Mastering the Obvious (a long winded story)
Mastering the Obvious (a long winded story)
(OP)
Warning: Long winded story, I won't be offended if you skip it. I just had to share this with all of you in the Drives industry with me. No help needed, but I thought some of you would get a kick out of this story.
I got a call this week to go out and help a contractor program a PID loop on a VFD he installed. He has used me for this in the past and we work well together, but usually he also uses me to help him convince his customers to use VFDs in the first place. This time it was different, his end user contracted with him to just supply and install a drive on a pump application that they had already purchased.
The PID loop was a breeze, extremely simple level control loop for a 100HP centrifugal pump emptying a holding tank of waste-water coming from a gravel wash plant. The water came off of the various processes by gravity into a holding tank, then was pumped up hill into a settling pond. The total dynamic head; only 13 feet above the bottom of the tank. The tank was 7 feet deep. After the fact I thought a VFD was overkill but I wasn't hired to consult on this in the beginning, just to make it work. They had an existing pump that was being replaced by a new "more efficient design" that would save them energy and were told the VFD would compound that energy savings by modulating speed. Remember, I did NOT get the consult contract on this marvel of engineering thought. The fact that there was no throttling valve to be replaced by the VFD nor a solid reason to even bother maintaining a level in the tank was apparently never even brought up (until I got there). Still, I went about the job I was hired to do and programmed the PID loop and the ultrasonic level transducer like a good little soldier. As I said, a piece of cake.
The only "minor" problem I encountered was that the tank overflowed! Why? Because at full speed, the pump was not pumping out as much fluid as was coming into the tank! Hello??? Rocket Science here? I think not. Bottom line, the pump was too small for the task. My assessment took all of about 38 seconds, the time it took for the tank to overflow with the new pump running full speed AFTER the old pump was turned off. So they all look at me and start pointing fingers that I had not "tuned the drive and level controller" properly. I explained that this had NOTHING to do with "tuning" anything. The VFD was at FULL SPEED the entire time (after about a 5 second ramp soak from the loop). For some reason, all these other genius mechanical engineers had a difficult time grasping my 38 second conclusion. I saw more graphing calculators getting whipped out in front of me than I have seen in one place in my entire life. All of them attempting to prove to ME that I am wrong about this. Meanwhile, THE TANK IS STILL OVERFLOWING BEHIND THEM!!!! I soooooo wish I had set up a video camera, the ironic humor video would have been a gold mine!
So I investigate, because obviously I have made some kind of grave error in my assessment of the dirty water flowing over the top of the tank. Something MUST be wrong with the VFD, motor etc. Even though I had nothing to do with the selection, I was having a bit of fun with this, so I started poking around. Here is what I found.
Old pump: 300HP 460V, rated capacity; 14,400 GPM
New Pump: 100HP 460V, rated capacity; 4250 GPM at rated speed
Apparently they thought that somehow the VFD was supposed to magically "fix" this disparity. Granted, the outflow pipe had been significantly shortened from the original design that needed the 300HP pump, so that was why the "geniuses" had come up with only needing 100HP. I said, "Why not just put a VFD on the existing pump and modulate the speed, using only as much power as you need? You would get the same net effect and not have to put in another pump." Their answer: "The 300HP VFD would have been way too expensive." Me; "Compared to a 100HP VFD, new service to feed it, 100HP motor and new pump?" No clear answer came to THAT piece of pretzel logic.
Instead they said "Can't the VFD run the pump faster than 60Hz?" Absolutely I said, but that will do you NO GOOD. "Do it anyway!" So I did, restarted the VFD and of course the pump accelerated, filled the outlet pipe, then promptly slowed down until the VFD took it off-line on OL trip. "See, that VFD is too small. THAT's the problem!" they said. Having not sold the VFD I technically shouldn't have cared, but now I was getting angry because they were implying that somehow this was an ELECTRICAL problem, essentially besmirching my chosen profession! I conducted an in-situ class on VFD technology and what you can and cannot expect from them. I finally convinced a few key management types (the crowd was growing bigger every minute) that maybe this WAS a mechanical problem. It was a tough sell, but the WATER CASCADING OVER THE TOP OF THE TANK helped my argument a lot. They finally let me leave, but I have to be on call all next week for when they call a big meeting on this, because the MEs are STILL convince that their calculations are correct, and this IS an electrical problem.
