Powder and Bulk Engineering, March 2001 73 Pneumatic points to ponder. .. Building on the information in his h t four seriesof columns,L2 Paul E. Solt, a private consultant with more than 44 years experience in- stalling and troubleshooting pneumatic conveying systems, presents a mth series, on the gen- eral application of pneumatic conveying. In a separate section, Solt also answersyour pneumatic conveyingquestions. n this ninth column on the general application of pneumatic convey- I ing, we’ll apply information from previous columns to improving the operation of a pneumatic conveying system. As you read this month’s col- umn, it may be helpful to review top- ics in previous Information in past columns has cov- ered conveying system types (such as dilute phase and dense phase), mate- rial characteristics, and the best way to convey a material. Now that you understand what you want to happen in a conveying system, how can you tell what’s happening inside the sys- tem, and how can you improve it? For example, plastic pellets, coffee beans, and wheat can be handled in dilute-phaseflow or dense-phaseper- meable piston flow (as discussed in the March 1995 and November 1998 columns), but they don’t convey well at velocities just below the saltation velocity. Below the saltation velocity, the material settles in the conveying line, partially filling it, and then flows in a surging wave that can overpres- sure the conveying system and cause a line plug. This is frequently called the unstable zone for conveying. To minimize fines formation and con- veying line wear and to savepower, it’s best to convey at the lowest velocity possible -just above the saltation ve- locity. So how can you tell at what ve- locity a conveying systemis operating? Understandingwhat’s happening inside the system Use a sight glass. Whether a pneu- matic conveying system is operating above or below the saltation point makes a real difference in trou- bleshooting the system, as discussed in the November 1997 column. You can install a sight glass in the convey- ing line to determine the conveying velocity, but where? In the acceleration zone, which is the first 20 feet of the conveying system (covered in the July 1993 column), material can slide on the line’s bottom as well as move in dilute phase at the line’s center. Although a sight glass in this area would confirm this action, it isn’t really necessary. At the system’s first vertical section, where there isn’t saltation, a sight glass also may not show anything un- less the conveying velocity is so low that the system is conveyingin perme- able piston flow. In either horizontal or vertical sec- tions, a sight glass can sometimes re- veal whether the material is moving in permeable or nonpermeable piston flow. But bear in mind that this would be a higher-pressure conveying sys- tem, and such a system may not allow you to use a sight glass. Listen to the material’s movement. Did I say “listen” to the material? Be- fore you think I’m out of my mind, let me reassure you: Each conveying phase has a different sound that’s easy to distinguish.How can you listen to a conveying line? As long as the line is accessible, clean, and cool, you can place your ear against it. The old me- chanic’s trick of placing a screw- driver’stip against the line and holding the handle against your ear will also work. If you want to look official, you can use a stethoscope. For continuous monitoring, you can install a micro- phone on the line and amplify the sound to any convenient location. Where to listen to the line and what you can expect to hear is detailed in the November 1997 column. To sum- marize, in dilute-phase flow, the sound of the moving material should be steady - like the sound of cars passing at high, uniform speed on an interstate highway. In conveying below the saltation velocity (two- phase flow), you’ll hear steady sound, again like a steady stream of cars passing on the interstate, but this time you’ll hear an occasional 18-wheeler passing too, which is the wave-like surgein two-phase flow. In permeable or nonpermeable piston flow, the sound will be like small, intermittent 0 0 73 ra 5 2. d 0 cn 0 77 m r_ 7i
Transcript
Powder and Bulk Engineering, March 2001 73
Pneumatic points to ponder. ..
Building on the information in his h t four series of columns,L2 Paul E. Solt, a private consultant with more than 44 years experience in- stalling and troubleshooting pneumatic conveying systems, presents a mth series, on the gen- eral application of pneumatic conveying. In a separate section, Solt also answers your pneumatic conveying questions.
n this ninth column on the general application of pneumatic convey- I ing, we’ll apply information from
previous columns to improving the operation of a pneumatic conveying system. As you read this month’s col- umn, it may be helpful to review top- ics in previous
Information in past columns has cov- ered conveying system types (such as dilute phase and dense phase), mate- rial characteristics, and the best way to convey a material. Now that you understand what you want to happen in a conveying system, how can you tell what’s happening inside the sys- tem, and how can you improve it?
For example, plastic pellets, coffee beans, and wheat can be handled in dilute-phase flow or dense-phase per- meable piston flow (as discussed in the March 1995 and November 1998 columns), but they don’t convey well at velocities just below the saltation velocity. Below the saltation velocity,
the material settles in the conveying line, partially filling it, and then flows in a surging wave that can overpres- sure the conveying system and cause a line plug. This is frequently called the unstable zone for conveying.
To minimize fines formation and con- veying line wear and to save power, it’s best to convey at the lowest velocity possible -just above the saltation ve- locity. So how can you tell at what ve- locity a conveying system is operating?
