Dear pristine,

Surely, there will be some energy loss when using flexible hoses in pneumatic conveying.

The inevitable slamming of the hose induces these extra energy losses.

Nevertheless, I never experienced significant influences on the pneumatic conveying efficiency as the hose lengths were always kept to a minimum in relation to the total conveying length.

In cases where the total conveying length was executed in hoses, it was always a short length and the capacity was never an issue.

If you are planning to lay out a long length of hoses, there might be some capacity loss, compared to pipe.

However, the excessive slamming will cause dangerous situations and excessive wear on the hoses, which can become quit costly.

Best regards.

Teus

PS.

Did you solve your thread Aerzen vs Roots?

https://forum.bulk-online.com/showthread.php?t=15748 ■

Hose lengths should be kept to a minimum & are usually limited to the stowed length of a single tanker hose, about 5m. Remember the EU Regulations limit manhandling to 25kg! Connecting 2 tanker hose lengths is illegal in Europe but commonplace in Australia where they make an adaptor to overcome the lockout.

If you are talking about a wandering shipunloader type situation then you would have to work the losses out from the manufacturers' published friction factors (try Linatex) and determine the bend radii for yourself.

Slamming is the way that tanker drivers know the tank is just empty and quite fascinating to watch...but not for long. I've never seen the slamming over a long hose length but I bet it's something to behold...or not hold! ■

About 15 or 16 years ago, I did not notice a slamming hose coming my way.

An 8 inch rubber hose (abt. 5 m) for conveying cement slammed me at an opening pressure of 2.5 bar against a wall.

It knocked me unconciuos for a few minutes and broke my right collarbone.

Since then I always kept arespectable distance of these killers.

Still alive

Teus ■

About 30 years ago I undertook a research programme to compare the pressure drop for rubber hose with that of steel pipe. The application was the transfer of drilling mud powders from supply boats onto off-shore drilling platforms. I had a 40 m length of rubber hose and a test pipeline having a similar length of steel pipe, together with several tonnes of oil well cement and barite. I conveyed both materials through the steel pipeline. I then strapped the rubber hose to the steel pipeline (straight lengths and bends) to get an identical routing and geometry and repeated the conveying trials with both materials in the rubber hose line.

On comparing the data I found that for dense phase conveying with a conveying line inlet air velocity of about 4 m/s the pressure drop through the rubber hose was about 10% lower than that for the steel pipeline for both materials tested. I put this down to the fact that there were no joins at all in my rubber hose but many in the steel pipeline with all the bends. For dilute phase conveying with a conveying line inlet air velocity of about 25 m/s the pressure drop through the rubber hose was almost 50% greater than that for the steel pipeline, also for both materials tested.

There was a marked correlation between this pressure drop ratio and conveying air velocity over the entire range of conveying conditions examined. I put this increase in pressure drop for the rubber hose down to the coefficient of restitution between the conveyed material and the pipeline wall. A ball impacting against a steel surface will bounce back readily but a ball bouncing against rubber will not and it is my contentiion that the increase in pressure drop is due to the increased retardation of the particles and the loss of energy going into the re-acceleration of the particles. I would be interested to know if anyone has any other ideas on this. ■

Dr Dr Mills,

Pressure drop 50% higher for rubber hoses instead for steel pipe.

I tried to figure out how to understand the background of this observation.

My first thought was that a 50% increase in total energy consumption is used for an increase in product losses, which are only a part of the total losses. The 50% energy increase is totally absorbed by the flexible hose, what has to have been noticeable in slamming and temperature.

I am sceptical about this extra loss of energy only in boundary effects.

I know that measuring in pneumatic conveying and evaluating the results is a very difficult issue,

It is only possible to measure the overall parameters s.a. pressure, capacity and air volume,

The problem is that these parameters are strongly interrelated in a very complex way.

The 50% pressure increase needs additional information.

50% pressure increase in relation to???

(constant capacity, constant velocity, constant SLR)

Suppose the following:

-pressure increases

-intake velocity is kept constant

then

-Airflow is increased in order to keep intake velocity constant

then

-SLR is changed

However, a changed SLR influences the pressure drop for product losses and thereby the total pressure drop.

The measured pressuredrop is now caused by 2 effects. The increased product loss factor and the SLR.

The effect we want to know is the changed product loss factor due to the application of a rubber hose instead of a steel pipe.

To evaluate the effect of a rubber hose, one should recalculate the product loss factors and compare those.

Comparing the total pressure drops is not accurate enough, although this could give an indication.

To avoid all those comparing problems, I think that only cold-blooded calculations can reveal the truth.

Best regards

Teus ■

In lean phase the pressure losses in rubber are higher due to a number of reasons.

1.The wall friction between particle and rubbers hose is very high as compared to steel. This causes increased losses.

2.Due to the soft nature of rubber hose when the particle impacts the hose the rebound energy is less then steel pipe.

