Phan Cong Thanh

7/25/2019 Phan Cong Thanh

1/6

PETROVIETNAM

45DU KH-S4/2015

1. M u

Phng php chng n mn kim loi bng sn ph

kt hp vi bo v cathode l mt trong nhng bin php

hu hiu nht nhm bo v cc cng trnh ng ng

ngm, y bn b cha v cc cng trnh bin Trn l

thuyt khi h thng bo v cathode c thit k hp l

v hot ng tt, cng trnh ng ng ngm s c

bo v chng n mn trong mt thi gian di. Tuy nhin,

di tc ng ca mi trng v iu kin vn hnh thc

t, h thng bo v cathode c th b xung cp v xy ra

cc s c khng mong mun nh: bong trc lp sn ph,

cc u ni tip xc in km, cp in t, anode hoc

chnh lu c s c y l nguyn nhn dn n in

th bo v khng m bo, s phn b in th khng

u, gy ra hin tng gii phng hydro lm gin kim loi

v bong trc lp sn ph Hu qu l cng trnh khng

c bo v nh thit k, thm ch xy ra trng hp in

th tng bt thng dn n tc n mn kim loi c

th ln hn nhiu ln so vi cng trnh khng c bo

v cathode, c nguy c gy thng ng ng, b cha.

K thut o phn b in th v o chnh lch in

th dc theo chiu di ng ng ngm cho php nh

gi kh nng bo v ca h thng bo v cathode cng

nh mc bong trc lp ph ca cc cng trnh ngm

m khng cn tip cn trc tip. Trong bi bo ny, nhm

tc gi gii thiu nguyn tc hot ng ca k thut o

phn b in th, o chnh lch in th; kt qu ng

dng k thut ny nh gi hin trng thc t ca

tuyn ng ngm ang c bo v chng n mn bng

h thng cathode s dng dng in cng bc.

2. Nguyn tc bo v chng n mn cho cc cng trnh

ng ng ngm s dng h thng bo v cathode

dng dng in ngoi

Qu trnh n mn cc cng trnh ng ng ngm

thng xy ra theo c ch n mn in ha. Do s khngng nht ca kim loi, lp sn ph trn ng ng xut

hin khuyt tt bong trc, s khng ng nht ca mi

trng t dn n s chnh lch in th trn b mt

ca ng ng ngm, gy ra qu trnh n mn cc b.

Phn c in th m hn, ng vai tr anode, b n mn

v ha tan. Phn c in th dng hn, ng vai tr

cathode, khng b n mn. Do , to thnh cc vng n

mn cc b trn b mt ng ng, theo thi gian c th

lm thng ng ng v gy ra cc hu qu kh lng.

Phng php chng n mn kim loi s dng h

thng bo v cathode dng dng in cng bc hot

ng da trn nguyn tc ni ng ng ngm vi cc

m ca ngun in mt chiu (chnh lu/bin p) v ni

cc dng ca ngun in vi in cc anode tr (Hnh 1).

Thng thng, in cc anode tr c ch to t

thp khng g ph mng hn hp oxide titan (Mix metal

oxide - MMO) hoc gang c (high silicon cast iron) bn

n mn trong mi trng kho st cho php pht dng

hiu qu. Vi thit k hp l, in th/dng in cung cp

bi chnh lu thch hp, ng ng ngm ng vai tr

NH GI HIU QUCA HTHNG BO VCATHODE CHO CCCNG TRNH NG NG NGM BNG KTHUT O PHN B

IN TH(CIPS) V O CHNH LCH IN TH(DCVG)

ThS. Phan Cng Thnh, KS. Phm Ngc HiuThS. Trng Quang Trng, PGS.TS. Nguyn Th L Hin

Vin Du kh Vit Nam

Email: [email protected]

Tm tt

K thut o phn b in th (Close interval potential survey - CIPS) c s dng nh gi hiu qu ca h

thng bo v cathode trn ton b chiu di ca ng ng ngm. Theo di s chnh lch in th (Direct current

voltage gradient - DCVG) bng cch o s chnh lch in p ti mt t pha trn ng ng c bo v cathode,

l phng php chnh xc nht xc nh v tr kch thc v khuyt tt lp ph trn ng ng ngm m khng

cn tip cn trc tip. Trong bi bo ny, nhm tc gi gii thiu nguyn tc kt hp k thut o phn b in th vi

o chnh lch in th v kt qu ng dng k thut ny nh gi hiu qu ca h thng bo v cathode cho cc

ng ng ngm.

T kha: Bo v cathode, o phn b in th, o chnh lch in th.


