Floating seat Ball Valves - Low pressure air 5

[roshane87]

Hi Guys

We are having quite a big engineering problem where 1/2" and 3/4" bore floating seat(Teflon) ball valves are seizing and also leaking when dry air at 60cm of water pressure (0.06 bar approx) is being sent though it.

These ball valves are actuated using a pneumatically controlled head which produces about 10N/m torque at 6 bar pressure. With use the time to cycle between opening and closing keep increasing and when investigating we found a black residue on the teflon seat with some signs of abrasion on the ball as well. The ball valves are cycled through open and shut positions about 500 times a day. They are only used in open and shut positions.

One specialist is of the opinion that there is galling between the ball and seat and that the teflon is producing an oily substance which is what we see as the black residue. I held a magnet against the ball and it stuck to it which means it has to be a plated metal valve. It could also be the plating coming off the ball and adhering to the teflon surface. People have told us these valves are mainly used for fluid applications and that dry air isn't lubricating enough. But what sort of ball valve material is used in such low pressure dry air applications?

I believe the solution would be a 316 stainless steel ball with a different seat material but I would like to get some input from you guys. Any experience with such an application?

All your input would be greatly appreciated.

Cheers

RN

 

[zdas04]

Are you under the impression that torque on the actuator has any impact on seating-surface seal in a floating ball valve? If you are, then you need to spend a few minutes understanding how a floating ball valve works. The ball is not attached to anything at all, and when you shut it the dP causes the ball to float to the downstream seat and seal. It doesn't take much dP to do this, but it does take some (which is why I hate to see floating ball valves in a double-block-and-bleed scenario)

The black stuff you are seeing could be any number of things. Guessing is actually quite counter productive. Get a sample to a lab and get it analyzed. That will lead you to a reasonable path. Guessing will lead you to an expensive path that may not solve the problem.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[rocky06]

If you are testing the valve at low pressure- Pressure must be 0.5 bar. (as per supplementary test of API6d)

what about valves performance in standard 6d testings. do they found leak proof? (even after cycle)

If so, then i would recommend to increase pre-compression of seats in assembly condition.(Bcos, Ball seems to be not floating enough under low pressure)

 

[EmmanuelTop]

Dry air at low pressures should be pretty much harmless application. Like David says, doing analysis of black deposits should be the way forward.

What leakage class valve belongs to? And is the air stream free of particles - which quality standard is supposed to meet?

Dejan IVANOVIC
Process Engineer, MSChE

 

[gerhardl]

I would not have selected the type of valve you describe for this application. The reason is fear of the complications you describe, which are not unknown.

A better type of valve wuold have been an angle seat valve or a solenoid valve if possible, which both are manufactured in materials and qualities suited for most applications.

In my work with pneumatic systems I have met a lot of engineers, all answering on question that their air system is correctly laid out, no water, no oil, no contamination, no dust, no faulty filtration or pressure regulation. In most cases system failures is anyway caused by a combination of this.

It is practically impossible in normal surroundings to avoid some leakage from outside and water or contamination inside a pneumatic system, and your residues will probably have a partly outside system origin, perhaps combined with loosend or dissolved materials fraom inside, not necessarily from the valves themselves.

In addition a ball valve is (as you know) very suspectibel to the tiniest fragment of residue, and will easily leak at low pressure (low sealing force). A softer sealing material might give an improvement, but I would have started changing to another valvetype.

 

[roshane87]

Thanks for the replies guys.

Been hunting down pictures to post here, hence the slight delay in responding. This is not where I work but for one of our main clients who we are trying to help out.

Regarding the actuator, what I meant was the valves get harder to turn with use and the actuator is able to provide the breaking torque required to open/close the valve at the beginning when the seals are all clean but is unable to provide the torque after a month or 2 of use.

I have done some research and I believe this valve will be of Class VI. I don't have the data for a 3/4" valve though but I found this article which was very helpful -

http://www.engineeringtoolbox.com/control-valves-leakage-d_484.html

Does anyone have leakage data for a 1/2" and 3/4" Class VI ball valve?

The air that passes is sent through filters. It is clean dry air.

Pressure is between 0.03 bar (gage) to 0.10 bar (gage). Flow rate is between around 120 l/min to 300 l/min.

These are some of the pictures -

https://www.dropbox.com/s/q0p9gmyzf9altpn/IMG_20150317_152328.jpg?dl=0

https://www.dropbox.com/s/busw8uqcz3f3wso/IMG_20150317_152413.jpg?dl=0

https://www.dropbox.com/s/qwu04mh89ognrap/Valve A+B+C.jpg?dl=0

https://www.dropbox.com/s/85gz7nsl43snkb3/White particals in valve - SN036 EM-1A.jpg?dl=0

Any experience with such issues guys?

