Blowdown valves

A blowdown valve relieves trapped pressure when the compressor is shut down due to a malfunction or for maintenance. The flowsheet shows an automatic blowdown valve. 

Most operators require automatic blowdown valves so that if the compressor shuts down due to a malfunction, the trapped gas will not become a potential hazard. On some small onshore compressors some operators prefer manual valves to make it easier to restart the compressor. The compressor is only blown down for maintenance. 

Often, the blowdown valve is routed to a closed flare system, which services other relief valves in the facility to ensure drat all the gas is vented or flared at a safe location. In such instances, a separate manual blow -down valve piped directly to atmosphere, with nothing else lied in, is also needed. After the compressor is shut down and safely blown down through the flare system, the normal blowdown valve must be closed to block any gas that may enter the flare system from other relief valves. The manual blowdown valve to atmosphere protects the operators from small leaks into the compressor during maintenance operations. 

The blowdown valves on the boilers were operated by a special key, which had a lug on it so that it could not be removed when the valve was open. It was therefore impossible, in theory, for two blowdown valves to be open at the same time. However, the boiler fitter kept and jealously guarded a private key w ithout a lug and had used this one to open the blowdown valve on the boiler that was under repair. He forgot to tell the process foreman what he had done or to close the valve. The presence of this key would appear to have been of little moment as long as the correct procedure of complete isolation was maintained, but as soon as it was departed from, the additional key became a menace, which eventually enabled the present tragedy to occur, the accident report said. 

Emergency blowdown valves are among those that should be tested regularly. Reference 5 describes in detail the measures necessary to test emergency isolation valves when very high reliability is needed. 

The switch operated a solenoid valve, which vented the compressed air line leading to valves in the feed and blowdown lines (Figure 14-3). The feed valve then closed, and the blowdown valve opened. This did not happen instantly because it took a minute or so for the air pressure to fall in the relatively long lines between the solenoid valve and the other valves. 

Blowdown Valve Ball Valve Butterfly Valve... 

AS—Air Supply BD—Blowdown BF—Blind Flange CBD—Continuous Blowdown CD—Closed Drain CH-O—Chain Operated CSO—Car Seal Open CSC—Car Seal Closed DC—Drain Connection EBD—Emerg. Blowdown Valve ESD—Emerg. Shutdown FC—Fail Closed FO—Fail Open HC—Hose Connection IBD—Intermittent Blowdown LO—Lock Open ML—Manual Loading NC—Normally Closed NO—Normally Open OD—Open Drain... 

The simplest and that applied to small boilers is for the main bottom blowdown valve to be opened for a set period at regular intervals (e.g. 20 seconds every 8 hours). This method may also extend to larger boilers where conditions are such that there is little build-up of solids. Such conditions could be high condense returns and good-quality make-up feedwater. 

The second method could be automatic intermittent blowdown. With this, a timer-controlled valve is installed at the bottom of the boiler prior to the main blowdown valve. A program is then designed to operate this valve in short bursts, which disperses any sludge and controls the levels of solids. This method is preferred for boilers having internal treatment. 

The third method would be continuous blowdown through a regulating or micrometer valve. The take-off position for this should preferably be about 250 mm below the working water level and may either be on the side of the shell or on the crown with a dip pipe down to the correct level. If a connection is not available, it is possible to install the valve on the bottom connection prior to the main blowdown valve. 

While the above methods control the level of dissolved and suspended solids in the boiler, it will still be an insurance requirement to operate the main blowdown valve periodically. 

Blowdown from the boiler(s) should always be taken to either a blowdown sump or blowdown vessel before discharging into drains. Both should be adequately sized to give cooling by dilution and be fitted with vent pipes to dissipate pressure safely. The boiler(s) should have independent drain lines for the main manually operated blowdown valve and the drains from a continuous blowdown system. Where more than one boiler is connected to either system the line should be fitted with a check or secondary valve capable of being locked. 

With a blowdown vessel, these may be installed at ground level, and thus the water in them can be above the boiler blowdown valve. In this case, a drain valve for maintenance purposes must be installed at the lowest point in the line between the boiler and the vessel. 

Blowdown on a boiler is mandatory. On small boilers, the required operation of the main blowdown valve may be sufficient to control the quality of water within the boiler. On medium and large plants, additional systems are employed. 

The simplest is a preset continuous blowdown valve to maintain a suitable water quality in the boiler. It is necessary for water quality to be checked frequently and the rate of continuous blowdown adjusted as may be found necessary. 

Thus, to control the buildup of sludge and scale (which may include the prevention of these foulants and deposits in the first place), it periodically becomes necessary to deconcentrate the BW by physically removing some of it from the boiler (blowing down or blowing off). This is achieved via a blowdown valve (BD valve) usually located at the bottom rear of the boiler vessel or WT mud drum. The BD water lost from the boiler is replaced by less concentrated FW. 

Apart from bottom blowdown valves (main BD valve), other types of valves are often employed. Blowdown valves also can be used to control high water levels, drain the boiler for cleaning or inspection purposes, and maintain chemical concentrations and water chemistry below maximum permitted levels. 

On a FT boiler, the main blowdown valve is always located at the bottom of the shell towards the rear of the boiler. On a WT boiler, blowdown valves may be located in the side of the top (steam) drum, the bottom (mud) drum, and the lower water-wall headers (sides, front, and rear). 

NOTE Water tube boiler water-wall headers should not be blown down while the boiler is under load because it disturbs the natural circulation and may result in an overheated and bulging or ruptured tube. Usually header blowdown valves are locked closed and are only blown down when the boiler goes offline. 

Blowdown boiler via bottom blowdown valve Blowdown water columns... 

A PV that receives boiler BD prior to any final discharge to waste. Blowdown valve ... 

They should be located near other emergency devices that may need immediate activation in an emergency (i.e., fire water monitors, manual blowdown valves, etc.). 

In some instances it may be beneficial to maintain process inventories of certain process vessels until the incident is actually threatening the container. The inventory of the vessel may be crucial to the restart of a facility or the contents may be highly valuable. Loss of the inventory may be criticized if frequent false trips of the ESD blowdown system occur. In these cases an automatic fusible plug blowdown valve could be installed which would only activate from the heat of a real fire incident. In this way, a false disposal of the inventory can be avoided. 

Sectionalizing block valves shall be accessible and protected from damage and tampering. If a blowdown valve is involved, it shall be located where the gas can be blown to the atmosphere without undue hazard. 

Blowdown valves shall be provided so that each section of pipeline between main-line valves can be blown down to open atmosphere only. The sizes and capacity of the connections for blowing down the line... 

I-2.2.3 A shutoff valve shall be installed in each takeoff line as near as practicable to the point of takeoff. Blowdown valves shall be installed where necessary for the safe operation of piping, instruments, and equipment. 

An operator working at the main boiler would open the boiler s blowdown valve. Some lime-softened water, about 10 percent of the boiler s total feedwater must be drained to maintain the total dis-solved-solids (TDS) content of the boiler s circulating water below a safe maximum. 

Blowdown valve To drain out water from boiler with high concentration of dissolved solids Deposits of salts may occur on tube affecting efficiency of heat transfer and life of tubes Regular blowdown and water analysis is neeessary... 

Assume that one critical DU-failure has occurred during three years of operation of 35 blowdown valves. 

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