We are sometimes asked to repair fluidized beds either in our workshop (where they are small enough to get through the door in the silo skirt) or in-situ on the client’s sites. Usually by the time we are asked, the beds have been in operation for more than 10 years and are in a pretty poor state of repair, showing signs of blinding, cracking, water damage and hammer rash. These problems are usually symptoms of poor operation and maintenance stemming from a lack of understanding.
The problem arises because when they are set up and used correctly they are literally a “fit and forget” technology and will provide many years of trouble free service. So much so that operation and maintenance staff often forget that they are part of the process and have specific needs like any other piece of plant.
It is therefore worth a reminder about a few points:
1) Fluid beds should not be subjected to water. They are porous and water will carry small particulates into the fluidizing medium which reduces the porosity of the bed. Such water ingress can also encourage bacterial growth creating contamination issues. Since they are so easily forgettable, water damage can often occur as a result of washing the silo which can only be done if the fluid bed is removed and protected. This can be difficult in the confines of a silo skirt but worth the effort if you want the fluid bed to work.
2) Sometimes moisture in silos can occur by condensation – particularly in cold and windy positions. On-board blowers of tanker delivery trucks do not typically have air dryers and the air is therefore warm and moist. The subsequent cooling of the air, especially at the silo top of uninsulated silos, causes condensation. This condensation has the effect of making powder stick to the walls of silos creating hard deposits that sometimes fall off in lumps and can cause damage or blockages as well as further bacterial contamination. Such condensation can be prevented by using de-humidifiers to provide a dry air blanket.
3) Fluid beds are intended to operate at a supply air pressure of 200-600 mbar. If the fluid bed material loses permeability then the pressure needed for air to flow through it increases and might go above the 600mbar limit. Operators or maintenance staff will typically crank up the pressure beyond recommended limits which can cause damage. It is therefore important to monitor the fluidization pressure during operation regularly, manually or with automated instrumentation. If you see the supply pressure has been increased it could be an indication that the permeability has been compromised, so investigate further. Continuing to operate at high pressures can crack the bed material.
4) The beds themselves are usually supplied with air via non-return valves. The operation of these valves can be effected by material leaks or general wear and because they are passive they are often neglected when it comes to service and maintenance. A valve that is not operating effectively could allow material to migrate into the air header and subsequently “blind” the beds on other silos. If the valves are maintained and operate effectively they provide a great investigative opportunity as they help identify where the problem is.
5) The material of a fluid bed comes in different forms but a common type is a sintered plastic. Although this is very durable it does not last forever and over time it becomes more brittle. When this happens, slight flexing can occur (usually in conjunction with reduced permeability) and creates stress cracking. If loss of permeability has effected the silo discharge rate you may also see “hammer-rash” where staff have beat the silo. This action can also increase the possibility of cracking by causing pressure points in the internal plenum supports. Hammer rash is a clear indication that something is wrong.
6) If supplied with the right conditions, fluid beds can typically operate effectively for more than 10 years, but over time very fine particles do start to blind the material of the bed and cracking can occur. So if you want trouble free silo discharge and consistent feed to your downstream process don’t “fit and forget”, show your fluid bed some love.
When was the last time you inspected your equipment? If you would like to discuss your existing equipment with one of our expert material-handling advisors, please give us call to discuss your needs on a no-obligation basis.
Telephone Zeppelin Systems on 01623 753291 or email email@example.com for more information.