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It's all about HP heaters (Feed water heaters) in Power plants

 1-Why do you use HP heaters in power plants?

HP heaters are used for heating the feed water, which will contribute in increasing cycle efficiency as well as reduction in fuel consumption

2-What is the design code of HP heaters?

HEI, ASME SEC VIII Div-I IBR

3-What are the advantages of using feed water heaters in power plants?

  • Fuel consumption reduces
  •  Reduce heat losses in the condenser
  •  Lower emissions as fuel use is reduced due to improved heat rate
  • Decreases the plant heat rate & hence increases the plant efficiency

4-What are the different types of feed water heaters?

Open type feed water heaters & closed type feed water heaters

5-What do you mean by open type of feed water heaters?

In open type feed water heaters steam directly mixes with feed water. Steam pressure used is lower < 5 kg/cm2

6-What do you mean by closed type of feed water heaters?

In closed type feed water heaters steam directly mixes with feed water. Steam pressure used is high > 5 kg/cm2, these are shell & tube type heat exchangers

7-What are the pipe lines connected to HP heaters?

  • Bleed steam inlet line
  • Feed water inlet line
  • Condensate outlet line
  • Feed water outlet line
  • Feed water box drain & vent lines
  • Shell zone drain & vent

8-What are the MOCs of Tubes & shells used in closed type of HP heaters?

Tubes: SA 213 TP 304 (Feed water pressure up to 170 kg/cm2)

Shell: SA 516 Gr.70

9-What are the different zones of HP heaters?



  • Desuper heating zone
  • Condensing zone
  • Sub cooling zone

10-Where does the maximum heat transfer occur out of all zones?

Maximum heat transfer occurs at condensing zones

11-Why the name Sub-cooling has come?

Here condensed steam from condensing zone is cooled by feed water entering by convective heat transfer method.

12-What is the function of drain coolers in HP heaters?

Drain Coolers are employed because of heat consumption improvement in case of drain introduction into the lower heater through the control valve.

13-Why the feed water inlet line connection is at the bottom & outlet line is at the top?

          

Feed water inlet line & outlet line are connected in such a way that to separate desuperheating zone, condensing zones & sub cooling (drain cooler) zones

14-Where do the fixed & sliding supports of HP heaters are located?



Fixed support: Towards feed water inlet & outlet line

Sliding support: Opposite side of feed water line connections

15-What would be the velocity of feed water in HP heaters tubes?

It’s around 0.6 to 0.8 m/sec

16-What is the effect of high/low condensate level in heaters shell?

Higher the condensate level lower is the performance of heater and vice versa. Heater level is always maintained in between 30–50%.

17-Briefly explain the condensate level control in HP heaters?

One of the most common causes of tube failures in a feed water heater (FWH) is the improper control of the internal liquid level, which also can cause operational and maintenance costs that might lead to premature replacement. These problems are not new, they have been experienced by many utility plants throughout the industry during the past 50 years. However in many cases, the resulting damaging phenomenon has seldom been totally understood and the loss of corporate knowledge and failure of some utilities to identify and rectify level control problems continues to bring this issue to the forefront of root causes of FWH operational failures.

In general, the performance of the Drain Cooler (DC) Zone is tied to the operational parameter of Drain Cooler Approach (DCA). DCA is a good indication of whether the DC Zone is operating properly or not, it is not the only parameter that should be considered. DCA is a measurement of temperatures

The pressure of the drains also must be known in order to determine the degree of sub cooling and whether there is a potential for flashing, either within the DC itself or the downstream piping before the level control valve. Flashing and two-phase flow in either of these areas can cause significant damage to the heater.

It is important to remember that the drain cooler is designed to be a water-to-water exchanger. It must remain that way to function properly. Any admission of vapour into the zone typically results in problems. This might be a result of a low liquid level in which steam is admitted directly from the condensing zone into the DC zone, the result of flashing within the DC zone itself, or can be the result of leakage into the zone via the endplate or shroud cracks.

18-What do you mean by drain cooler approach (DCA) in HP heaters?

DCA is the temperature difference between the drains (steam condensate) leaving the heater and the temperature of feed water entering the heater. For more cycle efficiency DCA value should be small.

19-A HP heater is used to heat the feed water from 160 °C to 180 °C by using turbine bleed steam at 15 kg/cm2 and 320 °C. The condensate returning from heater is at 170 °C, calculate the DCA of heater.

