Electrolyser Failure analysis report by Titan.pdf

Estd. In 1981. An ISO 9001 2008 Company

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FIELD ASSESSMENT REPORT

ON

DAMAGED SEAWATER ELECTROLYZER

Shuweihat S1 Water and Power Plant, Abu Dhabi, UAE

SCIPCO PO Number: FEP1/503/2011/AB TiTaN Job No: TSVX 001

Assessed by

R. Nagarajan TiTaN, India

G. Murugan TiTaN, India

Fazir T. Subair MTKhoory, U.A.E

Estd. In 1981. An ISO 9001 2008 Company


INDEX

1. Introduction

2. Objective

3. Brief System Description

4. Electrolyser failure

5. Failure Evidences

6. Possibilities for the failure

7. Remedial action to be taken

8. Recommendation

9. Conclusion

FIELD ASSESSMENT REPORT-DAMAGED SEAWATER ELECTROLYZER


1. INTRODUCTION

SCIPCO (Shuweihat CMS International Power Company, U.A.E) who is operating the Shuweihat

S1 Water and power plant along with a seawater based electrochlorination system made a request to

MTKhoory (Mohammed Tayyeb Khoory & Sons, U.A.E) / TiTaN (Titanium Tantalum products Ltd, India)

to assess their recently failed one of the electrolyser which is a part of the electrochlorination system

through service work order no: FEP1/503/2011/AB.

With TiTaNs vast experience on design and manufacturing of various capacity and types of

electrolysers in past 3 decades, TiTaNs Engineers accompanied by MTKhoorys Engineer visited

Shuweihat 1 water and power plant on 09th and 10th of May 2011 and made a detailed study on the

one of the failed electrode assembly (without shell housing) and on the operation of the

electrochlorination plant. According to the field assessment and observation, this report has been

prepared which has covered the findings on the failure, immediate remedial action to be taken and

recommendation as the outcome of the assessment.

This report has been drawn based on the assessment made on the available failed electrode assembly

at SCIPCO workshop with the help of the maintenance team of the SCIPCO and system operating

procedure explained by the operating team.



2. OBJECTIVE The purpose of this assessment and report is to find the possible root cause for the failure of the

electrolyzer and recommend the required remedial actions or suggestions to avoid further failure of the

electrolysers and smooth operation of the plant.

3. BRIEF SYSTEM DESCRIPTION The seawater based electrochlorination plant located at the SCIPCO water and power plant (Shuweihat

S1) is being in operation from 2003 after erection and commissioned by Siemens who is the total

contractor for the power & water plant. Electrochlorination plant has been supplied by Severn Trent

Denora SRL.

Technical Data:

Generation Capacity of the EC plant : 458 Kg/hr

Sodium hypochlorite Production concentration : 1850 mg/l

No. of installed generators : 3 Nos

No. of operating generators : 2 Nos.

No. of electrolyzer per generator : 3 Nos.

Generation capacity per generator : 229 kg/hr

Seawater Flow rate per generator : 123.8 m3/hr

Design DC Current per generator : 7200 A

Design DC Voltage per generator : 140 V

Raw seawater coming from seawater distribution system is filtered at two stages using gravel filter and

activated carbon filter before entering in to the hypochlorite generators. Polyelectrolyte is being dosed

in the raw seawater before entering in to the gravel filter to flocculate the suspended particles present

in the seawater and further it will be filtered and removed by gravel filter. Activated carbon filter is



removing organic compounds, residual chlorine and solid impurities. Both stage filtration is removing

all TSS from seawater size greater than 0.5 mm and up to a filtration level of 20 ppm.

Filtered seawater is entering into sodium hypochlorite generators. The plant consists of three

generators, which are connected hydraulically parallel to each other. Two sodium hypochlorite

generators can be operated at its full load continuously and remaining one is standby.

Each generator is connected with a dedicated transformerrectifier using DC bus bar which will supply

required DC power to electrolysers to generate sodium hypochlorite from seawater by electrolysis

process.

Each generator consists of 3 electrolyzer modules connected hydraulically and electrically series to

each other. After first and second electrolyzer modules, there is a hydrogen separator to remove the

generated hydrogen from the first and second electrolyzers.

Seawater flows from the first to the second and to third electrolyzers via hydrogen separator, which is

in the hydraulic path between the intermediate electrolyzers and most of the generated hydrogen is

separated from sodium hypochlorite + seawater and connected to the hypochlorite outlet header line

which is going to hypochlorite storage tanks.

When DC current is passed through electrolyzers when seawater passed through them the sodium and

the chloride are chemically dissociated and the chloride undergoes an electrolytic change and the

available chlorine is formed at the anode. Water also undergoes an electrolytic change with the result

that hydroxyl ions and hydrogen gas are generated at the cathode. The chlorine produced in this

process is termed as active chlorine. The hypochlorite generators are rated on their output of active

chlorine (Cl). However, in practice, the chlorine and hydroxyl ions do not remain in this form and there

is a consequential chemical reaction resulting in the formation of hypochlorite. It is in this form that

the chlorine is used.



