Mag-Drive/Seal less vs. Canned Motor Pumps 3

[pumped]

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

 

[TD2K]

Have you had the vendor(s) involved? How many of these pumps do you have? If more than one, have you had the problems with all the pumps or just some of them?

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.

 

[Artisi]

You seem to have a contradiction in your story
"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

 

[rmw]

YOu use the phrase "much dirtier than what we currently pump..." This infers to me that your pumpage is somewhat dirty by some standard.

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

 

[pumped]

First off, thanks for your replies. Yes, we have had the vendors involved, Chempump. They inspected the system, piping, flow, pressure, head, etc. Everything seemed to check out. We've had the same pumps in service for over 20+ years without much of a maintenance issue. Up until '96 (at least that's when the failures started to be documented), they seemed to have been failing quite frequently with an MTBF recently of 24 hours. I know, not good at all. We've managed to up the MTBF to 3-5 days by changing the routing of the glycol in the cooling jacket around the stator assembly. We do have them installed vertically but the vendor assures that the pump can operate in this orientation. We installed a mag-drive in 2003, Kontro, and it lasted about 10 minutes before it failed. Temp. of the system is approx. 20 deg. C. (which is room temperature). Not too many users carry Nickel Carbonyl in their process. Most vendors aren't familiar with it's properties.

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.

 

[vanstoja]

I'm a bit confused by your description of a "front bearing" on a hydraulically thrust-balanced Chempump canned motor pump that has a brake or input power of less than 3 horsepower. Is it a radial bearing or a thrust bearing? Does it have a "soft" bearing material like carbon-graphite or is it a metal surface intended to act as as temporary overthrust load bumper? Surface contact temperatures should be relatively low for carbon-graphite bearings and should not cause vaporization of the pumped fluid unless it is very close to its boiling point.In my experience, Chempump designs for such low output were usually overhung, end-suction pumps with no bearing in front of the impeller. Such a design would seem to be a solution to your front bearing-rub-induced bearing failures. For your present design, you need to know more about the transient axial thrust loading conditions that are apparently overpowering your automatic hydraulic thrust balancing system. Also if rotor weight is acting toward a front thrust bearing in your vertical mounting, perhaps you would be better off to mount your pump upside down so that rotor weight would act away from a front thrust bearing.

 

[TD2K]

"We've had the same pumps in service for over 20+ years without much of a maintenance issue. Up until '96 (at least that's when the failures started to be documented), they seemed to have been failing quite frequently with an MTBF recently of 24 hours."

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.

 

[checman]

trpalmq: With 43 deg.c boiling point vaporization is always a possibility. And it sounds like vapor is a problem. I was not able to find the specific heat or a vapor pressure vs. temperature chart but they would be helpful. I would look closely at your cooling system. Has the cooling system changed? Do you continue to run it after you shut the pump down? Have you added other equipment to the cooling system? Also going to a larger pump at 1700 rpm would help as far as heat generated vs. dissipated. I am not sure what type of vapor relief system you have on this pump but would assume some type of return to supply tank and or a flare system if the stuff can be safely sent to flare. Have you asked your supplier of the Nickel Carbonyl what kind of system the recommend? I am just throwing some ideas out there for you to think about but I would not do anything without an engineering review. This is nasty stuff and just changing out a pump is a major project that could prove fatal as I am sure you know. Take care and good luck.

Checman

 

[pumped]

When I refer to "front bearing", I mean closest to the impeller but inside the stator assembly. Yes, the design is an overhung impeller. The contact surface was with the front bearing housing and impeller (metal to metal). Sorry for the confusion. The bearings are carbon graphite so there is a better wear surface than metal to metal. We have seen contact between the faces of the rotor and front bearing as well.

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.

 

[jet1749]

I would agree with others that an investigation is needed to find out what is different now, when the pumps have operated ok previously. However if you decide that the present pumps are not coping in a low flow-high head situation, a pump to consider is a peripheral turbine mag drive. I have recently installed a couple to replace Flowserve CPX magdrives on caustic and sulphuric acid duties. These are ideally suited to low flow applications, also took the oportunity to fit bypass lines to ensure minimum flowrates to keep bearings lubricated and cool

 

[pumped]

jet1749; The Flowserve CPX mag drive pump is one of the models I am looking at (ISO overhung). Curious to know about pump reliability, flow conditions and why you changed out to peripheral (what make?). Thanks.

 

[jet1749]

The CPX mag drive pumps I replaced had been originally sized and specified by a Flowserve rep. The duty of the pumps was such that they operated at near shut-off condition permanently at MCSF on the curve (in this case 0.5m3hr)The pumps suffered from excessive erosion/corrosion of the wetted parts (316 bowl and back and CD4MCU impeller/shaft. This we suspected from the increased temperature generated. Also suffered severe corrosion of the inner magnetic rotor skin even after an upgrade to Hastalloy C. The price of upgraded parts exceeded the cost of the original pump as supplied. Another problem was the cost of replacing the impeller and bowl due to the complex compound curves machined on bowl and impeller (semi-open design)Repair options were non existent. My solution was to specify Caster peripheral turbine mag drives sourced from HMD pumps in the UK at a lower cost than upgraded CPX's. Fitted bypass lines from discharge back to tank to maintain the minimum flow rate required to cool/lubricate bearings. No failures since fitted approx 1 year ago. MTBF of the ISO CPX's 4 months.
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

 

[pumpkind]

I would like to bring one fact to the discussion.( This will help only if you experience variable duty points required on your pump )
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.

 

[howard1954]

Interesting problem. In a magnetic drive with a metallic barrier, heat is generated due to the induced electric fields produced in it. Ways to reduce this heating are decreasing the magnetic flux or increasing the resistivity of the barrier. If the magnetic flux has increased (increased rating of motor or coupling) this can cause increased heating of the barrier. Alternately, if the barrier has changed (thickness, alloy composition) this can changed the resistivity. One way to get around this problem all together is to use a non-metallic barrier that is compatible with the Ni(CO)4.

Howard Schwerdlin
Product Manger
Magnetic Technologies Ltd.

 

[bulkhandling]

Assigned pumpkind a star since his posting is just what i was thinking.

 

[pumped]

I'm curious to know if anyone has had the opportunity to use a peristaltic pump and if it would be of benefit in this case. Watson Marlow/Bredel has an SPX50 pump that would surely eliminate a lot of the problems we are seeing now with heat being generated nor would we require cooling of the pump components since they do not interact with the fluid. We'd just need to modify a bit of piping to install.

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.

 

[bulkhandling]

The peristaltic pump and diaphragm pump are all positive displacement pumps and similar in many applications. You should check if your fluid is compatible with the pump material. For peristaltic pumps, the only material that contacts with the fluid is the hose, normally natural rubber for your size.
I personaly do not quite like peristaltic pumps. Normally more expansive (than diaphragm), more room req'd and higher cost for replacing the hose.

 

[Artisi]

If you product is lethal organic, I would not recommend a peristaltic unit, the failure point between a sound hose and a ruptured or holed hose is an unknown factor.

Naresuan University
Phitsanulok
Thailand

 

[rmw]

I am giving artisi a star for the previous post. In the case of a lethal fluid I concur wholeheartedly with his point.

rmw

 

[Newkiwi]

I was wondering if the fluid, Nickel Carbonyl, is possibly affected by the magnetic field and heats up due to induction?