Stepped pipe = increase capacity?

Posted in: , on 11. Apr. 2007 - 20:21

hi all....

am reading the fine book [pneumatic conveying design guide] by david mills. on page 110. it was mentioned that by stepping pipeline, the author was able to increase capacity two fold with the same airflow rate and pressure drop as compared to a non-stepped line. can somehow correct my train of thought on this?

- stepping of line, i.e., increasing pipe size,

results in air velocity drop along the lines,

thereby reducing rate of pressure drop in the lines,

- as compared to a straight through line,

the velocity will get greater and greater as it travels,

subsequently rate of pressure drop will be steepest.

as since overall pressure drop is similar, only difference is rate of pressure drop between stepped lines and straight-thru lines, correct?

but why then having a different rate of pressure drop along the stepped lines will result in greater capacity?

the reason i am wanting to understand this, is recently i did a trial run of a new vac transfer line and i am getting random chokages at the straightest part of the transfer line. I am wondering if by stepping the straight section, i will reduce the air velocity, in effect reduce the pressure drop along the line to reduce chances of chokages. or i am running the risk of lowering the air velocity too much that may result it to be below saltation velocity and instead cause higher chances of chokages.

thank you for your kind comments.

Re: Stepped Pipe = Increase Capacity?

Posted on 11. Apr. 2007 - 08:17

In order to understand this principle, a profile of the operating parameters spanning the entire conveying route should be developed. This profile would indicate gas pressure and velocity for each segment of the route. From this analysis, the pipe diameter stepping location(s) can be located to ensure that the system does not choke due to excessive saltation. This profile can be presented graphically and compared to alternate designs and will help you understand how this principle works.

Given that, the idea of increasing solids throughput using this method involves increasing the loading (solids:gas ratio) of the system. By increasing solids loading, the rate of pressure drop increases. By increasing the pipe diameter along the way, the rate of pressure can be reduced. However, If at any location along the route, the gas velocity falls below the critical level, the operation may fail.

This method does not work in all applications, and therefore should not be used universally. Instead, we recommend that an analysis be performed in each case, and applied selectively. In some cases, we have demonstrated that efficiencies can be gained using this method. We have re-engineered systems that were previously over-designed by stepping the pipe diameter, reducing gas flow, and increasing solids throughput without increasing the power requirements of the system. The benefits of this are:

1. reduced investment

2. reliability improvements

3. energy savings

I would advise you to consult an expert that can perform this analysis before you start making changes.

Robert Reischl Manager, Process Technology & System Services Coperion Corporation

Re: Stepped Pipe = Increase Capacity?

Posted on 12. Apr. 2007 - 02:36

In a simplified form, the ability to increase throughput by stepping is based on the assumption that the system is operating at some maximum pressure. By increasing the diameter of a section of pipe, the rate of pressure drop for that section is reduced.

Therefore the system operating pressure is reduced. More material can then be added to the convey line until the maximum pressure is again reached.

Typically the amount of increased capacity is marginal. The case stated by Dr. Mills must have been an extreme example. As Mr. Reischell stated, stepping is more often used for other advantages such as increased efficiency or degradation reduction.

The system you have described will likely not benefit from stepping alone. It is possible that a larger pipe size through the trouble zone and on to the destination will help matters but only when accompanied by an increase in airflow that will create the proper velocity profile in the system.

Jonathan Thorn

MAC Equipment

Re: Stepped Pipe = Increase Capacity?

Posted on 12. Apr. 2007 - 06:12

In some cases, the pipe diameter towards the end of the existing route can remain the same and the diameter at the beginning of the route can be reduced. This technic may result in a higher operating pressure, but with the same or lower gas flow. This method has potentially more promising benefits since the existing blower and filter separator(s) require little or no modifications.

My first inclination would be to not increase gas flow. That has potentially significant consequences regarding modification costs.

Good Luck with your project. Please contact me directly if you need further assistance.

Robert Reischl Manager, Process Technology & System Services Coperion Corporation

Re: Stepped Pipe = Increase Capacity?

