Some pumps and pumping applications (e.g., grinding, chemical metering, and pneu-matic ejectors) do not fit neatly into the kinetic or positive-displacement categories.
GRINDING. There are basically two types of sludge-grinding pumps: comminut-ing and pumpcomminut-ing, and comminutcomminut-ing and grindcomminut-ing.
Comminuting and Pumping. Typically used to prevent oversized particles from damaging or plugging downstream piping and equipment, a comminuting and pump-ing device (Figure 8.30) both grinds and pumps liquids and solids. It also may be used to grind scum and screenings. In this device, the solids flow axially to a serrated, wobble-plate rotor, which grinds them and forces them through a matching set of dis-charge bars in the outlet. The size of the resulting particles depends on the size of the rotor’s serrated edges and the spacing of the bars. Because the device functions like a
TABLE8.4 Troubleshooting guide for progressing cavity pumps.
Tighten connections to stop leaks.
Air in suction pipe.
Check motor nameplate data; test voltage, phase, and frequency.
Tighten connections to stop leaks.
Air in suction pipe.
Pump does not discharge
Clean and rinse pump after each use.
Liquid settles and hardens after pump shutdown.
Decrease solids-to-liquid ratio.
Blockage due to solids in liquid.
Reduce liquid temperature or use an undersized rotor.
Stator swells due to high liquid temperature.
Fill with liquid and hand turn. If still tight, lubricate stator with glycerine or liquid soap (household commercial product).
If pump or stator is new, or too much friction.
Remove foreign matter.
Foreign matter in pump.
Check motor nameplate data; test voltage, phase, and frequency.
Incorrect power supply.
Pump does not rotate
TABLE8.5 Troubleshooting guide for rotary lobe pumps (continued on next page). change; if low, add oil.
Gear case oil quantity low or quality poor.
Replace timing gear.
Worn or unsynchronized timing gear.
Inspect bearing; if worn or damaged, replace.
Shaft bearing worn or failed.
Adjust packing gland for 5 to 10 drops/minute; if no adjustment left, replace packing.
Packing gland overtightened.
Sludge should not be above 35°C (95°F).
Sludge temperature too high.
Check sludge thickness; should not exceed 9%.
Check sludge thickness; should not exceed 9%.
Check thickness of sludge; should not exceed 9%.
Sludge too thick, unable to move through lines.
Adjust packing 5 to 10 drops/minute.
Packing leaking excessively.
Check and correct any leaks in inlet valve.
Air entering inlet line.
Check temperature of sludge; if above 35°C (95°F), correct temperature.
Sludge vaporizing in line to pump.
Discharge under capacity
TABLE8.5 Troubleshooting guide for rotary lobe pumps (continued on next page).
Shaft bearing worn or failed.
Should not be above 9%.
Sludge too thick.
Should not exceed 35°C (95°F).
Sludge temperature too high.
Add oil if needed; replace oil if contaminated.
Gear case oil quantity low or poor quality.
Replace timing gear.
Worn or unsynchronized timing gear.
Replace bearing.
Shaft bearing worn or failed.
Check for loose mounts; correct same.
Loose pump and motor mountings.
Properly align sheaves.
Sheaves misaligned.
Check sludge thickness; should not be above 9%.
Sludge too thick.
Check inlet lines for leaks; correct leaks.
Add oil if needed; replace oil if contaminated.
Gear case oil quantity low or poor quality.
Replace timing gear.
Worn or unsynchronized timing gear.
Inspect bearing; if worn or damaged, replace bearing.
Shaft bearing worn or failed.
Properly align sheaves.
Sheaves misaligned.
Check motor for speed; adjust as necessary.
Overspeed.
Adjust gland for 5 to 10 drops/
minute.
TABLE8.5 Troubleshooting guide for rotary lobe pumps (continued on next page).
Problems Causes Solutions
Check and replace as required.
Worn oil seals.
Install vent plug provided or remove and clean in solvent.
Vent plug not installed or is restricted.
Fill or drain to specified oil level.
Incorrect oil level.
Oil leaks
Drain unit and refill with new oil as specified.
Incorrect grade of oil.
Adjust bearing clearance.
Excessive bearing clearance.
Check oil level and fill.
Insufficient lubrication.
Provide shock absorbing coupling or replace unit.
High, repetitive shockloads.
Reduce load or replace unit with one having sufficient capacity.
