BOILER BLOWDOWN is necessary for two separate and distinct reasons:
1. To ensure that the concentration of total dissolved solids (TDS) is kept below a certain maximum allowable level.
2. To prevent the accumulation of suspended solids that collect at the bottom of the boiler drum.
BOILER BLOWDOWN should consequently be carried out in two distinct steps:
1. Continuous blowdown from just below the low water level for the purpose of control of Total Dissolved Solids of boiler water.
Continuous blowdown lends itself ideally to recovery of some 80% of heat content and 10 - 20% (depending on boiler pressure) of pure water in the form of condensed flash steam.
2. Bottom blowdown to remove suspended solids should be carried out on an intermittent basis from the bottom of the boiler drum. Specialised valves are available to handle this arduous duty of handling hot boiler water containing solid particles, with reliable shut-off for long periods. The intermittent blowdown can be automated via programmable cycle timers. Discharge of bottom blowdown is to a blowdown vessel at atmospheric pressure. A cooling system may be required to ensure that blowdown temperature going to sewer is within stipulated limits. An exhaust head and silencer may also be necessary to avoid nuisance water and noise emissions from the vent stack of the blowdown vessel
Blowdown vessels are a preferred alternative to blowdown pits. The following information is extracted from HSE Guidance Note PM60 and provides information that may be useful in places other than the UK:
Traditionally, blowdown vessels have had tangential inlets. However, this has meant that the vessels have been structurally weak at the point where the inlet enters.
A preferred alternative is to bring the blowdown line in radially, giving a structurally superior vessel, and then fitting a diffuser inside the vessel. This arrangement also reduces the erosion which could occur inside a vessel with a tangential inlet.
Construction standard
The vessel will need to conform to the European Pressure Equipment Directive (2002) for Group 2 gases. This directive instructs the manufacturer to conform to design and manufacturing standards. As this is a pressure vessel specification, the vessel also needs provision for inspection including an access door and a drain.
Design temperature and pressure
The blowdown vessel design pressure should be at least 25% of the boiler maximum working pressure and the design temperature should be greater than or equal to the saturation temperature for the vessel design pressure.
Fig. 3.14.4
A blowdown vessel installation on a single boiler
Size
This depends on the boiler pressure and blowdown line size, however:
The vent should be large enough, that pressure within the vessel does not exceed 0.35 bar g.
The volume of standing water must ensure that the overflowing water temperature does not exceed 43°C.
1. To ensure that the concentration of total dissolved solids (TDS) is kept below a certain maximum allowable level.
2. To prevent the accumulation of suspended solids that collect at the bottom of the boiler drum.
BOILER BLOWDOWN should consequently be carried out in two distinct steps:
1. Continuous blowdown from just below the low water level for the purpose of control of Total Dissolved Solids of boiler water.
Continuous blowdown lends itself ideally to recovery of some 80% of heat content and 10 - 20% (depending on boiler pressure) of pure water in the form of condensed flash steam.
2. Bottom blowdown to remove suspended solids should be carried out on an intermittent basis from the bottom of the boiler drum. Specialised valves are available to handle this arduous duty of handling hot boiler water containing solid particles, with reliable shut-off for long periods. The intermittent blowdown can be automated via programmable cycle timers. Discharge of bottom blowdown is to a blowdown vessel at atmospheric pressure. A cooling system may be required to ensure that blowdown temperature going to sewer is within stipulated limits. An exhaust head and silencer may also be necessary to avoid nuisance water and noise emissions from the vent stack of the blowdown vessel
Blowdown vessels are a preferred alternative to blowdown pits. The following information is extracted from HSE Guidance Note PM60 and provides information that may be useful in places other than the UK:
Traditionally, blowdown vessels have had tangential inlets. However, this has meant that the vessels have been structurally weak at the point where the inlet enters.
A preferred alternative is to bring the blowdown line in radially, giving a structurally superior vessel, and then fitting a diffuser inside the vessel. This arrangement also reduces the erosion which could occur inside a vessel with a tangential inlet.
Construction standard
The vessel will need to conform to the European Pressure Equipment Directive (2002) for Group 2 gases. This directive instructs the manufacturer to conform to design and manufacturing standards. As this is a pressure vessel specification, the vessel also needs provision for inspection including an access door and a drain.
