Feed Water System
Shown above is a typical feed water system for a modern steam plant. Water is pumped from the main condenser by special centrifugal pumps (Supercavitating) having an inducer to allow suction from the very low pressures without vaporisation of the water. The water passes through the air ejector cooler to the Condensate cooled evaporator. A recirculation valve is available to return some condensate to the main condenser. The purpose of this is to increase the overall flow though the evaporator cooler thereby increasing water production as well as to ensure a minimum flow through the Condenser extraction pumps.
The water passes via the gland steam condenser and LP heater which in this case are shown as a combined unit on to the Main Condenser level controller. This prevents the level in the condenser falling below a set level thereby causing the main condensate pumps to run dry.
Some times mounted after this is a deioniser and feed filter before the water is passed to the deaerator.
The deaerator is mounted as high as possible in the engineroom increaseing the suction head for the feed pump preventing vaporisation in the suction eye of the pump. Not shown is an automatic recirculation valve fitted to the main feed pump outlet to ensure a minimum flow through the pump. The boiler water then passes via the boiler water level controller to the economiser and then through to the boiler steam drum.
An second supply is available for use in emergencies to the drum either via or by passing the economiser.
The drains tank condensate is pump via the drains tank level controller into the main feed system.
System level Control
Control of the amount of water in the system is by level control of the deaerator. One of the purposes of the deaerator is to act as a reserve capacity of high quality feed for the boiler. Water may be spilled to the feed tank or made up to the drains tank. An emergency filling valve is available for the main condenser the use of which is avoided as it introduces large quantities of gasses into the condenser reducing efficiency
High speed , multistage centrifugal pumps are the preferred type due to their ability to supply large quantities of water and to provide it at steady flow avoiding shock loads on pipe lines and valves.
The impulse Curtis wheel ( velocity compounded ) rotates at speeds of around 7000 rpm. Velocity compounding means that there is very little pressure drop across the stages reducing the need for fine clearances. This allows the turbine to be run up quickly from cold and with its inherent strength allows for rapid speed/load variations
The balance chamber must have a diameter greater than the suction wear ring. The high pressure on the back side of the impeller would produce an axial loading pushing the impeller towards the low pressure of the suction eye and overloading the bearings.
Bleeding away the high pressure on the back side of the impeller removes this effect but looses pump efficiency through recirculation. The balanced piston design allows for the bleeding but restricts the flow to only that necessary.
As the pressure builds up the impeller is pushed towards the suction eye which increase the leakage into the balance chamber reducing the pressure at the back of the impeller. The high pressure on the forward face pushes the impeller back away from the suction eye and so the leakage to the balance chamber reduces and the pressure increases on the back face of the impeller
If the pump is designed to be supercavitating hard metal inducers are fitted which screw into the water. Any cavitation occurs on this which is made sacrificial. Supercavitating pumps are required where the feed water temperature is close to the saturation temperature so any pressure drop causes the water to flash off to steam
Inlet steam pressure around 60 bar, outlet around 3 bar. Expansion down to lower pressures would require excessively large casings and would lead to problems of centrifugal stresses due to the larger blading required.
Carbon seals are used instead of labyrinths for simplicity and to keep the length of the unit down to a minimum. Due to the different coefficients of expansions between the carbon and the steel a madrill must be used to set the correct running clearance. For multistage pumps an extra end bearing is required and hence additional packed gland.
The pump is dynamically balanced by means of the balance chamber leak off to the suction eye, and the dam edges on the back of the impeller
Balancing of two stage pumps
For two stage pumps it is possible to dynamically balance then by having the impellers back to back. More typically the balance piston assembly is fitted to the final stage
Feed pumps of this type of balance are best started against either a closed or spring loaded discharge v/v to ensure rapid build up of pressure
Pump side Bearings
Water lubricated bearings- Steel backing onto which is sintered a layer of porous bronze impregnated with PTFE (0.025mm thick). This PTFE is transferred to the shaft so providing very low coefficients of friction
Bearings operate at 115oC with water supplied at 5.5bar 70oC.
Bearing clearance- 0.15 mm
Max- 0.25mm must be replaced
Danger- 0.3 mm severe damage will occur
The bearings should also be replaced if less than 75 % shows on the surface.
The coffin feed pump balances the axial load of the pump end with the axial load of the turbine. The excess is absorbed in the thruster arrangement.
Inexplicably in his early career the author was let loose on one of these and can attest to the superb design including the arrangement of tooling for disassembly/assembly. Unfortunately the mix of left and right hand threads caused ‘issues’!
Coffin Feed Pumps
Flow characteristics of controlled feed pumps
Constant speed ( electric )- Below 10% turbulence becomes so great that the pump operates very inefficiently and must not be worked in this range. Hence, a recirc valve must be used. At full flow pressure is only just sufficient to feed the boiler
Pressure governed- Pressure droop designed in to give stable control.
