Relief and safety valves for thermal power plants | PPT - SlideShare

• 1. RELIEF & SAFETY VALVES FOR THERMAL POWER PLANTS SHIVAJI CHOUDHURY
• 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.
• 3. 2.TYPES OF PRESSURE RELEIVING VALVES  SAFETY VALVE  SAFETY RELEIVE VALVE  POWER OPERATED PRESSURE RELIEVING VALVE.
• 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. 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. 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.
• 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. 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. 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. 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. 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. 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. 12.Lever safety valve  Deadweight or weighted lever safety valves or safety relief valves shall not be used . (ASME SEC 1)
• 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.
• 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. 16.Sequential lift series  Sequential lift series applies when there is more than one pressure relieving device in the system.
• 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. 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. 18.SAFETY VALVE SETTING -500 MW BOILER  The valves are selected for following conditions  Maximum evaporation of boiler is 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 kg/hr
• 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. 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.
• 24. 18.3.SAFETY VALVE STTING-500 MW BOILER DRUM & SH
• 25. 18.4.SAFETY VALVE STTING- 500 MW BOILER –CRH&HRH
• 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. 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. 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. 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. 21.3.SAFETY VALVES AT TURBINE CYCLE  Aux steam after desuperheater.  LP heaters (shell side).  HP heaters (shell side).  Before BFP Turbine inlet (steam).
• 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. 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. 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. 26.Layers Of Protection- Safety valves are Part of mitigation System in a power plant as per IEC Typical risk reduction methods found in a Thermal power plants

All coils used on AC supplies will hum when energised but any vibrating or rattling should be investigated and rectified quickly. Solenoid coils should be firmly secured to their armature for efficiency and if not tight, could be the reason for any noise.

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The operation of the solenoid valve cannot directly heat the fluid but since the coil does get hot, it will radiate and convect heat through the air to the surrounding area. In a confined space or when a valve is inverted with the coil beneath the pipework, convection currents will carry warmed air to the pipework and could heat the contents. Note that any cold fluid passing through the valve would have a cooling effect on the valve and pipework and any increases in temperature would likely be negligible. However it would always be recommended to house the solenoid valve where free air can circulate to prevent excessive heat build up which could shorten the working life of the coil.

The clearance tolerance between the coil mounting hole and the armature is critical for reliable operation and the matching coil winding will also be specifically designed for efficiency. Since there is no international standard for coil and armature construction, every manufacturer designs them to suit their requirements and in the main, they are not exchangeable at all. However, some coils do have similar internal diameters and heights and can be seen swapped onto the wrong valves on many plants. This rarely works well or for long and for reliability, it would always be best to maintain the valve by using the recommended coil from the same manufacturer.

Not usually, no. Both the armature and coil designs are different for AC and DC voltages and doing this exchange is likely to produce reliability issues in addition to wasting energy. This can be done on some valve designs but always check with the manufacturer or distributor first.

Solenoid coils must never be energised without being secured to the valve armature. Doing this will cause excessive heat build up and the coil will fail, producing bulges within the mounting hole and /or externally. Loose coils will produce exactly the same result but over a longer period of time. If the coil used to fit and now doesn't, you need a new coil. If the coil cannot be removed because it has expanded onto the armature, you need a complete new valve.

Most solenoid coils are unpolarised so that it doesn't matter which of the two coil connections are positive/negative or live/neutral. If a coil is polarised, it will state it clearly in the installation instructions and would only apply to DC circuits. Commonly this would be for latching coils or ones with additional control circuitry.

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Some solenoid valves include a manual operator feature which is a push button or a screwdriver actuated twist head to manually lift the valve off the seat. Without a manual operator, the only other option is to remove the coil and fit a Commissioning Magnet in its place and remove it when finished. In this way, the magnetic field of the permanent magnet replaces the electromagnetic field created when the coil is energised.

There could be a number of reasons depending on the model involved. Perhaps the manual operator has been left in the open position if there is one fitted. If not, consider that when the valve opens, the distance the diaphragm or piston moves off the seat is only small and can become clogged with particulates or debris and prevent the valve shutting completely. Another reason could be damage to the diaphragm. These all assume that the valve was installed according to the manufacturer's instructions. Fitting a valve contrary to the marked flow direction will prevent the valve shutting. Some valves require a minimum pressure drop across the valve to ensure the diaphragm seats securely. With insufficient pressure, the diaphragm can float and the valve can open and close unpredictably. For this reason, pilot operated or servo type solenoid valves should not be used in closed loop or low pressure systems.

There are highly specialised valves on the market for some difficult media such as glues and heavy fuel oil but in general, highly viscous media are not suitable. Equally fluids with a large particulate content would quickly prevent reliable operation. Foodstuffs like milk for example could be used in solenoid valves because they are not too viscous but would not be recommended as the valves are not suitable for CIP or SIP cleaning processes and cannot easily be inspected for manual cleaning to prevent bacterial contamination.

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