MOC Approach for Open Cooling Water System

7/27/2019 MOC Approach for Open Cooling Water System

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1Essential Expertise for Water, Energy and AirSM

Essential Expertise for Water, Energy and Air

SM

MOC Approach forOpen Cooling Water System

Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]


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Agenda

Mechanical Cooling tower

Heat exchanger

Metallurgy

Operation

pH

Cycle

Chemical Makeup water chemistry



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Essential Expertise for Water, Energy and AirSM

Cooling Tower



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Cooling tower performance provide significantcontribution to the plant performance and efficiency

Poor cooling tower performance could limit the plant

production

Why cooling is important?



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Impact of Cooling Water Temperature

A 3oC (5.4

oF) increase in

approach temperature (on all

stages) is equivalent to a 1%increase in energyconsumption



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Function of Cooling Towers

Remove heat from process operation

Mostly by evaporation (80%)

partially by sensible heat loss (20%) contact of hot water with cooler air


Obt i d f EDMS A d El t i C [P i t d 08 J l 2011]


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Cooling Tower Process

As ambient air is drawn past aflow of water, a small portion of

the water evaporate. The energy

released by water for

evaporation reduces the

remaining water temperature

Evaporation results in saturated

air conditions and lowers the

temperature of the water to a

value close to wet bulb air

temperature


Obtained from EDMS Approved Electronic Copy [Printed 08 Jul 2011]


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Types of Cooling Towers


Obtained from EDMS Approved Electronic Copy [Printed 08 Jul 2011]


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Crossflow Cooling Tower




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Counterflow Cooling Tower




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12

Cooling Tower Fill

Purposes of Tower Fills

Increases the water surface

area for air contact

Increases contact time

Main Types of Fill Splash Fill

Film Fill

Tower Fill

pp py [ ]



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Fill Characteristics

SPLASH FILM

EFFICIENCY Medium High

DURABILITY Medium Low/Medium

FOULING Low High



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Problems with Film Fill

Easily fouled with

microbio and solids

Hard to clean once

fouled

Quick loss of

efficiency

Tower Fill



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Factors Affecting Cooling Tower Performance

Wet bulb temperature

Dry bulb temperature

Plant (heat) load

Cooling waterT

Cooling water flow No of pump running

Air flow

Fan (tip, angle, motor; etc)

Cooling water distribution

Distribution system Fill cleanliness



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Wet Bulb Temperature Measurement

A measurement of the lowest temperature that can

be achieved by evaporative cooling of a water

wetted ventilated surface.

Sling Psychrometer



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Liquid to Gas Ratio

TYPICAL L/G

L/G = 2.0 PERFORMANCE

CURVES

L/G = 1.5

L/G < 1, TOWER OVERSIZED

L/G = 1.0 - TURN OFF FANS OR DOWNSIZE

L/G > 2.5, TOWER UNDERSIZED

AND WILL NOT PROVIDE LOW

APPROACH TEMPERATURES

WET BULB TEMPERATURE, OC

14 16 18 20 22 24 26

27

37

25

29

31

33

35

COLDWELLTEMPERATURE,

OC

12



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Air Flow Issue

18

RO TA T I O N

Air Recycling(Incorrect Tip)

Fan Stall

(Incorrect Pitch)

Air In-leakage

From Fan Shaft

Air flow is a critical component to

ensuring the proper heat

rejection from a cooling system

Many cooling tower performance

issues may well be airflow

problems



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What is the Correct Way toEvaluate Cooling Tower Performance ?

1) CW temperature range (T)

2) Cooling tower heat load

M x Cp x T

3) CW supply temperature

4) Approach temperature

5) Evaporation rate



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Performance Monitoring Cooling Tower

GPSA Nomograph:

(require T& wet bulb

temperatures)

Performance Factor proportional to:

L/G ratio (L = CW flow, G = air flow

both in kg/hr)

RR (or L)

1/(Fan Power)1/3

Performance Factor > design means

performance deteriorated



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Essential Expertise for Water, Energy and AirSM

Heat Exchanger



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22

Why is This Important?

