Relief and safety valves for thermal power plants


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Relief and safety valves for thermal power plants

  2. 2. 1.Introduction  Pressure relieving valves (relief, safety and safety relief valves) are used throughout the thermal power generating industry to provide overpressure protection of pressurized systems  They are designed to mitigate pressure rise in the system to below a defined design value.
  4. 4. 3.SAFETY VALVE  An automatic pressure relieving device actuated by the pressure upstream of the valve and characterized by full opening pop action .  It is used for steam ,gas or vapor service  Safety valves may be spring loaded or pilot actuate
  5. 5. 4.RELIEF VALVE  Safety relief valves are pressure relieving devices actuated by the inlet static pressure and characterized by rapid opening or “popping” action, or by opening in proportion to the increase in pressure over the opening pressure, depending on application.  Safety relief valves can be used for either liquid or compressible fluid service.  The primary difference between a safety relief valve and a safety valve is that the safety relief valve has a fluid tight bonnet, allowing it to be used for liquid service.
  6. 6. 5.POWER OPERATED RELIEF VALVE  Power operated relief valves (PORVs) are pressure relieving devices which require an external power supply for actuation.  These valves are typically controlled by an electrical signal resulting from high system pressure or manually from the control room.  The electrical signal initiates the relief action by activating the valve actuator, either electrically (ERV) or pneumatically.
  7. 7. 5.1.Electromatic relief valve system
  8. 8. 6.REQUIREMENT OF SAFETY VALVE ( ASME SEC 1)  Each boiler shall have at least one safety valve or safety relief valve and if it has more than 47 m2 of bare tube water-heating surface, or if an electric boiler has a power input more than 1,100 kW, it shall have two or more safety valves or safety relief valves.
  9. 9. 7.Multiple valves  When only two valves are used, capacity of smaller shall not be less than 50% of the larger. (PG-71.1)
  10. 10. 8.SETTING OF SAFETY VALVE FOR BOILER  The safety valve or safety relief valve capacity for each boiler shall be such that the safety valve, or valves will discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 6% above the highest pressure at which any valve is set and in no case to more than 6% above the maximum allowable working pressure.
  11. 11. 9.CAPACITY OF VALVE  The capacity of the safety valve is the quantity of steam it can relieve when the valve is fully open.  The safety valves in a boiler are so selected that the capacity of spring loaded safety valves on drum and superheater put together will be more than the 100% steam generation of the boiler.
  12. 12. 10.Super heater valve  At least one valve shall be installed on the superheater outlet. it is good practice to size the valve to relieve a approximately 20% of the total boiler capacity to protect the tubes against overheating.  Drum valves must relieve a minimum of 75% of total steam generation of boiler.
  13. 13. 11. Reheater safety valve  Every reheater shall have one or more safety valves, such that the total relieving capacity is at least equal to the maximum steam flow for which the reheater is designed.  Boilers having reheaters must have at least one safety valve on reheater outlet capable of relieving a minimum of 15% of the flow through the reheater.
  14. 14. 12.Lever safety valve  Deadweight or weighted lever safety valves or safety relief valves shall not be used . (ASME SEC 1)
  15. 15. 13.SAFTEY VALVE CONNECTION  The safety valve connection to the Boiler shall be independent of any other connection and without any unnecessary intervening pipe or fittings.  The opening or connection between the Boiler and the safety valve must not be smaller than the valve inlet.
  16. 16. 14.Safety valves installation
  17. 17. 15.SAFETY VALVE DISCHAGRE DRAIN  The discharge drain outlet must be piped full size without any shut-off valves, independent of other piping.  Install piping with sufficient flexibility to allow for free expansion and properly support so there is no strain on the safety valve body.  Pipe to a safe point of discharge to prevent any possibility of personal injury and within 18" from the floor .
  18. 18. 16.Sequential lift series  Sequential lift series applies when there is more than one pressure relieving device in the system.
