Re: Rotary Valve Damage
If the spigot clearance excedes 0.15mm then it is likely that the end caps are losening slightly under high temperature cycling & the material download is taking disadvantage of that situation. There goes your alleged tip clearance. When you run cool the rotor does not charitably centralise itself. End plate wear could be due to the misalignment of the housing & rotor axes caused by the above drop not being equalised at each end face. Am I warm? ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Re: Rotary Valve Damage
Our ambient temperatures are between 16 and 20C. When we wash we were using temperatures as high as 80C but have recently lowered these to 50C. We have difficulty dropping below this due to the detergent foaming. The valves are relativley new and do not have much running time on them. Our operational temperatures do not rise above 35C.
What is the best way to check clearances? The endplates have alignment pins and multiple bolts that are torqued when installed and do not loosen up between cycles. Everything is tight from begining to end of a cycle with no play in the endplates. This dosen't mean that maybe there is play in the bearings. A few thousandths could wreck the whole day.
Are these temperatures "high"?
kaos1 ■
Re: Rotary Valve Damage
Operation is quite cool so exit the temperature business. With dowels locating the end caps; exit the axes out of line idea.
Everybody who uses rotary valves has some sort of trouble with them. A lot is being asked of them. In rougher processes tandem valves are mounted on a sliding frame where one of the valves is at standby, ready to be slotted in when its mate goes down out of service. In those cases it isn't so much the valve quality & life issue as an overall operating issue. Consider a suction ship unloader heavily reliant on a rotary valve. No ship in port: no rotation: all sorts of crud & rusty stuff building up from condensation, wind dislodging overhead accretions. Suddenly there is a ship on the horizon & soon after that the rotary valve is whirled into action. Failure is reasonably accepted, even if dragging powder out of a ships hold. So the standby principle.
I just mention it in case you can spot something similar with your washing activity. Your valve sounds well put together. ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Internal Rotary Valve Damage
Dear Kaos,
I have had quite a few cases such as yours during my 40 years of design and operation of pneumatic conveying systems. I learnt that the best way to prevent this kind of rotary valve damage was to make sure that the rotor to body clearances were maintained under the worst operating conditions. These conditions are the maximum operating temperature of the rotor and the minimum temperature to which the body is exposed to. Your 10 bar rotary valve are expensive. For such valves I used jacketed construction to maintain a constant temperature of the valve body.
I used feeler gages to measure the clearances at the worst operating conditions.
Regards,
Amrit Agarwal
Consulting Engineer
Pneumatic Conveying Consulting
Email: polypcc@aol.com
Ph and Fax: 304 346 5125 ■
Re: Rotary Valve Damage
I think he needs to know where & how to poke about with the feeler gauges! Inside an assembled spinning valve???
As described the main temperature difference would result from the washing if the casing had cooled to ambient. But the scraping happens when there is no (Measured;;;How???) temperature difference.
How do your investigations rule out trapping of particles? ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Re: Rotary Valve Damage
Excellent information but we have not resolved the issue yet.
The temperature difference does not have to be measured given the timing of the failure. The unit may sit unused for a couple of days and this would assure that the temperature would have equilibrated. Since we have had so many troubles with these, we have elected to "bump" the motors prior to full scale run up. When we turn on the rotary valve, we generally hear scraping within the first revolution. The valve that is giving us the most trouble turns at about 7rpm. There would be no time for differential temperatures to be generated.
Our investigations have shown that the particle size of all of the material passing through this particular valve are under 250um. The majority (90%) falling 50 to 100um. The particles are of common salts, sugars, amino acids that are relatively soft compared to the stainless steel of the rotary valves and would easily fracture to smaller particles if caught between the vanes and housing.
I believe that this is a clearance issue. Obviously if the vanes never touch the housing, then we should not sustain any damage.
What is the best method of checking clearances? If we pull the cover to gain access to the vanes, then the operating clearances are not maintained and the end of the vanes that was supported by the cover will drop until the vanes touch the housing.
kaos1
Originally posted by johngateley
I think he needs to know where & how to poke about with the feeler gauges! Inside an assembled spinning valve???
As described the main temperature difference would result from the washing if the casing had cooled to ambient. But the scraping happens when there is no (Measured;;;How???) temperature difference.
How do your investigations rule out trapping of particles?
■
Rotary Valves.
To overcome the grooves/scars/grazing you can have our installed
errosion, friction resistant coating system to the body / end plates
and veins.
All components are machined to exact tolerances.
All veins are adjustable so are not redundant after useage.
The system can be installed to any worn valve and can be
re-furbished to give a totally new item ready for use.
The life is 10 times stainless steel.
The maximum working temperature is 425 deg C.