A critical component of the cooling system

Affect efficiency

Fouled exchangers also decrease throughput and capacity

Leaking exchangers can force unscheduled shutdown

Also affect water treatment options

product selection and control limits

Water Distribution System



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Heat ExchangerWhat We Need to Know ?

Type of heat exchangers

Hydraulic, mechanical and

metallurgical

Mechanical stress to cooling water

treatment Performance monitoring



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There are several general types of heat exchangers:

Shell-and-tube

Plate-and-frame

Spiral flow

Type of Heat Exchangers



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Shell and Tube Heat Exchanger (Water in Tube)

Most common heat

exchanger type found

in cooling water

systems

Typically has

adequate velocity

Throttle and spatial

relationship could

create problem



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A shell side heat exchanger offers challenges

Low velocity

High skin temperatures



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Mechanical Stresses

Three main mechanical stress factors that impact theefficiency of heat transfer in heat exchanger are:

1)Cooling Water Velocity

2)Skin Temperature

3)Heat Flux



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Cooling Water Velocity Stress Range

Scale and fouling deposition is more prone to occur at low

velocity

Lower flow rate, lower turbulence that can lead to static

thicker film of water at metal surfaces reduces heat

transfer

Lower velocity = higher skin temperature

> 1 m/s

0.6-1 m/s0.3-0.6 m/s

< 0.3 m/s

Mild Stress Minimal effect of reliability

Moderate Stress Needs to be consideredHigh Stress Will typically be a problem

Severe Stress Typically a problem



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Heat Flux Stress Range

High heat flux will lead to high skin temperature

Typically

v 3.1515.8 kW/ M2 : mild steel

v 15.831.5 kW/ M2 : alloy

v 31.563.1 kW/M2 : copper or stainless steel

< 7.5 kW/m2

7.5-25 kW/m2

25-40 kW/m2

> 40 kW/m2

Moderate Stress Needs to be considered

High Stress Will typically be a problem





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Skin Temperature Stress Range

Reasons:

Corrosion rates ~double for each 10C increase in metaltemperature

Scaling tendencies become much more pronounced at high

temperature

Film boiling may occur as skin temperature

Some treatment chemicals break down at high temperature

< 500

C50 - 600C

60 - 700C

> 700C

High Stress

Severe Stress

Minimal effect of reliabilityNeeds to be considered

Will typically be a problem

Typically a problem

Mild StressModerate Stress



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Other Important Mechanical Factors

1) Spatial Relationship

2) Metallurgy



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Spatial Relationship and CW Distribution

127C

124C

101JC 2002JC

116C

128C

1st

2nd

3rd

101JCoolers

108C 146C115C

Primary Cooling Water Header Tertiary Cooling Water HeaderSecondary Cooling Water Header Cooling Water Return Header



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Metallurgy

Copper alloys

High thermal conductivity

Sensitive to ammonia and oxidant

Need film forming inhibitor

Titanium

Self passivating metal

Extremely brittle with very thin walls

Leaks common (especially during initial

commissioning and startup) ANY mechanical impingement can cause leaks

Stainless Steel

Self passivating metal

Subject to chloride stress corrosion

Must ensure cooling water chloride level

remains within limits

Under-deposit corrosion, MIC issues

Ammonia

Grooving

Under Deposit

Corrosion / MIC

Pitting / MIC



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Chloride and Stainless Steel

PLANT DATA

Duty 3,286,000 Btu/hr

Area 424 ft2

U 91.2Btu/hr.ft2.C

Heat Flux 7750Btu/hr.ft2

Velocity 6.111 ft/s

T CW in 91F

T CW Out 100F

Process In 637F

Process Out 300F

CALCULATION

T Skin 106.34F

41.3C

Max T Skin 146F

62C



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Importance of Heat Exchanger Monitoring

Prevent the loss of cooling

Predict Onset of Problems

Potential Surprises - scaled exchanger

Loss of production

Useful to troubleshoot problems

Methodology using appropriate data

Documentation



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Heat Exchanger Monitoring

Approach Temperature Generally could represent HE performance

Approach = T Process Exit (air) T Cooling Water Inlet(counter current)

Others: C Factor, U Value, Fouling Factor; etc



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Cooling Water OperationWhat We Need to Know ?

