By applying theory of liquid-solid two-phase flow, the location and process of wear on pump are analyzed. The results are consistent with the real wear situation. On this basic, base on the geometry non-linear theory and material non-linear theory, the wear process on flow components was simulated by ANSYS. The simulation results showed that there will be pits on the surface of flow components due to the impacting of coal particles with certain diameter, shape, impacting velocity and invasive angle; and the largest Von Mises stress will be at the pits. The distortion degree of pits and the largest Von Mises stress will increase with the increasing of coal particle diameter, impacting velocity and invasive angle, and with the decreasing of tip angle. There will be some accumulation at the front-end of impacting when the invasive angle is small.
Keywords: centrifugal pump; slurry pump; wear; finite element
1. Introduction
In recent years, with unceasingly increase of quantity of cleaning and processing coal, the
development of coal preparation process with dense medium in the coal preparation plant has been
promoted. In dense medium cyclone coal preparation, centrifugal slurry pump must always be used
for feeding materials[1]. When feeding materials with pump, the actuating medium is fluid-solid
two-phase slurry which is a mixture of water, coal, gangue and magnet powdered ore. Because the
working condition is very bad, serious partial wear to the flow components of pump is caused,
which not only reduces the operational reliability of pump, but also reduces the service life of
pump[2-4]. Therefore, besides energy saving and high efficiency, higher requirement to the
operational reliability of pump is also proposed at coal preparation plants. There will be great
significance for safe production to study the wear location and wear process of centrifugal slurry
pump used in dense medium cyclone, and to reduce the wear degree of the slurry pump.
2. Wear location and wear process
Wearlocation
Wear location on impeller
The main location of wear on impeller is in turn the impeller vane outlet and inlet, the intersect
point of the vane inlet with rear cover plate, internal surface of rear cover plate, and the
middle of vane[5-9]. In the flow channel of axial surface of impeller, when solid particles enter
the flow channel, the direction of motion of the particles from axial turns to the radial
direction. Due to centrifugal force, the majority of solid particles are transferred to the rear
cover plate, which results in the degree of wear at the rear cover plate larger than that at the
front cover plate. Especially, the wear at the intersect point of vane inlet with rear cover plate
is the most serious. In the plane flow channel of impeller, small solid particles have good
characteristics of following with fluids due to their small inertia. Therefore, in the inlet of
impeller vane, particles get into the flow channel of impeller along the direction which
approaches the liquid flow angle at vane inlet with low impacting velocity and light wear on the
vane inlet edge. In the flow channel of impeller, particles move along the vane working surface
with a curvature of motion trace close to the vane type curvature, and their radial velocity and
outlet liquid flow angle are small. As a result, wear on the outlet edge caused by small particles
is more serious than that on the working surface. Large particles have bad characteristics of
following with fluids due to their big inertia. Therefore, in the vane inlet, particles get into
the flow channel of impeller with direction different from the liquid flow angle of vane inlet, so
that many large particles impact on the vane inlet edge, and some of the big particles are pushed
to the back of the vane, which results in wear on the vane inlet edge and the back of the vane
outlet. In the flow channel of impeller, large particles loss momentum after impacting, there
will exist three different situations: the first one is that the reflect velocity of particle is
so large that the particle flows straightway out of impeller after impacting, without impact with
vane again; the second one is that the reflect velocity of particle is relatively large, so that
the particle does not flow out of the vane outlet immediately, and impacts with the vane working
surface again (may be more than two times), which produces wear on the middle part and the outlet
of vane working surface; the third one is that the reflect velocity of particle is so small or
zero after impacting that particles will not bounce off from the vane surface, but rolls or glides
along the vane surface, so that wear on the middle part and the outlet of vane working surface is
produced.
Location of wear in dischargechamber
The main locations of wear in the discharge chamber are in turn the Ⅷ cross section, pump tongue,
the I cross section and sidewall[8-10]. For small solid particle with small outlet liquid flow
angle out of impeller, its motion trace is approximately a concentric circle in the discharge
chamber. It forms circulation flow through the pump tongue, then the majority of solid particles
are returned to the discharge chamber. Therefore, uniform wear on the discharge chamber sidewall
and outer wall is produced. For large solid particles with large outlet liquid flow angle out of
impeller, particles will impact on the outer wall of discharge chamber and move to diffusion tube
along outer wall, and serious impact will be produced on the pump tongue. Therefore, it creates
wear on the outer wall of discharge chamber and the pump tongue.
For example, a centrifugal slurry pump is used to supply material for dense medium cyclone in a
coal preparation plant. There are three situations of wear on the impeller and discharge chamber.
The first is wear on the impeller vane working surface outlet and back outlet, which makes the
outlet edge became thin concave arc, and presents obvious trench at the outlet edge of working
surface. Four outlet edges of the five vanes largely pell off because of wear (Figure 1). The
second is that the inlet diameter of impeller becomes larger because of wear on the intersect
point of the impeller vane inlet edge with the rear cover plate. The third is the VIII cross
section region of the discharge chamber appears obviously linearity trench due to wear, the pump
tongue becomes non-arc, and its length gets shorter (Figure 2).
Wearprocess
There are three stages in the process of wear on flow components of the centrifugal slurry pump:
The first is saturation wear stage: the flow components of the centrifugal slurry pump are
founded; there is some roughness on the surface of components; the surface becomes smooth due to
wear caused by particles after first running of the pump; and the process of wear gradually
becomes steady from being faster at the beginning.
没有评论:
发表评论