If you are interested contact me on;
trevor@densit.co.uk ■
Re: Rotary Valve Damage
In my experience, the normal way to check clearances is with feeler gauges though the product inlet and product outlet.
Can you tell us something about the location of the contact on the housing?
If the damage is on the "rotor falling side" of the housing then non-friable materials are a possibility (possibly foreign bodies?). If they towards the bottom on the rising side, it is difficult to see how that could be hard material. Though with the materials you are handling It is difficult to imagine it is the material which is causing the problem.
Unless the hard particles are coming in with or are dislodged by your wash liquid? If you wash and drain through the rotary valve, it will act a great seive collecting any small particles. This may not be a problem in normal operation as the main powder flow will make it unlikely that a hard particle will get into a position where it gets jammed. Is your wash procedure dissolving all the soft particles and leaving the hard ones behind as the only solids which then get trapped in the rotor?
If the damage is towards the top of the housing then pressure below may be a problem. You mention that these are 10 bar rated valves. Pressure rated normally means that the casings will not fail if subjected to the specified internal pressure. Many valves are pressure rated to 10 Bar (for dust explosion reasons) but only rated to 1 bar as a pressure differential across the rotor. What are the pressures above and below the rotors in your case?
It is a common problem for valves handling hot materials to jam if clearances are too tight. The problem is that the thermal mass of the rotor is low and the heat transfer area is high compared with the housing, the rotors heat up quicker then the housings. Increased clearances or as Amrit suggests, a steam or hot oil jacket or tracing or electrical heating are the normal solutions. Your washing temperature of 80C is borderline as to whether it is necessary to take precautions though.
You have quoted the clearances but not the rotor diameter, by the way so it is not possible, to calculate the thermal expansion of the rotor to check if the clearance is adequate.
On a similar vein, don't forget that the rotor may not heat evenly. If the rotor is stationary and you drop hot material or wash liquid into the inlet and / or blow cold gas underneath it. The top of the rotor gets hotter than the bottom and the rotor bows (hogs) and again this is a known cause of contact with the housing. Do you wash with the rotor static or turning? ■
Re: Rotary Valve Damage
[QUOTE]Originally posted by RPD
In my experience, the normal way to check clearances is with feeler gauges though the product inlet and product outlet.
Can you tell us something about the location of the contact on the housing?
We have seven rotary valves through the plant. Between them, there have been failures at every location. Front and back cover plates, falling and rising sides of the housing, on the vanes edges with nicks out of the edge a couple of mm deep at times. There is generally damage to the vane and a matching spot on the housing that causes it. We grind, sand, polish, and buff until there is no remaining burr present.
[QUOTE]Originally posted by RPD
If the damage is on the "rotor falling side" of the housing then non-friable materials are a possibility (possibly foreign bodies?). If they towards the bottom on the rising side, it is difficult to see how that could be hard material. Though with the materials you are handling It is difficult to imagine it is the material which is causing the problem.
Unless the hard particles are coming in with or are dislodged by your wash liquid? If you wash and drain through the rotary valve, it will act a great seive collecting any small particles. This may not be a problem in normal operation as the main powder flow will make it unlikely that a hard particle will get into a position where it gets jammed. Is your wash procedure dissolving all the soft particles and leaving the hard ones behind as the only solids which then get trapped in the rotor?
We don't see any foriegn objects except sometimes metal that has been scraped from the valve or housing. A thought, we do use potable water (softened) to wash with detergent added. Could we be carrying something in from the municipal system?
[QUOTE]Originally posted by RPD
If the damage is towards the top of the housing then pressure below may be a problem. You mention that these are 10 bar rated valves. Pressure rated normally means that the casings will not fail if subjected to the specified internal pressure. Many valves are pressure rated to 10 Bar (for dust explosion reasons) but only rated to 1 bar as a pressure differential across the rotor. What are the pressures above and below the rotors in your case?
Below is near atmospheric, above is unmeasured but was designed to slightly negative to the area below.
[QUOTE]Originally posted by RPD
It is a common problem for valves handling hot materials to jam if clearances are too tight. The problem is that the thermal mass of the rotor is low and the heat transfer area is high compared with the housing, the rotors heat up quicker then the housings. Increased clearances or as Amrit suggests, a steam or hot oil jacket or tracing or electrical heating are the normal solutions. Your washing temperature of 80C is borderline as to whether it is necessary to take precautions though.
We have adjusted to 50C and still have had problems.
[QUOTE]Originally posted by RPD
You have quoted the clearances but not the rotor diameter, by the way so it is not possible, to calculate the thermal expansion of the rotor to check if the clearance is adequate.
Rotor diameters vary from ~6-10".
