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TechnicalExclusive
22

Januar y 2019

Pneumatic Conveying

through Inclined Pipe

By Jonathan Thorn, Process Technology Schenck Process 2) Vertical pipe angling more than 10 degrees

A 3) Horizontal pipe declining more than 10 degrees
As with many technologies related to bulk materials
handling, pneumatic conveying can be a mixture of
art and science. Along with many tangible physical
relationships (i.e. pressure, airflow, and velocity)
* the threshold angle ranges from 7 to 15 degrees
depending on the source
there are also several “rules-of-thumb” that drive
industry know-how. A few such rules relate to the
Primarily, we will deal with the first case of a
layout of piping that will make its way through the
facility:
• Choose the most direct route – shortest distance
10 degrees up to and including a 45-degree slope.
and fewest elbows horizontal pipe that rises at an angle greater than
Above a 45-degree slope would technically fall into
• Avoid back-to-back elbows the second category of an angled vertical, but we can
• Run pipe horizontally or vertically – avoid in- trust that the learnings will extend into this realm to
clines some degree. In both these cases, gravity is working
against the effort to move the material either hori-
The last one regarding inclined pipe is probably zontally or vertically. In the vertical arrangement the
the least understood and is sometimes at odds with travel is directly opposed to the gravitational force.
the other recommendations. The most direct pipe If the air velocity is above the aerodynamic terminal
routing will yield the lowest operating pressure and velocity of the material, then it will elevate with the
energy consumption. Provided there is a clear path associated rise in potential energy. In the horizontal
for the pipe, an incline would be more direct com- arrangement the particles oppose gravity indirectly
pared to any vertical and horizontal run. Likewise, either by creating lift and entraining the particles
for small changes in vertical height, two elbows will (dilute phase) or allowing the particles to rest on
need to be close to each other (back-to-back). The the pipe bottom and overcoming the weight fric-
way to overcome this is to use an inclined pipe in tion. When the pipe is inclined at an angle, it would
its place. Since contradictions are unavoidable, logic seem both mechanisms are in play, but to varying
suggests a lesser of two evils approach. Whichever degrees depending on the angle and the material
violation provides the least negative impact would characteristics. In the third scenario gravity is assist-
be the preferred choice. The difficulty is knowing ing in the flow of material as it moves through the
how to evaluate this comparison if inclined pipes are pipe. Initially as the angle gets steeper it aids the
not well understood. To help answer this question, horizontal movement. Eventually the decline angle
research was conducted to assess both the viability will be greater than the slide angle of the material
(when to use it) and the impact (how to account with the pipe wall and the material would flow with
for it). or without the airflow present. For this reason, we
can eliminate the third case (declined piping) as a
Background concern and focus on inclined piping.
Since no two applications are exactly the same,
each piping system will be unique with some level Experiment
of customization to the general routing, avoiding An experiment was created to analyze the case of
obstructions and a preferred path to end up at the the inclined pipe as compared to a traditional piping
correct elevation for the destination. Generally, pipes layout. In Figure 1, we can see the feed point for a
are more easily supported when laid horizontally pneumatic conveying system using an airlock fed by
or elevated straight vertical due to the interactions a weigh belt. The piping after going horizontal for
with the steel. They also display better in drawings a short distance then takes two routes to get to the
and isometrics, so designers will normally gravitate same destination – one of which is a 21-degree slope
toward this practice. However, when encountered, for 50 ft. By using the same feed source and routing
obstacles, tight spaces or subtle changes in elevation the material through the same connecting point and
or direction may require other solutions to be imple- operating at the system similarly we get a close com-
mented. Subtle changes (such as a 3-degree angle) parison for analysis. Four materials were selected
have little impact on the system so the angle must that have a range of density and particle size (See
be significant to warrant special consideration. An Figure 2). These materials were handled in either di-
inclined pipe may be considered in three ways: lute phase, dense phase, or both. Two different rates
1) Horizontal pipe rising more than 10 degrees* for each condition were targeted.