Choose the Right Oil Skimmer for Cost-Effective Oil/Water Separation

July 12, 2010 - PRLog -- The first three steps in selecting a suitable oil skimmer for your application were presented in Part I of this article. This is Part II, which presents the last two steps.

Step Four: Determine the Right Design. For industrial oil skimming, there are six basic designs commonly used in moving media skimmers. (Suction oil skimmers are excluded, as they are generally unsuitable except for relatively thick (1/4 inch) layers of oil; otherwise they tend to ingest large amounts of water.) In more demanding situations, oil skimming, such as with Abanaki oil skimmers, is a cost-effective means of removing most of the oil before using more complicated and costly treatments, such as coalescers, membrane filters and chemical processes.

Belt-type oil skimmers – Belt-type oil skimmers use an endless belt of stainless steel, elastomeric or polymer medium, which is lowered into the tank or vessel to be skimmed. The belt passes through the fluid, and then through the skimmer’s resilient wiper blades where oil is removed from both sides of the skimmer medium. The removal rate is not affected by belt length. The length is chosen to ensure contact with the liquid at its lowest level. The belt length should also allow easy mounting of the skimmer where oil discharge is convenient, and assure easy access for routine service. Lengths up to 100 feet or more are possible.

Corrosion-resistant stainless steel is for applications with high temperatures and harsh chemicals. Elastomeric media is used where abrasive particles are present, or when physical abuse of the belt is likely. Various engineered polymers can also be used for resistance to high temperatures and to many chemical types. Fuzzy (raised nap) polymers can pick up low viscosity hydrocarbons, such as fuel oils, gasoline and diesel fuel. Other media shapes, as described below, use some of the same materials.

Wiper blades made of standard nitrile material (also known as Buna-N) are suitable for about 80 percent of all applications. Other wiper materials, such as ceramic hybrid types, are designed for harsh operating conditions.

Mop oil skimmers – These oil skimmers use an endless medium shaped like a rope that has mop-like tendrils that pick up the oil. As the medium leaves the liquid and enters the drive unit, it is pressed and wrung out with pinch rollers. For higher viscosity oils, this medium tends to mat down and lose effectiveness. Generally, a decantation system must be used with these units, as water pickup can be quite high. Also, replacement mops tend to be more expensive than belt type media.

Large tube oil skimmers – Larger-diameter tube oil skimmers use a floating plastic hose that snakes out over the surface of the liquid and is then drawn back through the drive unit where oil is removed. This design requires a relatively large amount of fluid surface area for proper operation. Length considerations are the same as for belt oil skimmers; a typical tube diameter is 1 inch.

Mini tube oil skimmers – Smaller-diameter tube oil skimmers typically use a medium with an O.D. between 3/16 and 5/16 of an inch. Otherwise, they operate similar to larger tube diameter skimmers. In mini tube skimmer the larger diameter designs are preferred, as those tubes have enough stiffness to not drag on the housing and prematurely wipe off oil when drawn into the unit. Pickup rates vary from one quart/hour to 1.5gph, depending on diameter and speed. These units can be used where space is limited. Units that have the motor mounted underneath reduce the amount of space required over the rim of the tank to nearly zero.

Disk oil skimmers – Oil skimmers with disk shaped media rotate through the liquid. Oil is wiped off and discharged into a collection container in a manner similar to belt skimmers. When specifying disk diameter, it is important to consider reach, the portion of the disk that actually gets immersed. Less disk area in the fluid means less oil removed.

Obviously, fluctuating fluid levels can be a real problem. Also, it is important to look at how much of the disk the wipers are actually wiping. If they wipe only 2 inches from the edge, then any reach into the fluid beyond 2 inches is wasted. As a result, removal capacities for disks can be relatively low.

Drum/barrel style oil skimmers – Oil skimmers with cylindrical (drum/barrel-shaped) media operated similar to disk skimmers. Compared to disk types, they are usually more rugged and have higher removal capacity. As is the case with disk oil skimmers, drum-shaped media can be rendered ineffective by fluctuating fluid levels.

When selecting any skimmer design, the principal criterion is removal rate. Some examples of typical removal rates for different moving media skimmers are listed below.
Disk Style Oil Skimmers                  Up to 1gph
Tube Oil Skimmers                  Up to 1.5gph
Fixed-Installation Belt Oil Skimmers, Belt Widths 1”-2”      2 – 6gph
Portable Belt Oil Skimmers, Belt Widths 1” – 4”         6 – 20gph
Fixed Installation, 4-inch Wide Belt Oil Skimmer         20 -40gph
Fixed Installation, 8-inch Wide Belt Oil Skimmer         40 -80gph
Fixed Oil Skimmer with Multiple (2 – 5) 8-inch Wide Belts      80 -200gph
Grease Skimmer with 80-inch Wide Belt            120 – 160gph

Step Five: Consider All the Options. All moving media oil skimmers use motors to move the belt, tube, disk, etc. While many are designed with standard, industrially rated, continuous-duty motors, and may also use fully enclosed speed reducing drives, some can be specified with a number of other motor options. These may include:
·   Any common voltage or electrical requirement
·   Explosion proof
·   Drip proof
·   Tropical service rated
·   Continuous duty in a dirty environment
·   Food service duty
·   Water wash-down environment

In addition, consider any optional equipment you may need with the oil skimmer you are purchasing. Accessories allow customized systems that can ease installation and optimize performance. Some of the more common options include:
·   Special drive components – Tail Pulleys can help improve removal rates by stabilizing belt assemblies in the fluid. Yoke-and tether assemblies help prevent pulley loss due to human error or belt breakage.
·   Mounting Stands and Adapters – These include pre-engineered mounting stands for easier installation. Models are also available for pits, walls, and freestanding operation.
·   Shelters/Enclosures – Reinforced poly-shelters and below-grade enclosures provide protection from the elements.
·   Controls/Accessories – A float switch and warning light can be used to monitor fluid level in the oil collection drum, which helps prevent overflow. Other options include a timer, control panel, variable speed drive and power packs.
·   Concentrator – See the discussion on Skimmed Water Content in Step 3, Part I of this article.
·   Heaters – Many skimmers can be ordered with heating devices to keep skimmed product fluid in cold environments, which can improve the removal rate.

Conclusion. When properly selected and applied, oil skimmers are highly economical solutions for separating oil from water. They allow recycling and reuse of the water, or its safe disposal in a sanitary or storm sewer, depending on local regulations. In more demanding situations, skimming is a cost-effective means of removing most of the oil before using more complicated and costly treatments, such as coalescers, membrane filters and chemical processes.

For more information please visit www.abanaki.com