Performance Improvement of Cooling Tower (Minimizing bio growth and Air recirculation)

SPEAKERS:

ZUBAIR TALHA & AHSAN SARFRAZ

Fatima Fertilizer Company Limited

Sadiqabad, Punjab, Pakistan

Contents

Fatima Group Overview and History

Problem Background

Problem Analysis

Corrective Actions

Key Learnings

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Fatima Group-Overview

Our one of the largest business groups in

Pakistan with

diversified businesses

in fertilizers, textiles,

and sugar industries.

Fatima Group has experience in

undertaking large-scale

green-field projects in

Pakistan

Primary business includes manufacture

of compound

fertilizers-annual

capacity 2.1 M Tonnes

Fatima Fertilizer was conceived at a cost of

750 M USD in

December 2003

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Fatima Group-Overview

One of the largest business groups in

Pakistan with

diversified businesses

in fertilizers, textiles,

and sugar industries.

Fatima Group has experience in

undertaking large-scale

green-field projects in

Pakistan

Primary business includes manufacture

of compound

fertilizers-annual

capacity 2.1 M Tonnes

Fatima Fertilizer was conceived at a cost of

750 M USD in

December 2003

4

Production and Safety Statistics

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Fatima Fertilizer achieved ever best Combined TRIR of 0.075 and safe man hours of 22.1 Million at end of this year (Dec 31st 2014).

DuPont Process Safety Management (PSM) model was launched in 2013. Being implemented successfully. Target is to achieve the OSHA Compliant

Site in 2015. Final Products:

Urea 1,500 TPD

Calcium Ammonium Nitrate (CAN) 1,400 TPD

Nitro Phosphate (NP) 1,200 TPD Intermediary Products:

Ammonia 1,500 TPD Nitric Acid (NA) 1,500 TPD

Plant Revamps: Ammonia Plant revamp project is in hand. 1st phase of 1650 TPD will be implemented in Dec 2015; whereas 2nd phase of 1800 TPD will be realized in Dec 2017.

INTRODUCTION-COOLING TOWER

Fatima fertilizer installed and commissioned cooling tower in 2009 designed by SPIG S.p.A (Italy)

Cooling tower is induced draft, counter flow and consists of 12 cells installed in line with each other.

Recirculation rate is 49 000 m3/hr at a design supply temperature of 33 oC. High efficiency film type fills were installed in each cell.

Fig : FFL Cooling Tower

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PROBLEM DESCRIPTION

Underperformance of cooling tower was observed during increased heat load in summer 2011.

As first step, conventional Static distribution nozzles were replaced with ECOJET nozzles. However no noticeable improvement was observed.

Continuous deterioration in performance led to inspection of fills.

Very heavy bio fouling observed on middle layers of fills.

Fig : Fouled film type fills packs.

High air recirculation in cooling tower was also noted in summer. Average difference between ambient and entry wet bulb temperatures on both sides of cooling tower was 1.5 to 3 oC.

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PROBLEM ANALYSIS

Plant was kept in operation with reduced production, Cell by cell online cleaning of fills was also performed. But bio fouling re grown.

Bio fouling could not picked earlier as ATP and CFU were well within range and did not indicated any bio problem.

Besides elevated levels of Nitrates and Nitrites were indicating that excessive bioactivity is present somewhere in the system.

Top and bottom fill layers were inspected various times and found perfectly clear. Need for middle layers inspection was never envisaged, until accidentally discovered during cells inspection.

The primary factors involved for massive bioactivity were;

1. Type of installed fill

2. Nutrients ingress through atmosphere and plant leakages

3. Make up water quality

4. Insufficient dosage of biocide

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PROBLEM ANALYSIS Cont

1. Type of installed fill:

High efficiency, cross fluted film type fills were installed in the cooling tower.

Low water velocities and low flushing rates make Film type fills more prone to fouling.

Some bioreactors also use similar type of fills for nitrification and bio filtration.

For use of film fills in cooling water system, excellent control on micro bioactivity and scaling is required (Clean Bacteria Free Water, not practically possible in fertilizer complexes).

Fills were not compatible with actual water chemistry and environmental condition of plant location.

Fig : Film Fill pack 9

PROBLEM ANALYSIS Cont

2. Nutrients introduction (Atmospheric and plant leakages):

Few Ammonia plant heat exchangers leakages were adding Nitrogen and Hydrogen to cooling water (rich source of food for micro-organisms)

Ingress of Ammonia in cooling water through atmosphere was from plant vents and ponds.

3. Make up water quality:

Make up water to cooling tower was through an open reservoir with a residence time of >2.5 days.

Ground water was pumped to the reservoir through Wells installed at offsite area spread over ~ 22 km.

The conditions were ideal for bacteria growth. Make-up water was providing fresh supplies of healthy bacteria in cooling tower.

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PROBLEM ANALYSIS Cont

4. Insufficient dosage of biocide:

Standard biocide treatment program based upon sodium hypo and Bromine was in place.

