February 2006 - Boiler Maintenance
posted by Larry J Smith
Submitted by K L Petrocelly; RPA, CPE, CFM, PMP.
Lowered efficiencies, shoddy controls and substandard operations -- Three strikes and your boilers are out! Aside from the Chief Engineer, the most likely thing to blow up in your power plant is a poorly maintained boiler. Short of an explosion, unscheduled boiler outages during the heating season can close your facilities. At best, the resulting decreases in efficiencies will adversely impact your fuel and operating costs. To avoid these cold realities, the engineers’ credo should be “KEEP IT CLEAN”, “KEEP IT TIGHT”, “KEEP IT OILED”, and “KEEP IT RUNNING”!
BOILER TYPES
Boilers are classified by material composition, manufacturer, design, application, tube configuration, number of passes, furnace type, fuel burned, horsepower rating, heating surface, fluid medium; well, you get the idea. As an inspector years ago, I visited the interior of boilers running the gamut from miniature flue less units used to generate steam for pressing garments in a dry cleaning shop, to 20 story once-through, tri-fuel utility boilers operating in excess of 4500 psi: I even inspected a locomotive or two. And through it all, was manifest a common thread; ... if you don't take care of them, they won't take care of you! The boilers most often associated with commercial and institutional heating plants are the package fire-tube, water-tube and sectional types:
FIRE TUBE boilers are made of steel and manufactured in sizes up to 15,000 pounds of steam per hour with maximum allowable working pressures up to 250 pounds per square inch. Physically, they come in a variety of sizes and configurations and are principally used for heating systems. Low pressure units are limited to 15 psig for steam service and a maximum of 160 psig and 250 degrees Fahrenheit for water service. Their large water storage capacities are useful in controlling the affects of sudden load fluctuations, but considerable time can be lost in bringing them up to operating pressure from a cold start. Heat exchange is accomplished by passing hot gasses through metal tubes which are surrounded by water.
WATER TUBE boilers are made of steel and used when large steam generating capacities and high pressures are required. As a rule, they have better efficiency ratings than do fire tube boilers but cost more to install initially. Heat exchange takes place in the generating tube bank, wherein water is passed through metal tubes which are surrounded by hot gasses from the heat source.
SECTIONAL boilers are most often made of cast iron and used primarily for low pressure applications of 15 psig for steam service and 30 psig for water service. Operating and maintenance costs are much lower than that for fire tube and water tube boilers but initial erection costs can be high since fit up and assembly are usually performed on the premises. Heat is transferred in this unit by directing hot gases between cast iron sections in which water is contained. Though uniquely configured, each utilizes the same principles of heat transfer and, depending on design, can be used for either steam or hot water service, in high or low pressure applications. Each also requires pretty much the same attention to keep them in optimum operating condition.
GAUGING MAINTENANCE NEEDS
The fact that your boiler remains on-line in the dead of winter suggests you've prepared them for operation prior to the start of the heating season. How well they perform will indicate what follow up maintenance they'll require before you shut them down again in the spring. Unless you're blessed with the tri-dundancy to enable scheduled outages for full-blown dismantled inspections (most of us would be scared to death to try it with just two), you'll have to rely on operating indicators to determine the care they need. During operation, maintenance requirements can be ascertained in one of three ways; by, 1) measuring on-line efficiencies, 2) testing operating controls and devices, 3) observing boilers and auxiliaries in-service. Let's have a look at each of them:
Measuring Efficiencies
That's efficiencies (plural). Too often, engineers think of boiler efficiency only in terms of fuel consumption at the burner and/or percent increase in make-up feedwater. The reality is, there are a multitude of areas that are often overlooked which, if properly addressed, could vastly improve overall performance while significantly reducing operating costs. Below are listed four areas where significant improvements might be had and some means for exacting them.
Combustion Efficiency
reduce excess air to the furnace
adjust burners for optimum consumption
convert to more efficient fuels
switch from steam to air atomization
check boiler air casings for leaks
Waterside Efficiency
decrease continuous and/or bottom blowdown
install a blowdown heat recovery system
repair leaks in condensate return lines
preheat make-up feedwater
Heat Transfer Efficiency
clean boiler heating surfaces
discontinue on-off operation
perform daily feedwater analyses
maintain baffles in good repair
Steam System Efficiency
shut off steam traces during mild weather
repair/install insulation where needed
repair and/or replace faulty steam traps
re-stuff valve and pump packing glands
repair leaks in lines and reducing stations
Testing Devices
As important as a boiler's controls and safety devices are, their spontaneous functioning is often taken for granted; often with disastrous results. The most common boiler failure is from overheating due to operating the unit while it is suffering from a "low water" condition (water level in the boiler is below the normal operating level). Truth is, this problem was what led to the development of the automatic controls in the first place. The two controls most closely associated with the prevention of this condition are the feed water regulator, which causes water to be added to the boiler during operation to maintain the normal operating level and the low water fuel cut-off, which shuts the fuel supply off to the burner when the boiler water level drops below a predetermined, safe level. Each of these devices comes in a variety of types and actions. The imperative here isn't that you learn the particular way they work; better you understand why they fail.
No matter how good the design and construction of a control may be, its reliability depends on the maintenance and testing it receives. Failures fall into four categories: (1) installation, (2) mechanical, (3) electrical, or (4) human. For example:
1. Installation: (Mercury switches cannot make or break contact at the proper point if the control is not mounted in a level position.)