What tickles me most now is, I will get paid handsomely to sit in on that meeting as the MEs attempt to convince everyone in the room that the water was NOT flowing over the top of that tank, I guess! In addition, one of the managers said "From now on, we will hire you to evaluate all vendor proposals that include VFDs."
Bottom line, I win.
I got a call this week to go out and help a contractor program a PID loop on a VFD he installed. He has used me for this in the past and we work well together, but usually he also uses me to help him convince his customers to use VFDs in the first place. This time it was different, his end user contracted with him to just supply and install a drive on a pump application that they had already purchased.
The PID loop was a breeze, extremely simple level control loop for a 100HP centrifugal pump emptying a holding tank of waste-water coming from a gravel wash plant. The water came off of the various processes by gravity into a holding tank, then was pumped up hill into a settling pond. The total dynamic head; only 13 feet above the bottom of the tank. The tank was 7 feet deep. After the fact I thought a VFD was overkill but I wasn't hired to consult on this in the beginning, just to make it work. They had an existing pump that was being replaced by a new "more efficient design" that would save them energy and were told the VFD would compound that energy savings by modulating speed. Remember, I did NOT get the consult contract on this marvel of engineering thought. The fact that there was no throttling valve to be replaced by the VFD nor a solid reason to even bother maintaining a level in the tank was apparently never even brought up (until I got there). Still, I went about the job I was hired to do and programmed the PID loop and the ultrasonic level transducer like a good little soldier. As I said, a piece of cake.
The only "minor" problem I encountered was that the tank overflowed! Why? Because at full speed, the pump was not pumping out as much fluid as was coming into the tank! Hello??? Rocket Science here? I think not. Bottom line, the pump was too small for the task. My assessment took all of about 38 seconds, the time it took for the tank to overflow with the new pump running full speed AFTER the old pump was turned off. So they all look at me and start pointing fingers that I had not "tuned the drive and level controller" properly. I explained that this had NOTHING to do with "tuning" anything. The VFD was at FULL SPEED the entire time (after about a 5 second ramp soak from the loop). For some reason, all these other genius mechanical engineers had a difficult time grasping my 38 second conclusion. I saw more graphing calculators getting whipped out in front of me than I have seen in one place in my entire life. All of them attempting to prove to ME that I am wrong about this. Meanwhile, THE TANK IS STILL OVERFLOWING BEHIND THEM!!!! I soooooo wish I had set up a video camera, the ironic humor video would have been a gold mine!
So I investigate, because obviously I have made some kind of grave error in my assessment of the dirty water flowing over the top of the tank. Something MUST be wrong with the VFD, motor etc. Even though I had nothing to do with the selection, I was having a bit of fun with this, so I started poking around. Here is what I found.
Old pump: 300HP 460V, rated capacity; 14,400 GPM
New Pump: 100HP 460V, rated capacity; 4250 GPM at rated speed
Apparently they thought that somehow the VFD was supposed to magically "fix" this disparity. Granted, the outflow pipe had been significantly shortened from the original design that needed the 300HP pump, so that was why the "geniuses" had come up with only needing 100HP. I said, "Why not just put a VFD on the existing pump and modulate the speed, using only as much power as you need? You would get the same net effect and not have to put in another pump." Their answer: "The 300HP VFD would have been way too expensive." Me; "Compared to a 100HP VFD, new service to feed it, 100HP motor and new pump?" No clear answer came to THAT piece of pretzel logic.
Instead they said "Can't the VFD run the pump faster than 60Hz?" Absolutely I said, but that will do you NO GOOD. "Do it anyway!" So I did, restarted the VFD and of course the pump accelerated, filled the outlet pipe, then promptly slowed down until the VFD took it off-line on OL trip. "See, that VFD is too small. THAT's the problem!" they said. Having not sold the VFD I technically shouldn't have cared, but now I was getting angry because they were implying that somehow this was an ELECTRICAL problem, essentially besmirching my chosen profession! I conducted an in-situ class on VFD technology and what you can and cannot expect from them. I finally convinced a few key management types (the crowd was growing bigger every minute) that maybe this WAS a mechanical problem. It was a tough sell, but the WATER CASCADING OVER THE TOP OF THE TANK helped my argument a lot. They finally let me leave, but I have to be on call all next week for when they call a big meeting on this, because the MEs are STILL convince that their calculations are correct, and this IS an electrical problem.
