|The flow of bulksolid materials may be controlled with the aid of devices, mostly valves, designed for controlling the rate or stopping the flow, usually at the discharge section of a silo, hopper or pneumatic transport line.
A valve consists of a valve body, of varying shape, inside which a cut-off system is installed for reducing the section involved in the passage of the flow.
The choice of the valve to be used for a specific application must be made after taking into consideration the following
a) the physical and chemical properties of the material to be controlled;
b) the pressure of the stream, in the case of applications on pneumatic transport circuits;
c) required degree of sealing;
d) required flow section with the valve completely open;
e) any operations such as splitting the flow;
f) overall dimensional limits, if any.
The valves mostly used for stopping the flow of loose solid materials
- butterfly valve;
- slide valve;
- pinch valve;
- arc valve;
- diaphragm valve;
- ball valve.
The nature of the bulk solid material, especially such features as abrasiveness, corrosiveness, contaminability, degradability and temperature, apart from influencing the choice of material to be used for constructing the controlling device, or the need to carry out surface treatment on the parts which come into contact with the product and/or subject to wear, in some cases, strongly influences the type of valve to be used.
It must be remembered that:
- The specific weight influences the pressure generated on the plug element and therefore on the torque to be transmitted to the actuator.
- The low flowability of cohesive powders determines the use of sections with larger passage.
- Abrasiveness orients the selection towards systems which do not concern the flow, to avoid maximisation of effects of abrasion, and prefers systems which allow product unloading to systems which trap the material in mechanical couplings (e.g. ball valve).
- In the case of fragile products, the systems selected must have limited cutting action (e.g. slide, butterfly valves).
Controlling the flow rate of bulk solids materials by using a butterfly valve is achieved by placing a disk, which can be inclined inside the valve body, in the path of the flow. When the rotation axis of the disk is transversal to the direction of the flow, the valve is closed, as the border of the disc fits against the valve body, thus cutting off the flow. When the valve is completely open the flow section is reduced because of the presence of the disk, which acts like a blade, dividing the flow of material.
Controlling the flow rate of bulk solids materials by using a slide valve is achieved by inserting a blade perpendicular to the flow in the section of the valve passage. In this case also the sealing is perfect because of the contact between the blade and valve body.
This type of cut-off device has basically two advantages: the total absence of elements which can disturb the flow of material with the valve completely open, and its compactness in the vertical direction, which determines the limited overall dimensions.
The pinch valve consists of a cylindrical metallic body containing an elastomer sleeve, co-axial to the body; when subjected to suitable strain, it tends to reduce the section involved in the flow of material. This type of valve is used on pneumatic circuits and, in order to operate as a stopping element, it must be supplied with a service fluid (compressed air or pressurised water) which, thanks to a pressure that is a few bars greater than the circuit pressure, forces contraction of the sleeve on the flow of material passing through it, thus reducing or cutting off the flow.
Controlling the flow rate of bulk solids materials by using an arc valve is achieved by means of a spherical cap inside the valve body which, if positioned perpendicular to the direction of flow, with the convexity facing the material infeed, prevents the passage of material. These devices are constructed so that, when the valve is closed, the spherical cap is in contact with the valve body, thus ensuring high sealing performance. When the valve is open, although the cap remains inside the valve body, the passage section for the flow of material is totally free of obstacles.
A diaphragm valve basically consists of a tube made of flexible material held in cylindrical shape by two coaxial, equal-sized rings connected to the ends; one ring is fixed to the structure from which the material comes, the other is free to rotate. Rotation of the free ring modifies the passage surface according to a diaphragm-shaped figure, creating a sort of funnel, which maintains the passage surface in the central area. If suitably designed, this type of valve can be used not only as a cut-off device, but also as a device for extraction and dosing the flow of solid material. Depending on the distance between the two rings, it is possible to obtain different inclinations of the surface of downflow of the material, and by controlling the angle of rotation of the free ring, the passage of the section, and thus the flow of material can be regulated; in addition, the diaphragm valve has reduced overall dimensions.
A ball valve consists of a valve body containing a sphere with a hole through: when the axis of the hole in the sphere is parallel to the direction of flow, the material passes through the valve and is unloaded; otherwise the flow of material is reduced until it is stopped completely. When the valve is completely open, there is, however, a reduction in the unloading section, which limits the maximum capacity unloaded. The main advantages of this type of device are the possibility of obtaining very low flow rates of material and a high degree of sealing, thanks to the manner in which the cut-off device fits into the valve body. Its disadvantages are the reduced section presented to the passage of flow of material, and the considerable problems of abrasion on the surfaces facing the valve body and the plug, caused by the presence of solid material.
Depending on the unloading section of the silo or hopper, the valve inlet section may be circular, square or rectangular, and the connection between the two parts is generally achieved by flanging.
These devices may be activated manually, or automatically by means of electro-pneumatic or motor-operated actuators, depending on the process requirements.