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OH Boiler Operator practice questions

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Question 1Boiler Construction, Design, and Fittings

Under ASME code rules, the minimum allowable thickness for a boiler shell plate is generally:

  1. 3/8 in. (10 mm)
  2. 1/2 in. (13 mm)
  3. 1/8 in. (3 mm)
  4. 1/4 in. (6 mm) Correct answer.

Explanation

ASME Section I establishes a minimum shell plate thickness of 1/4 in. This minimum is not set by pressure alone; it provides a margin for corrosion, handling damage, and fabrication.

Question 2Boiler Construction, Design, and Fittings

The primary function of staybolts in a firetube boiler is to:

  1. Direct flue gas flow across the tube bank
  2. Support the weight of the boiler shell on its foundation
  3. Provide a path for heat conduction from the fire to the water
  4. Tie flat stayed surfaces together against internal pressure Correct answer.

Explanation

Staybolts connect two plates, such as a crown sheet to the outer shell, so the pressure load on a flat surface is carried in tension by the bolts instead of bending the plate. Without them a flat stayed surface would bulge and fail.

Question 3Boiler Construction, Design, and Fittings

In the shell thickness formula the symbol E represents joint efficiency. Using a welded seam with full radiography instead of an unradiated seam will:

  1. Reduce the required shell plate thickness Correct answer.
  2. Reduce the maximum allowable working pressure
  3. Have no effect on the required plate thickness
  4. Increase the required plate thickness

Explanation

Joint efficiency appears in the denominator of the thickness formula, so a higher E (full radiography gives the highest value) permits a thinner shell for the same pressure and diameter. The better the proven joint quality, the less thickness is needed.

Question 4Boiler Construction, Design, and Fittings

A steam boiler pressure gauge must have a dial range extending to at least:

  1. The set pressure of the highest safety valve
  2. The normal operating pressure of the plant
  3. Twice the maximum allowable working pressure
  4. At least 1.5 times the MAWP Correct answer.

Explanation

Code requires the gauge scale to read to at least 1.5 times the maximum allowable working pressure so the normal operating point falls near mid-scale and any overpressure remains visible on the dial.

Question 5Boiler Construction, Design, and Fittings

A siphon (pigtail) is installed between the boiler and the pressure gauge to:

  1. Prevent water from freezing in the gauge line
  2. Dampen needle vibration caused by pressure pulsations
  3. Keep steam from overheating the gauge mechanism Correct answer.
  4. Allow the gauge to be isolated for calibration

Explanation

The siphon loop traps a slug of condensate that transmits pressure to the gauge while keeping high-temperature steam away from the bourdon tube and movement. Overheating would distort the tube and destroy calibration.

Question 6Boiler Construction, Design, and Fittings

The water gauge glass on a boiler is connected to the shell at:

  1. One in the steam space and one in the water space Correct answer.
  2. A single connection at the normal water level
  3. Both connections in the steam space of the drum
  4. Both connections in the water space below the tubes

Explanation

The glass must see both the steam space and the water space so the liquid level in the glass equals the level in the boiler. Two connections in the same space would give a false, unusable reading.

Question 7Boiler Construction, Design, and Fittings

The water column attached to a boiler serves mainly to:

  1. Increase the steaming capacity of the boiler
  2. Steady the water level so the glass reading is not disturbed Correct answer.
  3. Remove dissolved solids from the boiler water
  4. Condense steam before it enters the gauge glass

Explanation

A water column is a vertical chamber that quiets the turbulent surface inside the drum and provides a stable reference level for the gauge glass, low-water cutoff, and try cocks. It is also used to blow down the level devices.

Question 8Boiler Construction, Design, and Fittings

The lowest try cock on a boiler is located at:

  1. The top of the shell
  2. The lowest safe water level Correct answer.
  3. The normal operating water level of the boiler
  4. The centerline of the gauge glass

Explanation

Try cocks are set so the lowest one is at the lowest permissible water level; if it blows water the operator knows the level is still safe. Upper cocks bracket the normal operating range.

Question 9Boiler Construction, Design, and Fittings

Steam boilers must be protected by safety valves of which type?