What tickles me most now is, I will get paid handsomely to sit in on that meeting as the MEs attempt to convince everyone in the room that the water was NOT flowing over the top of that tank, I guess! In addition, one of the managers said "From now on, we will hire you to evaluate all vendor proposals that include VFDs."
Bottom line, I win.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376





RE: Mastering the Obvious (a long winded story)
I got a phone call from that paper mill. "There's a problem with the drive on top roll in second press. We think that the controller needs retuning".
Went there. Found that there was no controller for the top roll. Hmm... No VFD or SCR stuff either. Hmm... Visited the machine. No motor on top roll!!! The press had only the bottom roll driven and that drive was rock steady.
Time for the Meeting with Production Manager and the MEs. Someone had put together a Nyquist diagram to educate me on loop stability. I told the assembly that Nyquist and Bode are fine and handy but we didn't need it in this situation.
"But the rolls slows down! We get web breaks!"
"There must be something wrong in the mechanics, then" I dared to say. "Or the process, perhaps". This meeting was a very long meeting. Suddenly, a teacher from a nearby school appeared. He was summonned to run a check on my abilities in loop tuning. He knew me very well, I had been lecturing at that school, and he was surprised to find me in a situation like this.
That took them down a bit. But just a bit. "So where is that wrong, then?" They asked. "I have no idea", I answered. "Have you studied what happens in the machine?"
They had not. Why should they? This was, after all, a loop tuning problem.
I talked to the operators. Those guys know their machine and know what is happeneing. It turned out that the flow meter for an additive (retention agent) had been replaced and, as they said, "We buy ten times more of that stuff now". "So you use ten times as much?" I asked. No, same flow rate. Finally found out that someone had put the decimal point in the display in the wrong place. That retention agent is more or less like wall-paper glue and is used in low concentration to bind the small fraction fibres. Using ten times more made the pulp slippery, which made the top roll slip. The End.
Gunnar Englund
www.gke.org
RE: Mastering the Obvious (a long winded story)
My favorite (not a motor story, but a blame the PID story)
Chrome plating tank upgrade puts in a more efficient heat exchanger for cooling the plating solution.
Complaint: the controller isn't working. It allows the plating bath to rapidly overheat.
Controller was spec'd by an engineering firm, but my company sold it, so I get called in because the tank is rapidly overheating and the controller isn't "working". The former controller was a pneumatic controller of 1950's vintage, the new one a digital PID controller. Interesting that they didn't blame the controller's tuning, they didn't know PID controllers needed tuning.
Sure enough, when I got there the factory tuning constants were the ones in use (10% PB, 1 min reset, some minimal rate value).
I cycled the valve, just to make sure the loop was functional and then they ran a load so I could see the response - tremendous heat input when the current turned. The PV rapidly went way beyond SP so the PV was outside PB within seconds and the controller was calling for 100%. At 100% output, it wasn't terribly obvious that any cooling was happening. I check the valve to make sure, it's wide open and it's cold, but I'm getting suspicious.
So I put in tuning constants to produce on-off action to see how much cooling the loop could actually produce. 0.1° off setpoint and the controller calls for 100%. I explain to the plant engineer who called me in, what I was doing. He did understand the basics of a thermostat and that a call for cooling at 100% means valve wide open and that that's all the controller is supposed to do or can do, is make the call. It can't provide the actual cooling, the heat exchanger does that.
We have to wait for the tank to cool below SP before we can run a second load. Painfully long time (that tank cools awfully slowly).
The second load starts. The temp skyrockets even with the on-off tuning constants. Not a lot of difference from the first load.
I'd set the display to read % output from the start and within a couple seconds after the current is turned on (just thermocouple sensing lag time), the controller calls for 100%. We check the modulating valve and it's wide open.
We start poking around (this plant engineer had not been involved in any aspects of the upgrade) and sure enough, the new "more efficient" heat exchanger is less than 1/2 the capacity of the old one. The operators said that the reason for the upgrade was that the old heat exchanger "couldn't keep up".
I listened to the plant engineer's end of the phone conversation when he called the engineering firm.