That explains the 50% higher pressure drops in lean phase as stated above. In dense phase the most materials will have very high wall friction with rubber hose and it will not move on the hose wall and instead will have an outside stationary layer and the plug will slide on this stationary layer. If you want to quantify this the increase in pressure drop will be the difference between the wall friction of the material with steel pipe and internal friction of the material. Which when correlated to pressure drop will be in agreement with Dr Mills observations. ■

Is there a consensus that rubber hose is largely unsuitable for anything more than a flexible connection? ■

Dear Mr Mills, Mr Mantoo, Mr louispanjang

Dense phase:

In Mr Mills observation, the pressuredrop was 10% lower in a rubber hose than it was in a steel pipe.

In the assumption of Mr Mantoo, the wall-friction of rubber is higher than the internal friction and the internal friction is higher than the wall-friction of steel.

That makes the product loss factor in steel the lowest and one should expect there the lowest pressure drop.

However, the pressure drop is 10% lower for rubber hoses, suggesting that the internal friction is lower than the wall friction of steel.

In that case there will also build a stationary layer in steel pipes and the product losses will be the same.

Quantifying the difference between wall friction an internal (fluidized) friction is then not necessary anymore. It is internal friction in both cases. (rubber and steel)

Lean (dilute phase)

Based on Mr Mills project, I made a calculation for barite.

I modelled a 40m pipeline of 6”

To get a 50% pressure increase I calculated a steel pipe for 1.5 bar and a rubber hose for 2.25 bar with a higher product loss factor.

The results were as follows:

====================================================

Resistance factor for steel pipe = 5 * 10^-12

Capacity 58 tons/hr at 1.5 bar

Air volume = 1.2 m3/sec

gas velocity at intake =32 m/sec and gas velocity at end = 66 m/sec

SLR = 10.9

product velocity at intake = 30.3 m/sec and product velocity at end = 62.2 m/sec

pressure drop for product intake = 100 mmWC

pressure drop for acceleration = 7840 mmWC

pressure drop for product losses = 3405 mmWC

pressure drop for elevation = 207 mmWC

pressure drop for suspension = 256 mmWC

pressure drop for gas =2339 mmWC

pressure drop for filter = 1287 mmWC

==============================================

Resistance factor for steel pipe = 20 * 10^-12

Capacity 58 tons/hr at 2.25 bar

Air volume = 1.2 m3/sec

gas velocity at intake =25 m/sec and gas velocity at end = 66 m/sec

SLR = 11.2

product velocity at intake = 22.3 m/sec and product velocity at end = 58.2 m/sec

pressure drop for product intake = 100 mmWC

pressure drop for acceleration = 7012 mmWC

pressure drop for product losses = 11615 mmWC

pressure drop for elevation = 255 mmWC

pressure drop for suspension = 333 mmWC

pressure drop for gas = 1907 mmWC

pressure drop for filter = 1316 mmWC

===============================================================

The conclusion of this calculation is that the product resistance factor must be 4 times as high in rubber hoses as in steel pipes.

However, the product losses are a combination of collisions against the wall and interparticle collisions.

The wall collisions are a function D and the inter-particle collisions are a function of D^2..

The wall collisions are less important than the inter-particle collision losses.

The wall effect between rubber and steel must therefore be more than the overall “measured” effect. (More than 4 times, depending on D)

Another effect is the slamming of the hose. The involved slamming energy is delivered by the kinetic energy if the material, which emerges as a product loss factor.

The, measured based, calculated friction factor is now depending on the flexibility of the hose and how rigid it is supported.

I believe that the above approach is the best way to evaluate the influence of the effect on pneumatic conveying of rubber hoses compared to steel pipes.

It is apparent, that a rubber hose instead of a steel pipe has a negative influence on the performance. Whether that influence is a factor higher than 4 in this case, depends on how well I modelled Mr Mill’s experiments.

There a consensus that rubber hose is largely unsuitable for anything more than a flexible connection.

Best regards

Teus ■

Dear Pristine,

I now understand that the rubber hose is conveying clean air.

From your threads, I conclude that the mentioned compressor is an Aerzen GM90S at 1190 rpm.

The air displacement of that blower is 0.614 m3/sec.

In a 6” hose this results in air velocities of:

-35 m/sec at o bar(o)

-13 m/sec at 1.5 bar(o)

In a 5” hose this results in air velocities of:

-50 m/sec at o bar(o)

-20 m/sec at 1.5 bar(o)

In a 4” hose this results in air velocities of:

-78 m/sec at o bar(o)

-31 m/sec at 1.5 bar(o)

A 5’ hose wil be a normal choice under these circumstances.

However, if the air displacement is reduced to 0.4 m3/sec (final calculations of the existing installation required), then a 4” hose can be used.

Replacing hose length by steel pipe is always Ok, because it saves you the money of replacing long damaged hoses.

In pressure drop, it will not be noticeable whether a rubber hose or a steel pipe is used.( only clean air)

all for now

Teus ■