  1. Weighted-lever relief valves
  2. Non-reclosing rupture discs
  3. Spring-loaded pop valves Correct answer.
  4. Thermostatic relief valves

Explanation

Boiler safety valves must be direct spring-loaded pop valves that open fully and snap shut with a defined blowdown. Rupture discs, weighted levers, and temperature devices do not meet the reclosing, capacity, and set-pressure requirements.

Question 10Boiler Construction, Design, and Fittings

The discharge piping from a boiler safety valve must:

  1. Drain into the boiler feedwater line
  2. Be fitted with a shutoff valve for testing purposes
  3. Be reduced in size to increase discharge velocity
  4. Be supported so no strain is imposed on the valve Correct answer.

Explanation

Safety valve discharge piping must be independently supported and arranged with drain openings at low points, so neither weight nor thermal movement is transferred to the valve body. Any strain can misalign the disc and change the popping pressure.

Question 11Boiler Construction, Design, and Fittings

A fusible plug is installed in a firetube boiler to:

  1. Relieve excess pressure if the safety valve fails
  2. Indicate the steam pressure inside the shell
  3. Prevent scale from forming on the crown sheet
  4. Melt and release steam when water level drops Correct answer.

Explanation

The plug is screwed into the crown sheet with its fusible core exposed to the fire. If water drops below the crown sheet, the core melts and steam escapes into the furnace, warning the operator of a low-water condition.

Question 12Boiler Construction, Design, and Fittings

The automatic low-water fuel cutoff on a boiler functions to:

  1. Automatically add feedwater when the level drops
  2. Shut off the fuel when the level becomes unsafe Correct answer.
  3. Sound an alarm only and let the burner keep firing
  4. Reduce the firing rate of the burner gradually

Explanation

The low-water fuel cutoff stops the burner when the water falls to an unsafe level, protecting heated surfaces from overheating. Feedwater makeup is handled by the feedwater regulator, which is a separate control.

Question 13Boiler Construction, Design, and Fittings

A hydrostatic test on a new boiler is conducted at:

  1. 1.25 times the safety valve set pressure
  2. Twice the maximum allowable working pressure of the boiler
  3. The maximum allowable working pressure
  4. 1.5 times the maximum allowable working pressure Correct answer.

Explanation

The standard hydrostatic test pressure is 1.5 times the maximum allowable working pressure, held long enough to inspect all joints for leakage. Safety valves are removed or gagged during the test.

Question 14Boiler Construction, Design, and Fittings

The ASME nameplate on a boiler must show, among other data:

  1. The date of the most recent hydrostatic test
  2. The water treatment program currently in use
  3. Design pressure, heating surface, and year built Correct answer.
  4. The name of the authorized boiler inspector

Explanation

The nameplate is the permanent record of the boiler's code rating, showing the maximum allowable working pressure, heating surface, year built, manufacturer, and the code symbol stamp. Operating and inspection records are kept separately.

Question 15Boiler Construction, Design, and Fittings

Compared with a watertube boiler of the same capacity, a firetube boiler generally:

  1. Reacts more quickly to sudden changes in load
  2. Holds more water and responds slowly to load changes Correct answer.
  3. Needs no safety valve protection
  4. Is built for higher steam pressures

Explanation

A firetube boiler contains a large volume of water surrounding the tubes, so its large thermal mass makes it slow to respond to load swings. Watertube boilers hold less water and can follow load changes faster.

Question 16Boiler Construction, Design, and Fittings

The purpose of a superheater in a boiler is to:

  1. Preheat combustion air using flue gas
  2. Remove moisture from steam before it leaves the drum
  3. Heat the incoming feedwater before it enters the drum
  4. Raise steam temperature above saturation Correct answer.

Explanation

A superheater adds sensible heat to saturated steam so its temperature rises above the saturation temperature for that pressure. Superheated steam improves turbine efficiency and reduces condensation losses in the piping.

Question 17Boiler Construction, Design, and Fittings

An economizer in a boiler installation uses flue gas to:

  1. Heat the combustion air before it enters the furnace
  2. Superheat the steam leaving the drum
  3. Dry the fuel before it is burned
  4. Heat the feedwater before it enters the boiler Correct answer.

Explanation

An economizer recovers heat from the leaving flue gas and transfers it to the feedwater, reducing the fuel needed to bring that water to saturation. It is placed in the gas path ahead of the air preheater.