"Let me understand this, you're saying the greater efficiency of the new digital electronic controller compared to the old pneumatic controller is supposed to make up the difference in the capacities of the new and old heat exchangers ? ? ?"
I was out of there. He realized the controller had no bearing on the inability to cool the tank. But it was amazing that someone at an engineering firm got their 'efficiencies' so mixed up.
Dan
RE: Mastering the Obvious (a long winded story)
More more more more more...
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
The soft starter was a very new technology for the client and was driving a pulp refiner which was two cones, one rotating inside the other. The inside surfaces of the cones had a series of ribs around them and the slurry was pumped between them.
A KW transducer was used to measure the power input to the refinner and the PLC used that information to control a valve feeding the refiner.
The reported problem was that the system was not working correctly because the harmonics produced by the soft starter were interfering with the PLC input. I was even sent a chart recording showing a very furry waveform at the PLC input. It was real bad, with "noise" swinging from about 10% load to 150% load. The problem was, that the sfot starter had in internal bypass contactor and so was not producing any harmonics at all. We discuyssed this by phone and fax to no avail. In the end, I jumped on a plane with an oscilloscope and flew over.
The native were very friendly, especialy with the distance that I had travelled to get there. I proceeded to explain that if there was a harmonic problem, we would see it on the voltage waveform of the scope. I also explained that provided that the bypass contactor was closed, the soft starter could not produce harmonics. They were very attentive and followed these arguments, expecing that when I put the scope on, I would discover some new criteria and then set out to solve the problem with the soft starter.
We agreed a plan and off we went. The starter was running, the bypass copntactor was closed and the waveforms were perfectly clear. - "Not the soft starter I stated."
All of a sudden, the natives became hostile. They could imagine me leaving site with a clean slate and them still with the problem.
Back to negotiation. - They would let me leave site when the soft starter was fixed. But it wasn't broken, but it must be, look at the input to the PLC. I pointed out that mechanically, the refiner would be presenting a rapidly changing load and I believed that the waveform that there were seeing reflected the instantaneous load changes. They could follow the argument, but that did not solve the problem. "fix the soft starter to stop it happeneing". The real problem was that the KW transducer was extermely fast and had no internal averaging or filtering. - wrong transducer! I suggested that the transducer was to fast. "No, the transducer is what the manufacturer recommended, and it has been back to the factory three times for checking". I pointed out that the transducer was not faulty, just too fast for the operation. Somewhere, there needed to be some form of filter, either in the output of the transducer, or in the PLC input. They could sense an escape attempt here and locked all exits.
I suggested that in order to prove the point, I could temporarily ad a capacitor to the output amplifier of the transducer and slow it down. This could possibly void the waranty, but it would hide the problem. The natives became friendly again as a possible solution was in sight. I added the capacitor, and the problem went away, I was then allowed to leave.
System problems are often blamed on "new" equipment or technologies.
Mark Empson
http://www.lmphotonics.com
RE: Mastering the Obvious (a long winded story)
I'd be willing to bet that to this day, those "natives" still tell people that soft starters are fraught with problems and use that example as proof!
Today is Saturday here, I'm in my office because I just got a call from the VFD user in my story. They split a pipe in the process system and now they are pointing the finger at the VFD again; after all, that is the only thing that has changed in the last 10 years. I pointed out to them that the unit was Off when I left it on Thursday. They checked it out (after the fact of course) and found it was indeed Off, in fact locked out by their electrician. I then asked which pipe split open. It was one of the SUPPLY pipes! It was everything I could do not to laugh at them on the phone.
Oh well, another billable hour, and at my weekend rates too!
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Mastering the Obvious (a long winded story)
Mark Empson
http://www.lmphotonics.com
RE: Mastering the Obvious (a long winded story)
I actually like it. No hard work. Money galore. And, if you tell them in a nice and friendly manner, you are their hero and friend. Happens all the time. Like that printing press. Got a phone call late evening. "The READY lamp doesn't light up". I asked all the questions. Changed lamp? Checked fuses? Any interlocks? Everything OK.
Went there. Found that there was voltage across the lamp. Changed to new bulb - and presto! - There was Light!
