Better On-Off Valves

This is a subject I have a lot of experience with. But to give you anything other than general advice I need more information. All valve styles and even versions withing a particular class from different manufacturers have significantly varying features. Some are better suited to a particular application. Because of the temperature, media and and cycle frequency, you need to be very detailed.

What size valves?

What is the particle size, texture or other description of the powder. Is it like talc or sand? Abrasives? This is critical.

Do the valves have cavity fillers?

I assume these are 3-piece valves?

Is your flowing media actually at 510C at the valves or was that just a class rating? (Need to confirm conditions at the valve)

Are elastomers permitted in the flow stream, temperature permitting?

Is Nitrogen the transport gas?

A leaking seal usually does not pass the solids, just the gas. But this compacts the solid, or creates a dam while the valve is closed. True here?

How many hours per day is the process running?

How long do the valves normally last before leakage becomes noticeable?

How long before they must be replaced?

Is "bubble tight" shutoff ( nearly zero) always required or could you accept a small leak rate that does not cause problems or is not excessively expensive in transport gas losses?

What type and size actuator is being used? Double acting or spring return? Solenoid?

Any regulations to adhere to such as FDA?

There are occasionally some very simple tricks that can lengthen the service life but it all just depends. If you answer the questions we can probably work it out.

Thanks for the note.

We are talking about 50NB/150NB valves/ 3 PC/ metal seated ball valves of Velan make, end flanges to Class 300#.

The valves are exposed to 510 Deg. C in closed position and to 300 Deg. C in open position. Nitrogen is the transport gas. Though the valves in closed position are subjected to operating conditions all the 24-hours, in the open position it is around 20 mins. in 8- hrs.

Process requires "bubble tight" shut-off, and the particles are in the range of 3 to 40 microns, abrasive in nature.

I have used Velan many times in the past but it's been a few years. My experience has been with fly ash and various combustion products. Because they have so many specialized variations, my recommendations might not make any sense at all unless I know the model number.

The easiest thing to try first ( If you haven't already) is to reduce the compression on the seat seals. The valves are designed to shut off against Class 300 pressures and that usually means a very heavy static compression load on the seals. For sealing against 1-BAR though, you can often get by on less. This reduces the wear on the ball and seals, sometimes substantially. But it's not always workable, depending on the valve seal design. In some designs the valve will not seal at all unless the compression is maintained at a very high level. Others have a wide tolerance despite what the literature might say.

My procedure is to bench test the valve at the process pressure. Gradually reduce the torque on whatever body parts compress the seal until it begins to leak and then tighten back a bit. (I use a torque wrench and log the force). It sometimes takes a bit of trial and error to compensate for the high process temperatures. The high temps will most always improve the seal via thermal expansion.

You might also want to try purging the body cavity behind the ball with a pressure slightly higher than the process pressure. The higher the purge pressure, the more effective it will be. But even 1 psig higher helps a lot. You would need to drill and tap a port on each side of the valve body and connect tubing to your purge supply. The idea is to prevent buildup and/or compaction of the powder behind the ball. Depending on conditions that might not exist in your application, solids buildup in this area can substantially increase the force necessary to stroke the valve. It also tends to keep the powder from scrubbing and abrading the seat seals and ball.

Remember that the purge gas, nitrogen I assume, flows into the process stream only while the valve is stroking. If you cannot tolerate cold nitrogen, wrap the supply tubing around your piping to preheat the gas. Or you may have a better solution for heating.

The purge also helps keep seat leakage down for another reason. If the purge pressure in the cavity is the same or slightly higher than the process, nothing from the process will leak past the seals. Will the purge pressure leak into the process? It all depends on the condition of the seals. The point being that the purge could prevent the seals from degrading as quickly as they are now now. We use limit switches activating solenoids for knife gate valves in fly ash service, to purge the chest area only when stroking. You could easily do the same.

Again, these suggestions might be worthless to you for many reasons. The problem is that you have a very demanding application. High temperature abrasive solids and gases requiring nearly zero seat leakage. I don't believe that anyone's off the shelf product will do it for you.

I always double, as a minimum, the manufacturer's torque specs for metal seated ball valves in pneumatic solids conveying service. Their literature is mostly geared to gases and liquids. That's not much of an issue for a 2" valve but if it's a 6 or 8", the cost can be a killer.

Lastly, you might be using an expensive valve when a cheaper one would do just as well. I used the above ideas on valves costing half that of Velan. If you want to pursue this let me know the model number of your existing valve. I'm not selling anything here by the way. If you prefer, you can email me at charliewolfe-comcast.net. Just replace the dash with the & symbol.

Good luck.

Thanks a lot. My experience goes somewhat in the same lines. Some one was suggesting a butterfly valve for this application.

Arun, remember that I don't actually know the details of your installation so my comments are just generalizations. If the lines are vertical, does the powder build up on the inlet side when the valve is closed? A butterfly would have problems moving the disc against a solid column.

Most metal seated BFV's have areas around the seat ring, or seals, where small solids get trapped and build up regardless of valve orientation. In some cases this will prevent the seal from deforming as needed for seal compression. And, since the disc is always in the flow stream, it will abrade quickly. Any solids trapped between the disc and seal will immediately cause a leak. But this all depends, of course, on the velocity and especially the amount of solids in the flow stream. All in all, a ball valve would "seem" to be a better choice though.

One thing that would help me is to know the valve size and how many are in service. Some of the things we can do with a 2" doesn't make sense for a 10". And are the valves in a vertical line? Let me know and I'll give you my valve recommendation and you can see if it makes sense. I mentioned earlier that very small differences in the application details make a major difference in what valve type and options work best.

Thanks for the note. As already mentioned, I am talking about 50NB and 150NB valves of Velan make and all of them are in verticle lines, placed in pairs and the central spool being constantly purged.

You may want to try Kolsterising the seats and balls if the material is 300 series stainless. Although your temperature may be too high, it is worth a look.

The treatment can produce a surface hardness of 70 HRc without dimensonial change.

www.kolsterising.info

Kolsterising is an ideal treament process for ball valves involved in fly ash applications.

Might be a good idea but the max temp for Kolsterised parts is < 500C according to the literature. Arun's process temp is 510C for long periods. Still, maybe worth taking a look.