VALVE & POSITIONER

How old is the unit?

I ask this as I have had numerous occasions where the pneumatic control valves of digital and proportional valves got stuck over time due to dirty air.

Sometimes the air quality is different for different parts in the compressed air line and some components require oil in air and others require dry air. If oil is administered before the dry components the oil will cause some seals to swell up and the pilot gets jammed. Any vibration from the engineer could have dislodged the pilot and it suddenly moved to the required position. If this is actually what is happening you can be assured that the problem will get worse over time and in the end it will affect more or all your components.

Please give us some feedback with new findings.

Thanks

Air is bone dry and clean. No oil or moisture can be present in the compressed air used, this is assured. I believe this is being seen only during the rainy season ???

How old is the unit?

Has the air been checked for dirt, water and oil or do you just rely on the system of the water separator to work correctly? Don't forget that compressors get older too and start to put rust and oil in the air over time so the check has to be done frequently to be sure.

Rainy season ??? could it be that humidity is too high and the seals are getting sticky due to this? What are the humidity levels and how do they compare with the specs for the pilot valve and the valve itself?

Is it only one unit with a problem or is this on more components?

Sorry about the questions but we need to have detail for correct answers, I hope you understand.

Thanks for getting back.

May be 7 years i think if its not changed. I have asked for the details i guess my guys will come back soon.

Actually we use the process air after the Prepurifiers which saperates the moisture and already we have around 2 micron dust filters in line so that's why said that the oil, water and dirt may not be there. we measure the process and hence i believe no dust , moisture etc may be there.

Well yes its a pretty humid coastal place & its rained recently, ill comeback with the humidity levels of the place and the workouts, I think its a good point uve made ,

Thanks..

Next time it happens, instead of opening the unit, try tapping the base or the box with the blunt end of a screwdriver to see if it is the vibration you can cause that makes the pilot jump position. If so you can be pretty sure the seals get stuck due to something.

If the air quality is as high as you say it is it must be wear or tear or the humidity.

Maybe the manufacturer does a rebuild kit comprising of all the seals and gaskets as well as pressure faces for the pilots or other delicate internal parts. Such kits are far cheaper than renewing the proportional unit as they can be pricey. This is only useful to you once you have decided what the problem is of course.

Good luck and looking forward to hear from you with regards to the humidity.

The cover might have been installed lopsided or is warped and was pressing against the cam rotor shaft...

Dear sir

I am ravivarma. This type of problems I practcally faced. this type of problems will come in only pneumatic positioners. some times positioner vent mess is chocked with dust particals.then after flapper and nozel venting air is not released from positioner due to positioner vent mesh is chocked. then after sometime later this valve positioners flopper and nozle vent air presser is increased due tocontroling signal.inthis time positioner inside and air relay outside some diff presser is created due to that diff presser air relay venting air is not vent outside of the relay. due to this relay back presser is effected to valve actuater .that s valve is not close properly . When you open this positioner cover is open immidiatly the positioner inside presser releases then after air relay back presser comes normal then after valve become close. for better perforfomence of valve you should clean positioner air venting mess. OK. I think your doubet become cleare. onemore thing due to this mess choke sometimes valve hunting also come make it sure.

thankingyou

RAVIVARMA(INST SUPERVISOR)

FLUOR

Its an electropuenmatic positioner. NE724, Could you explain the last part once again, i could not make out. AFter the pressure in the positioner increases , from there on,.

I believe he means that the positioner has a vent where the excess air during opening or closing is vented out. The vent itself may be in inside the positioner with another vent that will allow the air to pass out to the atmosphere. If the second vent is clogged, the excess air will not get out and the valve will not move. Removing the positioner cover released the air and allowed the positioner to move to the closed position.

Yep. Good one.

vents are ok, Its s Neles Jamesbury positioner model NE724. Ive never seen this kinda problem in 10-15 years of operation in these positioners. i think it could be with the pilot getting stuck more so. i did ask the site guys to check the vent they found it good, no probs on that front i guess.