2/6

CNG NGH- CNG TRNH DU KH

46 DU KH-S4/2015

lm cathode, c bo v v khng b n mn. Cc phn

ng c xy ra trn in cc anode v cathode nh sau:

Ti anode tr, phn ng oxy ha nc hoc cc tc

nhn oxy ha trong mi trng theo phng trnh:

H2O => O

2+ 4H++ 4e (1)

Ti cathode (ng ng ngm), thng xy ra phn

ng kh oxy ha tan theo phng trnh:

O2+ 2H

2O + 4e => 4OH - (2)

Tuy nhin, khi in th cathode qu m, c kh nng

xy ra phn ng kh nc to cc bt kh H2trn b mt

kim loi di lp sn ph (phng trnh 3), c th gy

nguy c bong trc lp sn ph.

4H2O + 2e =>H

2+ 2OH- (3)

Theo tiu chun NACE 0176 m bo chng n

mn cho cc cng trnh ngm bng thp, in th bo

v cng trnh phi m hn -850mV/ECS [1]. Tuy nhin,

trnh hin tng bong trc lp sn ph, tiu chun

ISO15589 quy nh in th bo v khng c m

qu -1.200 mV/ECS [2].

Trong qu trnh hot ng, nhiu yu t thay i theo

thi gian nh dn in ca mi trng, s xung cp

ca h thng bo v cathode, nh hng do dng in r

gia cng trnh c bo v v cc cng trnh ph cn...

dn n in th bo v khng m bo nh thit k ban

u. Do , cn thit phi theo di in th bo v cng

trnh ngm theo thi gian, nhm kim tra hiu qu hot

ng ca h thng bo v cathode, m bo cng trnh

ngm c bo v an ton.

3. K thut o phn b in th v o chnh lch in

th dc theo h thng ng ng ngm

Phng php kt hp o phn b in th v o

chnh lch in th cho php theo di in th bo v v

chnh lch in th dc theo h thng ng ng ngm

m khng cn tip cn trc tip. V nguyn tc, k thut

o phn b in th v o chnh lch in th da trn

nguyn tc o in th cng trnh ngm.

3.1. K thut o in th bo v [3]

S dng ng h vn nng hoc vn-k kim tra in

th bo v ng ng ngm theo nguyn tc o hiu

in th gia ng ng ngm v in cc so snh sulfate

ng trong mi trng t nh s mch in Hnh 2.

Khi h thng bo v cathode ang cp dng bo v,

gi tr in th o c hin th trn vn-k (Vm

) chnh l

gi tr hiu in th VON

:

Vm= V

ON= V

p+ IR (4)

Trong :

Vp: in th phn cc ca cng trnh ngm (in th

bo v);

IR: in th ri.

Vi:

I: Cng dng in trong h thng;

R: Tng in tr tip xc v in tr t... ca h thng.

xc nh in th bo v ng ng cn phi gim

in th ri IR

0, khi gi tr in th o Vm

Vp.Trong thc t, ti cc trm kim tra in th (Test

post/Test station), c th s dng 2 cch gim thiu

in th ri IR:

- in cc so snh c t ngay st ng ng

nhm gim thiu ti a in tr t (R0)

- Hoc/v s dng mu th (coupon) c th ngt

dng in tm thi (I0), in th o c ngay ti thi

im ngt dng (Instant off potential) tng ng vi in

th bo v cng trnh ngm.Hnh 2.S o in th bo v trn ng ng

Hnh 1.S nguyn tc h thng bo v cathode s dng dng in ngoi


3/6


4/6


48 DU KH-S4/2015

S dng thit b xc nh ng ng ngm (RD8000

Radiodetections universal precision locator) tm v

nh du v tr ca tuyn ng di mt t. Thit b ngt

dng t ng (GPS - Synchronized Current Interrupters -

M.C.Miller); thit b o in th v lu d liu t ng (Gx

Data-Logger - M.C.Miller) kt ni vi cc in cc so snhsulfate ng cho php o phn b in th bo v v o

chnh lch in th dc theo ng ng.

4.2. Kt qu o phn b in th v o chnh lch in th

o in th bo v ng ng Pipe 01-SA, Pipe 02-SA,

Pipe 03-SA ti cc trm kim tra tng ng TP1-SA, TP2-

SA, TP3-SA cho cc kt qu in th EON

, EOFF

v in th

t nhin (in th n mn) Ecorr ca mu thp (Bng 1).

Kt qu o in th cho thy 3 ng ng kho st

u c gi tr in th bo v m hn -850 mV/CSE, p

ng yu cu v in th bo v. Tuy nhin, gi tr o in

th thu c ch phn nh hiu qu bo v chng n mn

ca h thng bo v cathode cho cc v tr ng ng gn

trm kim tra. kho st in th ca ton b tuyn ng

phi o phn b in th v o chnh lch in th ca 3

ng ng trn (Hnh 9 - 11).

Kt qu kho st cho thy dc theo 3 ng ng kho

st, in th phn b khng ng nht. Cc v tr gn

anode c in th rt m cn cc v tr xa anode c nhiu

v tr in th dng hn -850mV, cha p ng c yu

cu v in th bo v. ng ng Pipe 01-SA in thphn b dc theo ng ng u m hn -850mV, trong

khi cc ng ng Pipe 02-SA v Pipe 03-SA vn c cc

v tr in th dng hn -850mV.

