Mag-Drive/Seal less vs. Canned Motor Pumps
Mag-Drive/Seal less vs. Canned Motor Pumps
(OP)
I'm in need of some assistance. We've been having trouble with our canned motor pumps for the past number of years.
Possible solutions at this point are mag-drive pumps, seal less pumps or displacement chambers. We've replaced the same canned pumps on another system with displacement chambers and with great succes since the 70's (much dirtier system than what we currently pump and slightly different failures than what we currently see). In order to do it on our current system, we need to use a very clean gas and at a specified pressure.
The pumped fluid is lethal but not terribly corrosive. We can't have mechanically sealed pumps due to the lethal nature of the fluid. Sealed or seal/less is a must. The required flow rate downstream is 25 USgpm at a TDH of 130ft (low flow, high head). We have sufficient NPSHa and the system is gravity fed. The pumps seem to cavitate which causes the impeller to hit the front bearing housing and we've also seen the rotor assembly contact the wear surface on the bearing. This generates heat which causes the fluid to vaporize and plate out on the bearing. When this happpens, the pump fails. All of this happens over a matter of seconds. Not sure if process measurement devices would help us in such a case. We have a glycol cooling jacket around the pump as well as glycol on the backflush piping coming into the rear end of the pump. The stator is oil filled to assist in temperature dissipation. A lot of cooling to say the least. The fluid is very temperamental in terms of temperature variation.
In order to acquire a successful prime, lately we've been bumping the pump about 5 times in 30 second intervals to ensure that the pump is flooded completely (suction, discharge and backflush). I don't have a great deal of experience with pumps. Just learning the ropes.
Looking for any help or advice that someone could pass on (i.e. pros and cons on mag drive vs. canned vs. mechanically sealed, displacement chambers, proper material selection and so forth).
Thanks,
TP
Possible solutions at this point are mag-drive pumps, seal less pumps or displacement chambers. We've replaced the same canned pumps on another system with displacement chambers and with great succes since the 70's (much dirtier system than what we currently pump and slightly different failures than what we currently see). In order to do it on our current system, we need to use a very clean gas and at a specified pressure.
The pumped fluid is lethal but not terribly corrosive. We can't have mechanically sealed pumps due to the lethal nature of the fluid. Sealed or seal/less is a must. The required flow rate downstream is 25 USgpm at a TDH of 130ft (low flow, high head). We have sufficient NPSHa and the system is gravity fed. The pumps seem to cavitate which causes the impeller to hit the front bearing housing and we've also seen the rotor assembly contact the wear surface on the bearing. This generates heat which causes the fluid to vaporize and plate out on the bearing. When this happpens, the pump fails. All of this happens over a matter of seconds. Not sure if process measurement devices would help us in such a case. We have a glycol cooling jacket around the pump as well as glycol on the backflush piping coming into the rear end of the pump. The stator is oil filled to assist in temperature dissipation. A lot of cooling to say the least. The fluid is very temperamental in terms of temperature variation.
In order to acquire a successful prime, lately we've been bumping the pump about 5 times in 30 second intervals to ensure that the pump is flooded completely (suction, discharge and backflush). I don't have a great deal of experience with pumps. Just learning the ropes.
Looking for any help or advice that someone could pass on (i.e. pros and cons on mag drive vs. canned vs. mechanically sealed, displacement chambers, proper material selection and so forth).
Thanks,
TP





RE: Mag-Drive/Seal less vs. Canned Motor Pumps
If you've been having a lot of bearing failures my first area to consider would be a misapplication of that pump in this service or an installation/operational issue.
We had Sundyne in our offices recently to put on a presentation. Now granted this is a sales pitch but they offer both conventional pumps as well as canned and mag drive and they seemed to have a good understanding of the application areas for canned, mag drives and conventional pumps. If your current vendor isn't/can't fix this problem, time to start looking for a new one. Make sure they can give you references for users with your fluid in your conditions and then talk to the users yourself.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
"We have sufficient NPSHa and the system is gravity fed. The pumps seem to cavitate which causes the impeller ---".
If the impeller is hitting the front bearing housing something else is amiss.
You don't say what temp. the system is operating at.
The old Allis Chalmers Pump Company - which became ITT A_C pumps and now not sure who or what they are called - manufacture a very nice mag.drive pump based on their chemical process pump model 731. This would be a good unit to investigate for your process -- from memory here were a couple of units that should fit the duty you have.
Naresuan University
Phitsanulok
Thailand
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Canned motor pumps and mag dirve pumps do not like dirty fluids. In my experience with canned and mag drive pumps, only clear fluids were permitted. Clearances for the gadgets to work at all were just too close.
Second, you mention the low flow high head nature of your system. Where is the actual pump point with respect to the BEP point of your pump curve?
It could be that you don't have constant flow or enough flow to guarantee minimum flow through the pump, required for cooling, and your fluid is overheating and flashing in the pump, even with adequate NPSH.
