Technical knowledge
Concentric versus eccentric reducers: choosing by what settles out
The functional difference between concentric and eccentric reducers, why horizontal pump suction lines need eccentric reducers, how to specify flat on top or flat on bottom and the layout mistakes to avoid.
The difference in one sentence each
A concentric reducer is a symmetrical cone with both ends on the same axis. An eccentric reducer has one side flat, so the two ends are offset. That flat side is what prevents a pocket forming where gas or solids can accumulate.
Selection by service and orientation
| Situation | Correct choice | Reason |
|---|---|---|
| Vertical line, any service | Concentric | No pocket can form on a vertical axis |
| Horizontal pump suction | Eccentric, flat side up | Prevents an air pocket entering the pump |
| Horizontal line with settled solids | Eccentric, flat side down | Allows solids to travel rather than collect |
| Steam or condensate horizontal line | Eccentric, flat side down | Allows condensate to drain |
| Clean fluid, horizontal, no pocket risk | Concentric | Simpler and cheaper |
| Instrument connections and vents | Concentric or eccentric per detail | Follow the piping detail |
Mistakes seen on site
- Installing the eccentric reducer upside down. This creates exactly the pocket it was meant to prevent.
- Marking the orientation only on the isometric. Add a note or an orientation mark on the fitting itself.
- Using a concentric reducer on pump suction because it was in stock.
- Ignoring the straight length requirement downstream of the reducer before a flow meter.
Specifying reducers
State ASME B16.9, concentric or eccentric, the two nominal sizes and the wall thickness or schedule for each end, the material specification and UNS number, and the end preparation to ASME B16.25. For eccentric reducers, state the orientation required on the drawing so the installer does not have to guess.
Questions about this topic
Why is a concentric reducer bad on pump suction?
A concentric reducer on a horizontal suction line can trap gas at the top of the reducer, which then enters the pump intermittently and causes cavitation, vibration and impeller damage.
Which way should the flat side face?
Flat side up on pump suction to avoid gas pockets. Flat side down where solids or condensate would otherwise collect. The correct orientation depends on what you are trying to drain or avoid.
Are eccentric reducers more expensive?
They can be slightly more expensive and are less commonly stocked, because they are asymmetric and must be oriented correctly. The difference is small compared with the cost of pump damage from cavitation.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email [email protected] or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
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