current

sources being instal led or report ing ones that you have instal led.

• Dur ing the des ign process for the new or add i t iona l CP systems, per form

an as sessm ent o f undergroun d ut il it ies that are in the v ic in i ty and per form

prel iminary interference test ing wi th those ut i l i t ies.

• Ut i li ze sound eng ineer ing judgm ent dur ing the des ign process to min imize

any stray current effects.

S U M M R Y N D C O N C L U S IO N S

The above c ase h is tor ies are actua l examples o f in ter ference tes ts per formed by

the author . For each tes t, la rge amoun ts o f data were taken before dur ing and

after the test ing. Interference test ing is not straight forward. Every interference

s i tuat ion is un ique. Som e s i tuat ions though un ique can be ana lyzed and

mi t igated us ing the wel l known techn iques. For most s ituat ions, exper ience a nd

know ledge w i th in ter ference tes t ing and m i tigat ion is needed to proper ly ana lyze

the interference si tuat ion.

Case 1 conc lus ions:

AC in terference ca n be as or m ore detr imenta l than D C in ter ference.

Addi t iona l in format ion or spec ia l ized tes t ing m ay be ne cessary to m i tigate the

interference condi t ion.

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Cas e 2 conclusions:

The amoun t of current necessary to m itigate interference with bare s ystem s can

be excessive.

Direct bonds are not the b est m etho d of m itigation for bare to coated systems.

Recom mende d reading abou t interference testing is in Reference 2.

R E F E R E N E S

1. Appalachian U nderground Corrosion Short Course, Ba sic T e x t- West

Virginia Universi ty, Morgantown, WV

2. A. W. P eabody, Co ntrol of Pipel ine Corrosion Houston, TX; N AC E, 1967).

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