I think vents are only of consequence in air operated pilot valves, not the electro pneumatic ones.

I stay with the humidity, if the seals inside are swelling due to high humidity you may find that the pilot gets sticky. Try to aggravate the problem and see if a soft knock will release it. If so you have to take the valve apart to see what parts are sticking and draw your plan accordingly.

I cant stop the plant on this account.And the valve operates every 30 mins approx, if the valve doesnt, plant trips( shutsdown). I do not think 24 mins will be sufficient to do much on the positioner,( even replacement).

i think humidity could be the reason. spoke to neles jamesbury guys and they too are bullish on the pilot valve sticking theory, i have seen wear on this and failure in one case long ago.

but i aint have any hard data thisway or that...

I didn't offer the vent as the culprit. I only explained what I thought Ravivarma said.

Your theory about the pilot getting stuck is valid. If you do PM on these valves, do you include everything? including the pilot? If so, you may have an issue with the quality of the materials used. Or it may be poor workmanship.

Since this is your first time to see this (first time I've heard of it myself), it could be a one-chance-in-a-million occurrence. Then again, if your plant's safety is at stake, you'll be wise to find the root cause.

I suggest you involve Jamesbury in the investigation and send it to them (you did replace the positioner, right?). Ask for a report and possible solutions.

Loose wire somewhere?

How often is the valve operated?

What fluid is being pumped through the system?

I have known, on more than one occasion, the valve itself can stick or seize, rather than the controller being the problem.

Most valves, Ideally, should have a manually operated handwheel, so that the valve can be closed in an emergency, or when automatic systems fail.

it operates nearly every 30 mins

dry air is the fluid

no , the valve got stuck on account of positioner , no conditions , no loose terminations, no water in jb.

i do not want the problem to repeat and have any downtime on that account. thats why i need to be ansolutely sure, hence i need some advices.

If it operates every 30 minutes you can try several times in a relative short time span to see if it sticks and what dislodges it.

As I can see it from here, you are not going to find out for sure from any forum, you need to take action and disassemble the unit to find out what is happening.

IT didnt operate only ones. otherwise its been fine. But this has a trip potential and hence iam worried and another reason is iam not able to find answer. No certain answers....

i guess its like that with pneumatics always..

You mentioned previously that the unit is about 7 years old.

Has it ever been completely refitted in this time....................if not, normal wear and tear could well be the problem and this is just the start of it...............letting you know something may fail in the not too distant future.

We have PM plans in place for each insts so , i think it would have got the refurbishment done,

Hey naveen it's no good thinking........you either have the PM schedule and records for it or you don't.

When was it last looked at?

What was done and by whom? e.g. were any adjustments carried out, was it checked for correct operation, were any new components fitted (if so what were they), etc.

Has any operating problem occured before?

Has any breakdown maintenance been carried out on it between scheduled PM periods?

OK ,

We have a very good PM program

It was refurbished in 2006 dec. Cal check done again in 2007 nov.

All the soft goods replaced. in 2006 dec

no operating problems occured later/earlier of 2006 dec.

No breakdown maintainance recorded on the valve/positioner

Iam not at the site and provide tech support from a central location, so need to get the info from the software/people at site hence i wasnt sure.

Hi

It is simple. The positioner breather is choked, while the cover was closed.

The moment you opened , this was released. Check and clean the mesh like filter

in your positioner.

Navaneethan

There are no seals around the spool valve on this positioner. Try having the guys at the site carefully remove the spool and check for straightness by rolling across a flat surface. Also inspect for any contaminates built up on shoulders of spool. If you see any dirt or build up run a pipe cleaner thru the spool valve body and wipe off the spool.

If the problem persist check the wires going to the coil assembly.

Due to how these operate I do not think that the humidity or seal have an impact on this problem.