- Blowdown valves in the boiler system control the concentration of dissolved solids in the boiler water.
- Blow-Down valves remove sludge and impurities from water in boiler systems.
- This valve maintains fluid-tightness and it is easily operated without the help of any wedging action.
- The water from the boiler system is blown out by the force of the steam pressure within the boiler.
- The blow-down speed is such that the solid impurities present in feed water are carried out with it.
- The boiler efficiency and integrity can be maintained by the elimination of impurities.
- These concentrated solid impurities cause many problems such as
- Reduces heat transfer.
- Boiler and piping corrosion.
- Reduces efficiency by increasing required fuel.
- Requires additional chemical treatment.
- The primary function of the blow-down valve is to control a continuous flow of steam when the differential pressure is high.
- These blow-down valves are used for continuous or intermittent use to drain dissolved solids from the boiler water in fossil fuel-fired boilers.
- These valves are fixed on boiler drain lines.
- Generally, only one blowdown valve is required, but multiple blowdown valves are installed based on design and purpose.
- To reduce erosion in the boiler the two blowdown valves are used in series.
- One valve acts as a sealing valve, and the other valve acts as a blow-down valve.
- The sealing valve opens first and closes last.
- Erosion on the disk faces and the seats can be eliminated by the simultaneous and rapid opening of both valves.
- During blow down the trapping of rust, particles are avoided within the valve by reopening a valve again and again, if any rust particles remained will be flushed out with the next steam passage through the pipe.
- The bottom blowdown valves can be rebuilt or replaced only during the boiler maintenance period.
- These blowdown valves are designed and manufactured based on boiler requirements.
- Removal of suspended solids through blowdown advantages the boiler system to generate high-quality steam.
Continuous Blowdown (CBD) valves:
- Continuous blowdown operates on a set of time intervals to perform the elimination of solid corrosive particles.
- Continuous Blowdown valves operate in the continuous open position by continuously releasing water through a blowdown tap near the water surface of the boiler.
- This maintains the Total Dissolved Solid level of feed water in the boiler drum.
- A continuous blowdown valve looks like an angle valve having a needle-shaped trim inside a venturi diffuser.
- This venturi diffuser provides sufficient area for the high-velocity flashing water to avoid choked flow.
- A continuous blowdown makes use of flash tanks and heat exchangers to stimulate the maintenance of water and heat levels.
- Its continuity replaces or removes the manual type that encourages the accumulation of solid impurities in the boiler.
- Continuous blowdown setting depends on:
- Boiler design
- Feed water properties
- Boiler operating conditions
- Control of the blowdown is essential to prevent explosions and other problems caused by flash steam and pressure differences.
- Intermittent blowdown is to remove undissolved solids from the boiler. Intermittent blowdown is known as bottom blowdown.
- Although small amounts of solids are diffused throughout the boiler
- In water tube boilers higher concentration occurs at the bottom points of the mud drum.
- But in fire tube boilers higher concentration occurs at the bottom points of the boiler shell.
- Large steam boilers consist of several bottom points of water wall section at its lowest part this intermittent blowdown connection is done
- The intermittent blowdown serves as a drain at the waterside of the boiler.
- The quality of the boiler feed water used defines the amount and frequency of intermittent blowdown.
- For utility boilers, an intermittent blow-down line is opened for a short period as a daily routine task.
- But for large boilers this line is opened only during start-up, shutting down, or at the time of boiler draining.
- Generally, if the working pressure of the boiler is high and is operating at the rated design parameters under these conditions the blow-down valves may not be opened.
What is a Blow-Off valve?
- It is known as a dump valve or compressor bypass valve
- This valve is mounted at the bottom side of the boiler.
- These valves handle the pressure drop from atmospheric pressure to boiler operating pressure.
- This valve provides safe control of hot water and sludge under higher operating conditions.
- The blow-off valve allows the boiler to drain out the water periodically.
- This valve allows for the discharge of mud, scale, sediments, and other solid corrosive impurities that are settled at the bottom side of the boiler.
- These valves must also have the capacity to resist corrosive environments during acid cleaning of boilers and by precipitated solids.
- These blow-down valves help to maintain the surge level of the boiler within desired limits during the initial start-up hours of these high-pressure systems
- Every high-pressure boiler has more than one blow-offvalve, one among them must be a slow opening and closing type with five turns of operating mechanism to change from full closed position to full open position and vice-versa.
- These valves are built in such a way that they can resist various problems in service where high-pressure results in higher velocities that may cause cavitation in the metal surfaces.
- The boiler code requires the valves to have dams or pockets to accumulate sediment particles that cannot be used in blow-off service.
- The special features of the blow off valves are that they don’t allow any sediment particles to settle and collect under the seat.
- An ordinary globe valve cannot be used as a blow-off valve.
- The boiler is isolated from the blow-off tank by the blow-off valves
- The blow-off valve arrangement has a specific valve opening and closing sequence to ensure safe operation and protect valves and piping.
Difference between Blowdown and Blowoff valves:
- A blowdown valve is referred to as a surface blow-off valve
- Blowdown is the process of eliminating sludge and solids particles that are deposited in the mud drum and the bottom side of boiler tubes.
- This blowdown is performed after every 6 to 8 hours or when the boiler experiences a large variation in steam demand.
- The procedure of blow down is opening the valve located away and near the boiler.
- The valve should be kept open for the prescribed time as per the operating procedure of the boiler.
- Blow-off is known as intermittent blow down or bottom blow down.
- It is the process of removing Total Dissolved Solids (TDS), oily materials, and unwanted solid products from boiler water.
- The procedure calls for the testing of the TDS in the boiler and adjustingthe valve surface to raise or lower the TDS.
- Testing and adjustment of TDS are performed as 1 per hour for a boiler operating in a steady state.
- For a boiler that experiences variation in demand and upset testing and adjustment of TDS is performed as more than 1 per hour.
What is the blowdown? How is it done?
Blowdown is the wasting of water from a boiler to avoid the concentration of impurities during the steam evaporation stage.
The water from the boiler is blown out with some force by steam pressure within the boiler.
Application of Blowdown valves:
- To remove sludge, and solids particles to avoid corrosion.
- These valves help to maintain fluid-tightness.
- These valves are used for continuous or intermittent use
Application of Blow Off valves:
- To remove Total Dissolved Solids (TDS), oily materials, and unwanted solids present in boiler feed water.
- This valve helps to maintain surge levels within limits.
- This blowoff valve allows the boiler to drain out the water periodically.