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A boiler drum is a unit which is rated at a certain temperature and pressure to force the boiler feedwater into the vapor phase (steam). As the vapor phase is formed the liquid level in the drum decreases. It is important to maintain control of the liquid level in the boiler drum to prevent damage to the unit. Complex control schemes have been developed control the level of the drum by adjusting the rate of flow, the inlet temperature etc. The steam formed in the boiler drum still contains a certain percent of liquid content. This liquid contained in the steam is referred to as the quality. The two phase system that is formed (liquid & vapor) in the boilerdrum is then cycled though the boiler several times before it is sent to the superheater unit.
The Mud drum is a unit which is located beneath the boiler drum to collect the solid materials which precipitate out of the boiler feedwater due to the high presssure and temperature conditions of the boiler. The process by which suspended solids are collected in the boiler is referred to as cycling. Cycling occurs because the boiler feedwater is sent throught the boiler drum a number of times. This is done to produce the maximum amount of steam per unit volume of feedwater. On each run through the boiler drum, a portion of the boiler feedwater is vaporized. This results in an increase in the solids concentration in the boiler drum. Eventually, the solids concentration hinders the ability to maintain steam generation efficiency. At this time, a stream of compressed air is used to blow the solids into the mud drum beneath the boiler. The mud drum then stores these materials for later disposal. This process of using compressed air to remove the collected suspended solids is referred to as blowdown.