Cc kt qu o chnh lch in th tng thch tt

vi cc kt qu o phn b in th. Dc theo ng ng

ngm, cc v tr ng ng c in th qu m thng

xut hin gn anode, ni tp trung dng in ln. Khi

in th qu m, qu trnh kh nc trong t c th xy

ra trn b mt ng ng, gy ra hin tng gii phng

hydro ko theo hin tng bong trc lp ph, tng ngvi gradient in th ln. Mt khc, nguyn t hydro to

ra c kch thc nh, c kh nng i vo mng li tinh

th lm gin kim loi v to ra cc vt nt ng lc i vi

cc cng trnh ngm. Khi lp ph b bong trc, din tch

TT

Trm kim tra

in th (V/CSE)Nhn xt

EON EOFF ECORR

1 TP1-SA -1,051 -0,867 -0,145 t in th bo v

2 TP2-SA -1,926 -1,035 -0,318 t in th bo v

3 TP3-SA -2,389 -1,205 -0,059 t in th bo v

Bng 1. Kt qu o in th bo v ti cc trm kim tra

Hnh 8.Phn b DCVG vung gc ti v tr lp sn ph khuyt tt

Hnh 7. K thut DCVG vung gc

Hnh 6. Phn b DCVG song song ti v tr lp sn ph khuyt tt


5/6

PETROVIETNAM

49DU KH-S4/2015

hot ha ca ng ng ngm tng ln, do phi cung

cp dng in cao hn c th p ng yu cu ca h

thng bo v cathode.

Trn c s cc kt qu o chnh lch in th c th

la chn c v tr lp ph c kh nng khuyt tt, bong

trc cao nht kim chng thc t. Hnh 12 cho thy lp

ph c khuyt tt, b bong trc v h kim loi nn. Kt quthu c khng nh chnh xc ca k thut o phn

b in th v chnh lch in th.

5. Kt lun

S dng kt hp phng php o phn b in th

v chnh lch in th gip kho st v nh gi nhanh,

chnh xc, hiu qu tnh trng bo v chng n mn ca

h thng bo v cathode i vi ng ng ngm.

Kt qu kho st thc t ti hin trng cho thy bn

Hnh 9. Kt qu o phn b in th (a) v chnh lch in th (b)ca ng ng Pipe 01-SA

Hnh 10.Kt qu o phn b in th (a) v chnh lch in th (b)ca ng ng Pipe 02-SA

Hnh 11.Kt qu o phn b in th (a) v chnh lch in th (b)ca ng ng Pipe 03-SA

Hnh 12.Kt qu kim chng thc t bong trc lp ph

(a)

(a)

(a)

(b)

(b)

(b)


6/6


50 DU KH-S4/2015

cnh vic kim tra thng xuyn in th ti cc trm

kim tra, cn kim tra nh k tng th in th dc

theo h thng ng ng ngm kp thi pht hin

s c (nu c), m bo h thng bo v cathode hot

ng hiu qu, h thng ng ng ngm vn hnh

an ton.K thut o phn b in th v chnh lch in th

c kh nng p dng rng ri nhm kho st v nh

gi hiu qu h thng bo v cathode chng n mn

cho cc cng trnh du kh, c bit i vi cc cng

trnh ngm.

Ti liu tham kho

1. NACE RP0169-2002. Control of external corrosion

on underground or submerged metallic piping systems.

2. ISO 15589-1-2003. Petroleum and natural gas

industries - Cathodic protection of pipeline transportationsystems - Part 1: On land pipelines. www.iso.org.

3. Ronald L.Bianchetti. Survey methods and evaluation

techniques. Peabodys Control of Pipeline Corrosion (2nd

edition). National Association of Corrosion Engineers.

2001; p. 65 - 100.

Summary

The Close Interval Potential Survey (CIPS) technique is targeted at assessing the cathodic protection effectiveness

over the entire length of the pipeline. Direct Current Voltage Gradient (DCVG) survey, based on measuring the voltage

gradients in the soil above a cathodically protected pipeline, is the most accurate method available to size and locate

pipe coating defects without direct access. In this paper, the authors present the principles of combining DCVG and

CIPS techniques for actual assessment of buried pipelines cathodic protection effectiveness and the results obtained

by using this technique.

Key words: Cathodic protection, close interval potential survey, direct current voltage gradient.

Effectiveness assessment of buried pipelines cathodicprotection system by CIPS and DCVG techniques

Phan Cong Thanh, Pham Ngoc HieuTruong Quang Truong, Nguyen Thi Le Hien

Vietnam Petroleum Institute

Phan Cong Thanh

Documents

Transcript of Phan Cong Thanh