Posted on 12. Apr. 2007 - 07:43

Dear Mr Samuelcsw,

A stepped pipeline causes lower velocities in the pipeline as compared to a single diameter pipe line.

In a single diameter pipeline, the convey air velocity increases, due to the decreasing absolute pressure in the pipeline.

The lower velocity limit is given by v-air = > factor times the suspension velocity of the conveyed particles at the given location in the pipeline.

To maintain this lower velocity limit, a tapered pipeline would be needed..

(I remember one pneumatic copra unloader in the past, which was equipped with a tapered suction pipe).

Providing that the lower velocity limit is reached in the beginning of a single bore pipe line, then, due to air expansion, the air velocity along the pipeline increases above the local lower velocity limit,

This higher air velocity causes higher velocity losses.

By stepping up the diameter at a certain location along the pipeline, the velocity is reduced and the energy losses are reduced as well, causing a lower pressure drop.

The location of the pipe diameter increase is related to the pressure drop along the previous pipe sections.

The decreasing absolute pressure in the pipeline induces higher velocities.

After the diameter transition, the velocity should still comply with the requirement of being above the factor times the local suspension velocity.

A too early diameter increase causes a blockage in the pipe section right after the pipeline step.

In this case the total pressure drop decreases, due to the lower velocity losses in the larger pipe.

To restore the total pressure drop, the capacity should be increased to achieve this

(Increasing the solid loading ratio SLR)

As you are referring to a vacuum system, it is also important to know the characteristics of the vacuum pump.

The air mass flow will vary with the vacuum in case of a positive displacement pump, causing an extra influence on the pipeline velocities, apart from the pressure drop in the pipeline.

A too high vacuum will cause the pipe line to block.

The Zenz diagram can be useful in this case also.

The stepped pipeline keeps the conveying regime close to the lowest point in the curve.

(see other threads in the pneumatic conveying section)

Because of the many (and interacting ) parameters in pneumatic conveying, all with different influences under different conditions, it is almost impossible to predict results of changes to an installation, without proper calculations..

F.i. an air volume increase causes a higher product resistance energy loss, but at the same time a lower suspension energy loss, due to the shorter residence time of the particle in the pipeline.

Externally, almost no or little effect is noticed.

If you can share the product- and installation data, may be we can discover the source of your problem.

best regards



Re: Stepped Pipe = Increase Capacity?

Posted on 15. Apr. 2007 - 06:43

thanks all for your kind patience in your explanation.

It would seem that the only way to solve my chokage issue is to increase blower capacity, or reduce feed rate, in order to reduce SLR as it seems the new setup with the extra bends are too much for the current setup.

I will summarise my setup and post it here for all to 'dissect' later in the day.

thank you all once more.

Setup Details

Posted on 29. Apr. 2007 - 05:27

hi all, included are details of my setup.

lobe blower (vac)


Gas - air

inlet conditions - 32degC , 600mbars(abs)

differential pressure 413 mbar

flow at inlet conditions - 400m3/hr

blower Speed - 2898rpm

max speed - 4200rpm

max differential pressure - 800mbar

Pipe setup


2 inch single bore polished pipe

horizontal - 20 meters

vertical - 5 meters

bends - 8 bends

Required Capability


material flow rate of 400-450kg/hr

minimum air velocity - assume 18m/s

Material - powder


inital bulk density - 450kg/m3

tapped bulk density - 750kg/m3

mean particle size - 40µm

initial calculation would yield air velocity to be approx 54m/s, for 2 inch pipe with flow of 400m3/hr.

is this too excessive?

during trials, chokages did not occur immediately nor at a fixed interval. chokages occur randomly and requires only slight hammering of the longest horizontal section of the pipe to clear it.

another observation, during trials, it would seems that feed rate of material did not have any impact on probability of chokages. at one instance, we were able to increase feed rate, thereby reducing the usual transfer time of 160mins to 120 mins.

first train of thought, not enough pressure to convey material with new longer setup. but there are instances of successful transfer and chokages are not 'consistent' if i may say.

second train of thought, too low velocity, especially saltation velocity, resulting in product buildup along the horizontal line, therefore resulting in 'random' occurances of chokages, possible?

third train of thought, too fast air/product velocity, resulting in too much pressure drop along the line, resulting in chokages. but why then the ability to increase feed rate?

can someone shed a light in my tunnel?

thanks in advance.........

