Crude oil held in storage tank farms, refineries and terminals usually contains varying amounts of sediment and water sometimes laden with paraffin, heavy ends, sand and silt. Left unchecked, these materials can accumulate and cause corrosion of the tank floor and lower shell plates. The accumulation can also seriously limit operational flexibility, damage the floating roof when the tank is being pumped down, damage transfer pumps and meters, and disrupt refinery process units if sediments break free.
To control or prevent this basic (bottom) sediment and water (BS&W) accumulation, the tank needs high-velocity circulation that will completely scour the tank bottom and keep sediment, water and solids in suspension.
A side-entry tank mixer with 60 degrees rotation is ideal for handling this problem. The mixer can be equipped with an electric motor with long-life sleeve-mounted bearings, one-piece cast iron drive housing, steel mounting flange with integrated swivel-angle assembly, mixer drive shaft with pre-lubricated, sealed-for-life shaft bearings, rugged transmission and special impeller to minimize maintenance and operating problems.
A tank shut-off device and hydraulic ball seal in the swivel-angle assembly can provide zero leakage to meet the stringent environmental requirements of the petrochemical industry. The mechanical shut-off device is engineered to ensure “no-oil-on-the-ground” environmental control. It includes a stain-less steel taper-to-taper shut off that ensures secure locking during bearing and mechanical seal change out. Maintenance can be done under full tank conditions.
Since the rate of BS&W accumulation in an oil storage tank varies with the quality of the crude oil, one to six mixers per tank may be recommended for effective control. Installation of swiveling side-entry mixers in the recommended quantity and orientation to deal with the rate of bottom accumulation can provide highly effective control. In addition to limiting corrosion and other damage to equipment, control of BS&W also helps ensure tanks are available for maximum storage capacity, allows quicker and less costly tank inspection, repair, alteration or reconstruction and limits personnel exposure to hazardous manual tank cleaning.
Controlling the accumulation of BS&W, especially in crude oil storage tanks, demands high-velocity circulation in order to completely scour the bottom of the tank. To achieve the desired process results, the mixers should be located so maximum flow can be induced across the tank bottom to keep it free of BS&W and maintain a clear suction line. The swivel-angle tank mixers should be placed opposite the outlet to allow the free range of movement across the full 60-degree swivel. Where more than one mixer is installed on a crude tank to control BS&W, locate the mixers between 15 degrees and 30 degrees apart, but generally within a quadrant of 90 degrees. The use of computational fluid dynamics facilitates the mixer vendor with the ability to prove out his selections and give proof to successful operations.
For more information about the National Institute of Storage Tank Management, visit www.nistm.org or call (800) 827-3515.
For more information about this topic, contact Hastings at (281) 224-2720 or chastings@philadelphiamixers.com, or visit http://philamixers.com.