What is the fluid being pumped? (Or can you tell us without having to kill us?)
rmw
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
The pumpage is very clean. Little to no solids whatsoever. In order to maintain hydraulic balance in the impeller, the pump requires 7.5-50 gpm. The pump is running at about 26% efficiency w/ a 6" impeller (@ 22usgpm and 130ft head). I agree that there could be insufficient flow to the pump (i.e. line blockage) that would allow the fluid to heat up and plate out causing failure. We've snaked both the suction and discharge piping - both were okay. We're investigating a blockage at the tank's outlet port to rule that out.
I appreciate your help and comments.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Something has changed. I know that is a long time ago but I'd start to focus what has changed in the process or the equipment or the installation back around 96. Maybe Chempump made what they thought was a minor change with consequences to you, maybe the process changed but the point is something changed.
Although it's old, it would be an excellent problem to do a Kepner Tregoe problem analysis on. Basically, you list all the facts known about the issue. You need to involve a wide range of people from Ops to Maintenance to Engineering. The facts are covered by looking at what is and isn't for the following areas: what, where, when and extent. At that stage, no 'problems' are allowed to be identified which is the hard part as people want to jump ahead to the solution. If someone isn't sure about a fact, write it down but make a note it needs to be verfied before the next stage.
For each step, you need to look at true and false statements. For example, you would note it is the bearing that is failing. The bearing shows blah, blah, blah. On the 'not' side, It's not the motor failing, it's not corrosion of the containment system, it's not shaft failure, whatever.
Once all the facts are listed, you then get people to start proposing what could be the problem and write them ALL down. Just write them down at this point, don't evaluate them, don't discuss them, don't say 'it can't be this because .....'.
Then, go back and compare each proposed 'cause' against the known facts to see if it explains ALL of them. If not, it can't be the problem and is discarded.
I've done these on a couple of problems where everyone had their pet theory or idea but until we listed all the facts and reviewed whether each theory explained all the facts, we never did get agreement on the problem. It works.
You might want to look at bringing a facilitator in who is trained in the process to do it for you and make sure you stay on track because it's easy to get off track (either jumping to solutions or starting to evaluate a solution someone proposes). I'm sure it's not cheap from one point of view but your pump repairs have got to be killing you now.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Checman
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
We have started to perform a PHRED analysis of the system which is a painstakingly slow process but it gives everyone a better overall picture to what we are working towards which is a solution. Rather than put miscellaneous pumps in such a lethal (and explosive) fluid, this process must be completed and Operations/Maintenance/Engineering must come to an agreement as to the next step to take. PHRED sounds close to Kepner Tregoe.
We've questioned the manufacturer as to any materials or methods of construction that may have changed over the years especially in the late 90's which could have impacted our system. They haven't been all that forthcoming with information. All I've really noticed is the weight differences in the two pumps from now and the 1970ish pump (128lbs vs. 150lbs). Not sure if this would have an impact or where the weight reductions were. Good question to ask though.
We don't continue to run the cooling after the pump shuts down. We haven't modified the cooling system too much other than adding the cooling jacket to the pump which is designed into the pump casing ($$$). With the vertical installation, we have glycol circulating in the jacket from the bottom up. The pump contains a relief valve on the stator for oil (set at 17psi). During initial startup we vent to the tank while we flood the system.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Have since pulled several other long coupled CPX's out because of prohibitive repair costs and replaced the with ISO standard Durco Chemstar (enclosed impeller)
regards
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Canned pumps and magnetic driven pumps have pretty much the same field of applications and the same advantages and disadvantages.
The main weakness of the two ( apart from dirty liquids) - they do not like variable duty points.
The bearings are specifically designed for a set of operating parameters - any significant change in the operating conditions will cause overheating due to unproper lubrication of the hydrobearings.( irrespective if the operating point moves to the right or to the left of the design point )
As it appears that the pumped fluid is pretty close to the vapour temperature - the extra heat generated by the bearings can explain why 'cavitation' symptoms appear.
Therefore - if in your application there are significant changes in the pumping duty - you are having now the wrong pump for the duty.
Option 1 : If a pulsating discharge flow can be accomodated - you should look maybe at replacing the pump with a positive displacement pump with a hydraulically actuated diaphragm. (due to the lethal service)
Option 2: To accomodate the existing pump and deliver a variable discharge - investigate a three way control valve in the discharge - recirculating the balance of the flow not needed in your process as to keep the pump on the same operating point.
Personally in situation of low flow high head I would look seriously at the option 1.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Howard Schwerdlin
Product Manger
Magnetic Technologies Ltd.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
If any of you have had experience (good or bad) with these type of pumps, please advise. Our liquid is lethal organic so I'm uncertain at this point if this would be a solution having no experience with these pumps.
Thx.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
I personaly do not quite like peristaltic pumps. Normally more expansive (than diaphragm), more room req'd and higher cost for replacing the hose.
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
Naresuan University
Phitsanulok
Thailand
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
rmw
RE: Mag-Drive/Seal less vs. Canned Motor Pumps
RE: Mag-Drive/Seal less vs. Canned Motor Pumps