Question 18Boiler Construction, Design, and Fittings

Ligament efficiency in a stayed flat surface refers to:

  1. Ratio of metal left between holes to the pitch length Correct answer.
  2. The ability of the plate to conduct heat into the water
  3. The spacing of the staybolts measured in inches
  4. The percentage of plate area covered by staybolt heads

Explanation

Ligament efficiency compares the width of solid plate between adjacent stay or tube holes to the full pitch distance between hole centers. It determines how much of the pressure load the remaining metal can carry.

Question 19Boiler Construction, Design, and Fittings

Handholes are provided on a boiler mainly for:

  1. Mounting the pressure gauge and the water column
  2. Adding chemicals to the boiler water
  3. Cleaning and inspecting internal surfaces Correct answer.
  4. Attaching the safety valve to the shell

Explanation

Small oval or circular openings with covers let the operator wash out scale and sludge and visually inspect tube ends and waterside surfaces between major inspections. They are not used for mounting instruments.

Question 20Boiler Construction, Design, and Fittings

A corrugated (Morrison-type) furnace is used in firetube boilers because it:

  1. Stiffens the furnace and allows for expansion Correct answer.
  2. Increases the grate area for burning coal
  3. Eliminates the need for a refractory lining
  4. Allows the use of thinner shell plate

Explanation

The corrugations act like circumferential ribs, greatly raising resistance to external collapsing pressure, and they flex to absorb the differential expansion between the hot furnace and the cooler shell.

Question 21Boiler Construction, Design, and Fittings

A dry pipe installed in the steam space of a firetube boiler is used to:

  1. Collect steam while excluding water droplets Correct answer.
  2. Dry the steam by superheating it
  3. Drain condensate from the steam header
  4. Distribute feedwater evenly across the drum

Explanation

The dry pipe runs along the top of the shell with slots or holes facing upward or sideways so steam is drawn from the driest part of the space. It reduces carryover of moisture into the steam header.

Question 22Boiler Construction, Design, and Fittings

The vent connection on top of a boiler is used to:

  1. Add chemicals to the boiler water
  2. Vent air on fill and break vacuum on drain Correct answer.
  3. Carry steam to the plant header
  4. Act as a backup relief if a safety valve fails

Explanation

An open vent lets air escape as the boiler fills so water can enter, and it allows air in when the boiler is drained so a vacuum cannot collapse the shell. It is always opened before filling or draining.

Question 23Boiler Construction, Design, and Fittings

One end of a boiler is usually left free to move on its supports in order to:

  1. Permit thermal expansion without overstressing the shell Correct answer.
  2. Allow the burner to be aligned with the furnace
  3. Allow the boiler to be slid out for inspection
  4. Make room for the blowdown piping runs

Explanation

A boiler grows in length as it heats up; fixing both ends would develop large stresses in the shell and in the supporting structure. One end is anchored and the other rests on sliding or roller supports.

Question 24Boiler Construction, Design, and Fittings

In a boiler feedwater line, the check valve is normally installed:

  1. Between the feed pump and the feedwater heater
  2. In a bypass line around the feed pump
  3. Between the boiler and the feedwater stop valve Correct answer.
  4. Downstream of the stop valve, away from the boiler

Explanation

The check valve is placed closest to the boiler, with the stop valve outside it, so backflow out of the boiler is blocked even if the stop valve is left open. This arrangement also lets the stop valve be serviced with the check valve holding the boiler.

Question 25Boiler Construction, Design, and Fittings

The ASME code requires that the blowoff valve on a boiler be connected:

  1. Through a tee fitting so that a second gauge glass can be supplied there
  2. Directly to the shell, or to a nipple with no fittings between Correct answer.
  3. To the lowest point of the feedwater supply line to the boiler
  4. At the end of a long pipe run that cools the blowdown before discharge

Explanation

The blowoff valve must attach directly to the boiler shell or to a very short nipple, with no fittings between the shell and the valve. This keeps the blowdown path short, rigid, and free of joints that could plug or fail under the scouring flow.

Question 26Boiler Construction, Design, and Fittings

Staybolts in a firetube boiler are used mainly to

  1. support the boiler tubes inside the shell
  2. tie flat plates together to resist pressure Correct answer.
  3. guide the flue gases along the shell
  4. hold the furnace against the tubesheet

Explanation

Flat surfaces such as tubesheets, crown sheets and side sheets cannot carry internal pressure by their own shape, so staybolts are screwed and riveted through them to hold the plates at a fixed spacing and carry the load in tension. The stayed surface behaves like a series of small supported plates rather than one large unsupported area.

