Water Treatment Plant

Floating Suction Units for Storage Tank Offtake

A floating suction unit (FSU), also known as a floating suction assembly or floating offtake arm, is a device installed inside a storage tank that draws product from just below the liquid surface rather than from the fixed bottom outlet. Because water and sediment settle to the tank bottom while the cleanest, lightest product rises to the top, floating suction ensures that pump-out and transfer operations consistently deliver high-quality, water-free product — maximizing the useful volume of the tank and protecting downstream equipment from contamination. Wanan Technology manufactures stainless steel and carbon steel floating suction units for crude oil, refined product, and chemical storage tanks worldwide.

In large atmospheric storage tanks holding crude oil, gasoline, diesel, jet fuel, or similar hydrocarbons, even a small volume of water at the bottom can cause significant product quality issues, corrosion in pipelines, and damage to refinery equipment. A properly designed floating suction unit eliminates these risks by ensuring product is always drawn from the cleanest layer of the tank inventory, regardless of filling level.

Key Features of Wanan Floating Suction Units

• Buoyancy-controlled float arm keeps the suction inlet at a fixed depth below the liquid surface as the tank level rises and falls, ensuring consistent product quality regardless of inventory level.
• Stainless steel and carbon steel construction — SS 304, SS 316L, and carbon steel options matched to the stored product, corrosion environment, and tank shell material.
• Flexible hose and articulated arm configurations — flexible stainless steel hose is standard for most refined product service; articulated rigid arm designs are available for high-temperature or high-pressure service.
• Custom inlet depth control — adjustable float position sets the suction inlet at an optimum depth below the surface to avoid drawing in entrained water or foam at the interface.
• API 650 / API 620 compatible nozzle connections allow direct installation on new or existing API standard tanks using standard 6-inch or 8-inch bottom outlet or side shell nozzles.
• Low maintenance design — no moving parts in the product stream other than the pivoting arm joints, which are designed for long service life in hydrocarbon service without lubrication.

Types of Floating Suction Units

Wanan supplies floating suction systems in configurations to match tank size, product type, and operational requirements:

• Flexible hose floating suction — the most common and cost-effective design for refined product tanks. A stainless steel braided flexible hose connects the floating suction head to the fixed tank outlet, accommodating the full range of tank level variation with minimal stress.
• Articulated arm floating suction — rigid stainless steel or carbon steel pipe arms connected by swivel joints, preferred for crude oil tanks, high-temperature service, or applications where flexible hose is not suitable. Provides higher flow capacity than hose designs for the same connection size.
• Multi-arm configuration — two or more suction arms connected to a manifold for very large tanks (diameter above 40 meters) where a single inlet cannot achieve adequate drawdown of the full tank cross-section.
• Combined floating suction and mixer — integrates a tank mixing nozzle at the suction head to homogenize product near the offtake point, improving blending efficiency and reducing stratification.

Floating suction units are typically installed in conjunction with fixed bottom outlet piping as a permanent internal tank fitting. They can also be combined with pressure vacuum relief valves and emergency relief valves on the tank roof for complete tank protection.

Specifications

Applications

Wanan floating suction units are installed in storage tanks across the petroleum and chemical industries:

• Crude oil storage tanks (AST) at pipeline terminals and refineries
• Refined product tanks (gasoline, diesel, jet fuel, kerosene, naphtha)
• Chemical and solvent storage tanks where product purity is critical
• Bunkering fuel oil tanks at marine terminals and fuel depots
• Edible oil and specialty liquid storage requiring clean product offtake
• Tank farm upgrades: replacement of corroded or undersized existing floating suction assemblies

Certifications and Standards

Wanan floating suction units are designed to comply with the standards most commonly specified by tank owners and operators:

• API 650 — Welded Steel Tanks for Oil Storage: floating suction designs are compatible with API 650 tank shell and bottom nozzle configurations.
• API 620 — Design and Construction of Large, Welded, Low-Pressure Storage Tanks: applicable for low-pressure and large-diameter tank installations.
• API 2000 — Venting requirements that are considered in the overall tank design, complementing the floating suction installation.
• ISO 9001 — Quality management system covering design, fabrication, inspection, and testing of all floating suction assemblies.

How to Specify a Floating Suction Unit

1. Define tank dimensions. Tank diameter, shell height, and maximum/minimum operating level determine arm length, hose length, and swivel joint reach.
2. Specify stored product. Product type, density, viscosity, vapor pressure, and corrosivity determine materials, float design, and inlet depth below the surface.
3. Select connection size. Match the FSU inlet size to the existing or planned bottom outlet nozzle size and the required pump flow rate.
4. Choose arm type. Flexible hose for standard service; articulated arm for high-temperature, high-flow, or large-tank installations.
5. Review tank internal layout. Confirm clearances for internal heating coils, mixer impellers, and other internals that may restrict arm travel.

Provide tank drawings and product data to our engineering team for a detailed floating suction unit design and quotation.

Frequently Asked Questions

Contact Wanan Technology for a floating suction unit quotation or technical support.