Told them to switch from that obscure lamp manufacturer to a better one (the other one). They bought it wholehartedly and (as I said before) I was their hero and friend. Good pay, too. Papers are produced in the wee hours.
Gunnar Englund
www.gke.org
RE: Mastering the Obvious (a long winded story)
Thx, jraef
RE: Mastering the Obvious (a long winded story)
The town I'm in had the last leather tannery west of the Mississippi. (Had been for years.) I ended up there because some obscure power supply in the most important piece of machinery they had had failed. Their 'cash register' the machine that measures the infinitely odd sizes leather comes in, as it's being wrapped, for shipping. Well, after this 'save' they realized they had someone they could call. I got called for problems on block-long sprayers, waste water PH control, multimillion BTU burner problems, 200HP vacuum dryers, re-tanning drums, PA systems, and once I hand made 57 PCs and installed them all(shoulda seen my living room).
Anyway, one day I was there working on a network problem and I notice bells going off and people frantically running in all directions. I asked a guy what's up? He said, "there's a big f*&*&^% water leak!!" I trotted down there and indeed it was a big f*&*&^% water leak!! I was on the second floor and as I got closer there was water ankle deep... Not good. An 8 inch fire main's riser coming up from the first floor and meeting at one of those lame muffler clamp joints had just pulled in two. The water was coming out horizontally 360 degrees at about 250PSI. Of course people were trying to shove pallets covered with in-process leather away from the scene of the disaster. Other people were running towards the area carrying huge power drills. They wanted to drill holes to drain the water before the floor collapsed. I pointed out that you should turn the water off first! Think how many holes you'd need to drill to beat that flow!!
I pressed them to turn off the water. They screamed at me that no one knew where the valve was. The only guy who actually knew was at lunch... So I just did the logical thing and started following pipes. It wasn't easy as all the fire lines came to that one area and disappeared into the concrete floor. But after about five minutes I found a sump next to the plant transformers that now had four feet of water in it but had a red valve in it. I got a huge wrench from maintenance and gave it a few turns. I could hear throttling sounds and so continued. Turned out to be the correct guess. I went back in the plant and it was amazing as in an area of about 30k square feet there was water pouring out of the ceiling, out of all the fluorescent fixtures, into the production offices, into computers, onto desks, onto machinery, and onto hundreds of leather dollies stacked with leather. And now through holes being drilled in the steel flooring.
I went back to my original job elsewhere. Two days later I ran into the company pres and he said, "you missed all the fun! The fire main broke!". I said, "Hey I'm the one who turned off the water..." That got me a steak dinner.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
The history behind the problem was that almost every time Operations went to test the pump, the 170MW turbine would trip. The trips appeared to be caused by a severe sag on the 110V DC supply during motor starting. There was a lot of speculation about the problem and a lot of witchdoctor engineering taking place: replace the battery! replace the cables! replace the power supplies! Quite amusing looking back, seeing the pseudo-engineers grasping at any straw they could find.
I was thinking along the lines of "1400AH battery? How much current do you have to draw to cause a deep sag on a supply that big?" I asked "How much current does the motor draw during starting". No one knew for sure: it was over and done with by the time the clamp-on meter responded, and the panel meter was trying to complete a second full rotation of the needle.
First step was to find out what the starting current did: a fast current probe on the armature, a tacho added to the motor, isolation amp on the battery and on the amrmature voltage. Results: full resistance in, about 400A and no rotation; first stage resistor out, about 500A and still no rotation; no resistance in, about 1700A then a rapid drop to normal running current. Holy shi... 1700A into a 15kW motor! Was the brushgear and commutator ok? A quick check showed the pump running ok and no signs of distress to the motor.
I reported back my findings, and being the new guy was told that the senior management had decided to swap the motor anyway. Being the new guy I didn't argue too much but I was convinced the motor wasn't too blame. The 1.1 tonne motor was duly swapped and the pump test re-run. Sure enough the meter buried its needle into the pin, then there was an almighty 'BANG' as the starting resistor blew up. Someone had decided that the timestarter needed longer between each step and adjusted the pneumatic timing relays. They either didn't think, or didn't understand, that the starting resistor only has a short-term rating.
I proposed that the cubicle wiring be checked against the OEM drawing. "Already been done!" I was told. I'll do it myself anyway. The wiring was checked and it was per the drawing. Credit to the orignal checker because the cubicle is a dreadful design with lousy access and sharp things everywhere. Much like the engine of my old Renault, but that's another story!