Question 27Boiler Construction, Design, and Fittings

In a watertube boiler, the mud drum is located at the lowest point of the tube bank and serves mainly to

  1. collect sludge and connect tube bottoms Correct answer.
  2. store steam for sudden load demands
  3. support the boiler setting brickwork
  4. superheat the steam leaving the tubes

Explanation

The mud drum is the lower header of the generating bank; it receives the lower ends of the tubes and, because it sits at the lowest point of circulation, precipitates settle in it and are removed by bottom blowdown. Sludge and scale-forming solids are concentrated there rather than in the upper drum.

Question 28Boiler Construction, Design, and Fittings

Refractory material lining a boiler furnace is installed to

  1. increase the heating surface of the tubes
  2. carry the weight of the boiler tubes
  3. seal the shell against steam leaks
  4. protect steel from heat and cut heat loss Correct answer.

Explanation

Refractory shields the pressure parts and the structural steel from radiant furnace heat and insulates the setting so that heat goes into the water rather than out through the casing. It also helps shape the combustion space and keep gases in the furnace long enough to burn.

Question 29Boiler Construction, Design, and Fittings

Tubes are secured in the tubesheets of a firetube boiler by

  1. packing the tubes with graphite rope
  2. welding the tubes into the shell plate
  3. bolting each tube to the tubesheet
  4. rolling and beading the tube ends Correct answer.

Explanation

The tube end is expanded (rolled) into the hole until it is tight against the seat, then the projecting end is beaded or flared over so that it cannot pull out under differential expansion. Both steps are needed because rolling alone gives no mechanical key against longitudinal movement.

Question 30Boiler Construction, Design, and Fittings

The siphon (pigtail) fitted at a boiler pressure gauge is provided to

  1. keep live steam out of the gauge element Correct answer.
  2. dampen needle vibration during firing changes
  3. allow the gauge to be checked while in service
  4. isolate the gauge when the boiler is pressurized

Explanation

The loop of the siphon fills with condensate and holds a water seal, so the Bourdon tube sees water at close to saturation temperature instead of live steam. This protects the tube and keeps the reading steady on a boiler where steam would otherwise overheat the element.

Question 31Boiler Construction, Design, and Fittings

Steam separating equipment inside a boiler drum is used to

  1. raise the temperature of the steam before it leaves
  2. knock entrained water droplets out of the steam Correct answer.
  3. measure the amount of steam being produced
  4. distribute feedwater evenly along the drum

Explanation

Water droplets thrown up from the boiling surface carry dissolved solids with them, so separators, baffles and scrubbers change the direction of the steam and let the heavier droplets drop back into the water. Clean, dry steam is essential to avoid carryover, priming and deposits in the superheater and turbine.

Question 32Boiler Construction, Design, and Fittings

Baffles in a boiler are used mainly to

  1. support the tubes against vibration only
  2. route flue gas across the tubes in passes Correct answer.
  3. hold the refractory in contact with the tubes
  4. direct the flow of feedwater in the drum

Explanation

Baffles turn the gas stream so that it crosses the tube bank several times on its way to the outlet, increasing gas velocity across the tubes and the heat transfer per unit of surface. Their arrangement sets the number of gas passes and the path of the flue gas.

Question 33Boiler Construction, Design, and Fittings

The huddling ring (adjusting ring) on a boiler safety valve is used to

  1. prevent the valve disc from rotating in service
  2. control blowdown and give a crisp pop action Correct answer.
  3. change the pressure at which the valve is set to open
  4. lock the valve so that it cannot be adjusted

Explanation

The huddling ring alters the direction of the escaping steam so that it acts on the disc area, changing the amount of pressure the valve must lose before it reseats. Adjusting it sets the blowdown (the difference between popping and reseating pressure) and sharpens the opening action, but it does not change the set pressure.

Question 34Boiler Construction, Design, and Fittings

Supports are fitted at intervals along a long boiler tube bank in order to

  1. allow free thermal expansion of the tubes
  2. prevent sagging and vibration of the tubes Correct answer.
  3. increase the effective heating surface
  4. hold the tubes clear of the refractory

Explanation

A hot horizontal tube loses strength and will sag between its headers, and the gas flow can set it vibrating; either effect leads to cracking at the seats or to tube-to-tube rubbing. Tube supports and spacers keep the tubes straight, evenly pitched and restrained.

