Jack,

I have some 34 years experience in exactly this area that says the following:

1) Shot peening is effective in minimizing streamers / floss. Spiral grooving has no effect one way or the other on streamer / floss generation.

2) Shot peened aluminum piping is effective for approximately three years. Shotpeened stainless steel piping is effective for approximately five years.

The spiral groove vendor claim of 15 years of life is a moot point if it is ineffective in reducing streamers / floss. Ask the vendor for independent test data that proves the effectiveness of spiral grooved pipe and see what you get.



Kind regards,

Dennis Hauch ■

Jack, Dennis

Independent test data ??????

From a supplier ???????

Hahahahahaha!

After years of generating independent test data for users of systems, and trying but mostly failing to engage suppliers in this activity, I have come to the conclusion that most are fundamentally either opposed to or at best ambivalent to independent evaluations.

Too many people project pneumatic conveying as a "black art" and that's why there are so many systems that don't fulfil the users' expectations.

Mike Bradley. ■

Dennis, Mike

There is clearly a lack of scientific and peer reviewed publication on this topic. Otherwise, we would not resorting to experience.

Definitely the vendors have an axe to grind & we all understand that.

I have talked with a couple of spiral groove pipe users (who have had these lines for almost 10 years), and they have indicated that they see short streamers and floss but never long ones. Shot peened lines work but they need treatment every 3-5 years.

I am hoping that various readers of this forum will contribute their experience in this discussion. ■

Jack, Dennis

From a fundamental understanding of the mechanism of production of streamers, it is clear to see that helical grooving will eliminate long streamers because the rubbing of the particles around the extrados of the bend, depositing a "skid mark" of plastic and building these up on each other until they become unstable, is interrupted.

The pitch of the helix should be short to keep the streamers as short as possible, then you will have something which will be effective for a longer period of time, provided you can tolerate short streamers.

In our pipeline designs we usually try to overcome the problem by using different bend geometry - blind tees or "Vortice-Ells" are very successful in controlling streamer production. They also last indefinitely. Provided you have only a small number of bends, this is the way to go. The problem which arises if you have a large number of bends, is that because they give a higher pressure drop you need a higher air mass flow rate to give the same pickup velocity, and velocity is higher at the end of the pipeline. However this can also be overcome by stepping the bore of the pipeline - increasing the diameter to keep velocity under control.

Almost all the pipelines we design incorporate steps in bore size if the pressure is above about 0.9 bar, because it reduces energy consumption as well as particle damage and wear.

We have a test rig specifically set up to do particle damage analysis, so if someone could find some money we could produce the critical independent data!

Mike. ■

Of course you're perfectly right about dense phase conveying - reduced velocity is always beneficial - as indicated in my discussion of using stepped bore pipelines.

But many plants are stuck with what they already have - and are unwilling to make such radical changes in spite of the theoretical payback period. In which case, changing the bends is about all they can do.

Another factor to be considered is that in my experience, many people do not obtain the payback of the theoretical reduction energy consumption because they use plant air to drive the dense phase system. The energy needed to compress the air to mains pressure - usually seven bar - is mostly wasted as the air pressure is let down through regulator and orifice plates or choked flow nozzles. We have analysed many existing plants using dense phase conveying and especially for plastic pellets, most of the systems actually use more energy than an equivalent well designed lean phase system. I emphasise the term WELL DESIGNED here - many lean phase systems are not well designed!

If the theoretical savings are to be realised in practice, it requires the use of a dedicated compressor providing air at only just sufficient pressure to run the conveyor, and the purchase of the compressor and its electrical supply is a major addition in capital expenditure - so actual payback can be a lot longer.

Consequently the question lean phase versus dense phase is not as clear cut as you would seem to suggest, in my experience. We have designed and troubleshot many of both, so have an even-handed viewpoint!

Mike. ■