All your comments are much appreciated.

Cheers

RN

 

[EmmanuelTop]

If the air is "dry and clean" I doubt you would be seeing this amount of dirt inside the valve body, as shown in the photos. If some of this white stuff had migrated up the stem packing and the gland, that could possibly explain difficulties in turning the valve on/off.

I agree with Gerhard's comments - we normally used solenoid valves in this service. I also remember seeing dual diaphragm valves for compressed air distributed through dirty/corroded pipework. Is your piping clean, have you inspected the upstream filters? I don't find them very reliable, at least those I encountered so far. Frequent issues with automatic water dumping, and with solids as well. The filters get damaged and sometimes even rupture quite fast.

Have you found what is the white stuff composed of?





Dejan IVANOVIC
Process Engineer, MSChE

 

[gerhardl]

After looking at the pirctures and running discussion I will stand by my first comment. Another type of valve would be far better.Be sure to check actual flow/pressure data (cv's) if changed.

Additional comments:

1. Increasing torque is a well-known problem, manufacturors recomendations of actuator sizing will reflect 'ideal conditions' eg. without the inner containment, which you have here. Upsizing the actuator one or two sizes might solve the torque problem, but not the containment problems.

2. The black residue is a clear indication of impure air conditions inside the pipeline, possible coauses as explained before. This is in no way 'pure air', its getting polluted some way or the other. Difficult to find direct cause, might need different layout and additional filters and drainage points. Oilfree, filtered air? No original greasing of valves or other components?

3. Pictures of the white powder is unclear, what does this look like microscopic, and what does it feel like when touched? It will have to come from ether:

a) dissolved or abrased products from inside the pipeline system. From the (unclear) picture this is probably less possible.

or

b) This looks perhaps like sawdust or similar(???) intruded from outside, perhaps introduced when cutting or adapting the white plastic pipeline.

Generally impurities intruded under service or maintenance operations is a well-known problem for all pipelines. I have seen it all from dust and water up to metal cuttings, welding droplets, earth, sand, gravel, large stoens, twist and T-shirts(!) for larger sizes pipelines, all claimed to be cleaned, inspected and pure pipelines.

 

[bcd]

OK, this all looks pretty normal.

The black you are seeing is most likely residue from polishing the ball. Buffing compound plus some oxidized metal surface. The black color can appear even when there is no air or other media applied to the valve.

The ball is most likely stainless steel but is magnetic because it has been cold worked during machining.

The white powder you see inside the valve is PTFE. That is what it does when it wears. It produces white flakes because of how it is produced. The PTFE is compression molded powder that is then sintered to help fuse the particles together. So what you are seeing is the particles, or parts of the particles that have not fully bonded to adjacent particles breaking off from the surface.

If you need this type of valve for low pressure sealing, with low pressure being less than 4 bar or less, then you are asking for trouble. Floating balls only seal at low pressure because of the compression between the ball and seats at assembly. As the valve cycles and the seats wear, you lose this compression, and the pressure is not enough to push the ball into the downstream seat and make a good seal. Frequently, valves like this have grease on the seats to help reduce wear and improve low pressure sealing ability. But grease has temperature limits, compatibility issues with potential media inside the valve, and tends to collect debris in the system, making a nice abrasive compound which quickly scratches and wears parts whenever the valve cycles.

 

[roshane87]

Bcd I agree with you. Thing is the client hasn't still done laboratory tests to confirm what the black stuff is, I'm pretty sure its the ball being polished by the seat. I did some reading on Class VI valves which this falls to and they usually are rated to leak around 0.15ml/min at 3.44bar dP for a 1" valve. Given this is a 3/4" one, based on the data it should be less as leak decreased with decreasing ball diameter.

When used below 4 bar, I believe the ball and seat aren't close enough to create a good seal. These leak around 0.6ml/min or more which is way above the rated leak.

Does anyone have details for a Class VI 3/4" Ball valve?

Might it be that the dry air is actually drying the grease out of the ball valves and causing them to seize?

Gerhardl - The air passes through bacterial filters which I believe filters almost all contaminant. I don't have a data sheet to give a micron value but it should be pretty small.

Cheers

RN

 

[rocky06]

Try to Keep 0.20-0.25 mm pre-compression per seat.You will get better results.