Steam generator (boiler)

A monotube boiler, a type of steam generator (early 1900s White Motor Company)

Safety Valve

A steam generator is a form of low water-content boiler, similar to a flash steam boiler. The usual construction is as a spiral coil of water-tube, arranged as a single, or monotube, coil. Circulation is once-through and pumped under pressure, as a forced-circulation boiler.^[1] The narrow-tube construction, without any large-diameter drums or tanks, means that they are safe from the effects of explosion,^{[note 1]} even if worked at high pressures.^[2] The pump flowrate is adjustable, according to the quantity of steam required at that time. The burner output is throttled to maintain a constant working temperature. The burner output required varies according to the quantity of water being evaporated: this can be either adjusted by open-loop control according to the pump throughput, or by a closed-loop control to maintain the measured temperature.

They are used as auxiliary boilers on ships.^[3]

Inspection

Steam generators (boilers) have the potential to injure personnel or cause large amount of damage to property. Boiler Inspections are required by law to ensure proper safety mechanisms are functioning to specific standards. A few safety mechanisms inspectors check are the nameplate, relief or safety valve, control safety devices, expansion tanks, and visual inspections.^[4]

Nameplate

Inspectors inspect the nameplate to ultimately determine what safety mechanisms that specific boiler contains. The nameplate also states what year the boiler was made, who certified the boiler, type of boiler, max operating temperature, max operating pressure, input firing rate, output firing rate, and blow off pressure of the relief or safety valve.^[5]

Relief or safety valve

The relief or safety valves function ability is important due to the possible chance of overheating or pressurizing a boiler. Overheating or over pressurizing a boiler can cause a boiler to explode.^[6]

Control safety devices

Another safety mechanisms of boilers is the control safety devices. One control safety device is the low water cut off switch. This device detects the amount of water in the boiler. A low amount of water flowing to a boiler could potentially cause a boiler to overheat ultimately damaging the boiler. The safety mechanism used to control firing rate of boiler is called the operating switch. This operating switch also has the ability to turn the boiler on and off. This switch is inspected to ensure proper functioning ability. Pressure control devices include the operating switch and the high temperature limit switch. These prevent over pressurizing of the boiler by controlling firing rate and providing an automatic shut off if temperature reaches beyond safe operating conditions. Expansion tanks are used in boilers to allow the expansion and contraction of water to travel somewhere.^[7]

Components

The components of boilers are necessary for the safe and efficient functioning of a boiler. A few of those components include a burner, superheater, economizer, and a deaerator.

Burner

A burner mixes fuel and air to a specific ratio and sends it into the boiler. Many different fuels are used in boilers. A few include diesel, coal, oil, wood chips, natural gas, and others. Burners control the efficiency of which the fuel and air and burning along with the control of the flame produced.^[8]

Superheaters

Superheaters heat saturated steam to a temperature above saturation point to ensure dry steam is being produced. Superheaters are necessary for when steam is being discharged to a turbine. Moisture in steam can damage the turbine blades, therefore superheated steam (very dry steam) is necessary.^[9]

Economizers

Economizers improve the overall efficiency of a boiler. Economizers use the exhaust gas from the boiler to transfer heat to a heat exchanger containing water. This water can then be heated and turned into steam to run another turbine. Economizers can also use a heat exchanger to heat the feed water for a boiler. Deaerators purpose is to remove non-condensable gases out of the feed water for boilers. These gases cause corrosion in which can affect the safety of the boiler. The deaerator uses trays in which feed water runs off. Steam is used to lift the non-condensable gases out of the water. Therefore allowing the feed water to be free of non-condensable gases.^[10]

Steam traps

A few different types of traps include thermostatic, mechanical. Traps are designed to capture a desired substance and allow another substance to transfer into its desired location. During operation of a boiler, a trap would be used to capture condensate and allow steam to flow free of any unwanted condensate.^[11]

Thermostatic traps

Thermostatic traps capture condensate from steam by opening the valve only when the condensate has cooled below steam temp. Examples of thermostatic traps include bimetallic and bellows. The bellows trap uses the temperature change in the substance to activate the evaporation or condensation of the alcohol inside the trap.  This evaporation or condensation allows for the valve to close or open, therefore controlling the phase of the substance. Bimetallic traps use two strips of dissimilar metals to close the valve when steam is present. The valve will then reopen once the steam has cooled allowing the condensate to flow.^[12]

Mechanical

Unlike thermostatic traps, mechanical traps use the different density of steam and water to control the position of the valve. Mechanical traps commonly use a bucket or float to correlate the position of the valve. A few different types of traps include float and thermostatic, and open bucket traps. Open bucket traps rise as condensate rises, this allows for the valve to open. Once the bucket starts to fill with water it begins to close. Float and thermostatic traps use a float to correspond to a condensate level. When the condensate level rises, the valve opens. When the condensate level lowers the valve closes.^[13]

Types

Stone-Vapor

One of the best-known designs is the Stone-Vapor.^[3][14] The inner casing of the boiler forms a vertical bell, with an outer airtight cylindrical casing. The oil or gas burner is mounted at the top, above the coils, and facing downwards. The heating element is a single tube, arranged into a number of helical cylinders. The first helices (in the flow direction) are small-diameter tubes, wrapped in large diameter turns. Succeeding turns are coiled inside this and the tube is of progressively increasing diameter, to allow for a constant flow rate as the water evaporates into steam and forms bubbles. The steam outlet is from the final turn at the bottom of the inner helix. The outlet is approximately 90% steam (by mass)^[14] and residual water is separated by passing it through a steam-water separator. The exhaust gases turn upwards and flow over the outside of the bell, usually passing additional helices that are used as an initial feedwater heater.

Clayton

The Clayton steam generator is similar to the Stone-Vapor, but the burner and flow directions are reversed. The heating coil is mounted within a simple cylindrical casing. Rather than helical, cylindrical layers, the Clayton coils are arranged as layers of flat spirals. Water is pumped into the top layers and forced downwards. Again, the tube diameter increases in steps, as evaporation takes place. The final turns form a single closely spaced helical cylinder around the burner as a water-wall furnace and is heated by radiant heat. The steam output is passed through a centrifugal separator and a dry steam quality of 99.5% is claimed.^[1]

References