Originally posted by RPD
On a similar vein, don't forget that the rotor may not heat evenly. If the rotor is stationary and you drop hot material or wash liquid into the inlet and / or blow cold gas underneath it. The top of the rotor gets hotter than the bottom and the rotor bows (hogs) and again this is a known cause of contact with the housing. Do you wash with the rotor static or turning?
Due to the difficulty of cleaning, we have come up with the process of pulling the non-drive end coverplate, pulling the vanes out 1/2", and using a wand to wash each pocket and housing. This is followed by reassembling and washing using spray devices above and below as the rotor turns.
■
Re: Rotary Valve Damage
Kaos1,
Your clearances are on the tightish side but not worryingly so. On those sizes of valves, the clearances you have sound reasonable with no additional allowance for temperature. As I mentioned before it is borderline whther an additional allowance for temperature is needed. Your clearances are approx 0.18 mm, I would expect a carbon steel valve of that size to have clearances of about 0.125 mm, SS valves always need a greater clearance and a 50% increase for SS sounds reasonable. The 30C differential between ambient and your 50C wash equates to a reduction in clearance of about 0.06 mm.
I am not sure we are going to fully understand what causes the probalem but I have a possible solution. You mentioned in your last note that the pressure differential above and below the valve are very low. Bearing in mind that valves of this type have clearances set to minimise leakage rates at up to 1bar differential, it sounds as if the increase in leakage rates on your process would be negligible. It isn't uncommon on low pressure differential duties to use increased clearances so that parts don't need to be assembled so accurately.
Even if we don't understand the root cause of the problem, if there are no process reasons to have the tight tolerances having increased clearances should help a great deal.
If you want to try this I would suggest trying a clearance of approx 0.3 mm. The manufacturer of the valve should be able to give you some information on the affect on leakage rates if the effect of that on your process is a concern for you.
One final thought, some manufacturers use tapered rotors and housings and you can control the clearances by moving the rotor axially in the housing. This isn't a maintenance problem is it? When the rotors are removed and replaced for maintenance and valve cleaning, are they put back in the correct axial position ? ■
Re: Rotary Valve Damage
Hi,
what kind of "seal strips" do you have on rotor vanes? Is it of flexible material like PEEK or metallic? Or you have direct metal to metal contact? Is there steam tracing available on casing with insulation? ■
Re: Rotary Valve Damage
316 stainless throughout. ■
Rotary Valve Body Problems.
As I mentioned in my last post to you, to eliminate the problems you have you should consider a material which will reduce the
friction between product and the 316 stainless.
The properties you have are only corrosion resistant.
You need abrasion resistance and low friction/ stick properties.
Your end plates score with grooves because the 316 is NOT
wear resistant to the product you convey.
We specialise in such applications and can eliminate you situation
with tip wear/scoring and prevent any damage to the vein tip
edge.
If you want to explore such a solution please contact me on;
trevor@densit.co.uk
Regards. ■
Re: Rotary Valve Damage
Is your installation compliant with the manufacturer's allowable nozzle loading? ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Re: Rotary Valve Damage
Here is an update:
We bought the rotary valves through a system manufacturer who bought the rotary valves from a supplier. The system manufacturer provided the specification that the rotary valves would be operating at 20-25C. They did not account for the cleaning process raising the temperatures as high as 80C. This, the rotary valve supplier states, causes the clearance to be reduced to the point where the vanes touch the housing. After that they will gall.
The manufacturer recommends that the clearance be increased but we can only go so far due to these valves be explosion containing and must keep a flame from propagating past them. The greatest gap we can have is 0.2mm all around. I think that we are ~ 0.16mm now. Not much to play. Hopefully when open the gap so that no contact is made when hot, the gap will not open up too far when cool.
This reworking of the valves is going to take awhile so I'll try to give another update later.
In regard to nozzle loading, the loading is a small percentage of the capacity.
Kaos1 ■
Rotary Valve Damage
Greetings,
We process fine powders from fines up to 250 microns. In our system of milling and blending, we institute several rotary valves. The valves are supposedly rated at 10 bar. The clearance between the vanes and housing is supposed to be ~0.007". There is one that meters raw chemicals into our mill and one under each of two cyclones. They are constructed of 316 stainless.
We have had much difficulties with the rotary valves being damaged.
Some have argued that temperature differentials play a part by the vanes expanding when heated by wash solutions but I would argue that the housings expand also when they are heated. We have had the valves start scrapping at ambient temperatures when there is no delta T.
Some have argued that there is non-friable material getting caught between the vane and housing that causes damage but this has not been the case to the best of our investigations.
Has anyone else had trouble where the vanes are damaged (nicks and burrs) and the housings, including the endplates, wind up with gouges in them?
Has the source of the damage been determined?
Help,
kaos1 ■