Question of the Month – 2018

Question of the Month for November 2018

QUESTION:

What is meant by Stoichiometric Combustion?

 

ANSWER:

Stoichiometric Combustion is the perfect combination of air and fuel that result in perfect combustion! Sounds good doesn’t it? Unfortunately, it is impossible to achieve in burners that you would commonly find on a boiler.

So why toss this term about? For the simple reason that it gives us a target in which we might compare our combustion conditions against. For example if we supply too little air, the burner will run “rich”. This means that not all the fuel was burned. Not only is this inefficient it also results in sooting that will decrease the heat transfer in the boiler.

Introduce too much air into the process and again, you reduce efficiencies. Not all the fuel is burned. This is why we strive for the perfect balance  – Stoichiometric Combustion.

 Stoichiometric Combustion

For a methane/oxygen flame the chemical reaction can be written:

CH4 + 2 O2 ? CO2 + 2 H2 O + Q

where Q is the heat of combustion. In this reaction, atoms are conserved and the equation balances for each of the species. The reaction is a stoichiometric reaction, and the coefficients multiplying each of the chemical species are known as the stoichiometric coefficients.

If air is used in the reaction rather than pure oxygen, the inert gas nitrogen will be present. Air is about 79% nitrogen by volume and 21% oxygen by volume. For a stoichiometric combustion reaction in this case each mole of oxygen will be accompanied by 3.76 moles of nitrogen. The reaction equation is then written:

CH4 + 2 O2 + 2(3.76 N2) ? CO2 + 2 H2 O + 2(3.76 N2) + Q

Question of the Month for October 2018

QUESTION:

Where Does Water Hammer Occur?

 

ANSWER:

Water hammer can occur in any steam or condensate line. Its effects can be even more pronounced in heterogeneous or condensate bi-phase systems. Condensate bi-phase systems contain two states, the liquid (condensate) and a vapor (flash or generated steam). The bi-phase condition exists in a steam system where condensate coexists with generated or flash steam. Typically examples include heat exchangers, tracer lines, steam mains, condensate return lines and sometimes, pump discharge lines.

A common example is water hammer occurring during the start-up or energizing of a steam main. If the steam line is energized too quickly and the condensate created during the startup is not being properly removed; water hammer will be the result.

Effects of Water Hammer

The effect of water hammer cannot be underestimated as its forces have been documented to result in many of the following:

  • Collapse the float elements in steam traps
  • Overstress pressure gauges
  • Bend internal system mechanisms
  • Crack steam trap bodies
  • Rupture pipe fittings
  • Cause valve failures
  • Cause heat exchanger equipment tube failures
  • Break pipe welds and even rupture piping systems
  • Failure of pipe supports.

Question of the Month for September 2018

QUESTION:

In order to determine the heating value of coal in BTU per pound, what is used?

 

ANSWER:

Ultimate Analysis – Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulphur, oxygen, nitrogen.

Ultimate analysis is also known as elemental analysis, it is the method to determine the Carbon, Hydrogen, Nitrogen, Sulphur and Oxygen content present in solid fuel.

Question of the Month for August 2018

QUESTION:

A volute is part of a:

a. centrifugal pump.

b. gear pump.

c. hose pump.

d. plunger pump.

ANSWER:

 a. centrifugal pump

Volutes are designed to capture the velocity of liquid as it enters the outermost diameter of an impeller and convert the velocity of the liquid into pressure.

In the picture below, notice that the impeller is not located in the center of the volute. This is intentional. The portion of the volute that extends closest to the impeller is called the cutwater.

Note that starting from the cutwater and proceeding in a counter-clockwise fashion, the distance between the volute and the impeller increases gradually. This has the effect of causing pressure to build within the volute as the distance increases. Once the point of greatest separation is reached – directly next to the cutwater moving in clockwise direction – the pressure is at its greatest, and water is forced out the casing when it encounters the cutwater.

 

Question of the Month for July 2018

QUESTION:

What is ductility?  What is malleability?  What is resiliency?

ANSWER:

Ductility = Ductility is when a solid material stretches under tensile stress. If ductile, a material may be stretched into a wire.

Malleability = Malleability, a similar property, is a material’s ability to deform under pressure (compressive stress). If malleable, a material may be flattened by hammering or rolling.

Resiliency = In material science, resilience is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading.

Question of the Month for June 2018

QUESTION:

A boiler develops 5000 BHP.  The temperature of the feedwater entering the boiler is 225 deg. F and the enthalpy

of the steam leaving the boiler is 1200 Btu/lb.  How many pounds of steam per hour is this boile4r generating?

ANSWER:

W = BHP X Btu/BHP

         Hs – Tfw -32

 

W = pounds of steam/hr

BHP = Boiler horsepower

Hs = heat per pound

Tfw = Feedwater temperature

 

Btu/BHP = 33,475

W = 5000 X 33,475

         1200 -(225-32)

Answer (W = 166,211.5 lb/hr)

 

Question of the Month for May 2018

QUESTION:

What materials are used in the construction of water columns?

ANSWER:

Water columns are made of cast iron, malleable iron, or steel.  According to the ASME Code, cast iron can be used for pressures up to 250 psi, malleable iron is used for pressures up to 350 psi, and steel is used for pressure in excess of 350 psi.

Question of the Month for April 2018

QUESTION:

What are the cold clearances on an average steam turbine?

ANSWER:

Steam turbine clearances are comparatively small.  Radial clearances are 0.180″ to 0.250″, which is the distance between the top moving blades and casing.  Axial clearances are 0.100″ to 0.200″, which is the distance between the nozzle exits and the leading edge of the blade.  Diaphragm gland clearances are 0.002″, which is the distance between the shaft and the bottom of the diaphragm.  Bearing clearances are 0.001″ per inch of diameter of shaft, with a minimum of 0.005″.  A 4″ shaft would rotate in a bearing with an inner diameter of 4.005″, whereas a 10″ shaft would have an inner diameter of 10.010″.

Question of the Month for March 2018

QUESTION:

How should horizontal steam lines be pitched? 

ANSWER:

Horizontal steam lines must always be pitched in the direction of the steam flow and have traps installed at the end of the run so condensate can be removed.  If the lines were piched back toward the boiler, the steam would pick up the condensate and cause water hammer and possible line rupture.

Question of the Month for February 2018r

 

QUESTION:

Can plant efficiency be increased if a closed feedwater heater is used? 

ANSWER:

Yes, it is possible to increase plant efficiency when heating feed water in a closed feed water heater.  A good example of this is when steam is extracted or bled from a high-pressure stage of a steam turbine and used in a closed feed water heater.  The latent heat of the steam is recovered in the feed water instead of being lost in the condenser.

 

Question of the Month for January 2018

QUESTION:

What is the difference between a flush-front and an extended-front horizontal return-tubular boiler?

 ANSWER:

In the extended-front type horizontal return-tubular boiler the front tube sheet is set in line with the front of the boiler setting.  The lower part of the shell extends beyond the tube sheet.  This extension forms part of the smoke box and is known as the dry sheet.

The shell of the flush-front type horizontal return-tubular boiler does not extend beyond the front tube sheet.  The front of the boiler is set back from the front of the setting to allow a space which forms the smoke box.  An arch at the front of the furnace prevents the gases from entering this space and going directly to the stack.