Understanding what’s happening inside the system
Use a sight glass. Whether a pneu- matic conveying system is operating above or below the saltation point makes a real difference in trou- bleshooting the system, as discussed in the November 1997 column. You can install a sight glass in the convey- ing line to determine the conveying velocity, but where?
In the acceleration zone, which is the first 20 feet of the conveying system (covered in the July 1993 column), material can slide on the line’s bottom as well as move in dilute phase at the line’s center. Although a sight glass in this area would confirm this action, it isn’t really necessary.
At the system’s first vertical section, where there isn’t saltation, a sight glass also may not show anything un- less the conveying velocity is so low that the system is conveying in perme- able piston flow.
In either horizontal or vertical sec- tions, a sight glass can sometimes re- veal whether the material is moving in permeable or nonpermeable piston flow. But bear in mind that this would be a higher-pressure conveying sys- tem, and such a system may not allow you to use a sight glass.
Listen to the material’s movement. Did I say “listen” to the material? Be- fore you think I’m out of my mind, let me reassure you: Each conveying phase has a different sound that’s easy to distinguish. How can you listen to a conveying line? As long as the line is accessible, clean, and cool, you can place your ear against it. The old me- chanic’s trick of placing a screw- driver’s tip against the line and holding the handle against your ear will also work. If you want to look official, you can use a stethoscope. For continuous monitoring, you can install a micro- phone on the line and amplify the sound to any convenient location.
Where to listen to the line and what you can expect to hear is detailed in the November 1997 column. To sum- marize, in dilute-phase flow, the sound of the moving material should be steady - like the sound of cars passing at high, uniform speed on an interstate highway. In conveying below the saltation velocity (two- phase flow), you’ll hear steady sound, again like a steady stream of cars passing on the interstate, but this time you’ll hear an occasional 18-wheeler passing too, which is the wave-like surge in two-phase flow. In permeable or nonpermeable piston flow, the sound will be like small, intermittent
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74 Powder and Bulk Engineering, March 2001
truck convoys passing on the inter- state late at night - the sound of pis- tons moving through the line.
Watch the pressure gauge. If a pres- sure (or vacuum) gauge is located on the conveying line near your system’s air mover, use it to check the system’s operating pressure during conveying. Then turn off the material feed to the system and check the pressure again. If the pressure drops rapidly and steadily, the system is probably con- veying in dilute phase. This is because in dilute phase all the material is sus- pended and moving toward the line’s end, and when the feed is stopped, the pressure rapidly falls to the light line pressure (the pressure level when no material is being conveyed in the air- flow) in less than 15 seconds. For ex- ample, if you’re conveying material at 4,000 fpm and the conveying distance is 200 feet, the material should clear the conveying system in 200/4,000 = 0.05 minutes, or 3 seconds.
By listening to material flowing in the line, you can help determine how many pistons per minute pass that loca tion.
However, if you’re conveying mate- rial in two-phase flow, as the pressure starts to drop the airflow entrains some of the settled material, causing the pressure to rise a little. This causes the pressure to drop, rise, drop, rise, while all the time slowly decreasing. Thus it may take several minutes - depending on your conveying line length, conveying velocity, and con- veyed material - for the pressure to drop to the light line pressure.
Estimate a dense-phase system’s pis- ton size and velocity. You need to en- sure that your dense-phase conveying system has strong enough pipe and support structures to withstand the forces applied by pistons flowing around the line bends. By listening to material flowing in the line, you can
help determine how many pistons per minute pass that location. You can then determine how large they are by using your conveying rate. For example, if two pistons pass per minute and you convey material at 500 lb/min, you can safely assume that the average piston weighs 250 pounds. By correcting the system’s airflow to the line’s actual pressure conditions: you can closely estimate the airflow volume passing through the pipe, and by dividing this volume by the line’s cross-sectional area, you can determine the pistons’ ve- locity. You can calculate each piston’s kinetic force as F = %MV2, where F is the kinetic force, Mis the piston’s mass, and Vis the piston’s velocity. This gives an idea of how much strength your con- veying line’s support structure requires.
Improving your system’s operation
Let’s consider how you can apply what you’ve learned in this and past columns to improve your conveying system’s operation. We’ll use the ex- ample of a dilute-phase conveying system operating under pressure.
A practical way to improve system operation. By listening to the line and checking out the calculations, you know that this system is operating well above the saltation velocity. But you think the system’s current airflow volume is wasting horsepower and causing material degradation and line wear. So you want to see whether re- ducing the airflow can improve the system’s operation - without perma- nently changing the conveying phase, without spending much money, and with minimal system downtime.
Start by heading to the air mover. In this example system, the air mover is a lobe-type blower. But you find that it’s driven by a 100-horsepower motor directly connected through a gearbox, making it difficult to change the blower speed for your test without purchasing new gears, which would be costly and time-consuming. So how can you test your theory that re- ducing the blower speed - and thus, the airflow volume - will improve the system’s operation?
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Installing a tuningpipe. You’re going to install a simple tuning pipe on the system to help you measure the effect of incremental airflow reductions on the system’s operation. Here’s how it works:
In this system, the air flows from the blower’s discharge to the material feedpoint. Located between these are a silencer, pressure-relief valve, and check valve. This section of the system layout looks like that in Figure la.