During the above main reaction, series of other side reactions occur in the electrolyzer due to the

other constituents such as Calcium, magnesium, sulphate, carbonates present in the seawater. This will

lead to the generation of calcareous particles such as calcium / magnesium carbonates, hydroxides and

sulphates. These calcareous deposits whose settling velocity more than the seawater velocity inside

the electrolyser will deposit over the electrode assembly which needs periodic cleaning using diluted

HCl acid.

The produced sodium hypochlorite + seawater along with hydrogen collected in the hypo tank.

Hydrogen is diluted using air and vented to atmosphere and hypochlorite is dosed at dosing locations

using suitable hypochlorite dosing pumps.

4.0 ELECTROLYSER FAILURE Though the design capacity of the each sodium hypochlorite generator is 229 Kg/hr, it was not operated

at its full design load of 7200 ADC at any point of time from the date of its commissioning. As per

operating team feedback, so far all three electrolyzer modules are being operated at a load of 4000 A to

6000 A DC only. Moreover, to meet the shockdosing requirement, anyone of the generator is being

operated for short duration (15 mins in every 6 hours interval) and it is being stopped after shock dosing

is completed. Thus peak DC amps load was given to electrolyser in short duration.

After 7 years of operation from the date of commissioning, one electrolyzer out of three from generator

A found to be damaged in the month of April 2011. The damaged electrolyser was removed from the

generator and damaged electrode assembly was replaced with new assembly which was available as

spare. Damaged electrode assembly had been kept at the maintenance work shop for study and same

was dismantled and inspected by TiTaNs engineers with the help of MTKhoory and SCIPCO. From the

inspection and study, following are the observations



1. Bus bars 4. End cathode assembly 2. Base plates 5. Bipolar assembly 3. End anode assembly 6. Electrode studs with Teflon spacers

Visual examination of the dismantled electrode assembly revealed severe damage on the

electrode assembly and its base plates.

Base plates of end anode assembly found to be totally damaged due to heaving heating and

further burning

Coating on the end anodes assembly plates is totally consumed and few plates looked like

bare plates without coating.

In the first anode side bipolar plates as well as many more other bipolar plates, cathodes are

totally damaged and a thick oxide films have been formed over the surface of the plates.

Most of the cathode plates have lost their strength and plate thickness has come down to less

than 0.5 mm from its original thickness of 1.5 mm

In the first cathode side bipolar plates as well many more other bipolar plates, anodes are lost

their coating and got damaged heavily.

Most of the middle bipolar plates are got punctured with more than 5 to 10 holes

Many of the plates in the overall assembly are got short circuited and lost their strength and

coating.

Most of the titanium studs connecting the bipolar plates and PVDF spacers are got damaged.

1

2 3

4

5 6 6 6 5 5 5



5.0 FAILURE EVIDENCES

Burning of PVDF spacers due to burnt particles from end anode assembly, short circuit and high heat generation.

Electrodes short circuit due to scale as well as burning particles



Note : Titanium is a highly reactive metal, combining readily at elevated temperatures with oxygen and hydrogen to form interstitial solid solutions, which can cause embrittlement and cracking in heat affected zones at base plates of welded joints.

Heavy burning happened at the anode end side due to heavy scale build up and hydrogen accumulation. High busbar heat would have initiated the explosion / burning. Heavy thick titanium base plates was burnt in heavy heat.



Tiny pieces from the burnt anode end base plates (top side) have spilled over the electrodes and created the further burning of electrodes

Short circuit of the electrode plates and burning of the PVDF spacers created by the tiny particles from Anode end plate burning.



This is a burnt end anode plate. Coating made on the anode is almost consumed

In the same plate coated is totally gone at the one end which was welded to the anode end base plate and also weld fracture due to chloride residues.



This is end cathode assembly. Copper Bus portions remains good but Cathode plates are lost their strength and properties due to hydrogen embrittlement.



This is one of the first cathode side bipolar plate anode end. Coating is almost consumed from this plate and passive layer formed due to TiO2

This is cathode end of the same plate. Hydrogen embrittlement occurred in the bare titanium plate.

This is one of the first anode side bipolar plate cathode end. Plate has lost its strength and properties due to anode end plate side explosion and further heating.



This is one of the middle bipolar plate which has lost its strength and properties due to hydrogen embrittlement.

All the middle bipolar plates are punctured and plates are totally damaged due to short circuit and burning.



Most of the Titanium studs used in the bipolar assemblies are got damaged and broken



5. POSSIBILITIES FOR THE FAILURE

There have been many possible causes for the failure of the electrolyzer. Some of the failure possibilities have been tabulated as below.

S.No. Occurrence Possible Causes

1. Heavy heating and burning

of Anode end assembly

base plates

1. Heaving scaling build up at the anode end which was not acid cleaned properly.

2. Hydrogen accumulation at the anode end which was not

fully removed from the electrolyzer may be because of obstacle due to heavy scale formation.

3. DC busbar heating which might have initiated the heavy

heating and subsequent burning of the end anode assembly base plates.

2. Short circuit of the

electrodes

1. Tiny pieces from the burnt end anode assembly base plates spilled over the other electrode plates which has formed a short circuit between anodes and cathodes and it initiated burning of the other plates in the assembly.