Relative to Oxidizing biocide, Two benchmarks are important.

1. System Volume: It indicates system demand. Larger holding time provides sufficient time for microorganisms to decay organic matter.

2. Recirculating rate: It provides information on how fast chlorine will be lost from a system.

System survey with nearby industries indicated that based upon system volume oxidizing biocide dosage was not sufficient.

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PROBLEM ANALYSIS Cont

4. Insufficient dosage of biocide:

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Corrective Actions

After fouling of fills in 2012, a number of hardware modification and operational practices were implemented to keep plant in operation and resolution of problem in subsequent years.

Following hardware changes and operational practices has been implemented;

1.Fills replaced with anti-clogging type

2.Fan stacks height increased

3.Fan blades pitch angle increased

4.Heat exchangers leakages fixed

5.Make up water chlorination started

6.Online monitoring tools put in effective use

7.Thorough inspection regime of tower internal components put in place

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Corrective Actions Cont

1. Replacement of Fills:

Film type fills replaced with Low clog Hybrid splash fills.

Benefit: Fills Bio fouling issue resolved

Drawback: Reduced efficiency (0.6 oC in our case for same fill height in tower)

Film Fill Hybrid Splash Fill

.

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High efficiency with low water velocities

(Increasing fouling potential)

Moderate efficiency with higher water

velocities

Required clean bacteria free water.

Suspended solids

Corrective Actions Cont

2. Replacement of fan stacks:

Existing fan stack of 2.44 m height were replaced with higher ones of 4.2 m to increase air flow through the tower by improving Fan efficiency.

3. Increase in Fan pitch Angles:

Pitch angles of all fans were increased to 13.2o from 12o to further increase air flow and its velocity.(Better plume dispersion and maximum out put from installed motors & gear boxes)

Fig : Plume dispersion at different pitch angles

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4. Replacement of leaking heat exchangers:

System oxidant demand was high but maximum biocide dosage was limited by maximum chlorides limit of 200 ppm in cooling water system to protect SS-304 & 316 metallurgy against stress corrosion cracking.

Syn gas coolers leaking nitrogen & hydrogen replaced with new ones

Synthesis converter effluent coolers leaking ammonia to cooling water attended (one bundle replaced, other bundle fixed)

Methanator effluent cooler leaking nitrogen, hydrogen, methane replaced with new one

After replacement of leaking heat exchangers, ORP level of cooling water return got normal in positive range, Free chlorine started appearing frequently while chlorides also remained within limit.

Corrective Actions Cont

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4. Replacement of leaking heat exchangers:

Corrective Actions Cont

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5. Make up water chlorination:

Chlorination of make up water with free halogen of 0.2 to 0.5 ppm reduced ATP counts in make up water from 4000 to

7. Inspection of tower internal components:

Cooling water temperatures in summer 2013 were still higher and performance of tower was not as intended and forced plant load reduction

An extensive regime was developed for thorough inspection of tower internals in winter season. This included;

1. Establishment of proper showering pattern by cleaning all nozzles.

2. Cleaning of clogged drift eliminators posing unwanted air resistance.

3. Internal inspection and cleaning of cooling water showering headers.

Fig : Internal view of headers before and after cleaning

Corrective Actions Cont

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Key Learning's

Selection of film fill design was not as per actual plant water chemistry and environment conditions.

Replacement of fills with hybrid splash type fills provided less surface area for growth of bioactivity and increased efficacy of biocide on biofilm due to higher flushing rates.

Carefully select type of fills while taking all local environmental/operating conditions into account.

Make up water chlorination provided less microbes to cooling tower.

Resolve problems before they get complex.

Less nutrients contribution after rectification of leaking heat exchangers and online monitoring practices provided good control on bio growth problems.

Leaking heat exchangers in fertilizer plants -> Bio fouling is taking place somewhere. Find it to monitor and control. Plan HXs repair at the earliest possible.

Develop regime of pH, Ammonia & ORP profiling of critical/poor health heat exchangers for early detection of leakages.

Localized bio-dispersant & anti-scalant dosing give breathing space till next opportunity.

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Key Learning's

A cumulative cooling area of one complete cell was recovered by remedial measures. In 2014, plants operated at max loads with 2 000 m3/hr additional circulation with 1.09 oC less cooling water temperature than previous year.

Efforts put in inspection and maintenance of cooling towers pay back well.

However design cooling water supply temperature (33.3 oC) could not be achieved as margin of high air recirculation was not considered in the design (WBT at CT louvers was 1.5 oC higher than ambient WBT).

Always give due consideration to air recirculation impact on WBT while developing CT design basis. Provide good year long data of air directions, DB & WB temperatures and CT layout to CT designer.

Demand him to:

assess air recirculation impact,

base CT design at worst case,

specify ambient and entry WBT separately.

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Thank you for your attention! Any Questions ?

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