I was sitting looking at the schematic when it hit me: the shunt field was connected AFTER the starting resistors! With the starting resistors in, the shunt field was reduced to about 20% of normal value, plus whatever the series field contributed. The motor could not develop enough breakaway torque agaisnt such a weak field. The timestarter was essentially behaving as a plain contactor, applying full voltage to start the motor. I swapped the shunt field connection to a point before the starting resistors and re-ran the test: perfect start!
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Sometimes I only open my mouth to swap feet...
RE: Mastering the Obvious (a long winded story)
Gunnar Englund
www.gke.org
RE: Mastering the Obvious (a long winded story)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: Mastering the Obvious (a long winded story)
<Intentional misspellings intended to fool my spell checker to point out that even then it isn't perfect!>
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Mastering the Obvious (a long winded story)
Anyways, the steel coil straightener for this machine would stall from time to time even though it was rated for the steel thickness. During the summer of 1996 or so I decided while this press was slow on business to check the generator and motor brushes. They were about 50% worn but I convinced the plant owner that it would be worthwhile to change out the brushes and soapstone the commutators while we had some natural slow time.
After putting in the new brushes, sanding them to fit the commutators and then soapstoning the brushes, maximum torque went way up and this steel coil straightnener never struggled again.
About a year later we figured outn why we had poor performance with the die cushion air piston of this press and why we had to use supplemental rubber pads. Turns out that the pressure regulator was not putting out enough air and the pressure gauge was inaccurate. After sticking a part kit into the regulator and putting in a new pressure gauge, the die cushion piston was leaking expletiveloads of air. I went under the press and the cylinder had a big crack in it. Sent out the cylinder for some brazing and reboring and it worked like a champ too.
I have another it-was-so-stupid nobody else could figure it out story for later.
RE: Mastering the Obvious (a long winded story)
Here's mine. I like this one because the problem was honestly fixed with a "flick of the finger".
My dad (at the time lab manager of a hospital) was showing my fiancee at the time (married 30yrs now), a different lab where it was "much more automated" than the hospital lab.
The machine in question was a "SMAC 12". A 12 channel chemistry analyzer for basic chem panels. An old monstrosity with tons of tubing and filters, etc.
The tech on duty was having trouble with one of the channels. He fiddled and farted around, changed the filter, and generally wasn't having a fun time with it.
The display for the device was an old oscilloscope. One of the channels was showing a .1 HZ (10 second) waveform that could best be described by a full wave rectifier unregulated (you know the absolute value of a sine wave).
Anyway, I asked him which channel that signal represented and which colorimeter it was. He pointed to the critter and I walked over and flicked the colorimeter with my finger, disloging the air bubble that had formed in it..........
The channel settled down and there were 3 pairs of very wide eyes from the three techs........
Sometimes, 'ya get lucky.
Cheers,
Rich S.
RE: Mastering the Obvious (a long winded story)
Mick: "Control room, call back?"
Control: "Go ahead Mick"
Mick: "This bus coupler that won't close..."
Control: "Yeah..."
Mick: "Have any of you guys considered racking the f___ing breaker into the f___ing board before trying to close it? F___ing moron."
Control (muttered): "Can we discuss this in the Control Room?"
Anonymous caller: laughter
The whole episode was broadcast across site for anyone listening on Channel 2 to hear. There were a lot of knowing smiles as we walked back.
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Sometimes I only open my mouth to swap feet...
RE: Mastering the Obvious (a long winded story)
I was looking around the engine room to see what switches and sensors were related to the clutch pressure. I noticed the owner, and three or four others climbing over one of the engines and associated clutch. I asked the owner what they were doing, and, as I suspected, he answered that they were looking for the problem with the clutch.
Then I told him this story about the ship captain and his mate.
The two had sailed together for years and been through many tight spots together. Whenever the situation was the bleakest and panic was setting in, the captain would go to his cabin, open his desk, take out a small box, open it with the key he wore on a chain around his neck, and take out a piece of paper. He would read the paper, replace it, relock the box, and replace it in his desk. He would then return to the bridge, completely calm and the master of any situation.
