Answer EKM Question 48

With reference to main boiler super heater arrangements:
A. Compare the advantages and disadvantages of contra flow with parallel flow design.
B. How the element tube banks are supported yet allow for expansion?
C. How boiler carryover affects super heater effectiveness?
Answer:A. Advantages and disadvantages of contra flow with parallel flow design:
Parallel flow:
(a) High thermal stress due to high temperature differential at inlet, possible stress corrosion.
(b) Low temp differential at outlet, therefore inefficient use of tube area.
(c) Low mean metal temperature allows low grade metal to be used since the temperature troubles are avoided.
Cross flow or contra flow:
(a) More uniform temp gradient gives better overall heat transfer.
(b) High outlet temp of cold fluid may be useful for heat recovery.
(c) High mean metal temp requires high grade of metal element tube banks are supported yet allow for expansion:
Integral furnace and D-type water tube boilers usually have their superheaters placed between the steam and water drum, behind two or three rows of 50 mm diameter screen tube, thereby protecting them from the direct radiant heat of the furnace flame. The superheater consists of series of U-tubes which project horizontally across the stream of hot gases leaving the furnace. In many cases the tubes are given a slight slope, just sufficient to allow for drainage.
As these boilers operate with superheat temperatures below 455°C, solid-drawn mild steel tubes can be used. These tubes, 32 mm in diameter, are expanded and bell-mouthed into two separate, cylindrical or rectangular section, forged steel headers which are arranged in a vertical position, outside the gas stream, at the rear of the boiler.
Internal bases welded inside these headers, cause the steam on its way through the superheater to make several passes across the gas stream. Small holes in these baffle plates allow for drainage. Internal access to the benders provided by means of hand holes: these are closed by plugs kept in position, by external dogs.
B. The superheater tubes are supported by plates made of heat-resistant alloy steel. In some versions these plates are suspended from lugs welded to the underside of the steam drum, with a locating slot, and plate at the bottom as shown in Fig above Other designs use a water-cooled support tube fitted between the steam and water drums, to carry a number of support plates which are damped onto it.

 

Gas burners are fitted to direct the gas flow over the superheater as required. No Special means of control over the superheat temperature is provided as the normal operating temperatures are sufficiently below the limit for mild steel to allow some temperature fluctuations to occur while manoeuvring etc. However care must be taken with rate of firing at these times to prevent excessive variation in superheat temperatures.
As with all superheaters arrangements must be made to prevent overheating by allowing for the circulation of steam at all times when hot gases are flowing over the superheater. Thus some steam driven auxiliary machinery, such as turbo-feed pumps, are advisable to ensure a sufficient minimum circulation when the main engine throttle is dosed. De-superheaters are also usually fitted.
The safety valve fitted to the super heater outlet header must be set at a pressure sufficiently below that of the safety valves to ensure that it opens first and so provides steam flow through. the superheater in the event of the boiler blowing off. Due to the risk of internal deposits these superheaters are not flooded when raising steam. Drains and air vents are left open and the rate of firing kept low, until enough steam is being generated to circulate the superheater; the steam is allowed to blow off to the atmosphere until some steam demand exists to provide circulation through the superheater.
C. Boiler carryover:
Along with steam water droplet gets carried away in the steam line.
Reason:
(a) Rate of evaporation too high.
(b) Excessive chloride or chemicals.
(c) This wet steam will enter super heater.
(d) It will evaporate and leave scale forming compound i.e. scale inside super heater tubes will deposit.
(e) Once this happens heat transfer will reduce.
(f) Overheating ,drop inefficiency of super heats and ultimately it will down.
(g) Effective efficiency will drop.
(h) Won't get superheated temperature.
Effects of carryover:
Boiler water solids carried over with steam form deposits in non return valves, super heaters, and turbine stop and control valves. Carryover can contaminate process streams and affect product quality. Deposition in super heaters can lead to failure due to overheating and corrosion.
Superheated steam turbines are particularly prone to damage by carryover. Sticking of governor and stop valves due to deposits can cause turbine over speed and catastrophic damage. Solid particles in steam can erode turbine parts, while deposition on turbine blades can reduce efficiency and capacity. Losses of 5% in turbine efficiency and 20% in turbine capacity have occurred due to deposition. When large slugs of boiler water carry over with steam, the resulting thermal and mechanical shock can cause severe damage.
Loss of production may result from reduced capacity or equipment failure caused by carryover. In some instances, the effect of carryover on production overshadows all other considerations.
Steam can be contaminated with solids even where carryover is not occurring. Contaminated spray attemperating water, used to control superheated steam temperature at the turbine inlet, can introduce solids into steam. A heat exchanger coil may be placed in the boiler mud drum to provide attemperation of the superheated steam. Because the mud drum is at a higher pressure than superheated steam, contamination will occur if leaks develop in the coil. Failure to check for these possible sources of contamination may necessitate time-consuming steam purity studies.