Basic mass balance calculation

Cycle management

Operational stress factor



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EvaporationDrift

Windage Windage

Blowdown

LossLeaks

Makeup Cycled Water

Cooling Tower Mass Balance



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Blowdown to Reduce Cycles

500 ml

150 umhos

500 ml150 umhos 500 ml100 umhos 1000 ml125 umhos

1000 ml

150 umhos

Cycles = 125/100

= 1.25 Cycles



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Blow Down and Cycle

Blow down portion of the water ejected or drained from a

system to control solids in cooling water

Cycle number of times that dissolved minerals in cooling

water are allowed to concentrate. Cycle is controlled by

blowdown

Cycle = Dissolved Solids Concentration in Cooling Water

Dissolved Solids Concentration in M-Up

In general, the cycle of concentration allowed depends on both

the level of solids in the make-up and the level of solids that

can be tolerated in cooling water



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Basic Cooling Tower Calculations



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Holding Time Index (HTI)

HTI is indicate the time required to reduce the

chemical added to the system to 50% of its original

concentration

HTI = 0.693 x Holding Volume

Blowdown Rate

It is important to select the right chemical program



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Operational Stresses

Variation of operational and control parameters cause

stress in cooling water systems. The key examples of

operational stresses as follows

1)Variation of cycle

2)Variation of pH

3)Variation of HTI



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Variation of Cycle

80%




Low High Total

17.7% 5.3% 23.0%

% Out of Spec



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Variation of pH

+ 0.1

+ 0.2+ 0.4

+ 0.6




Control Lower Upper

Parameter Spec Spec LCL Mean UCL STD

pH 7.50 8.00 7.16 7.72 8.28 0.19

Statistical Calculations



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Variation of HTI

< 40 hrs40 - 100 hrs

100 - 200 hrs

> 200 hrs



Moderate Stress Needs to be considered

High Stress Will typically be a problem



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Cycle and pH Operating Window



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Cycle Management



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Main Problems in Cooling Water OperationRelated to Water Treatment

There are basically four inter-related problems in the operation of acooling water system as depicted by the Cooling Water Treatment

Triangle below. Each problem affects and is affected by the other

problems.



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Corrosion

A natural process convertingprocessed metals to their

native states

Factors affecting:

- Water chemistry

- Physical environment

(temperature, velocity,

hydrodynamic)

- Dissolved gases

- Halogen or other oxidizers

- Deposit



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Scale

A dense, adherent layer ofminerals tightly bound to

itself and to metal surface

Factors influences

- pH

- Minerals concentration (Ca,

Mg, SiO2, Alkalinity;etc)

- Temperature



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Fouling

Deposits that formed frommaterial suspended in water

(clay, silt, iron, manganese,

microbiological)

Factors affecting

Suspended solids

concentration

Hydraulic and flow velocity

Spatial relationship

Scale, corrosion and

microbiological

Mi bi F li



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Microbio Fouling

The most prevalent problems inindustrial cooling water systems

A good control of microbial

growth is essential if we are to

control the other water

chemistry problems

Factors affecting:

Suspended solids

Contaminants (organics,

ammonia, phosphate; etc)

Physical factor (velocity and

hydrodynamic) pH



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General Methods forCorrosion Inhibition

Use Corrosion Resistant Materials

Apply Inert Barrier or Coating

Use Cathodic Protection

Adjustments to Water Chemistry

Application of Corrosion Inhibitors



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Corrosion Inhibitors Cathodic & Anodic

Combinations of both anodic and cathodic inhibitors generally provide the best

protection.

These combinations are called synergistic because the combination provideslower corrosion rates than either inhibitor could alone,

Cathodic inhibitionPrevents the transfer of electrons

prevent the reduction of oxygen.

prevents the dissolution of the base metal,

iron.

