nalco at orp™ sensor to measure and control corrosion...

Essential Expertise for Water, Energy and AirSMEssential Expertise for Water, Energy and Air

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Essential Expertise for Water, Energy and Air SM

Presented by: Philip Dixon

October 2011

Nalco AT ORP™ Sensor to Measure and

Control Corrosion Stress and Corrosivity of

Boilers Feed water Systems


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DETECT DETERMINE DELIVER

CONTROL OF CORROSION AND CORROSION PRODUCT TRANSPORT IN BOILERS CONDENSATE AND FEED-WATER SYSTEMS IS CRITICAL TO MAINTAINING RELIABLE, EFFICIENT OPERATIONS.

HIGH CORROSION RATES CAN LEAD TO A VARIETY OF SYSTEM PROBLEMS, WHICH INCLUDE LOCALIZED CORROSION AND FAILURE OF FEED-WATER HEATER TUBES, BOILER TUBE CORROSION AND FAILURES, BOILER DEPOSITS AND FREQUENT CLEANING


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• Control of the corrosive environment through the use of passivator is a widely accepted practice and a part of the EPRI recommendation is to utilize oxidation-reduction potential (ORP) for passivator control in AVT-R programs.

• However, the use of low-temperature ORP for monitoring and control of redox stress in the feed-water system has been quite problematic for most users as the low-temperature ORP systems cannot detect the true reduction/oxidation stress that is found under operating temperatures in feed-water systems.


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• There have been new developments in automation technology for boiler feed water treatment monitoring and control to help meet these challenges.

• This paper discusses Nalco’s best practice for boiler feed water treatment automation technology using AT ORP probe, with the 3DTrasar boiler technology.


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• Traditional FW REDOX stress management philosophy is to:

• Feed an oxygen scavenger/reductent at a constant feed rate to the FW.

• Analyze FW or boiler for residual levels of scavenger perhaps once per shift

• Scavenger feed rate is adjusted accordingly to maintain a desired residual amount in the water.


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• The drawback to these REDOX stress management control scenarios is that they are incapable of true system REDOX stress control, because REDOX stress is not being measured and systems themselves rarely operate at steady state.

• To address the need for improved monitoring and control of boiler FW REDOX stress events, Nalco Company in 2000 began a research effort to develop a technology that could measure the actual FW REDOX stress at the temperatures and pressures of the operating boiler systems.


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• After 10 years extensive R&D effort we developed an

innovative @T ORP sensor technology that measures the

net oxidation-reduction potential (ORP) of the water at

actual FW system temperatures and pressures

• The @T ORP provides a quantum improvement in FW

REDOX stress management greatly improving preboiler

corrosion control with continuous, real-time monitoring

and diagnostics

• Measuring and reacting to the changing corrosivity of the

system at actual operating conditions. The @T ORP

technology automatically feeds chemicals on demand

based on true at-temperature and pressure REDOX

changes.


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• Nalco @T ORP provides a quantum improvement (and paradigm shift) in FW REDOX stress management by:

greatly improving preboiler corrosion control

with continuous, real-time monitoring and diagnostics

by measuring and reacting to the changing corrosivity of the system at actual operating conditions.

The @T ORP technology automatically feeds chemicals on demand based on true at-temperature and pressure REDOX changes.


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What is ORP?

Oxidation Reduction Potential (REDOX)

[DO] ORP

[Scavenger] ORP

Solution

Voltage

Noble Metal Reference ElectrodeAg + Cl- AgCl + é

EPBRE vs sat KClExternal Pressure Balanced Reference Electrode

Type

Temp.


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WHAT DOES AT ORP DO?

• The AT ORP program feeds reductant to maintain the system in an AT ORP control zone

• It responds to AT ORP stress, which correlates with system corrosion events

• The AT ORP probe responds quickly and with the appropriate magnitude / sensitivity to the actual redox stress event

• Action is taken immediately 24/7 to resolve the issue within the MOC constraints of the plant

• Chemistry is fed on demand

NCSM with AT ORP technology: Provides a paradigm shift in corrosion stress measurement, understanding and control

ORP Cell Internal Configuration

Active PlatinumElectrode

Reference ElectrodeCeramicJunction

3/8” Stainless Steel ORP Cellwith 1/4” Reducers

Water In

Water Out

ORP Cell Internal Configuration

Active PlatinumElectrode

Reference ElectrodeCeramicJunction

3/8” Stainless Steel ORP Cellwith 1/4” Reducers

Water In

Water Out



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NCSM – A Custom-Built ORP

Analyzer

• ORP Oxidation Reduction Potential

• Considerations:

– Corrosion is linked to REDOX potential

– REDOX potential is indicated by ORP voltage (mV)

– ORP voltage can be used to assess bulk FW

corrosivity

• FACT: Reducing conditions, as indicated by more negative ORP values, tend to result in

lower corrosion. Oxidizing conditions,

similarly, tend to be more corrosive.

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NCSM: How does it work?