2. Mechanical: (A float in a water-control float bowl cannot rise and drop as it was designed to if the float chamber is filled by sediment.)
3. Electrical: (Switches used for making the feed pump circuit or breaking the fuel supply circuit won't function if the circuitry is grounded along its path.)
4. Human: (Failures occur when operators, whether through negligence or lack of orientation, inappropriately close water feed valves or fail to ascertain that there is sufficient water in the boiler before firing it off.)
Both of these important devices should be tested frequently in accordance with the boiler and control manufacturer's recommendations and as required by your inspector. When testing controls on boilers in operation, the following precautions should be taken:
Slow drain tests by shutting off feed supply and blowing down the boilers should not be made when operating at high capacity.
When actual working tests of low-water fuel cutoffs are to be made and interruption of steam supply may affect plant production, make sure all departments concerned are notified that such tests are to be conducted.
Open blowdown valves should never be left unattended. Make certain all blowdown valves are closed tight upon completion of tests.
Boilers should never be blown down below the minimum safe water level. Some water should always be visible in the gage glass.
When making actual working tests of low-water fuel cutoffs, observe proper safety precautions in restoring the fuel supply.
Make certain that the water level is restored to normal and the burner equipment is operating properly before leaving the boiler room.
Basically, there are two ways of testing these controls. One is by the quick drain method and the other, the slow drain method. My recommendation? On low pressure steam boilers, controls should be quick drain tested at least weekly and slow drain tested quarterly. On high pressure steam boilers, they should be quick drain tested daily and slow drain tested monthly. Here's the procedure for each method as exacted on a low water cut-off.
Slow Drain Test - The slow drain test simulates a gradually developing low-water condition and is applicable to all types of low-water fuel cutoff and alarm devices on steam boilers. The test should be made with the burner in operation. Shut off the condensate return and feed supply so that the boiler will not receive any replacement water, and permit the water level to drop. The test is expedited by opening the blowdown valves. The gage glass should be closely watched and the water level noted at the moment of cut off and/or sounding of the alarm. If the device fails to function and shut off the valve at the proper level, close the blowdown valves immediately and restore the water level to normal. Do not operate the boiler unattended until the cause of the malfunction has been corrected. Each cutoff device should be tested independently of the other.
Quick Drain Test - The quick drain test is applicable to low-water fuel cutoff and alarm Devices having actuating elements (float or electrodes) located in a drainable chamber external to the boiler shell. The test consists of blowing down the chamber at a time when the burner is operating. If the device is functional, it should cause the burner to shut off and the alarm to sound. It will also flush the chamber and connections of accumulated sediment. As with the Slow Drain Test whenever dual cutoffs are installed, each device should be tested independently of the other. A quick drain test as described above is not applicable to so-called "built'-in" low water cutoff or alarm devices having the actuating elements (float or electrodes) extending inside the boiler shell. Such tests are not practical on an operating hot water heating boilers, since the systems would have to be drained in order to perform them.
Observing Operations
Much can be learned from the thumps, groans and whistles an operating engineer feels and hears during the course of a watch. Once alerted, operators can often avert catastrophe by the simple turning of a valve or throwing of a switch that had been left in the wrong position. During their work shifts, engineers should continually survey their plants for:
excessive noise/vibration
overheating of system components
broken belts/pulleys
improper pressures/temperatures/levels
inadequate guarding
improper valving
lack of lubrication
misalignment of components
faulty operation of auxiliaries
bulging/blistering of heating surfaces
deformation/discoloration of metal
blockage of inlet/discharge piping
COMMON CAUSES OF FAILURE
There are a lot of ways that your boilers can bite the big one but time has shown that the majority of failures can be classified as bulging, rupture, collapse, cracking, overheating or leakage due to corrosion, erosion, wear or loss of joint or seam integrity. Reviewing the possible causal factors you'll find that most failures can be directly attributed to:
abnormal pressure
abnormal temperature
structural fatigue
ignition of gasses/vapors
external mechanical damage
indirectly traced to:
misapplication
abnormal loading
faulty workmanship
failure of controls
failure of safeties
improper assembly
foreign substances
thermal shock or contributed to by:
failure to perform routine maintenance
untimely removal from service
inadequate maintenance or supervision
insufficient operator training
mis-operation of support devices
inadequate or incomplete inspection
failure to carry out recommendations
THE ROLE OF MAINTENANCE
However automatic the controls or failsafe the interlocks, nothing technological survives this world mechanically that isn't properly maintained; boilers and their auxiliaries included. Boiler maintenance falls into four basic categories - cleaning, calibration, lubrication and parts replacement. Preventive Maintenance efforts should focus on the cleaning of fire and water-sides, testing of controls and overpressure protection devices, calibration of combustion safeguards and renewal of seals and gaskets. Maintenance of electrical devices should consist of checking wiring insulation, insuring proper closure of switching mechanisms, calibration of meter/instrumentation, physical tightening of connections, integrity of grounding and protective devices, and cleanliness of busses and component parts. Mechanical maintenance calls for bearing and gear replacements, cleaning and lubrication of moving parts, cleaning of filters and strainers, calibration of instrumentation and testing of operating controls and safety interlocks. In all cases, maintenance activities should begin with the recommendations outlined in the manufacturers operating instruction manual, and only end when the system is decommissioned.