Single arm, double arm or roller type: choosing a floating suction layout

Every floating suction does the same job: it holds the tank outlet’s inlet just below the product surface, where the product is cleanest. How it does that depends mainly on the shape of the tank and the type of roof. This guide explains the four common layouts and how to choose between them.

At a glance

• Single arm: tanks wider than their filling height.

• Double arm: tanks taller than they are wide.

• Roller type: tanks with an external or internal floating roof.

• Riser type: horizontal, mounded and underground tanks entered through a manhole.

Single arm

A straight arm pivots on a swivel joint at the outlet nozzle and carries the float at its far end. As the level rises the arm swings up; at the top level it must still be short of vertical, and at the bottom it must fit inside the tank. So a single arm works when the tank diameter comfortably exceeds the filling height above the outlet, which is the case in most large product tanks.

Double arm

In a tall, narrow tank a single arm long enough to reach the top would not fit across the tank at low level. Two shorter arms joined by an intermediate swivel solve this: the assembly folds as the level falls and opens out as it rises. The geometry and stops are set so the arms can never lock straight, which would leave the float stuck. Very tall tanks can take three arms.

Roller type for floating roof tanks

Under a floating roof the float cannot rise to the surface freely, because the roof is there. The roller type carries rollers that bear on the underside of the roof and roll across it as it moves, keeping the inlet below the product surface. Aluminium and stainless steel internal floating roofs need a track under the deck for the rollers. A position indicator outside the shell is strongly advised so operators can see the unit is following the roof.

Riser type for horizontal and underground tanks

Horizontal and buried tanks are usually entered only through a manhole, and the outlet is often a pipe through the top. A short arm with a compact float connects to a riser that comes up through the manhole cover. A foot valve keeps the riser full so the pump keeps its prime.

Choosing: worked examples

Tank	Layout	Why
Fixed roof, wide and low, side outlet near the floor	Single arm	Diameter is well above the filling height, so one arm fits through its whole swing.
Fixed roof, tall and narrow	Double arm	One arm long enough for the top level would not fit across the tank.
External floating roof crude or product tank	Roller type, single or double arm	The float works under the roof.
Fixed roof with aluminium internal floating roof	Roller type with deck track	Rollers need a hard track under a light deck.
Underground or mounded tank, manhole access only	Riser type with foot valve	All parts pass through the manhole and the outlet is on top.

Product and materials

• Diesel, kerosene, lube oils: carbon steel or stainless steel.

• Jet fuel: aluminium or stainless steel, no copper or zinc in contact with the fuel; see our aviation fuel floating suction unit.

• Chemicals and water: stainless steel, with seals chosen for the liquid.

Vens floating suction units are designed by engineers who studied at MIT, Stanford, Cornell, the Indian Institutes of Technology, Arizona State University and the University of Cincinnati, and who have worked in the refineries, depots and power plants that use them. They size each unit for the tank, the product and the duty, choose materials and seals for the product, and design to proper factors of safety.

Questions

Is a hose-type floating suction an alternative?

Flexible hoses are used in some small tanks, but rigid arms on swivel joints keep their shape and do not kink or wear in the same way, which is why they are preferred in storage tanks.

Which layout do I need?

Send us the details on our enquiry data sheet and our engineers will propose the layout.

Not sure which layout suits your tank? Send us your details or e-mail the completed data sheet to venshydro@gmail.com, and we will come back with a proposed layout.