Page 125 - Root Cause Failure Analysis

Conveyors 113
performance

Pneumatic conveyor performance is determined by the following factors: primary-
driver output, internal surface of the piping or ductwork, and the condition of the
transported material. Specific factors affecting performance include motive power,
friction loss. and flow restrictions.

Motive Power
The motive power is provided by the primary driver, which generates the gas (typi-
cally air) velocity required to transport material within a pneumatic-conveyor system.
Therefore, the efficiency of the conveying system depends on the primary driver's
operating condition.

Friction Loss
Friction loss within a pneumatic-conveyor system is a primary source of efficiency
loss. The piping or ductwork must be properly sized to minimize friction without low-
ering the velocity below the value needed to transport the material.

Flow Restrictions
An inherent weakness of pneumatic-conveyor systems is their potential for blockage.
The inside surfaces must be clean and free of protrusions or other defects that can
restrict or interrupt the flow of material. In addition, when a system is shut down or
the velocity drops below the minimum required to keep the transported material sus-
pended, the product will drop out or settle in the piping or ductwork. In most cases,
this settled material will compress and lodge in the piping. The restriction caused by
this compacted material will reduce flow and eventually result in a complete blockage
of the system.

Another major contributor to flow restrictions is blockage caused by system backups.
This occurs when the end point of the conveyor system (i.e., storage silo, machine, or
vessel) cannot accept the entire delivered flow of material. As the transported material
backs up in the conveyor piping, it compresses and forms a solid plug that must be
removed manually.

Installation
All piping and ductwork should be as straight and short as possible. Bends should
have a radius of at least three diameters of the pipe or ductwork. The diameter should
be selected to minimize friction loss and maintain enough velocity to prevent settling
of the conveyed material. Branch lines should be configured to match as closely as
possible the primary flow direction and avoid 90" angles to the main line. The area of
the main conveyor line at any point along its run should be 20 to 25 percent greater
than the sum of all its branch lines.