Eccentric reducer: flat side on top – when the rule applies and when it does not

Rule of thumb

So that no air pocket forms in the horizontal suction line in front of the pump nozzle.

Where does the rule come from?

From pump engineering. If a horizontal suction line narrows concentrically to the smaller pump nozzle, a pocket forms at the top of the larger pipe where air and vapour collect. Sooner or later the bubble travels into the pump: loss of prime, noise, impeller damage. An eccentric reducer with a continuous top line ("flat on top", FOT) has no pocket at the top; the gas can move on with the flow. The rule is as old as the centrifugal pump in industry and appears, in one form or another, in every pump installation manual.

What is technically correct?

The eccentric reducer to EN 10253 is a fitting with one straight and one inclined generatrix; the installation position FOT or FOB ("flat on bottom") results solely from turning it through 180 degrees – it is the same part. The offset of the axes is (D − D1)/2. Physically two things are at stake that contradict each other on the same fitting: air collects at the top, solids and condensate at the bottom. Making the top line continuous (FOT) takes the pocket away from the air but gives one to the solids at the bottom; making the bottom line continuous (FOB) drains completely and lets nothing settle, but creates the air trap at the top. Hence the rule applies to the suction side with solid-free liquids in a horizontal line – the most common case, and there it is right.

Where does the rule end?

• Supply from above (liquid arrives from a higher tank, vertically or falling, to the pump): here a pocket that does not vent can form in front of the nozzle in the FOT position; FOB is then common so that the line falls continuously towards the nozzle. The piping layout governs, not the rule.
• Media with solids or several phases (slurries, suspensions): FOT lets solids settle in the lower pocket; FOB or an inclined line is often chosen. Clarify with the pump manufacturer.
• Vertical lines: no top line, no bottom line – a concentric reducer is right there.
• Discharge side: the air trap no longer matters; what decides is whether the line must drain completely (FOB) or whether supports and pipe rack need a continuous bottom line.
• Straight length: even the right reducer does not replace the straight run in front of the pump nozzle; manufacturers give guide values in multiples of the diameter (KSB and HI 9.6.6: several nominal sizes as orientation, depending on the disturbance upstream).

What does the standard say?

For the fitting, EN 10253-2 (steel) and -4 (stainless): dimensions, wall thicknesses, length L, type A/B – nothing about installation position. The installation rule is plant-engineering practice; it appears in pump manufacturers' documentation (KSB) and in the Hydraulic Institute recommendation ANSI/HI 9.6.6 on pump piping. There is no standard that prescribes FOT – and the page does not present the rule as one.

DIN/EN versus ASME/ASTM

The eccentric reducer exists in both worlds (ASME B16.9 "eccentric reducer"); the installation rule is the same, and the FOT/FOB vocabulary comes from English-language plant engineering and has established itself here. One difference is the length: ASME B16.9 and EN 10253 fix L differently per size – replacing one part by the other moves the pump nozzle.

Worked example

Eccentric reducer 114.3 × 88.9 (DN 100 to DN 80, core range) in front of a pump with a DN 80 suction nozzle in a horizontal DN 100 supply line, medium water: install FOT, axis offset 12.7 mm – the reducer shifts the pipe axis downwards by that amount, which must be shown on the isometric, otherwise the flange at the nozzle will not fit. The same part from the same box, turned through 180 degrees, would be the FOB position for the fully drainable discharge line behind the same pump.

Checked today

Traffic light: applies under conditions (class B). Right for the horizontal suction line with clean liquid – the normal case. With supply from above, solids, vertical lines and on the discharge side the piping layout decides, not the rule.

Further reading

• Reducers explained: concentric or eccentric, FOT/FOB and standards EN 10253, DIN 2616 – reducers: concentric or eccentric, FOT/FOB and standards (with drawing)