Overload.
Gear wear or failure
Provide coupling with ability to absorb shock or replace unit with one having adequate service factor.
High, repetitive shockloads.
Reduce overhung load or replace unit.
Check oil level; fill if necessary.
Insufficient lubrication.
Adjust tapered roller bearing to provide proper axial clearance.
Excessive or insufficient bearing clearance.
Reduce overhung load or replace unit with one having sufficient capacity.
Overhung load rating exceeded.
Reduce load or replace unit with one having sufficient capacity.
Overload.
Bearing failure
Drain unit and refill with correct grade of oil.
Incorrect grade of oil.
Adjust tapered roller bearing to provide proper axial clearance.
Excessive or insufficient bearing clearance.
Install vent plug provided or remove and clean in solvent.
Vent plug not installed or is restricted.
Fill or drain to specified oil level.
Incorrect oil level.
Reduce load or replace unit with one having sufficient capacity.
Overload.
Overheating
TABLE8.5 Troubleshooting guide for rotary lobe pumps (continued on next page).
Replace belts in matched set only.
New belts installed with old belts.
Mismatched belts
Check drive design. Check equipment for solid mounting.
Consider use of banded belts.
Excessive belt vibration.
Align idler. Reposition on slack side of drive close to drive sheave.
Flat idler sheave.
Replace set of belts correctly.
Broken cord, caused by
Not enough arc of contact.
Apply proper tension.
Shock load.
Drive squeals
Replace belts; apply proper tension.
Not enough tension.
Belt slip (sidewallks glazed)
Reduce load or replace unit with one having sufficient capacity.
Drain unit and refill with new oil.
Worn or damaged gears.
Reduce load or replace with one having sufficient capacity. Reduce overhung load or replace unit with one having sufficient capacity.
Adjust tapered roller bearing to provide proper axial clearance.
Check oil; fill if necessary.
Worn bearings.
Check and tighten as required.
Loose mounting or coupling.
Noise
Check and tighten as required. If problem still exists, remove clean mating surfaces, apply new sealant, and reassemble.
Loose pipe plugs, backstop cover, or input bracket.
TABLE8.5 Troubleshooting guide for rotary lobe pumps (continued on next page).
Replace set of belts correctly.
Belt pried over sheaves.
centrifugal pump, the influent should be dilute unthickened solids; however, it can break up large trash particles. The unit typically can handle flows ranging from about 1.5 to 20 L/s (25 to 300 gpm).
Comminuting and Grinding. A comminuting and grinding device (Figure 8.31) is not really a pump because its primary purpose is shredding solids. It only produces enough head to force the solids through the grinder. The device uses a high-energy im-pact blade to shear and shred solids before they flow through the slotted openings; de-pending on the application, capacity, and grinding configuration, it can reduce particle sizes to between 6 and 10 mm (0.25 and 0.38 in.). The device typically is used to com-minute previously thickened solids, scum, and screenings to protect downstream de-watering devices from clogging. To reduce the pressure on its seals, the device typi-cally is installed on the suction side of a solids pump. It requires intensive and thorough maintenance.
METERING. Similar to positive-displacement reciprocating pumps, metering pumps are designed to provide controlled discharges. The discharges can be controlled via the
FIGURE8.29 Rotary lobe troubleshooting guide (Courtesy of Vogelsang USA, Inc.).
FIGURE8.29 (continued from previous page).
FIGURE8.29 (continued from previous page).
FIGURE8.30 Trash and sludge grinder combined with a progressing cavity pump.
stroke speed or volume displaced per stroke. Wastewater utilities typically use three types of metering pumps: packed plunger, mechanically actuated diaphragm, and hy-draulically actuated diaphragm.
Overall, metering pumps’ O&M needs are similar to those of positive-displacement pumps. But they also have suction and discharge check valves, which should be in-spected and maintained about once every 6 months to ensure proper discharge metering.
PNEUMATIC EJECTORS. Pneumatic ejectors (Figure 8.32) have been used for years in collection systems—primarily in residential areas with low flows—but they re-quired intensive maintenance, so many utilities replaced them with submersible pump-ing systems. However, the devices are excellent for conveypump-ing scum and floatables in wastewater treatment plants. The Anthony Ragnone Treatment Plant in Genesee County, Michigan, has used one to convey scum for 30 years.
FIGURE8.31 Sludge grinders.
