Mud drum

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

Construction is a water-cooled wall combustion chamber connected to a steam drum at high level. The bottoms of the walls are connected to headers. Sometimes a bottom or mud drum is incorporated, but improved water treatment now available does not always necessitate this. 

NOTE Modem power boilers are designed for complete water-wall cooling (no boiler bank). They have very large superheaters, employ balanced draft systems, and typically do not have a mud drum. 

In addition to a top drum, most industrial WT boilers employ a bottom drum mud drum) to assist in the steam-water circulation and to provide for the separation of mud and sludges. 

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. 

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). 

Alpha quartz (aSi02) is found throughout the boiler and post-boiler sections. It is a very hard, adherent scale, found in the mud drum, on boiler tubes, and on turbine blades as a hexagonal crystal structure. 

A further options is to fill the dry, sealed boiler with nitrogen at 3 to 5 psig or dry steam at 5 to 15 psig. If steam is employed, traps on the mud drum or other lowest point remove the condensate. Steam storage has the benefit of keeping the boiler warm, which limits the risk of fireside dampness. 

Many types of WT boiler are of a one-drum design, but where mud drums are fitted, these should also be inspected. As its name suggests, much of the mud, sludge, dislodged scale particles, and other general debris in the boiler ends up in the bottom or mud drum. This material should be removed and the drum inspected for underdeposit corrosion, wall thinning, erosion, and other problems. 

Phosphates typically are found as tan to brown boiler tube deposits but also are often present as sludge in the FT shell or WT mud drum. They almost always are found in combination with carbonate, and some silicate most usually is present as well. 

Major Gen. tubes External corrosion of generating tubes under accumulated dust on mud drum... 

Figure 3 — Angle iron installed in mud drum. vill aid in removing particulates...

Mechanically, an inverted angle-iron of suitable design in the mud drum will aid in removing unwanted particulates from the boiler. Such a device is shown in Fig. 3. 

Steam can be contaminated with soHds even when carryover is not occurring. Contaminated spray attemperating water, used to control superheated steam temperature at the turbine inlet, can introduce solids into steam. A heat exchanger coil may be placed in the boiler mud drum to provide attemperation of the superheated steam. Because the mud drum is at a higher pressure than superheated steam, contamination will occur if leaks develop in the coil. 

The Stirling boiler has four transverse drums, one a bottom mud drum, with bent tubes expanded directly into the riveted sheet metal drums. 

The inclusion of a drop-out area (i.e., mud drum) for steam boilers to aid in accumulating and removing waterside impurities. 

After leaving the autoclaves the ammonia gas is washed and dried by being passed through a series of mud drums and an installation of 7 ammonia columns each 9 feet in diameter by 19 feet high connected by 14 inch pipe to 7 de-phlegmators each 7 feet 2 inches in diameter by 8 feet 3 inches high and 7 condensers similar In size to the dephleg-mators. P rom the condensers, the ammonia gas is stored in two 60,000 cubic feet gas holders. 

INTERMITTENT BLOWDOWN - The blowdown is taken from the mud drum, waterwall headers or the lowest point of circulation. 

MUD DRUM - A pressure chamber of a drum or header type located at the lower extremity of a water tube boiler and fitted with blowoff valve. 

Generating bank tubes have experienced corrosion adjacent to the mud drum in some recovery boilers. Thompson et al. [263] reported field and laboratory results. First, an in situ conductivity probe was used to determine when corrosive salts formed in the boiler at the mud drum/tube area. Then, erosion testing in the laboratory indicated that removal rates were insufficient to account for the tube wastage. Guzi [264] described an automated ultrasonic inspection procedin for use during boiler outages to detect near mud drum corrosion of generating tubes. 

A water-tube boiler consists of an upper steam-generating drum and a lower mud drum connected by three sets of tubes downcomers, risers, and steam-generating tubes. A furnace surrounds and provides heat to the drums and tubes. As heat is applied to the water-generating tubes and drums, the water circulates around the boiler, down the downcomer tube, into the mud drum, and back up the riser tube and steam-generating tubes of the furnace. 

A Steam generator or boiler is used by industry to boil water and produce high-, medium-, or low-pressure steam. The symbol for a boiler closely resembles that for a large water-tube boiler. Boilers are composed of an upper steam-generating drum, a lower mud drum, downcomer tubes, riser tubes, steam-generating tubes, an economizer section, a water make-up system, a stack, a fan, and burners. All of these devices are neatly enclosed inside a refractory-lined shell designed to reflect heat back into the furnace. 

The TDS content of BFW should be minimized by upstream treating. In former days, hot lime water softening was employed. In most modern process units, ion exchange resin is used to demineralize BFW. "Blowdown" is the water that is drained out of the boiler to control the accumulations of TDS or silicates in the boiler. My old design value for this flow was 10 percent of the makeup BFW flow. Two types of blowdown from a boiler are used continuous blowdown from the steam drum, and intermittent blowdown from the mud drum both will reduce silica. Note that with the continuous blowdown, heat recovery equipment may be economically justified. 

Intermittent Blowdown— This blowdown from the mud drum of a boiler is usually once a shift via manually operated valve. If this is automated it is important to have the valve full open for short periods to keep the valve clear and unblocked. 

Figures 22.1, 22.2, and 22.3 illustrate the three types of steamgenerating equipment that are used in process plants. Most process imits do not have the boiler shown in Fig. 22.1. Water flows via thermosyphon circulation (i.e., the density difference between the steam drum and the mud drum). This is the kind of design used for a large boiler of the type found in the Utilities Department of a refinery or power plant.

Mud drum—Bottom part of boiler using thermosyphon water circulation. 

Intermittent blowdown—Used to control suspended solids from mud drum of power plant boiler. 

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