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.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376![[pirate]](/data/assets/smilies/pirate.gif "[pirate] [pirate]")
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
