A dewatering decanter centrifuge, also known as a horizontal bowl centrifuge or solid bowl centrifuge, is a machine used to separate solids (sludge, slurry) from liquid(s).
- 2-phase separation separates one solid from one liquid in one process step
- 3-phase separation separates a solid from two liquids of different densities in one process step
To fully understand the importance of service and maintenance for decanter centrifuges, it is essential to know the basics of how a decanter centrifuge operates.
The following steps are a simplified outline of the dewatering process with a decanter centrifuge:
- Sludge or slurry is fed through the decanter centrifuge through a stationary feed pipe.
- Through the internal feed chamber a scroll (screw conveyor) is used. That along with centrifugal force moves the feed through the scroll ports into the centrifuge bowl.
- High speed rotation separates the solids and the liquid materials. Due to centrifugal force, anything heavier than the liquid settles on the inner diameter of the bowl wall.
- The scroll conveyor continuously scrapes the solids off the inner bowl wall and moves the solids in the direction of the centrifuge conical’s solids discharge end.
- The liquid (centrate) moves in the opposite direction of the solids due to Pascals law and because the solids discharge end is sealed by the solids.
- Bowl rotation and the scroll rotation run at a slightly different speed (differential speed). The differential speed of the centrifuge controls the solids retention time.
- Lower differential speed (more retention time) = drier cake
- Higher differential speed (less retention time) = cleaner centrate, higher throughput (the solids are removed faster)
- The choice of a decanter centrifuge design in a particular industry is determined by the characteristics of the sludge, slurry or solids.
Routine Centrifuge Predictive and Preventive Maintenance
To keep a decanter centrifuge operating at its optimal level, a maintenance schedule should include daily, weekly, monthly, and annual measures. Trained plant operators can perform many if not most of these on-site. Some more critical centrifuge service procedures may require additional equipment and expertise of a dedicated centrifuge service and repair facility or a Centrisys field service technician.
Download a helpful guide with a full decanter centrifuge predictive and preventive maintenance schedule here: Suggested Plant Maintenance Guidelines.
Decanter Centrifuge Cleaning Modes
Cleaning or flushing a centrifuge is a simple but critical maintenance process. It's used to remove as many solids as possible from the bowl and the scroll after operation, and/or before the next start-up. If solids are left inside the bowl, they will stick to the internal bowl wall. This creates an unbalanced centrifuge. When imbalance occurs, its common to hear louder than usual noise and centrifuge vibrations will be more agitated.
There are two types of centrifuge cleaning modes for typical wastewater applications:
1. Auto Stop Wash Cycle
Auto Stop Wash Cycle occurs during a normal Auto Stop shutdown. It’s a shorter wash cycle, initiated while the centrifuge is running and used when it will be down for a brief period of time. The auto-stop sequence begins one of two ways:
- If a system shutdown alarm goes off, the centrifuge controls automatically begin the sequence
- An operator manually starts the sequence from the auto-stop sequence screen
CIP (Clean in Place) is a cleaning process used typically in food and beverage processes. It’s an automatic flush sequence that operates in a similar manner to the Auto Stop and Auto Start sequence, except the sludge and polymer feed do not turn on. It is an extra, low-speed flush that’s programmed as a separate sequence used when the centrifuge is not running.
Learn more about decanter centrifuge cleaning modes auto stop and CIP here.
Should Chemicals Be Used to Clean a Decanter Centrifuge?It’s important to understand the effect cleaning chemicals may have on equipment. In some cases, chemicals used to clean a decanter centrifuge may do more harm than good. For example, a concentrated acid solution should never be left in a decanter centrifuge overnight, or for a longer period. The bonding material used to hold the tungsten carbide tiles to the centrifuge scroll and backup plates will dissolve. This occurs because of the low pH of acidic cleaning solutions. Over time, even highly chlorinated effluent used for flushing the centrifuge can weaken the solder bond. In a typical wastewater application, warm water is sufficient to clean a decanter centrifuge.
Check out more do’s and don’ts of cleaning a decanter centrifuge here.
Damaged flights due to chemicals.
How to Power Wash a Decanter Centrifuge:Failure to follow proper cleaning protocols during and after power washing may cause damage to the centrifuge, its parts and ancillary equipment.
Before power washing:
- Follow all lockout-tagout procedures
- Make sure the centrifuge is not running and is at stand-still
- Ensure the operator is using the proper PPE
To prevent bearing damage after power washing, it's important to run your centrifuge every week, even if just for a few minutes.
If your centrifuge is put into storage or not operating for a period of time after cleaning, it’s important to keep the bearings lubricated by running the centrifuge for a few minutes each week. Rotating the bowl by hand for a few turns each week also helps. This procedure moves the lubricant around; it keeps the rolling elements in the bearings coated in lubricant and shields them from damaging moisture.
Ongoing Decanter Centrifuge Preventative Maintenance
How often are you performing routine decanter centrifuge maintenance? Knowing the condition of your centrifuge or sludge thickener will reduce repair and operating costs and enhance performance reliability and efficiency. In addition, putting a forward-thinking centrifuge maintenance plan into place allows for proactive budgeting, extending the life of the centrifuge and creating a safer environment for your plant operators.
For these reasons and more, Centrisys has developed the Centrifuge Service Inspection (CSI) Program, a 40+ point inspection for your decanter centrifuge or sludge thickener. Learn about this program here.
Do you have questions about the best ways to maintain and clean your decanter centrifuge? Centrisys/CNP’s experienced technicians are always available to provide recommendations and guidance. Contact us today.