A Hydrocyclone, or more commonly referred to as a cyclone, is simply a configured pipe tee. There are certain elements in the design that are required to make the cyclone perform a separation of material based on size gradation, but a cyclone is straightforward. The mystery of a cyclone comes from the inability to see what is happening inside. The first subject to look at is a way to get some insight to what a cyclone is doing.
1. Pressure at the inlet of the Hydrocyclone can tell you a lot
Pressure at the inlet of the cyclone is an important indicator of where the cyclone separation point (also called cut point or d50) will be. The separation point is the size at which a particle has a 50-50 chance in reporting either to the underflow or overflow. When the pressure is below the target pressure, the d50 will be coarser then desired. Higher pressure than the target will send more fines into the underflow, thereby making a finer d50 then desired. The pressure should remain stable to keep the cut point constant.
Sudden changes in pressure should always be investigated because it indicates a change in the flow rate to the cyclone. This may be caused by upstream equipment failure such as a pump, damage to the cyclone or obstruction. If swings in the pressure are observed, it indicates surging in the flow, which could be caused by issues with a sump. When troubleshooting a cyclone performance, the pressure is normally the first place to look since dismantling the unit can be difficult due to location.
2. There are other ways to change the separation point besides changing the pressure
Since changing the pressure is not always an option or desirable, there are other methods to change the cut point of a cyclone. These changes are not always feasible once a cyclone is installed. Three of them involve the cyclone itself, while the first one to look at comes from the altering the feed.
While it is not always practical to do so in an existing application, changing the feed density will affect the cut point of the cyclone. The higher the density in the feed, the coarser the cut will be. As noted in the graph, different applications target a particular density, which fits their objective. Keep it in mind when the same performance is wanted and there needs to be a change in the application, changing only the flow rate or tph will affect cyclone separation. To maintain the same cut point, both the flow rate and tph need to be kept in proportion and the internals and/or number of the cyclone(s) adjusted to maintain the same pressure.
The cone angle plays an important role in the cut point of a cyclone. Generally, for fine cuts with a cyclone, a 10-degree angle cone angle is used. To make a coarser cut, the angle is increased as needed. Twenty degree cones are common in a lot of aggregate applications, but 40 degrees and even 60 degrees have been utilized.
Not seen as much, the installed angle of the cyclone will also change the cut point. From the vertical plane, laying the cyclone down will cause it to make a coarser cut. The cone angle must be taken in to account so the cyclone is not tilted too far. A 10- to 15-degree tilt will not have significant impact, but a 75-degree tilt can almost double the cut size.
Multiple, smaller cyclones operating at the same pressure as one large cyclone will have a finer cut point. Without going too deep into the math, the two forces that have the most impact on the cut point are centrifuge (CF) and drag (DF). With the CF directly affected by the radius of the cyclone, the cut point changes. The cut point is where a particle has equal amount of CF and DF, thereby giving it a 50-50 chance of going either way.
3. A Hydrocyclone has no moving parts
It may be obvious to state that a cyclone has no moving parts, but many times it is overlooked when troubleshooting. A cyclone is composed of a housing, liners and assembly hardware, so there are only so many things that can go wrong with the cyclone itself. Foreign debris, lining failure or improper assemblies are the main issues that can be traced back to the cyclone. Arguably speaking, the foreign debris is caused by the feed but can only be found by looking inside the cyclone or seen at the discharge points. Dismantling a cyclone is not always the easiest task because they tend to be installed in hard-to-reach locations, so it can pay off to ensure all equipment prior to the cyclone is operating properly.
A cyclone operates at a required pressure, which is usually provided by a pump. Anything that causes instability in the flow rate, as shown by the pressure reading, will affect the performance of the cyclone. A poorly designed sump can send entrapped air or surges in materials. A properly operating pump will go a long way to ensure that a cyclone is performing correctly. In contrast, a pump with issues creates a cyclone with issues.
4. A Hydrocyclone can be modified to make a dry underflow while maintaining a high separation efficiency
5. There will always be some fines in the underflow as part of the bypass
A Hydrocyclone uses an air core, which forms at the apex and extends up to the vortex finder. The formation of this air core is paramount to making a separation, and if it collapses, the separation efficiency dramatically drops. Therefore, the idea of modifying a cyclone to completely removing the air core should be doomed to failure. A Separator™ or siphon0assisted cyclone does exactly that and maintains a high separation efficiency. The cut point shifts slightly coarser. There are a number of reasons for this shift, but one that is easy to see is the change in the water split. As mentioned in the next point in more detail, fines go with the water and a Separator™ decreases the amount of water reporting to the underflow. The name itself comes from the idea of separating the water from the solids. In addition to lower percent moisture in the underflow, a Separator™ is able to handle larger swings in percent solids in the feed with minimal effects on performance.
The modification consists of an underflow regulator, also called fishtail, lip seal and numerous other names, to eliminate the forming of the air core and a set length overflow pipe to create the siphon. The siphon pulls the underflow regulator closed to allow the solids to build up. When enough solids gather in the spigot, the weight will force the underflow regulator open. As long as there are enough solids to open the underflow regulator, the Separator™ will discharge. A properly operating Separator™ will run on water with no underflow discharge.
Simply put, fines will always follow the water. Since the underflow of a cyclone and a Separator™ will always contain some water, there will be fines. The amount of bypass is related to the water split reporting to the underflow. By reducing the amount of water, the amount of bypass fines will be reduced. With a cyclone, this is accomplished by adjusting the spigot size. Too small of a spigot will cause roping in the cyclone and decrease separation efficiency, while too large a spigot will increase the bypass. For these reasons, cyclone work best when the throughput or tons per hour is consistent. The bypass in a Separator™ is less because of the extra water removal nature of the siphon-assisted cyclone and is less affected by throughput swings.
When targeting low fines in the underflow, it may be necessary to have multiple stages of cyclones if there is a significant amount of fines in the feed. This is especially true when there is a large amount of the size fraction near the cut point.
As mentioned at the beginning, cyclones are simple ,but there is still a lot to know about them in order to get the most out of them.