Steam Boiler: Failure Analysis

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Failure Modes and Effect Analysis (FMEA)

Failure Modes and Effect Analysis (FMEA) is a technique to analyze danger qualitatively that can be used to identify how an equipment, facility or system can be failure and analyze effect that may occur. Analysis of FMEA can be recommendation to enhance reliability and safety of equipment, facility or system. FMEA state failure modes, cause of component can experience failure or damage.

Component that is included in analysis FMEA is component or machine potentially causes function failure from previous maintenance note. FMEA data is obtained based on machine history record and interview with maintainer. Procedures of Failure Modes and Effect Analysis (FMEA) are performed according to following steps:

Determining problem

Determining problem on an equipment, facility or system is carried out by sort parts of equipment, facility or system so the explanation can be more focused.

Review

Review is done on working sheet of FMEA. How to fill working sheet of FMEA is as follow:

a. Item (ID)

In the item column is filled serial number of system which will be analyzed.

b. Identification (component)

In the identification column is filled equipment data which be identified.

c. Function

In the function column is filled data of equipment type, operating configuration and other specified character.

d. Failure Modes

1. Failure mode: it is defined as failure component to apply one of component functions.

2. Failure mechanism: it is possibly generated by failure mode that had been identified.

3. Detection of failure: All possibility of failure mode detection.

e. Effect

1. Local: occurred failure will influence performance of equipment / component.

2. System: occurred failure will influence overall system function.

3. Plant: failure that occurred will influence performance of plant.

f. Safety

Safety features or procedures in the system can eliminate failure that will occur.

g. Action / risk reducing measures

Possibility of action which is performed to prevent seriously effect of failure.

h. Comment

Record other information that cannot be entered in previous column.

Documentation

Documentation is performed by filling working sheet of FMEA to monitor consistently implementation of recommended correction.

Figure 1: Working Sheet of Failure Modes and Effect Analysis (FMEA)

Failure in Superheater Tube

Superheater tube is one of parts in steam boiler. Superheater is used to heat saturated steam from steam drum to be superheated steam. Failure in superheater tubes is rarely caused by corrosion except the condition of steam boiler is being maintaining or stands by in long term. Failure in superheater tubes is often occurs because of overheating either short term overheating or long term overheating.

Overheating is caused by attached deposit in internal tubes which is carried over by flow of bad quality steam which bring suspended solid or dissolved solid (hardness). The hardness can be carried over with steam because separation process of steam-water mixture can not executed perfectly in steam drum. Deposit can be removed by adding chemical injection (phosphate) in steam drum to make hardness be soluble then the next step is discharge it through blowdown process.

Failure in superheater tubes may also occur during bad start up process. If combustion temperature is increased rapidly without doing water and steam balance flow, the superheater will feel overheating in short term because there is not steam flow which has function as cooler. Steam flow through superheater tubes act as cooler because temperature of steam is lower than temperature radiant combustion. So start up procedure shall be established to perform good combustion.

Erosion, Corrosion Water Hammer and Leaks in Economizer

Economizer is one of heat exchanger equipment in steam boiler that absorbs heat from flue gas then transfers it to feedwater. Economizer can help steam boiler to increase efficiency. Some of failure regarding to operation of economizer can be occur because of erosion, corrosion, water hammer and leaks. So, the preventive maintenance should be performed.

Possibility of corrosion can be occur if there is concentration of oxygen either high or low concentration. Corrosion can be prevented by removing oxygen content in feedwater. Deaerator has important role to make feedwater free from oxygen.

Steam boiler which uses coal as fuel combustion will has a lot of fly ash particle. It will impact to metal tube erosion even steam boiler is operated with high excess air and on maximum continuous rating; possibility of erosion is highly occur. If certain amount of fly ash is localized in certain area then forming plugging, this condition will reduce flow area and increase the velocity of flue gas, so potential erosion on economizer tubes will mostly occur and the result economizer tubes will leak and rupture. Preventive maintenance to clean plug and fly ash attached on surface tube must be executed routinely. Rotary soot blower type is suitable applied to this preventive maintenance in economizer.