Flow + Speed and/or Pressure- The extra element must be added with the flow otherwise the system becomes unstable. Characteristic can be level or slightly rising giving low pressure at low flow rates conserving energy and preventing water being forced at high pressures through partly open feed control valve.
Can be operated at very low flow rates due to reduced speed
1. Bush bal drum
2. Bal drum
3. Bolts dis. Cover
4. Bearing HSG-NDE assy. (only Top and Bottom
5. Bearing HSG-DE-assy. (only Top and Bottom)
6. Diffuser assy.
7. Dis. Cover
8. End diffuser assy.
9. Floating seal
10. Gasket MS/CU
11. Gland housing
12. Impeller assy. 1st stg.-assy.
13. Impeller assy- 2nd stg.-assy.
14. Impeller assy. 3rd stg
15. Impeller assy. -4th stg
16. Impeller assy. Kicker stg
17. Inlet guide assy.
18. Journal bearing (DE)
19. Journal bearing (NDE)
20. Key coupling nut
21. Key impeller
22. Key ball drum
23. Key mech. Seal sleeve
24. Key thrust collar
25. Mechanical seal (DE)
26. Metaflex gasdet (casing dis. Cover)
27. Metaflex gasket (dis. Cover/gland hsg)
28. Metaflex gasket (dis. Cover/outlet cide)
29. Metarex gasket (casing/Suction guide)
30. Metaplex gasket (casing/int. stg.tube)
31. Nut ball drum
32. Nut thrust collar
33. Nut coupling
34. Nut dis. Cover bolt
35. Outlet guide assy
36. Oil guard (DE&NDE)
37. Oil thrower (DE)
38. Piston ring set (int. stg. tapoff)
40. Split ring
41. Suc. Guide
42. Thrust collar assy
43. Thrust bearing
44. Thrust pad set
45. Washer bal drum nut
46. Washer thrust collar nut
47. 'O' ring
48. Spring disc
49. Spare catridge
50. Mechanical seal NDE
51. 'O' ring backup ring
52. Oil thrower
The purpose of feed heating is to increase plant efficiency. With no feed heating steam gives up three times more heat to the cooling water as it does in doing useful work in the turbine. This major loss occurs in Condenser with the low quality exhaust steam being condensed to enable it to be returned to the boiler/
As the steam expands through the turbines so its specific energy content falls and the volume increases.
A proportion of the low quality steam is bled off from the turbines to a feed heater where it can give its energy ( its latent heat mostly) to the feed and so a higher proportion of the energy is reclaimed by the system as would have been gained by expanding it through the turbine to the condenser.
Feed heating can take two forms:
Condensate Feed Hearing
Theoretical practical cycle where a portion of the steam is bled off in stages and used for feed heating
It should be noted that although thermal efficiency increases with the number of stages, it is governed by the law of diminishing returns and the improvement is reduces with successive stages. Hence the cost of the increased plant becomes a factor.
Bleeding off a portion of the steam gives the added advantage that the steam volume that has to be accommodated in the final stages of the turbine and condenser is reduced.
Theoretical cycle with all steam expanded through the turbine
Theoretical cycle where all the steam is continuously used in Feed Heating 100% efficiency
This could only be possible where the turbines where fitted with a water jacket through which the feed water flows.
HP Feed Heater
HP feed heater as fitted to cascade feed heater system instead of economiser
For efficiency it is important that the bottom of the heater does not become flooded.
As a large difference in pressure can be accommodated between the feed and steam , this type of heater can be fitted on the discharge side of the main feed pumps; in a cascade feed heating system this replaces the economiser and the heat in the flue gas is recovered by a regenerative air heater ( Lungstrom ). This system allows the feed to be heated to a high temperature
The change in temperature is normally about 30oC
The heater, when new, must be able to withstand either or both of the following;
The purpose of the economiser is to increase plant efficiency by removing heat that would otherwise be lost in the flue gas and use it to indirectly feed heat the water. By heating the feed water it is also helping to prevent thermal shock as the water enters the steam drum.
The flow of water is general counterflow. The exception to this is in the radiant heat boiler where the economiser is mounted immediately above the superheaters. The water flow in these so called 'Steaming economisers' is then parallel flow to decrease the tendency for the economiser to steam excessively above the design limits and lead to steam blockage , this is why these economisers are bare tube with no extended heating surface.
Vent and drains are fitted to header where isolating valves are fitted a safety valve must also be added.
The modern design involves the fabrication of the header and stub tubes which can then be heat treated. The tubes are then site welded to the stubs.