Re: Stepped Pipe = Increase Capacity?

Posted on 29. Apr. 2007 - 06:24

For a 2” (53mm) Id pipe 180 m3/hr will be in the right velocity range. This does not include any RV leakage. You are using too much air this not only reduces the solid flow rate but also gives high system pressure drops.

8 bends in 25m also are not recommended try to have at least 5m distance between two bends as series of bends is close succession always cause problems .

Blockages could also be caused by material building up in conveying pipes since you have not mentioned what material you are using I cant comment on it.


Re: Stepped Pipe = Increase Capacity?

Posted on 29. Apr. 2007 - 06:44

hi mantoo,

the reason that i am having so much bends in my proposed change is due to the construction of the builiding in which i am moving material.

the 400m3/hr data is from the manufacturing data sheet, is there a way to verify it? i am also surprise by the large air flow, which results in high air velocity. but the interesting part, is with the current driver, and feed mechanism, i am having no problem transferring my material in my existing setup of 6bends and roughly the same amount of horizontal and vertical pipes. guess maybe 400m3/hr is not right. can taking pressure readings along the line help me draw the actual air flow rate? and if yes, how?

thank you......

Re: Stepped Pipe = Increase Capacity?

Posted on 29. Apr. 2007 - 08:00

Measuring air flow rate is very simple just stick an air velocity meter at the end of the line (at atmospheric conditions) and

Air vel. = Air flow rate / X- sectional area of the pipe.

Once you calculate air flow rate at atmos. conditions and you know the pressure at the start of the system form there pick up velocities can be calculated. Air velocity in between is of no importance.


Vacuum The Thing Of Suction

Posted on 29. Apr. 2007 - 08:11

vacuum is referred to as HG inches of mercury ,not pressure.

How often do you change the air filter on the suction side/inlet of the blower?

The only reliable way to guarantee accuracy in a vacuum system is to obtain a DELAVAL milk house gauge kit to verify vacuum velocities and reajust the reading for elevation or declination in relation to the sea level base reading for vacuum suction.

This allows you to properly set the inlet/relief valve for the vacuum system.

Measuring inches of vacuum HG mean nothing unless you know the velocity of the vacuum.

Every elbow you have is resistance, I repeat every elbow you have is resistance and slows the velocity of the product and vacuum.

The product, any powder product for that matter is lazy and likes/loves gravity and the bends only aid the lazyness of the powder and buildup of material. powders are lazy just like electricity and water if they can escape to a ground or a hole in a pipe they will.

in the case of a vacuum the suction is creating all the work and every bend or gasket is an opportunity for a leak from a pin hole or gasket failure.

Re: Stepped Pipe = Increase Capacity?

Posted on 29. Apr. 2007 - 10:46

Dear Mr Samuelcsw,

Some observations based on your info.

Hausner Ratio 750/450=1.67

Geldart classification A

This product should be poor flowing when settled, but easy fluidizable

That means that your feeding could be inaccurate and flow surges can occur, causing a blockage.

The solid loading rate you indicate is 0.12 kg/sec material on 0.08 kg/sec air = 1.6

Considering the high air velocity in the pipeline, the air only pressuredrop could be close to the maximum allowable vacuum, leaving very little pressure drop available for material conveying.

This could explain the very low SLR and the high sensibility for over loading the flow, causing the blockages.

If you can inform us about the suspension velocity of the material or the material density, then the appropriate velocity can be indicated.

Also the properties of the material itself (s.a. stickiness ) are important.

When your installation chokes, what is the vacuum at that moment?

best regards