Design temperature and pressure
The blowdown vessel design pressure should be at least 25% of the boiler maximum working pressure and the design temperature should be greater than or equal to the saturation temperature for the vessel design pressure.
Fig. 3.14.4
A blowdown vessel installation on a single boiler
Size
This depends on the boiler pressure and blowdown line size, however:
The vent should be large enough, that pressure within the vessel does not exceed 0.35 bar g.
The volume of standing water must ensure that the overflowing water temperature does not exceed 43°C.
Operation
The vessel should operate with a quantity of standing water, and the water quantity should be at least twice the quantity of blowdown water. Approximately half of the tank's volume should be occupied by standing water and the remainder as air space.
The vessel should operate with a quantity of standing water, and the water quantity should be at least twice the quantity of blowdown water. Approximately half of the tank's volume should be occupied by standing water and the remainder as air space.
Vent
The vent should ensure that flash steam is vented safely and there is no significant carryover of water at the exit to the vent pipe. The vent should be as straight as possible and ideally terminated with a vent head.
Tapping for a pressure gauge
The vessel must have a tapping for a pressure gauge, as the vessel is manufactured to a pressure vessel specification and regular testing and inspection are required.
Cooling system
A cooling device should be fitted to the vessel if the hot water temperature causes the outlet temperature at blowdown to exceed the permissible limit. The most cost-effective choice for this application is a self-acting control valve. If the temperature exceeds the set temperature, the valve will open and allow cold mains water into the vessel.
Multi-boiler installations
The piping arrangement for multi-boiler installations is covered in the UK HSE Guidance Note (PM60); the following points are made:
The vent should ensure that flash steam is vented safely and there is no significant carryover of water at the exit to the vent pipe. The vent should be as straight as possible and ideally terminated with a vent head.
Tapping for a pressure gauge
The vessel must have a tapping for a pressure gauge, as the vessel is manufactured to a pressure vessel specification and regular testing and inspection are required.
Cooling system
A cooling device should be fitted to the vessel if the hot water temperature causes the outlet temperature at blowdown to exceed the permissible limit. The most cost-effective choice for this application is a self-acting control valve. If the temperature exceeds the set temperature, the valve will open and allow cold mains water into the vessel.
Multi-boiler installations
The piping arrangement for multi-boiler installations is covered in the UK HSE Guidance Note (PM60); the following points are made:
Operation
Only one boiler can be blown down at any one time. In fact, sizing of the blowdown vessel will be based on the highest pressure boiler with the biggest blowdown line size. Reference is also made to the UK Factories Act (1961) which states the same thing.
Piping
Figure 3.14.5 shows the recommended layout for multiple boiler installations where the bottom and TDS blowdown lines are taken back separately to the blowdown vessel. Manifolding should be at the vessel and not at the boiler. Separate connections are required on the vessel for bottom blowdown and for TDS blowdown return lines.
A third connection is also needed on the vessel to comply with UK Guidance Notes (BG01 and INDG436) regarding water level control in boilers. This requires a connection for the blowdown from control chambers and level gauge glasses.
Valving
Where blowdown lines connect into an inlet manifold on the vessel, each must be fitted with either a screw down non-return valve or, a non-return valve and an isolating valve. This is to prevent the possibility of steam and pressurised hot water being blown from one working boiler into another (inside which personnel may be working) during maintenance.
The preference is for two separate valves. The check valve will have to work regularly, hence wear on the seat is inevitable.
Fig. 3.14.5
A blowdown vessel
on a multi-boiler installation
Figure 3.14.5 shows the recommended layout for multiple boiler installations where the bottom and TDS blowdown lines are taken back separately to the blowdown vessel. Manifolding should be at the vessel and not at the boiler. Separate connections are required on the vessel for bottom blowdown and for TDS blowdown return lines.
A third connection is also needed on the vessel to comply with UK Guidance Notes (BG01 and INDG436) regarding water level control in boilers. This requires a connection for the blowdown from control chambers and level gauge glasses.
Valving
Where blowdown lines connect into an inlet manifold on the vessel, each must be fitted with either a screw down non-return valve or, a non-return valve and an isolating valve. This is to prevent the possibility of steam and pressurised hot water being blown from one working boiler into another (inside which personnel may be working) during maintenance.
The preference is for two separate valves. The check valve will have to work regularly, hence wear on the seat is inevitable.
Fig. 3.14.5
A blowdown vessel
on a multi-boiler installation
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