To install the tuning pipe, stop the conveying system. Then take a piece of pipe that is threaded at both ends, about 8 inches long, and the same di- ameter (typically 2 or 4 inches) as the relief valve pipe. Drill and tap about eight %- or %-inch-diameter holes in the pipe. Modify the relief valve with a pipe tee, as shown in Figure lb, and install the pipe on the tee. Then place a pipe plug in each of the holes and a pipe cap on the pipe’s end.
Powder and Bulk Engineering, March 2001 75
Testing system operation at different airflows. Now, start the conveying system, operating it at its normal air- flow volume. Once the system’s oper- ation has stabilized and is up to its full capacity, record the operating pres- sure. Make sure the pressure gauge is operating properly.
Then follow these steps to measure the effect of reducing airflow volume in the system:
1. Remove one pipe plug to bleed some air from the system. (The small-diameter holes ensure that the resulting change in airflow is small, which prevents a large air- flow change that could plug the system.) Caution: Be very careful when removing a pipe plug from the pressurized pipe. To protect yourself from injury, don’t remove a plug if the system is operating above 15 psig.
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2. After the system operates another 5 minutes or at a stable level, record the operating pressure.
3. If the new pressure is lower than the previous pressure, the system is conveying in dilute phase. (If the new pressure is higher, the system is conveying below saltation in either two-phase flow or dense-phase pis- ton flow. Because this means the system is conveying at relatively low speed, it’s less likely to cause material degradation and line wear, so you won’t need to modify the system.)
4. Continue to remove one plug at a time and measure the operating pressure after each, until the pressure doesn’t increase with a plug’s removal. This is the opti-
mal operating condition for di- lute-phase conveying, providing the maximum conveying capac- ity with the minimum airflow volume, minimum conveying ve- locity, minimum material degra- dation, and minimum line wear.
5. Allow the system to operate at this condition for an extended time, even though it’ll be noisy. If the operation becomes unsatis- factory for any reason, replace one pipe plug and continue to monitor the system’s operation.
6. After you’ve determined that the system is operating satisfactorily, use the information in Table I to es- timate how much air in standard cubic feet per minute (scfm) has been bled from the system.
7. Use blower airflow charts (or simi- lar information) from your blower supplier to determine how much slower (in revolutions per minute) the blower should be operated to slow the airflow by the estimated standard cubic feet per minute you determined in step 6.
8. Calculate how much horsepower you can save at this slower blower speed and lower operating pressure to determine if it justifies the cost of a new drive for your blower.
PBE
Endnotes 1. Columns in Paul E. Solt’s first “Pneumatic
points to ponder ...” series (analyzing relationships among pneumatic conveying system operating parameters) appeared in the March, July, and November 1989 and 1990 issues of Powder and Bulk Engineering. Second series (how to apply pneumatic conveying basics to designing a conveying system): March, July, and November 1991, March, July, and December 1992, and March 1993. Third series (design criteria for a pneumatic conveying system): July and November 1993 and March, July, and November 1994
and 1995. Fourth series (troubleshooting pneumatic conveying systems): March, July, and November 1996 and 1997 and March 1998 issues. Previous columns in the fifth series (the general application of pneumatic conveying) appeared in the July and November 1998 and March, July, and November 1999 and 2000 issues. See endnote 2 for information on ordering reprints.
Powder and Bulk Engineering, March 2001 77
2. Three volumes of “Pneumatic points to ponder ...” reprints are available from Powder and Bulk Engineering: Volume 1, 1989 to 1993, Volume 2, 1994 to 1996, and Volume 3, 1997 to 1999. For more information, contact Mary Watt at 612-866-2242, fax 612- 866-1939 ([email protected]).
3. Here’s an example of how to correct your airflow volume to the line’s actual pressure conditions. If you know the airflow volume (scfm) your system uses, determine the actual airflow volume (acfrn) by dividing it by the absolute pressure ratio. Thus, if your system uses 200 scfm at 29.4 psig, use the calculation:
200 X 14.7 (standard pressure) (14.7 + 29.4)
If your system has 4-inch-diameter schedule 40 pipe, the pipe’s cross-sectional area is 0.088 square feet, and the piston velocity would be
66.7 = 757 fpm 0.088
Paul E. So& is an in- dependent consul- tant specializing in pneumatic convey- ing topics for both the American Insti- tute of Chemical En- gineers (AIChE),
New York, and the Center for Profes- sional Advancement, East Brunswick, N.J. Solt has a BS in mechanical engi- neeringfiom Lehigh University, Beth- lehem, Pa., and holds several patents forpneumatic conveying devices. Ifyou have questions about this column or your conveying system, contact the au- thor at Pneumatic Conveying Consul- tants , 5 2 9 South Berks Street, Allentown, PA 18104; 61 0-437-3220, fax 610-437-7935 (e-mail: pccsolt Qentelrnet).
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