2. Heavy scaling formation or accumulation of scaling

between anode and cathode plates which was not able to dissolute and remove from the electrodes while acid cleaning is being done. This has caused short circuit on some of the electrodes plates.

3. Uneven Coating

consumption from the end

anode plates and other

intermittent anode plates

1. Localized heavy heating on the anode plates due to heaving heating and burning which has initiated the loss of coating.

2. Due to short circuit intermittent anode plates have lost

their coating.

4. Cathode plates dissolution

and loss of strength

(passive layer formation)

1. Due to short circuit initiated by the scaling and burnt metal particles, cathodes were dissolved and reduction in thickness has occurred.

2. Due to hydrogen penetration (Embrittlement) on the

cathodes, titanium has lost its properties and cannot be used further for electrolysis application.



6. REMEDIAL ACTION TO BE TAKEN

As per above findings and evidences, the electrolyser found to be fully damaged with least chances for

repair. Hence, complete electrode assembly needs to be replaced with new assembly. Moreover, in

generator A, the failed electrolyser has been replaced with new assembly which was available in stock.

Since presently new electrolyser is in operation along with other two old electrolysers, a chance of

further failure is possible in the same generator due to improper load sharing. Hence, it is strongly

recommended to replace the other two old electrolyser assemblies also with new assemblies to ensure

the smooth operation and long running without problems.

5. Color change on the cathode

plates.

Due to heavy heating oxide film has been formed over the surface of the titanium plates, which has reduced the properties of the plate.

6. Electrode deactivation

1.The electrodes undergo a natural consumption of the catalytic material so that, after years of operation, a deactivation of the anodes appears and the electrolyzer voltage increases above normal values. 2. Permanent deterioration caused by scale buildup due to inadequate acid washing over a period or prolonged duration.

7 Bipolar assembly studs damage 1. Calcareous deposits over the studs has eaten away the titanium material due to which thickness and strength of the studs has come down.

2. Due to heavy heating by short circuiting of

electrodes, many of the studs were burnt and broken in to pieces.

8 Burning and damage of PVDF

spacers

Due to shortcircuiting of electrodes.



7. RECOMMENDATIONS:

Based on this joint field assessment by TiTaN / MTKhoory and above findings, following are the

recommendations which may be followed by the operational team which will be helpful to avoid

further failure of electrolyser or any other equipment in the electrochlorination plant.

Condition of the gravel filters and Activated carbon filters needs to be monitored

periodically and ensure the proper periodical back wash and healthiness of the filter

media. This is to avoid entering of any foreign particles or any other dissolved

contaminants in to the generators.

Tightness of the bus bar connection in each electrolyser needs to be checked

periodically.

Temperature of the bus bar end of the each electrolysers needs to be monitored

periodically to ensure that no unnecessary heating while in operation.

DC voltage of the individual electrolysers of each generator needs to be recorded to

ensure the healthiness of the individual electrolysers.

Performance of the each generator needs to be recorded periodically (as per format

given during assessment) to monitor the healthiness of the electrolysers and records to

be maintained.

The monthly performance and acid cleaning report shall be prepared and maintained.

Supply seawater quality shall be checked periodically to ensure there is no abnormal

things in seawater and it is as process requirement specified by OEM.

The frequency and duration of acid cleaning shall be strictly followed as per OEM

recommendations.

Operator should ensure the start up and shut down procedures are followed as per

OEM instructions/manual.(For eg, Gradual increase/decrease the amperage, proper

flushing etc.,)



Presently an unusual operating philosophy is being followed in SCIPCO as electrochlorination plant is

concerned. That means one hypochlorite generator out of three is in operation to meet the continuous

dosing requirement. While shock dosing, one more generator is being taken in to line for additional

hypo generation for short duration (15 mins once in 6 hours). Once shock dosing is completed, one

generator is being stopped. This is an unusual practice compare to general electrochlorination plant

operating procedure. During assessment, it was told that this operating procedure was recommended

by OEM. However being a high capacity plant (7000 DC Amps load per generator), it is not advisable to

start and stop the hypo generator very frequently. This sudden loading in to the electrolyser will spoil

the strength of the coating and reduce the life of the electrolysers. Moreover, this will reduce the life

of the thyristors present in the transformer rectifier unit.

As per global practice, to meet the shock dosing requirement excess hypochlorite has to be generated

and stored in the hypo storage tank and once in 6 hours (or as per design requirement), this excess

hypochlorite has to be shock dosed at the dosing locations. To meet this operating philosophy, only

hypo storage tank shall be adequately sized to hold the excess hypochlorite generated.

With our vast experience in electrochlorination field, TiTaN is strongly recommending to operate one

or two generators continuously depends no of desalination plants in operation and avoid intermittent

operation which is being followed presently. TiTaN can help SCIPCO for this change in operating

procedure if SCIPCO provides all necessary information.

6. CONCLUSION From the field assessment, it is collectively concluded that electrode assembly taken out from

generator AElectrolyser 1 fully got damaged beyond repair due to various reasons as described above.

This needs to be replaced with new assembly.

Documents

Electrolyser Failure analysis report by Titan.pdf