Question 35Boiler Construction, Design, and Fittings

Surface (skim) blowdown is taken from the water level in the drum in order to

  1. lower the drum water level quickly in an emergency
  2. cool the drum after a hydrostatic test
  3. dump sludge from the lowest point of the boiler
  4. remove dissolved solids and scum from the surface Correct answer.

Explanation

Dissolved solids and oily scum concentrate at the water surface and promote foaming and carryover, so a skim or surface blowoff draws from a collector at the normal water level. Bottom blowdown, taken from the lowest point, is the method used to remove settled sludge.

Question 36Boiler Construction, Design, and Fittings

The capacity marked on a boiler safety valve is expressed as

  1. pounds of saturated steam per hour at the set pressure Correct answer.
  2. the weight of steam that passes the valve in one minute
  3. the boiler horsepower the valve is able to control
  4. the volume of steam discharged at 60 degrees F

Explanation

Safety valve capacity is a mass flow rating, stated in pounds of saturated steam per hour that the valve will pass at its set pressure with a specified accumulation. The rating is established by flow tests, so the valve can be matched to the maximum evaporating capacity of the boiler.

Question 37Boiler Construction, Design, and Fittings

An air preheater in a boiler installation uses flue gas to

  1. cool the boiler water ahead of treatment
  2. dry the coal before it enters the furnace
  3. warm the combustion air entering the furnace Correct answer.
  4. heat the feedwater before it reaches the drum

Explanation

The air preheater transfers heat from the leaving flue gas to the incoming combustion air, recovering heat that would otherwise go up the stack and raising flame temperature so less fuel is burned. Feedwater heating is the separate duty of the economizer.

Question 38Boiler Construction, Design, and Fittings

A boiler drum is made from plate with a longitudinal butt weld that receives full radiographic examination. The joint efficiency used in the shell thickness calculation is

  1. 0.80
  2. 0.55
  3. 1.00 Correct answer.
  4. 0.95

Explanation

A butt-welded longitudinal seam that is fully radiographed is credited with a joint efficiency of 1.00, because the weld is accepted as having the full strength of the plate. Lower efficiencies are assigned to welded seams without radiography or with only spot examination, and to riveted seams.

Question 39Boiler Construction, Design, and Fittings

An opening cut in a boiler drum for a nozzle must be reinforced because the opening

  1. removes metal that carries the hoop stress Correct answer.
  2. interferes with the flow of water along the shell
  3. prevents the drum from being radiographed
  4. makes the drum harder to roll during forming

Explanation

Internal pressure puts the shell in circumferential (hoop) tension, and a hole cuts out the metal that was carrying that load, so the stress rises at the edge of the opening. Reinforcement by extra thickness in the nozzle neck, a pad, or both restores the lost strength.

Question 40Boiler Construction, Design, and Fittings

Where a superheater is fitted to a boiler, the superheater safety valve is set so that it opens

  1. before the drum safety valves lift Correct answer.
  2. only after the drum safety valves have lifted
  3. at the same pressure as the drum safety valves
  4. only when the superheater outlet is blocked

Explanation

The superheater safety valve is set at a slightly lower pressure so that it lifts first, guaranteeing a flow of steam through the superheater tubes whenever the drum valves are discharging. Without that flow the superheater would be starved of cooling steam and could overheat.

Question 41Boiler Operation and Water Level Management

What is the primary function of a low-water fuel cutoff on a steam boiler?

  1. To drain sediment from the boiler bottom
  2. To open the safety valve at the pressure set point
  3. To add feedwater when the level rises too high
  4. To stop burner firing at an unsafe low level Correct answer.

Explanation

A low-water fuel cutoff is a safety device that interrupts the burner when the water level falls below a safe point. It protects the heating surfaces from overheating and possible damage. It does not regulate feedwater or relieve pressure.

Question 42Boiler Operation and Water Level Management

When blowing down a gauge glass, what is the correct first step?

