Page 126 - Root Cause Failure Analysis

114 Root Cause Failure Analysis
When vertical runs are short in proportion to the horizontal runs, the size of the riser
can be restricted to provide the additional velocity, if needed. If the vertical runs are
long, the primary or a secondary driver must provide sufficient velocity to transport
the material.

Cleanouts, or drop-legs, should be installed at regular intervals throughout the system
to permit foreign materials to drop out of the conveyed material. In addition, they pro-
vide the means to remove materials that drop out when the system is shut down or air
velocity is lost. It is especially important to install adequate cleanout systems near
flow restrictions and at the end of the conveyor system.

Operating Methods
Pneumatic-conveyor systems must be operated properly to prevent chronic problems,
with the primary concern being to maintain constant flow and velocity. If either of
these variables is permitted to drop below the system’s design envelope, partial or
complete blockage of the conveyor system will occur.

Constant velocity can be maintained only when the system is operated within its per-
formance envelope and when regular cleanout is part of the normal operating practice.
In addition, the primary driver must be in good operating condition. Any deviation in
the primary driver’s efficiency reduces the velocity and can result in partial or com-
plete blockage.

The entire pneumatic-conveyor system should be completely evacuated before shut-
down to prevent material from settling in the piping or ductwork. In noncontinuous
applications, the conveyor system should be operated until all material within the con-
veyor’s piping is transported to its final destination. Material that is allowed to settle
will compact and partially block the piping. Over time, this will cause a total blockage
of the conveyor system.

MECHANICAL
A variety of mechanical-conveyor systems are used in chemical plants. These systems
generally incorporate chain- or screw-type mechanisms.

Chain
A commonly used chain-type system is a flight conveyor (e.g., Hefler conveyor),
which is used to transport granular, lumpy, or pulverized materials along a horizontal
or inclined path within totally enclosed ductwork. The Hefler systems generally have
lower power requirements than the pneumatic conveyor and, in addition, prevent
product contamination. This section focuses primarily on the Hefler-type conveyor
because it is one of the most commonly used systems.