The previous use of expanded joints has now fallen out of failure due to the requirement of a multitude of hand hole doors with associated joints and hence possible area of leakage. The materials used are governed the materials susceptibility to cold end corrosion (see later notes), any metal having a surface temperature below the dew point will tend to have acidic deposits forming on its surface caused by the water absorbing sulphur trioxide and dioxide from the flue gases. Some metals are more resistant to this form of corrosion at lower temperatures, choice of material will initially depend on the minimum metal surface temperature and is calculated as the feed water temperature that is passing through the economiser plus 5oC. For temperatures greater than 138oC solid drawn mild steel tubing is used. Fitted to these are welded on extended surface steel fins or studs.
For temperatures between 115oC to 138oC shrunk on or cast iron gills must be used.
The temperature should not be allowed to fall below 115oC as this can lead to heavy fouling as well as corrosive attack.
Efficient sootblowing is absolutely essential to ensure that surface are kept clear of combustion products which can not only lead to heavy corrosion and a drop in efficiency , but also to the possibility of an economiser fire with potentially disastrous consequences. With this in mind it is not unusual to find provision for water washing, something which is carried out on a very regular basis on a Motor ship with waste heat recovery.
If due to failure it is required to run the economiser dry then the maximum gas inlet temperature should be limited to about 370oC, vents and drains should be left open to ensure that there is no build up of pressure from any water that may be still located in the tubes.
Cast iron extended gill economiser
The unit is of the multiple pass 'melesco' style, the sleeves may be slid on before the bends are welded on.
The header is made out of mild steel.
Mild steel fin extended surface economiser
In marine water tube boilers it is essential to keep water free of dissolved gases and impurities to prevent serious damage occurring in the boiler
In a closed feed system the regenerative condenser removes the bulk of the gases with a dissolved oxygen content of less than 0.02 ml/l
However, it is recommended for boilers operating above 30 bar and essential for those operating above 42 bar that a dearator be fitted.
Purpose of fitting a Deaerator
There are four main purposes;
Feed water enters the dearator via the vent cooler, here the non condensible gasses and a small amount of steam vapour are cooled. The condensed water is returned to the system. The feed water is sprayed into the mixing chamber via nozzles, for systems with large variations of flow, two separate nozzle boxes may fitted with two independent shut off valves to ensure sufficient pressure for efficient spray.
Alternatively, automatic spray valves may be fitted.
By varying the spring tension the pressure at which the nozzles open can be set at different levels.
The water in a fine spray, and so high surface area to volume, is rapidly heated to corresponding saturation temperature in the mixing chamber by the heating steam. This steam is supplied from the exhaust or IP system and is at about 2.5 bar.
The heated water and condensed steam then falls onto a series of plates with serrated edges, the purpose of these is to mechanically remove any gas bubbles in the water improving the efficiency of the process.
Finally the water falls into the buffer tank, before exiting to the feed pumps; as the water is now at saturation temperature any drop in pressure (such as in the suction eye of the impeller ) will cause vapour bubbles to form. Hence, the deaerator must be fitted well above the feed pumps or alternatively an extraction pump must be fitted to supply the feed pumps.
Hot water drains are led to the dearator where they are allowed to flash into steam adding to the heating steam.
The non condensible gas outlet is limited so that there is only a small flow of water from the drain of the cooler. This water should be discarded as it contains not only high quantities of oxygen but also ammonia.
Requirements for efficient operation
With these parameters met the water at outlet should have an oxygen level of not more than 0.005 ml/l.
A thermometer is fitted to the shell, the temperature should be kept to within 4oC of the saturation temperature of the pressure indicated on the pressure gauge. This can be governed by the quantity of heating steam added. If the difference is always greater the partial pressure due to the non-condensable gasses is high and the possibility of redissolving increases. Where fitted the valve on the air outlet should be opened more to limit of too much steam being lost up the vent.
The fitting of vent condensers is not universal and it is not uncommon to see the vent led up the funnel where a small wisp of steam can be seen when correctly adjusted.
A well designed feed tank should be designed to minimise the oxygen within the feed system. This is especially important with open feed systems.
The following are taken as parameters for a well designed tank
As a rule of thumb chemical reaction rates double for evey 10'C rise in temperature. For feed system this remains true upto about 80'C for open system. After this due to reducing solubility of oxygen the rate of corrosion reduces. Thus steam heating on the open feed tanks have thermostats set at 85'C or higher
Steam - Steam Generators
Found on steam propulsion plant and used for the production of low pressure steam for tank heating purposes. This has the major advantage in reducing the possibility of damage due to oil contamination where the feed system is segregated.
Generally the heating steam is supplied from the Intermediate Pressure system when under way, additional ‘Live’ steam make up may be required under low system load conditions.