External Pressure Balanced

Reference Electrode

(EPBRE)

Inert Metal (Pt)

Bulk

Water

Voltage

External Pressure Balanced

Reference Electrode

(EPBRE)

Inert Metal (Pt)

Bulk

Water

Voltage

Inert Metal (Pt)

Bulk

Water

Voltage

The NCSM compares a reference electrode

(EPBRE) to an inert, platinum electrode.

The inert electrode does not participate in any

corrosion reactions , unlike a corrator probe.



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ORP indicates the potential for

water to corrode

Corrosion = REDOX Reactions

REDOX Reactions = Electron Flow

Electron Flow = ORP (mV) (Oxidation

Reduction Potential)

ORP = bulk FW corrosivity



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Reducing conditions

minimize corrosion

(more negative ORP)

-600

-400

-200

0

200

400

0.1 1 10 100 1000

OR

P (

mV

) 400F

, 204C

Dissolved Oxygen (ppb)

Oxidizing

Reducing

Mo

re R

ed

ucin

g

-600

-400

-200

0

200

400

0.1 1 10 100 1000

OR

P (

mV

) 400F

, 204C

Dissolved Oxygen (ppb)

Oxidizing

Reducing

Mo

re R

ed

ucin

g



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How does NCSM

control corrosion

stress?

• Detects REDOX stress in a boiler feed water

system (ORP at operating T & P)

• Determines correct response to REDOX stress

• Delivers the correct amount of scavenger to

minimize the corrosivity of boiler feed water



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Oxygen Corrosion Control

• Mechanical Deaeration

primary means of O2 removal

• Chemical Oxygen Scavenging

removal of trace amounts of O2 remaining

after deaeration

• Maintain Reduced Conditions

minimize corrosivity

promote passivation



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MU water flow variations and

their effect on REDOX stress

-400

-300

-200

-100

0

0

100

200

300

400

8/2

3/2

00

7

8/2

4/2

00

7

8/2

5/2

00

7

8/2

6/2

00

7

8/2

7/2

00

7

BF

W M

U to

V-9

73 (k

lbs/h

r)

AT

OR

P v

s E

BP

RE

(m

V)



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18

-600

-500

-400

-300

-200

-100

0

100

0 100 200 300 400 500 600 700 800

AT

OR

P v

s E

PB

RE

(2

44F

) -

mV

Oxygen Scavenging Equivalents Fed (ppb)

AT ORP on Scavenger addition(Deaerated DIW: DO

i = 6 ppb)

Plot 2 AT ORP_1.qpc

Scavengers AT ORP performance

is a function of DO scavenging and

specific reductant type within all

PBS variables


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Example of NCSM operating ranges for

sulfite & carbon steel system (400F)

NCSM (mV)

+300

+200

+100

0

-100

-200

-300

-400

-500

-600

Good

Deaerator

4-7

30-50

Dissolved Oxygen (ppb DO)

>100

0.8 mpy

0.2 mpy

0.5 mpy

2500

Sulfite residual(ppb DO scavenging

equivalents)

0

60

30

Pitting Attack

Confidence Line


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Current Oxygen Corrosion

Monitoring

• Check mechanical deaeration

Plume

Dome & Storage Section Temperature Differential

• Monitor & Control product residual in BW

• Oxygen Testing

Online DO Analyzer

Chemets

• Total Fe & Fe+2 Testing



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Corrosion & @T ORP

Correlations

• Corrosion

Higher, more positive @T ORP

Soluble species such as Fe2O3

• Lower Corrosion

Lower @T ORP

Soluble species such as Fe2O3 minimized

Low oxygen, reduced state

• Lowest Corrosion - Passivation

Lower @T ORP with higher pH

Solid phases such as Fe3O4

Solid, protective oxide film inhibits further metal dissolution

Corrosion will be minimized



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Example: Engineering Alloys and

Corrosion Potentials

0

2

4

6

8

10

12

14

16

-0.7

-0.6

-0.5

-0.4

-0.3

-700 -600 -500 -400 -300 -200 -100 0

Dis

solv

ed O

xy

gen

(p

pb

)

EC

orr C

arb

on

Steel v

s EP

BR

E(T

) - V

High T ORP vs EPBRE(400oF) - mV

In this case notice how the corrosion potential of carbon steel declines as oxygen is removed from the system with the oxygen scavenger erythorbic acid. The data is plotted with the @T ORP data at 400F.



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Case Study #1

Gulf Coast Refinery

• Utilities Unit

NaZ MU

600# Boilers


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3D TRASAR Pre-Boiler Corrosion Control

with the NCSM (AT ORPTM)

Begin Control Mode

Optimized Control

Excursions still happen


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Dissolved Oxygen

Before & After Automatic Control

Begin Control Mode Optimized Control

Excursions still happen


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Dissolved Oxygen

Before & After Automatic Control


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Summary

• Poor operating Graver unit results in excessively high FW DO levels which cause severe corrosion stress conditions

• NCSM AT ORP detects & responds immediately to changes in BFW REDOX/corrosion stress conditions caused by poor operating DA

• Manual feed of scavenger provides poor BFW corrosion stress control

• The use of AT ORP to automatically control feed of EliminOx significantly reduces the DO & corrosion stresses

