System Diagnostics and Testing

“The best laid plans o' mice an' men gang aft agley" (Robert Burns). Loose translation: Despite your best efforts, some things just go wrong.

This book focuses on selecting the proper air pollution control equipment for a given application. What happens if, however, you are “stuck" with what you have? Perhaps you do not need to replace it (or can delay replacing it). Maybe a repair or upgrade is in order.

Readers of the first edition of the Air Pollution Control Equipment Selection Guide requested that a chapter be added on problem solving for one of the most popular pieces of gas cleaning equipment, wet scrubbers. Given the popularity of that section, it was retained and updated based upon recent site-specific experiences. It is still true that the best of designs simply fail to work properly. Sometimes the process changes. Sometimes the economy changes and there simply are not enough funds to make a change. The pressure drop may be too high or the efficiency too low, chemical may be wasted, or assorted miscellaneous problems could arise. How does one go about correcting these problems? In this chapter, some proven ways of diagnosing and correcting some common and not-so-common problems with wet gas cleaning systems will be discussed. Many techniques mentioned also apply to dry scrubbers and may also be instructive for the reader.

Tools

First, you need the right tools.

To diagnose most scrubber problems, you could need any or all the following:

  • 1. Copy of operating manual (or, lacking that, a copy of the purchase order and quotation for the equipment)
  • 2. Copy of all stack tests on the system (particularly the most recent ones)
  • 3. Copy of operating records, charts, datalogger output, etc.
  • 4. Name and contact information of the person(s) operating the system
  • 5. Name and contact information of the person(s) maintaining the system
  • 6. A Dwyer Magnehelic™ gauge, or equal, of a range greater than the total differential pressure of the system (see Figure 24.1)
  • 7. A pitot tube (such as the type S) and manometer (see Figures 24.2 and 24.3)
  • 8. A pH probe and meter
  • 9. An oxidation reduction potential (ORP) probe and meter if it is an odor control system
  • 10. A 4-20 ma signal injector or calibrator if the system uses a 4-20 ma control loop.
  • 11. A volt ohm meter (VOM)
  • 12. Portable thermometer or temperature gauge
  • 13. If available, a clamp-on liquid flow meter (rentable if you do not have one)
  • 14. Tachometer (to check fan speed)
  • 15. Clip-on ammeter (or another suitable meter to measure fan and pump amperage)

FIGURE 24.1

Dwyer Magnehelic for pressure measurement. (Dwyer Instruments, Inc.)

FIGURE 24.3

Manometer for velocity pressure measurement. (Dwyer Instruments, Inc.)

The Dwyer gauge allows you to measure the pressure loss across individual system components or, if the gauge has sufficient range, the pressure drop across the entire system. Changes in pressure can be indicative of mechanical difficulties (worn internal components, scaling, etc.) and are critical for your diagnosis. These gauges come in various ranges, and common ones for testing include a range of 0-2 inches water column (w.c.) and 0 to the maximum static pressure of the fan or other prime mover.

The one shown is a type S pitot tube. It is used for measuring the gas velocity pressure from which the gas flow rate can be calculated. A thermometer is also used to obtain the gas temperature. Given a measurement or an assumption of the gas humidity and the internal dimensions of the ductwork, the gas volume at the measuring point can be obtained. Accurate gas volume readings require at least four duct diameters ahead of the pitot tube and at least two duct diameters after the pitot tube. This is often not possible for diagnostics. Instead, a traverse of the duct is made (simply sliding the pitot tube from near duct wall to far duct wall) on the same plane and the peak velocity pressure is recorded. The average gas velocity pressure is usually about 80%-85% of that reading. If you are consistent in your measuring technique, the relative gas velocities through the system can be used to compare the gas flows at various points in the system.

The velocity pressure itself is measured using a liquid-filled inclined manometer as shown in Figure 24.3 or an electronic manometer. The readings are typically only in terms of less than 2 inches w.c., so a precise instrument is needed. In a pinch, a 0-2-inch Magnehelic may be used. The measuring device needs to be zeroed and in the case of the liquid-filled be leveled.

Isolate and Correct the Problem

A simple, multiple-step procedure is suggested to isolate and correct the problem:

  • 1. Define the problem.
  • 2. Inspect the scrubber and/or system.
  • 3. Baseline the system.
  • 4. Search for a detailed solution to the problem.
  • 5. Fix the problem.

These steps are detailed in the remaining sections of this chapter.

Define the Problem

This obvious first step is often overlooked. Sometimes, we tend to run headlong toward solving problems rather than pause to think about what the problem really may be.

One way you can define the problem is by asking the right people the right questions. Who better to ask than the people running the system day after day?

Determine from the person responsible for operating the system the exact, if possible, behavior of the system that is unacceptable. Visible emission? High pressure drop? Excessive chemical consumption? Scaling? Interview both the operating person and the maintenance person (sometimes one and the same individual) to find out the type of problem at hand. Did the problem occur suddenly? Is it recurring? If so, at what intervals? Does it seem process related (triggered at a certain time of a process cycle)? Did it occur before and was temporarily fixed? If so, how was it fixed? How long did the "fix" last? What is the maintenance history? When was the last time any control instrumentation (usually pH and ORP meters) was calibrated? Any fan noises? Vibration? Odd sounds or other abnormal events (such as overheating, running pump dry, etc.)? Did the process change recently? Any major adjustments made recently? If so, what, and why?

After you get this information, find out what the system is supposed to do. Read the quotation and purchase order if available. Read the operating manual (Note: The number one cause of operating problems is the failure to read the operating manual). Did it ever do what it was supposed to do? If not, was the original vendor called in to fix it? If so, what was done? Who did it? You may have to call that person.

Sometimes, what the system is supposed to do is different from what the user expects or what the designer intended. This does not mean the system cannot do what might be expected, but it might mean that major changes are needed. Try to define the problem in the context of what the user expects.

Inspect the Scrubber and/or System

Make a visual inspection of the system. A “walk around" is the best method. Is what you see the same as described in the operating manual and/or quotation? If not, get an explanation from the people operating the system. If the unit or system was modified, find out why, when, how, and by whom. Get a phone number of the person(s) who made any changes (you might need to call them later to explain unusual changes in the system).

Baseline the System

To baseline the system, check all instrumentation and controls to make certain that they are within the intended operating parameters outlined in the quotation, purchase order, or operating manual. If possible, remove and calibrate all pH and ORP controls. Take a liquid sample and compare the readings to a calibrated pH and ORP meter (see Section 24.1, “Tools," above). If the readings differ, check the temperature compensation of the process probe (you may have to contact the vendor). Many modern pH controllers allow you to access the solution temperature from the controller's panel. Take a manual temperature reading using a thermometer or portable instrument. Do they agree? Many times, a false pH reading is caused by a failure of the temperature compensation circuit. If the readings do not agree, check the probe-to-controller wiring. Often a loose wire from the temperature compensation circuit can cause a false reading.

If the system has a recycle flow and/or blowdown flow indicator, does this agree with the design? A clamp-on Doppler type flow meter can be a handy check of these flows. If the flow rate is out of specification, note if the flow is higher or lower than normal. If lower, the problem could be a simple valve setting, line plugging, or pump impeller problem. If higher than normal, a valve may be too far open, or some flow resistance (such as a spray nozzle or two) that had been present is now gone.

Try to set all operating parameters as near to the design as possible. Many times, after doing so, the problem will resolve itself. If the problem is still present, you need to get into the details.

 
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