Reducers: concentric or eccentric, FOT/FOB and standards

What is a reducer?

A reducer is a butt-welding fitting that joins two pipelines of different nominal size. In practice, at most two or three nominal size steps are usually bridged per part; larger jumps are often solved by connecting several reducers in series. This is not a general rule of the standard – what governs are medium, flow, range and the applicable piping code.

Concentric or eccentric?

• Concentric: conical, both ends lie on one axis – the reduction runs evenly all round. Use: vertical lines and wherever the centre line is to run through.
• Eccentric: the two ends are offset against each other, one side runs flat and straight through. Use: horizontal lines. Advantage: the bottom or top edge of the line stays flush – so no air pockets form in liquid lines and no liquid collects in gas lines. The flat side can be placed deliberately on top or at the bottom:
  • Flat side on top (FOT): frequent on horizontal pump suction lines – prevents a gas or vapour pocket collecting at the top, which can contribute to better suction conditions; the pump’s cavitation suitability (NPSH) has to be checked separately.
  • Flat side on bottom (FOB): typical on pipe racks and in drain lines – the line rests squarely on its support and drains almost completely.

Why conical at all? Every abrupt narrowing of the cross-section constricts the flow: the jet does not follow the edge, becomes narrower than the cross-section itself and costs pressure – and in the constriction the pressure can drop locally so far that vapour bubbles form (cavitation). The conical transition of the reducer makes the change of cross-section gradual and keeps pressure loss and the tendency to cavitate small.

And why the installation position matters: with a concentric reducer in a horizontal line the axis stays at its level – at the bottom as at the top a step of half the difference in diameter results. Residual liquid stays in front of the lower step (it would have to run uphill to the small end), and a gas or vapour cushion collects at the upper one. The eccentric reducer puts the whole step on one side: flat side at the bottom (FOB) means a continuous floor – the line drains completely; flat side on top (FOT) means a continuous ceiling – no gas cushion. Hence the rule from installation practice: make reductions in horizontal lines eccentric – and where they have to be concentric, give the line a fall towards the larger nominal size so that the residual liquid can drain in front of the step.

A frequent installation error: eccentric reducers installed without a look at the installation position. On horizontal suction lines FOT is usually right, for draining usually FOB – the correct orientation, however, depends on medium, direction of flow, venting and draining concept, solids content, pump manufacturer and piping code.

The length L is identical for the concentric and the eccentric version of the same size.

Standards: from DIN 2616 to EN 10253

Reducers of steel are standardised today in EN 10253-2 (type A = reduced, type B = full pressure resistance (wall thickness to EN 10253); commercial stock of reducers is frequently type B), of stainless steel in EN 10253-4 (for example 1.4541/1.4571, seamless or welded). The predecessor standard was DIN 2616. What the pressure factor means is explained on our elbow pages (German so far); the sister fitting of the same standard family is explained on the page Tees.

DIN 2616 part 1 and part 2: what was in which?

The two parts of the old DIN 2616 were – unlike often reported – not simply separated by design:

• DIN 2616 part 1 stood for the reduced pressure factor and contained eccentric reducers only – it corresponds to today's type A of EN 10253-2.
• DIN 2616 part 2 stood for the full pressure factor and contained concentric and eccentric reducers – today's type B.

A concentric version with reduced pressure factor therefore did not exist under DIN 2616. The two short formulas circulating on the market – “part 1/part 2 = pressure factor” on the one hand, “part 1 = eccentric, part 2 = concentric” on the other – each pick up only half the truth: the division followed the pressure factor, but concentric reducers occurred in part 2 only.

Tolerances (brief overview EN 10253)

Flow at least 80 % of the theoretical inside diameter; minus tolerance on wall thickness −12.5 %; length tolerances of the installation dimensions ±2 to ±5 mm depending on diameter. From 3.2 mm wall thickness with welding bevel (30°, root face 1.6 ± 0.8 mm). Marking of reducers always with both ends: D and T as well as D1 and T1.

EN ↔ ASME

ASME B16.9 standardises concentric and eccentric reducers with inch-based lengths (for example 8″ reducer: L = 152.4 mm). The EN lengths are practically identical for inch nominal sizes (152 mm); EN adds metric sizes. Material counterparts as for all butt-welding fittings: A234 WPB ↔ P235GH, A403 WP316Ti ↔ 1.4571. On US drawings the abbreviations FOT/FOB are usual – on the German market this corresponds to “gerade Seite oben/unten” (flat side on top/at the bottom).