Preliminary Startup. Before starting a pneumatic ejector, operators should do the following:
1. Check, clean, and oil all external pivot points around upper bell rods, linkages, and level arms;
2. Confirm that all air, inlet, and discharge valves are still closed;
3. Open the gate valve on the air exhaust line at the piston valves; and 4. Open the inlet valve and half fill the injector pot.
If the counterweight tends to trip when the fluid enters, hold it up to allow the lower bell to fill with fluid. Then, close the inlet gate valve and balance the counter-weight against the counter-weight of the upper and lower bells. (This is the approximate loca-FIGURE8.32 Pneumatic ejector (in ⫻ 25.4 ⫽ mm).
tion for the counterweight when starting the ejector. Minor counterweight alterations may be necessary once the ejector is in full operation.)
Startup. To start up a pneumatic ejector, operators should do the following:
1. Allow compressors to build pressure to the required amount (the amount will depend on the head needed to convey fluid through the particular pumping system) and leave the controls in the “automatic” position.
2. Open the valve on the air pressure line.
3. Open the valve on the air exhaust line.
4. Open the valve on the discharge line.
5. Open the valve on the inlet line.
6. Close the inlet valve.
7. Bring the ejector up to full operation.
Shutdown. To shut down a pneumatic ejector, operators should do the following:
1. Close the inlet valve.
2. Hold down the counterweight to empty the injector receiver.
3. Close the discharge valve.
4. Close the air exhaust valve.
5. Close the air inlet valve.
Note that the valve in the air pressure line is the first to be opened for start-up and the last to be closed for shutdown.
Testing Procedures. There are several tests for pneumatic ejectors.
Inlet Check Valve Test. To test whether the inlet check valve is properly seated, close the discharge valve and allow the injector to fill and the air pressure to be applied as usual.
If the ejector receiver empties—indicated by falling bells and rising counterweight—
then the inlet check valve is malfunctioning.
Discharge Check Valve Test. To test whether the discharge check valve is properly seated, close the inlet valves, pull down the counterweight, and allow the ejector to empty. If it fills again—indicated by rising bells—then the discharge check valve is malfunctioning.
This test only applies when the discharge pipe can hold enough fluid to fill the ejector when it backflows. Otherwise, the ejector must be half or three-quarters full before the inlet valve is closed so the discharge pipe’s content will be sufficient to fill the ejector.
Piston Valve Test. To test whether the piston valve is functioning properly, remove the heads and note whether the pistons move freely in both directions. (Do this only if the ejector receiver cannot be discharged and exhausted.)
Slide or Pilot Valve Test. To test whether the slide or pilot valve is functioning properly, close the inlet valve, move the piston valve’s piston toward the exhaust end (closing the main pressure port), and turn on a little air. The 6-mm (0.25-in.) ports in one end of the piston valve should emit a blast of air. Then, move the piston toward the inlet end; the first blast should shut off, and the port in the other end of the piston valve should emit a blast of air. These blasts should alternate as the pilot valve lever is moved back and forth. (A little air may blow past the main piston on the pressure end because, with the heads removed, the piston retains less pressure than it would during actual operations.) If the discharge pipe or pump is empty, the air may blow straight through the ejec-tor into the discharge pipe. If the piston valves are positioned so the main pressure ports are open, the pressure would be insufficient to reverse the valve and close these ports, no matter which way the lever is moved. If this happens, leave the lever with the bell end down (the proper position if the ejector is empty and allows air to blow through). Quickly shut off the globe valve on the air pressure pipe. This typically re-verses the piston valve. If it does not, close the inlet and discharge gate valves and al-low air pressure to build up in the ejector. The valve will then reverse, cutting off the air. Open all valves, and repeat this process several times.
Upper Gear Assembly Cleaning. The ejector receiver should be blown out manually at least once a week to dispose of any accumulated scum, floatables, and solids. The easi-est method is to hold down the counterweight for 2 minutes, clearing out all foreign matter from the ejector receiver.
If the counterweight sticks, remove the cotter pin from the upper bell rod and turn the nut to within one thread of the top. Then, tighten the lower nut against the trun-nion. Doing this will postpone the need to completely disassemble the gear for about 6 to 8 months, depending on existing conditions.
When the counterweight sticks again, remove the gear assembly, clean the upper bell and the area around the receiver’s throat, and reassemble.