Anodic inhibition

Saturation Index as a Tool for Scale Control



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Saturation Index as a Tool for Scale Control

WATER

CHEMISTRY

pH TEMPERATURE SYSTEMDESIGN

SATURATION INDEX

PROGRAM DESIGN

VARY

CYCLE VARY

pHVARY

TEMPERATURE

VARY

CHEMISTRY

Saturation Index Reference Chart



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Saturation Index Reference Chart

Calcite 6 80 140

Aragonite 7 80 140

Anhydrite 1.2 3 4

Gypsum 1.2 4 6

Tricalcium Phosphate 50 1000 1500

Hydoxyapetite Insufficent Data Insufficent Data Insufficent DataFlourite Insufficent Data Insufficent Data Insufficent Data

Silica 1.1 1.2 1.5

Brucite 1.1 1.2 1.5

Magnesium Silicate 6 7 8

Iron Use of SI not indicative of iron fouling problems

Mineral Scale Mild Stress ModerateStress

High Stress



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Performance Comparison Scale Inhibitors

Capability AMP PSO HEDP PBTC PAPEMP

Corrosion Inhibition 2 5 3 1 1

CaCO3 SI 105 80 100 180 200+

CaSO4 734 483 130 257 574+

Cl2 Stab 2 5 3 5 3

Thermal Stability 3 5 2 5 4

Note:

5 Best, 1 Worst

Nalco Chemical Programs for



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Nalco Chemical Programs forScale and Corrosion

Under-saturated(corrosive)

Over-saturated(Scale)

PSO-Phosphate

Calcium Carbonate(Calcite)Saturation

Stabilized Phosphate

All organic

pHreedom

Low pH

Low AlkalinityHigh environmental impact

High pHHigh Alkalinity

Least environmental impact

Alkaline Zinc

Function of Dispersant



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Function of Dispersant

Prevent deposition commonly found

minerals & inorganic particulates such as

Calcium phosphate

Iron,

Silt

Zinc

Keep corrosion inhibitor soluble To be able function the inhibitors must be

dispersed in the water otherwise localized

deposition will occur


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Mechanism of Chemical Scale & Fouling Inhibitors



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Scale Inhibitor Mechanisms Treshold Inhibition & Crystal Modification

Sequestration

Scale conditioner (dispersant)

Fouling Inhibitor Mechanisms

Dispersant

Surfactants

Calcium Phosphate Crystal




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Microbiological Control



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The goal is not to STERILIZE the system, but

to MANAGE microbial fouling to a level that

minimizes mechanical, operational, and chemical

problems at an acceptable cost.

Microbiological Control

Bio-Control Practices in



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Bio Control Practices inRecirculating Cooling Water

Oxidizers (are usually the primary biocide)

Chemically oxidize organiccomponents of the cell

Effective against nearly allmicroorganisms

Chlorine: as gas or bleach Bromine: as NaBr or other bromine

donors

Chlorine Dioxide




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Recirculating Cooling Water

Non-Oxidizing biocides

Organic compounds, react

with cell components

Disrupt cell wall, metabolism,

or reproduction

Effective to control specificorganisms

Algae control

Biofouling control

Pathogen control

Improved Bio-Control / Bio-

Manage practices

Isothiazoline DBNPA

enyzm enyzm

Glut

amino

Quat




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Recirculating Cooling Water

Biodispersant & Bio-detergentsDisperse deposited particles

Improve the efficacy of biocides

Foaming could limit its application

Before addition of biodetergent

After addition of biodetergent

Other Scale/Fouling Control Methods



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Mechanical

Providing more cooling water flow HE modification

Air rumbling, reverse flow and ball cleaning

Install side stream filter

Operational factor Cycle management

pH control

Summary



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ASUs offers water treatment challenges High skin temperatures

Low flow

Control capability

Water chemistry

Contamination Program limitations

Understanding the MOC enables a complete

understanding of the system stresses that need to be

addressed

Summary



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M knowing the system CT performance, H/E data, flows, temperatures, locations, etc.

Determine potential trouble spots.

Determine chemical program limitations.

O Operational variables

Control is very important for getting performance. Automationis best.

C Chemical variables Scale and corrosion modeling with programs available.

Determine control limits.



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Thank You!

MOC Approach for Open Cooling Water System

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