One day he had a heart attack and fell dead at the wheel. His mate couldn't wait any longer. He grabbed the key ran to the captains cabin, and unlocked the box. He took out the paper and read:
"PORT IS LEFT, STARBORD IS RIGHT"
The owner gave a polite laugh and then a look that said;
"Why are you wasting time when we have a clutch to fix."
Then I asked him;
"Isn't this the port clutch over here?"
Long silence...
(A hydraulic line had been inadvertently drained, and the air wasn't making it past a pulsation damper to the pressure switch. There was pressure to the clutch but not to the pressure switch. I had them ready to go in 30 minutes.)
respectfully
RE: Mastering the Obvious (a long winded story)
I received a call from the group electrical engineer saying a site in North Yorkshire (a drive of about 3~4 hours for me) wanted me to come and look at an application on a slurry pump with a view to fitting a VSD. The motor kept burning out as it was starting and stopping constantly.
So off I went and arrived at the site. The site manager already had an idea where he was going to mount the drive, had cleared a dusty room out etc. I said, let's look at the application first and see what is going on.
The slurry pump is the final stage of the sand/gravel process when all sand is filtered off and left with a large tank of cloudy water to be pumped off to a lagoon about 1km away. I watched the process and as the tank was emptying, it hit a level switch and turned off. Immeadiatly, the tank started to fill again very fast. The top limit went and the motor started again. I managed to get away from the site manager and speak with an operator. I asked if there was a non-return valve in the outgoing pipe. "no, we took that out. Every month or so it kept getting blocked with silt and we had to clear it out so we locked it open".
I then had a long discussion with the site manager (who knew nothing about the fact they had locked the valve) and tried to explain that every time you pump the slurry out, you put a cost to it due to the energy spent to move it. About 80% of all the pumped water was flowing back into the tank to be pumped out again (the motor was a 75kW rating, so an expensive beast). I said: fix the non-return valve (maybe have two spaced evenly along the length of the pipe), adjust the top/bottom limits and introduce a better maintenance process. "But what about the VSD?" he said. FIx the basic process first, see if your motors last longer than 4 months and then we can look at possible improvements. He was quite sad that he couldn't spend £1000's on a 'magic' product that will save him money.
Moral of the story: always speak to the operators. Keep an open mind on the process and never, ever, forget to take your steel-toecapped wellington boots to a quarry (as I did).
Also, the sales person (ours) who came along for the ride (whose wellies I borrowed) was quite upset that I hadn't sold them a VSD...hmmm
RE: Mastering the Obvious (a long winded story)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
In actual fact, we did some calcualtions a fews months later and whilst it is a good application for a drive, the payback time and ROI was very long (4~5 years) as they reduced output at the quarry so were only operating for typically 5 hrs/day 5 days a week. The cost of a 75kW drive in a 'quarry-proof' enclosure was quite high making it an expensive proposal too.
The ROI on fixing the non-return valve was about 3 hrs and then the savings were quite apparant as the pump was not actually running for about 50% of the time.
I think we did supply a softstart eventually so the salesman got a return on having to clean his shoes..
RE: Mastering the Obvious (a long winded story)
We got the tool installed and ran all of the mechanical motions of the process with deionized water to avoid working around harmful chemicals. All four plating cells functioned as they should: they began spinning, dropped into the water-filled cells, then lifted out again and stopped spinning so that the pick-and-place robot could remove the product and advance it to the next stage of the process.
We then dumped the water and replaced it with electrolyte solution. During the first plating run, cells 1, 2, and 3 functioned normally, but cell 4 had an error associated with the lift motor that would not allow it to lift from the cell. We retrieved the product, then tried it again with the same result. We put the motors through the steps under manual control and everything worked fine.
As this occurred right at the end of our 12-hour night shift as well as our work week, we turned it over to the day shift crew and the lead engineer. The lead (who was really into programming, but also trained in electronics) decided at the turnover meeting that it was a software problem. I tried my best to convince him that it was an EMC issue, as the only thing different between the water runs and the plating runs was the addition of plating current. I explained (in vain, of course) that the plating current conductor from the cathode ran parallel with the motor cables for both the lift and spin motors through a metal tube so that the whole head assembly could be swung out for maintenance, and that the collapsing magnetic field when the plating current turned off was wreaking havoc with the lift motor by inducing a voltage spike somewhere, most likely in the encoder cable. I noted that the motors sequenced correctly with water in the cells, which meant it most likely wasn't a software problem or it would have shown up during earlier sequencing.