We have,

DCA = Temperature of condensate leaving the heater – Temperature of feed water entering the heater

DCA = 170 - 160 = 10 °C

Note: For best performance, heaters are designed to get DCA 3 to 5 °C at full operation capacity.

20-What do you understand by Terminal Temperature Difference (TTD)?

It is the difference between the saturation temperature at the operating pressure of the inlet steam to the heater and the temperature of the feed water leaving the heater. For more cycle efficiency TTD value should be small.

21-A HP heater is used to heat the feed water from 110 °C to 160 °C by using MP steam at pressure 13 kg/cm2 at temperature 280 °C, calculate the TTD.



We have,

TTD = Saturation temperature of inlet steam - Feed water outlet temperature

Saturation temperature of inlet steam at 13 kg/cm2g pressure = 195.6 °C

TTD = 195.6 - 160 = 35.6 °C

Note: For best performance, heaters are designed to get TTD 3 to 5 °C at full operation capacity.

22-How the DCA does affects condensate level of HP heaters?

An increase in DCA,HP heater level decreases & vice versa

23-What is flashing in heaters? How does it occur?

Flashing, by definition, is the change in state of liquid to vapour. While in most cases this change of state results from the addition of heat (as in the boiler) in a FWH the most common cause of flashing is a result of a reduction in pressure (or pressure drop). Pressure drop might be a result of the geometry of the Drain Cooler Entrance window, the fact that the drains must travel around the tubes and change direction many times due to the baffling arrangement and also due to changes in elevation and elbows in the downstream piping. If the liquid drains are not sub cooled enough, any one of these pressure drops could result in flashing and two-phase flow. Two-phase flow is known to cause problems to piping, tubing, the cage and the shell, especially in the case of carbon steel components.

24-What do you mean by fouling in heat exchangers?

Deposition of any undesired material on heat transfer surfaces is called fouling. Fouling may significantly impact the thermal and mechanical performance of heat exchangers. Fouling is a dynamic phenomenon which changes with time. Fouling increases the overall thermal resistance and lowers the overall heat transfer coefficient of heat exchangers. Fouling also impedes fluid flow, accelerates corrosion and increases pressure drop across heat exchangers.

Different types of fouling mechanisms have been identified. They can occur individually but often occur simultaneously.

Scaling:

  • Particulate/Sedimentation Fouling
  • Corrosion Fouling
  • Chemical Fouling
  • Freezing Fouling

25-What are the problems associated with HP heaters?

  •  Initial two phase mixture & hammering
  • Tubes failure due to wrong operation
  • Level fluctuation & leakages
  • Overfeeding of steam & feed water
  • Operating the heaters above the operating & design parameters

26-Write down the initial charging process of feed water heater (HP heater)

Steps:

  • Ensure all the maintenance activities on HP heaters are completed
  • Ensure all inlet & outlet valves of heaters are healthy
  • Ensure all field instruments are healthy
  • Keep open all water box & shell side vents & drains are  open
  • Ensure steam condensate outlet valve is open

Water side

  • Crack Open the feed water outlet valve
  • Then crack open the feed water inlet valve
  • Allow to vent out the air
  • Then gradually open the outlet & inlet feed water valves
  • Then close the water box vents & drain valves

Steam side

  • Ensure steam line drains are in opened condition
  • Ensure steam parameters are as per desired values
  • Ensure no water in steam line drains
  • Crack open the steam inlet valve to HP heater
  • Ensure there are no water particles in drain & vent line of shell. If found clear, then close the valves
  • Then gradually open steam inlet valves & allow for stabilization

27-A HP heater is used to heat the 200 TPH feed water from 160 °C to 180 °C by using turbine bleed steam at 15 kg/cm2 and 320 °C. The condensate returning from heater is at 170 °C, calculate the quantity of steam used.

 Given that,

Qf = 200 TPH

Tf1 = 160 °C

Tf2 = 180 °C

Hg  at pressure 15 kg/cm2 & temperature 320 °C =735.3 kcal/kg

Enthalpy of condensate water Hf = 171 kcal/kg

Heat lost by the steam = Heat gained by feed water

Ms X (Hg-Hf) = Mw X (Tf2-Tf1)

Ms X (735.3-171) = 200 X (180-160)

Ms = 7.08 TPH

28-What are the precautions to be taken for safe operation of HP heaters?

Precautions:

  • Operate the HP heaters as per SOP
  • Take utmost care during initial charging
  • Do not operate the heaters beyond the operating pressure & temperature
  • Bypass the HP heaters during Boilers Hydraulic tests
  • Conduct routine preventive maintenance




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