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Search for Detailed Solution to the Problem

You should have enough background information at this point that you can focus on the details and isolate the problem. In doing so, you will likely find that you have multiple small problems, effectively in layers. Go after the most obvious ones first. Sometimes, fixing the obvious cures the hidden problem as well.

Inspecting certain subsystems can isolate common problems. These subsystems include the following:

  • 1. The gas cleaning device itself (the inlet flange to outlet flange unit)
  • 2. The liquid circuit (pump, valves, piping, nozzles, etc.)
  • 3. The instrumentation (pH and ORP controls, level controls, etc.)
  • 4. The gas moving device (usually a fan)
  • 5. The gas discharge device (usually a stack)

Gas Cleaning Device

It would take many additional chapters of this book to describe various minor problems with wet scrubbers that could cause improper performance. Some common problems are addressed in this section. If you compare the suggestions to the symptoms you find during your Interview and Inspection, you may find the solution to at least part, if not all, of your problem. With certain exceptions (such as mist chamber scrubbers), most wet scrubbers exhibit a gas pressure loss through the unit. The loss of pressure is the result of changes in direction or acceleration or deceleration of the gas in the scrubber.

24.2.4.1.1 Lower than Normal Pressure Drop

If the pressure loss is lower than design and the gas and liquid flow conditions are at or near design, this indicates a change in an internal device. For example, a baffle that once was in place may now be corroded or eroded away. In cyclonic separators, the loss of an inlet baffle could be a contributor to an entrainment problem. An internal inspection of the scrubber would reveal the condition of such a baffle. For packed towers, a lower-than-normal pressure drop could indicate gas channeling in the packing caused by improper liquid distribution. An improper or insufficient spray of scrubbing liquid into the unit could cause a lower-than-normal pressure drop in a dynamic scrubber (such as a sprayed fan design).

If the gas flow is inherently lower than the original design, contact the original vendor or a scrubber consultant to have them estimate the expected pressure drop at that new flow condition. Most scrubbers have pressure drops that are in proportion to the gas flow rate. If you cannot find someone to estimate the expected pressure drop, factor the pressure drop by the ratio of the current gas volume to the design gas volume. If the result is a pressure drop that is still too low, this indicates a problem with an internal component that alters the direction of the gas or is intended to accelerate it (such as a Venturi throat, orifice plate, impingement plate, etc.). Focus your attention on that type of device.

An example is a tray type scrubber that exhibits a lower-than-normal pressure drop after having operated close to design for years. Many of these tray towers have removable trays consisting of small, perforated openings (often less than 3/16-inch diameter). If these trays lift or shift (say, from a sudden puff of gas or other upset), they may not return to a seated and sealed condition. The pressure drop will be abnormally low. The remedy is to remove and straighten (or even replace) the trays, making certain that they are fully seated.

A low pressure drop in a tray scrubber that uses internal liquid management weirs or downcomers (seal boxes) could be caused by a failure of those devices. Weirs are sometimes used at the end of a tray to hold up a minimum depth of liquid. If the weir corrodes, the liquid depth will decrease, yielding a lower pressure drop. If a downcomer (which lets liquid descend from an upper tray but normally prevents gas from bypassing the tray) loses seal, the pressure drop will be lower. The remedy is to increase the seal leg depth. This often requires an internal inspection. Some firms offer X-ray or ultrasonic inspections of such towers "on the run," thus allowing such problems to be more easily diagnosed.

A sprayed fan scrubber might exhibit a lower pressure drop if the inlet spray nozzle pattern changes or the nozzle plugs. Usually, the gas volume will increase along, yet the fan amperage may drop (since the fan is not moving the water as before). Using a suitable ammeter, check the fan amperage versus design. Check the liquid flow (see next section). You could find that you have a nozzle with an altered spray pattern (say, from scaling). Replacing or cleaning the nozzle may restore the scrubber to its design performance. Usually, the sound of these scrubbers will change if the wheel is improperly loaded with liquid.

A packed tower exhibiting gas channeling usually results in poor removal efficiency since the gas bypasses through the scrubber essentially untouched by the liquid. If you open the top inspection door (if provided) or can otherwise see the top of the packing, you may see that it is displaced (you will see a mountain or a valley of packing—it will not be level). High gas velocities can throw packing from the center of the tower toward the wall, leaving a center valley. Excessive liquid pressure in towers equipped with spray type liquid distributors can blast the packing toward the center, leaving a mountain of packing there. The solution to the problem may involve improved liquid distribution and/or lower liquid pressures or the addition of a holddown grating to hold the packing in place. If the tower has more than 8 feet of packing, the problem may be caused by the lack of a redistribution to move liquid from the wall back to the center of the tower.

On Venturi scrubbers, wear, or damage to a component in the throat zone (restriction) usually causes low pressure drop. Often, a simple plate is used (see Chapter 19, "Venturi Scrubbers"), and it wears away over time. Replacing this element often solves the pressure-drop problem. If the throat is intact and at or near specification, the problem usually is liquid-distribution related. For throats greater than 12 inches in width, a center spray is often needed to fill the center area of the throat with liquid.

For cyclonic separators, an incorrectly sized tangential inlet baffle is usually the cause of low pressure drop. These baffles also wear away with time. Another symptom is entrainment from the separator. Often, on induced draft systems, the drain leg from the separator may not be sealed under liquid. This can allow air to be drawn up through the drain, reducing the overall pressure drop of the unit and usually causing erratic pressure drops (sometimes, even noise from the vessel). Sealing the drain under liquid usually solves the problem.

24.2.4.1.2 High Pressure Drop

Higher than normal or expected pressure drop usually means that a restriction has been created, perhaps suddenly or over time, in the scrubber. A plate could shift and restrict a gas passage. Scaling from water hardness or other chemical reactions inside the scrubber could cause gas flow restricting effects that raise pressure drop.

On Venturi scrubbers, if the throat is adjustable, the movable portion may have been set too far closed. Obviously, the solution is to open the throat until a suitable pressure drop is obtained.

On cyclonic separators, if the sump level rises to the level of the gas inlet, the liquid could offer additional gas flow resistance, resulting in excessive pressure drop. Lowering the liquid and/or installing a sealed overflow connection to prevent recurrence of the problem usually is a solution. Other times, the swirling of the liquid in the separator sump can cause the liquid to rise at the wall, restricting airflow and usually causing entrainment. Antiswirl baffles under the liquid level are a typical solution to that problem. These baffles are normally 6-8 inches high and run from the periphery of the separator sump toward the drain opening such that any swirling liquid must jump over this baffle. Most often, only one or two such baffles are needed.

Packed towers usually exhibit high pressure drop at design gas and liquid flow rates if the packing is plugged. Some systems can be acid or caustic washed (sometimes on the run if the operating permit allows such maintenance) by adjusting the recycle pH temporarily. Others must be shut down entirely, the packing removed, and replaced with new or cleaned material. Investing in a water softener for the make-up water can sometimes reduce chronic scaling. This is common in hydrogen fluoride scrubbers where a carbonate and silicate reaction can produce troublesome hard scale.

Dynamic scrubbers showing high pressure drop usually have restrictions in the gas outlets from the devices or exhibit higher than normal liquid rates. The motor amperage usually is also high if the liquid is to blame.

Tray towers have high pressure drops if the tray openings begin to plug. Sometimes the trays can be removed and be cleaned. Often, however, the tray pluggage requires replacement of the trays with new ones.

 
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