Pumps & Systems, April 2013
The ability to increase the life of a mechanical seal can be as easy as adding one simple design element to the equipment. Cyclone abrasive separators provide an effective way to keep solids found in liquids from contaminating and damaging mechanical seal faces. They can also help improve the seal life and reduce maintenance costs.
The separator is part of the seal support system that allows a clean flush to be provided for the mechanical seal when an external source of clean fluid is not available or is too costly. With a properly installed cyclone separator, no fluid is lost, and it will remain almost maintenance free. The separator is piped from the discharge side of the pump where the abrasive laden fluid is found. It is then circulated through the separator, which removes a large quantity of the abrasive particles and leaves a cleaner fluid to be directed to the seal chamber. This provides better circulation and reduces the temperature at the seal faces.
When used properly, the solids are removed with little maintenance—unlike with strainers and filters, which require cleaning, cost and time. The lightweight and simple separator also allows for no supports or pipe stress in its inline design.1
Figure 1. A feed is placed on the discharge end, which leads out of the discharge pipe to a tap piped directly to the intake on the side of the cyclone separator.
Cyclone separators range in design from a unitary simple construction to a more complex design that uses gaskets, bolts and multiple parts. The cyclone separator discussed in this article is a one-piece design that uses its simple construction to allow for low temperature, low costs and clean flow rates.
How It Works
To fully understand the benefits of using a cyclone separator and how it works, an explanation of the complete process is needed. Liquids can be laden with suspended abrasives—such as sand, dirt, rust and particulates—that can easily damage seal faces in a short time period. The process begins as the fluid is initially cycled through the pump. A feed is placed on the discharge end, which leads out of the discharge pipe to a tap piped directly to the intake on the side of the cyclone separator (see in Figure 1).
Inside the separator, fluid rotates centrifugally in a cone-shaped chamber. The velocity and pressure differential is critical to the design and success at which the abrasive particles cycle through the separator and use gravity and force to fall from the liquid. The particles remain closer to the wall of the chamber where they are eventually funneled to the bottom of the separator and then discharged through an outlet at a lower pressure point and returned to the suction end with little of the process fluid. Meanwhile, the cleaner liquid is pushed from the outlet at the top of the separator and returned via piping to the seal chamber through a flush point in the gland plate and directly to the seal faces.
This process runs consistently as long as the equipment is running continuously, cycling the cleaner liquid to the faces and keeping them cool and lubricated (see Figure 2). This process has a high efficiency rate in extending seal life.
Certain conditions must be met for cyclone separators to work properly. One of the fundamentals for success is familiarity with the characteristics of the fluid. To begin, the abrasive laden fluid that is being pumped must suspend the particles to be removed in the process. As mentioned before, these are particles similar to sand, dirt, rust or any grit that can damage seal faces. Unfortunately, any substance that is dissolved in the fluid cannot be removed by an abrasive separator. These particles must:
- Be larger than 1 micron in size
- Have a specific gravity close to twice the actual fluid
- Not exceed a ratio of more than 1/10 of the fluid
A method for determining whether a cyclone abrasive separator will work is to insert a sample into a small beaker or clear container. Shake well to mix thoroughly, and let set for 10 to 15 minutes. Then see if the solution settles and separation occurs. If separation is observed, in most circumstances, the separator will work effectively.
Pressure is also a necessity for proper operation of a cyclone separator. The minimum pressure differential must be around 20 psi/1.4 bar between the suction and discharge. This could be the most crucial aspect of having the separator function properly. A pressure differential lower than listed will not effectively circulate and remove the particles.
Figure 2. The separator allows for cycling the cleaner liquid to the faces.
This is simple hydraulics. Liquid naturally needs to follow the lesser path of resistance. Aside from gravity, pressure is a common way to alter resistance. Pressure must maintain a 20 psi differential and not exceed standard curve limitations for the separator to work as planned. A lesser differential will mean that the liquid will bypass separation, and 100 percent will be sent directly back to the suction end.