  19. 19. 17.1.Capacity of Safety & Relief Valves for Supercritical Boilers  Spring loaded safety valves:  Separator and superheater- combine capacity 105% BMCR( minimum)  Reheater – combine capacity 105% of reheater flow BMCR (minimum)  Electromatic relief valves (ERV):  Superheater- 15% BMCR  Reheater -15% BMCR
  20. 20. 17.2.Safety valves in supercritical boiler -660 MW power plant s.n description Spring loaded ERV 1 Separator outlet 6 2 SH outlet 4 4 3 CRH 6 4 HRH 2 4
  21. 21. 18.SAFETY VALVE SETTING -500 MW BOILER  The valves are selected for following conditions  Maximum evaporation of boiler is 1675000 kg / hr  Maximum allowable boiler drum press is 207 kg/cm2 (g) and operating press is 194 kg/cm  SH steam operating temperature is 540 c deg.  Maximum flow through reheater is 1445530 kg/hr
  22. 22. 18.1.Total % of Evaporation of safety valve-500 MW boiler S.N LOCATION No of safety valves TOTAL % OF EVAPORATION 1 BOILER DRUM +SH Drum- 6 SH - 2 109.14 2 SH SH - 5 30.75 3 CRH+HRH CRH- 4 HRH - 4 104.39 4 HRH HRH - 4 33.34 ERV ERV
  23. 23. 18.2.Relieving capacity of drum & SH safety valves -500mw  relieving capacity from drum safety valves is 86.76% of total evaporation of boiler.  relieving capacity from superheater safety valves is 22.38% of total evaporation of boiler.  Total relieving capacity of drum and SH safety valves -109.14% =(86.76+22.38) of total evaporation of boiler.
  26. 26. 19.MATERIAL SELECTION  Every safety valve used on a super heater or reheater discharging superheated steam at a temperature over 230 deg c ,shall have a casing ,including the base ,body ,and bonnet and spindle ,of steel alloy or equivalent heat resisting material.  Materials used in body to bonnet or body to yoke bolting shall be in ASME B 16.34.  Cast iron seats and disks are not permitted.
  27. 27. 20.blowdown  Blowdown is the difference between the set pressure (“popping” pressure) and the resetting pressure of a pressure relieving valve. This pressure is commonly expressed as a percentage of the set pressure such as 5% Blowdown.  Another way of describing blowdown is to say that it is the difference between set pressure of the valve and system pressure when the valve recloses.
  28. 28. 21.1.Safety valve for tanks and pumps  Low-pressure storage tanks must be protected when liquid is pumped into or out of the tank. This is required to prevent overpressurizing or collapsing the tank when liquid is being moved from or to the tank.  Positive displacement pumps and reciprocating compressors should have pressure relief valves on their discharges to relieve the fluid if the discharge should be blocked.
  29. 29. 21.2.Safety valves for heat exchangers  Heat exchangers that have valves on both the inlet and outlet can be isolated if both valves are shut.  Safety/relief valves should be provided to protect the heat exchanger from the effects of thermal expansion of the liquids that may be isolated within the heat exchanger.  Consideration should also be given to protection of equipment on the low pressure side if a tube within the heat exchanger should rupture.
  30. 30. 21.3.SAFETY VALVES AT TURBINE CYCLE  Aux steam after desuperheater.  LP heaters (shell side).  HP heaters (shell side).  Before BFP Turbine inlet (steam).
  31. 31. 22.1.SAFETY VALVES
  32. 32. 22.2.Safety valves installation
  33. 33. 23.Muffler  If a muffler (silencer) is used on a safety valve or safety relief valve; it shall have sufficient outlet area to prevent back pressure from interfering with the proper operation and discharge capacity of the valve.  Mufflers shall not be used on high-temperature water boiler safety relief valves.  Silencers on all lowest set pressure safety valves.
  34. 34. 24.Feedwater supply and safety valves setting  Boilers having more than 500 ft2 (47m2) of water heating surface shall have at least two means of feeding water.  Each source of feeding shall be capable of supplying water to the boiler at a pressure of 3% higher than the highest setting of any safety valve on the boiler.
  35. 35. 25.STANDARDS  1. ASME- Boiler and Pressure Vessel Code Section I, Power Boilers, and Section VIII, Pressure Vessels.  2. ASME- Performance Test Code PTC-25, Safety and Relief Valves.  3.ASME/ANSI power piping B31.1
  36. 36. 26.Layers Of Protection- Safety valves are Part of mitigation System in a power plant as per IEC 61511 Typical risk reduction methods found in a Thermal power plants
  37. 37. 27.ASME Stamp
  38. 38. THANKING YOU