  1. Open the upper valve wide, then close the drain
  2. Close the lower glass valve, then open the drain Correct answer.
  3. Open the drain cock fully with both valves open
  4. Close both gauge glass valves, then open the drain

Explanation

Closing the lower valve first and then opening the drain lets steam from the upper connection blow down through the glass. This clears the upper connection and glass before the lower connection is blown down. The sequence prevents a false level reading during the procedure.

Question 43Boiler Operation and Water Level Management

In normal operation, the three-way cock on a gauge glass should be positioned so that the glass is connected to which spaces?

  1. The glass is connected to both steam and water spaces Correct answer.
  2. The glass is connected only to the water column drain
  3. The glass is connected only to the steam space
  4. The glass is isolated from both the steam and water spaces

Explanation

The gauge glass must see both the steam space at the top and the water space at the bottom to show the true drum level. The three-way cock is kept in the normal operating position that keeps both connections open. Isolating either connection gives a false reading.

Question 44Boiler Operation and Water Level Management

During normal steam boiler operation, the water level in the gauge glass should be near what part of the glass?

  1. Near the middle of the visible glass Correct answer.
  2. Below the bottom fitting of the glass
  3. At the very top of the visible glass
  4. Slightly above the top nut of the glass

Explanation

The normal operating level is maintained near the center of the visible gauge glass. This gives the operator room to see both high and low level conditions. Operating consistently at the top or bottom of the glass reduces safety margin.

Question 45Boiler Operation and Water Level Management

After a low-water fuel cutoff trips the burner, what must an operator typically do before restarting?

  1. Raise pressure to the set point and open the feed valve
  2. Blow down the gauge glass and restart immediately
  3. Open the safety valve and wait for the burner to relight
  4. Restore a safe water level and reset the cutoff manually Correct answer.

Explanation

Most low-water fuel cutoffs are manual-reset devices that require the operator to verify a safe water level before resetting. The boiler should not be restarted until the cause of the low level is corrected. Automatic restarting after a low-water trip is not permitted on most steam boilers.

Question 46Boiler Operation and Water Level Management

Which condition most commonly causes foaming in a steam boiler?

  1. A feedwater temperature that is too low
  2. High dissolved solids in the boiler water Correct answer.
  3. Excess air in the combustion chamber
  4. Low steam pressure and a closed main steam stop

Explanation

Foaming is usually caused by high concentrations of dissolved solids, organics, or other contaminants in the boiler water. These substances stabilize bubbles at the water surface and can lead to carryover. Blowdown and water treatment are used to control the condition.

Question 47Boiler Operation and Water Level Management

Priming in a steam boiler is best described as which of the following?

  1. Steam condensing inside the main steam line
  2. Boiler water leaving with the steam Correct answer.
  3. Scale forming on the fire-side tubes
  4. Air entering the boiler through the feed line

Explanation

Priming is the carryover of boiler water with the steam leaving the drum. It can be caused by high water level, foaming, or sudden steam demand. Priming can deposit solids in steam lines and damage equipment.

Question 48Boiler Operation and Water Level Management

Which condition can cause water hammer in a steam header?

  1. A burner firing at a steady low fire rate
  2. A safety valve set slightly below design pressure
  3. Condensate slugs in the steam header Correct answer.
  4. Feedwater entering the boiler at a constant rate

Explanation

Water hammer occurs when slugs of condensate are pushed through steam piping and strike fittings or changes in direction. Steam lines must be properly trapped and drained to prevent condensate accumulation. Water hammer can damage piping and supports.

Question 49Boiler Operation and Water Level Management

A modulating feedwater regulator controls boiler water level by doing which of the following?

  1. Adjusting feed flow as the water level changes Correct answer.
  2. Changing the burner firing rate directly
  3. Opening the safety valve when level is high
  4. Bypassing the low-water cutoff during firing

Explanation

A modulating feedwater regulator senses drum level and adjusts the feedwater valve to match feed flow to steam demand. This maintains a steady level without frequent start-stop operation. It does not control combustion or bypass safety devices.

Question 50Boiler Operation and Water Level Management

Continuous blowdown is used mainly to do what?

  1. Control dissolved solids in boiler water Correct answer.
  2. Cool the gauge glass before testing
  3. Lower the boiler pressure during firing
  4. Remove sludge settled on the crown sheet

Explanation

Continuous blowdown draws water from near the water surface, where dissolved solids concentrate. It is used to maintain boiler water chemistry within limits. Bottom blowdown is used for sludge and sediment removal.