The lead spent the whole day on the phone with the software people, who couldn't duplicate the problem on the lab prototype, and wouldn't allow the day shift crew to work on the machine until the software folks figured out the problem.
When the new night shift crew came on that night, they read the troubleshooting log. One of the engineers who really trusted my troubleshooting ability and judgment decided to look at the hardware, since there was nothing he could do about testing the software. He found a problem with the encoder cable. At the motor end, the pins for the power supply common wire and the cable shield had been installed reversed inside the connector. Every time the plating current shut off, the EMI from the collapsing magnetic field was inducing a voltage spike into the common of the encoder power supply via the shield drain wire pin. The encoder would freak out, the servomotor would lose track of its position, and the machine would fault. The pins were swapped, and everything worked just fine.
An interesting note: the lead assumed the problem was in his area of expertise and neglected to examine everything else, so he was only guilty of wearing blinders, not of trying to avoid responsibility. I'm much more used to the "It's not a [insert discipline here] problem, it's a [insert some other discipline here] problem" type of finger-pointing.
xnuke
"Do you think you used enough dynamite there, Butch?"
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Mastering the Obvious (a long winded story)
I can't believe it. You may have just described what the probelm is in MY application! It never occurred to me that when they shut down the big pump, there might not be a check valve and the backwash is what is overwhelming the smaller pump! Maybe THAT's why the pump guy's calculations are so far off! I'm going to have to "volunteer" to check that out before the finger pointing session starts this week.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Mastering the Obvious (a long winded story)
I just read properly your original post. You did say skip it and I guess I did skim through it without taking in some of the detail, that's probably what prodded the memory of my site adventure a few years ago (you did say you wouldn't be offended if it's skipped!
It does sound mighty similar. I think I'd check the backwash.
RE: Mastering the Obvious (a long winded story)
Under the category of "blame the PID", because we build complex positioning servo controllers used by a lot of people who don't have a good understanding of what's going on, we often get the blame first. Many years ago, a customer using our controller for a large positioning table complained that our PID could not stabilize his application. Phone support was fruitless (and we didn't have the diagnostic and communications tools we have now), so we had to send someone across the country to "fix it". Sent a guy who wasn't a degreed engineer, but with lots of experience and horse sense.
He gets to the facility, takes one look at the operation of the table, wipes down the slides with a rag and sprays them with WD-40. PID problem fixed -- in 5 minutes!
One of my favorites, though, came from a student user (which makes it a little more excusable, I think). He calls up one day, saying he just cannot get his (bare) motor to spin at all with our controller. So we walk him through step by step to break down the problem. First, we have him open the servo loop and just force an open loop 50% output on the DAC. The motor still does not move. We have him read back the number in the DAC register to confirm software operation. It reads back 16K, half of the 32K range, so we conclude that the software is operating properly, and he must have a hardware problem. Next, we have him pull out a voltmeter and read the output voltage. He gets 5V, half of the 10V range.
So we tell him that our controller is working fine in both software and hardware, so the next thing to check is his amplifier. To which his response is, "Amplifier???"
Curt Wilson
Delta Tau Data Systems
RE: Mastering the Obvious (a long winded story)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
xnuke
"Do you think you used enough dynamite there, Butch?"
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Mastering the Obvious (a long winded story)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Mastering the Obvious (a long winded story)
I learned that well over 35 years ago
RE: Mastering the Obvious (a long winded story)
When we arrived, before we could investigate, the operator immediately reset the breaker and hit the start button. Sure enough, the breaker tripped.
Electricians locked it out, closed all piping valves, and meggered the motor. No problems.
"It has to be the motor", claims the operations supervisor.
Come to find out that this is the second motor, recently installed.
I asked them to reset the breaker and open the valves.
Once the valves opened, the motor started to run backwards. None of the valves, nor the start/stop station was in line of site of the motor shaft.
I had learned enough in the past to watch and listen, so no one else saw what was going on.
It was a pleasure to tell the operations supervisor on my way out with the electricians that the check valve had failed, causing the pump and motor to spin backwards while off.
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