• Feeding EliminOx instead of sulfite reduces the overall solids loading on the boiler

• Opex Reduction: chemical oxygen scavenger spend ~ $75K

• Net Result – Improved Reliability Decreased potential for pitting corrosion

Decreased potential for super heater tube fouling


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Case Study #3

West Coast Refinery

#600 WHBs at Isomerization Unit

RO & Condensate MU


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System Layout


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Results

-400

-300

-200

-100

0

100

0

5

10

15

20

25

30

AT ORP and DO for Monitoring and Control TestsA

T O

RP

vs

EP

BR

E (

mV

)

LP

BF

W D

O (p

pb

)

70 Days

Monitoring

Mode

Control Tests

10, 12, 13

Control

Tests

3, 4, 8

Unit Series AT ORP and DO_1.qpc

Control Tests –Tuning PID

Control Mode

Monitoring Mode


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0

10

20

30

40

50

60

70

7/3

0/2

00

7

8/3

0/2

00

7

9/3

0/2

00

7

10

/30

/20

07

11

/30

/20

07

12

/30

/20

07

1/3

0/2

00

8

3/1

/20

08

3/3

0/2

00

8

4/3

0/2

00

8

5/3

0/2

00

8

6/3

0/2

00

8

7/3

0/2

00

8

8/3

0/2

00

8

9/3

0/2

00

8

10

/30

/20

08

11

/30

/20

08

12

/30

/20

08

Scav

en

ger

pro

du

ct

use

d,

GP

D (

V-9

73

)

Scavenger ActivesNalco ChemistryDate RangePhase

CarbohydrazideELIMIN-OX8/21/07 - 1/3/081

Catalyzed bisulfiteN17201/3/08 - 2/12/082

Diethylhydroxylamine and hydroquinoneCNQR34752/12/08 - 3/30/083



Catalyzed bisulfiteN17207/31/08 - present4

O2 Scavenger Usage

0

10

20

30

40

50

60

70

7/3

0/2

007

8/3

0/2

007

9/3

0/2

007

10

/30

/20

07

11

/30

/20

07

12

/30

/20

07

1/3

0/2

008

3/1

/20

08

3/3

0/2

008

4/3

0/2

008

5/3

0/2

008

6/3

0/2

008

7/3

0/2

008

8/3

0/2

008

9/3

0/2

008

10

/30

/20

08

11

/30

/20

08

12

/30

/20

08

Dis

solv

ed O

xy

gen

- C

hem

ets

(pp

b) (

V-9

73)

Scavenger ActivesNalco ChemistryDate RangePhase







Dissolved Oxygen

Results- Grab Samples

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

7/3

0/2

00

7

8/3

0/2

00

7

9/3

0/2

00

7

10

/30

/20

07

11

/30

/20

07

12

/30

/20

07

1/3

0/2

00

8

3/1

/20

08

3/3

0/2

00

8

4/3

0/2

00

8

5/3

0/2

00

8

6/3

0/2

00

8

7/3

0/2

00

8

8/3

0/2

00

8

9/3

0/2

00

8

10

/30

/20

08

11

/30

/20

08

12

/30

/20

08

Total IronFe (II) Low Level

Iro

n -

pp

m (

V-9

73

)Scavenger ActivesNalco ChemistryDate RangePhase







Iron


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Results

-400

-300

-200

-100

0

100

Statistical Data for Sulfite Scavenger

Median

Min.

All Percentile Plots.qpc

AT

OR

P v

s E

PB

RE

(m

V)

MedianMedian

Min.Min.

Max.

Max.

Max.

Monitoring

Mode

Control Tests

10, 12, 13

Control Tests

3, 4, 8Monitoring

ModeControl Tests –

Tuning PIDControl Mode

Best System Specific Scavenger found & utilized

Determined Optimum AT ORP Zone

Ability to Stay in Optimum ORP Zone

Minimize Pre-Boiler Corrosion


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3D TRASAR Corrosion Control

3D TRASAR Boiler Automation with AT ORPhelped the refinery:

• Understand the system stresses

• Diagnose & troubleshoot during upset conditions

• Determine the optimum REDOX operating zone to minimize pre-boiler corrosion potential

• Pro-actively feed the optimum amount of chemical regardless of system stresses

Improve Boiler System Reliability!


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Traditional feed/controlbefore NCSM After NCSM control

ORP measurements made with the NCSM correlate tightly with measurements made with a particle counter.

NCSM-based control of scavenger feed delivers less variability and less corrosion product generation.

Less corrosion, less corrosion product

Particle Counter

ORP


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Understanding the benefits of

AT-ORP™ - We Expect:

• The deferral of feed water heater replacements

• Cleaner boilers, less tube damage and lowered cleaning costs

• Cleaner superheaters, reheaters and turbines with reduced

repairs

• Better plant availability (e.g. thermal loading limited on startup)

and longer plant life

• Better recognition of and response to feed water system

mechanical and chemistry problems

• Reduced treatment chemical wastage

• Continued growth in the understanding of our systems

• Potential savings of millions of dollars each year and millions

more in unit life extension

•



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nalco at orp™ sensor to measure and control corrosion...

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