The general relationship between gas velocity and pressure gradient 
p
/ L for a horizontal transport line is shown in Figure 4 and is
in many ways similar to that for a vertical transport line.
Fig. 4: Phase diagram for horizontal pneumatic transport
Line AB represents the curve obtained for gas only in the line, CDEF for a solids flux,
G1, and curve GH for a higher solids feed rate, G2.
At point C, the gas velocity
is sufficiently high to carry all the solids in very dilute suspension.
The solid particles are prevented from settling to the walls of the pipe
by the turbulent eddies generated in the flowing gas. If the gas velocity is reduced
whilst solids feed rate is kept constant, the frictional resistance and
p / L decrease. The solids move more slowly
and the solids concentration increases.
At point D the gas velocity is insufficient to maintain the solids in suspension
and the solids begin to settle out in the bottom of the pipe. The gas velocity
at which this occurs is termed the saltation velocity. Further decrease
in gas velocity results in rapid "salting out" of solids and rapid increase in
p / L as the area available for flow
of gas is restricted by settled solids.
In the region E and F some solids may move in dense phase flow along the bottom
of the pipe whilst others travel in dilute phase flow in the gas in the upper part
of the pipe. The saltation velocity marks the boundary between dilute phase flow
and dense phase flow in horizontal pneumatic transport.
Once again, it is not possible to theoretically predict the conditions
under which saltation will occur. However, many correlations for predicting
saltation velocity are available in the literature. The correlation by Zenz (1964)
is frequently used but is entirely empirical and requires the use of a graph.
It is reported by Leung and Jones (1978) to have an average error of ±54%.
The correlation of Rizk (1973), based on a semi-theoretical approach,
is considerably simpler to use, and has a similar error range.
It is most unambiguously expressed as:
(Eq. 3) 
where (The units are S.I.)
Xs,g is the solids loading = [mass flowrate of solids] / [mass flowrate of gas]
Mp is the mass flowrate of solids
USALT is the superficial gas velocity at saltation
(superficial velocity is defined in section 4)
D is the pipe diameter
x is the particle size