Another warning regarding to economizer operation is water hammer. At certain condition, level water gauge can not read water level in steam drum correctly, the result is boiler control allow feedwater is not distributed into steam drum for a while. So steam will flow enter and become trapped in economizer. When feedwater is distributed again through economizer although with low pressure, it will impact to water hammer because of steam is encountered immediately with feedwater on certain pressure. To overcome this condition, venting should be installed to make the steam out from economizer or supply feedwater constantly.

Short Term Overheating in Steam Boiler

Steam boiler failure is not only caused by long term overheating but also short term overheating. Short term overheating can occur if temperature of pressure part (tube, drum, pipe and header) increases rapidly in short period exceeding limitation of design temperature. It can occur at the pressure part which has high operating temperature such as superheater, reheater, water wall, roof wall, boiler bank, screen tubes and downcomers and rarely occurs in low operating temperature of pressure part such as air heater and economizer.

When the tube has insufficient cooling from water flow inside and receives excessive heat input from combustion process will lead to short term overheating. To identifying that steam boiler is occurred this condition; operator may identify it with microscopic examination on the failure section tube (see figure 1).

Effort to reduce possibility of short tem overheating should be performed such as ensure feed water before supplied into steam boiler is fulfill the requirement, inspect on area which occur debris accumulation such as U bend-tube, drum and header. Circulation of water, blowdown, corrosion, and all of potential matter which disturb heat transfer should be monitored.

Effect of Scale in Steam Boiler

Water treatment should be performed to feedwater before to be supplied into steam boiler to remove scale content in feedwater. Bad treatment will cause a lot of scale content in feedwater. It can give bad effect in the steam boiler process as follow:

1. Influence amount of heat loss in overall boiler.

Scale which be attached in inner surface of pressure part (tube and header) will block heat transfer by conduction from metal tube or header to water because scale in the middle position between metal and water has role as isolator. So, the blocking heat transfer will impact to amount of heat loss. Practically, 2 mm thickness of scale attached in the surface will make heat loss approximately 2% until 3%.

2. Reduce efficiency in the steam boiler

Higher heat loss will influence lower efficiency of steam boiler because of less heat absorb in the furnace will make excessive fuel consumption and temperature of stack outlet

3. Overheating in boiler tube metal

Water flow in inner tube also has function as cooler because temperature of water is very lower than temperature of firing. If scale occurs in inner surface of tube, cooling function will be reduced, so metal temperature will be higher than temperature design and will impact to overheat. This condition will lead to failure and damage in pressure part.

4. Influence to corrosion forming

The deposit of scale will cause corrosion forming because scale composition can compound with metal tube or header to form corrosion.

Long Term Overheating in Steam Boiler

Long term overheating can occurs in all of pressure part in steam boiler such as water wall tubes, header, superheater, reheater, economizer, steam drum, water drum, roof tubes, screen tubes, generating bank, etc. Long term overheating may affect failure in pressure part. Almost 90% failure can occurs in superheater, wall tubes near with the combustion process, reheater and rarely occurs in economizer and floor tubes.

Long term overheating can caused by deposits attached on inner surface of tubes, thus insulate the tubes from cooling by water resulting in reduced heat transfer and make excessive heat input, so affect subsequent failure. Definition of long term overheating is a condition in which pressure parts have metal temperature higher than temperature design in long term or even exceed many months without doing routine maintenance. The higher temperature will lower the allowable stress in material (table allowable stress material based on temperature can be checked in ASME BPV Section II Part D).

Material which has more alloying element such as molybdenum and chromium will increase maximum allowable stress. Alloy material is often used as superheater and reheater. To protect the tubes from overheating, wall tubes and floor tubes usually use refractory tiles and refractory castable and make sure feed water before supplied into steam boiler is treated well, free from scales and deposits.