Questions relating to proper reducer application in centrifugal pump suction lines date back many decades. Until his death (at age 84, in 1995), world-renowned pump expert Igor Karassik frequently corresponded with the writer and other pump users on pump-related subjects. We rarely pass up an opportunity to highlight some of his experience-based comments.
Once, a pump user referred to Fig. 1 and noted that this was quite typical of illustrations found in many textbooks. In essence, Fig. 1 indicates that, with a suction line entering the pump in the horizontal plane, the eccentric reducer is placed with the flat at the top. Available texts often give no indication as to whether the pumpage came from above or below the pump.
Fig. 1. Illustration of eccentric reducer mounting from
Hydraulic Institute Standards.
Igor Karassik agreed that, if the supply source was from above the pump, the eccentric reducer should be installed with the flat (horizontal) surface at the bottom. Entrained vapor bubbles could then migrate back into the source instead of staying near the pump suction. If the pump suction piping entered after a long horizontal run or from below the pump, the flat of the eccentric reducer should be at the top.1
Still, in many older texts it has been assumed that the pumpage source originated at a level below the pump suction nozzle. Karassik reminded us that older Hydraulic Institute Standards commented on the suction pipe slope:
...Any high point in the suction pipe will become filled with air and thus prevent proper operation of the pump. A straight taper reducer should not be used in a horizontal suction line as an air pocket is formed in the top of the reducer and the pipe. An eccentric reducer should be used instead.
This instruction applies regardless of where the pumpage originates. Depending on the particulars of an installation, trapped vapors can reduce the effective suction line cross-sectional area. Should that be the case, flow velocities would tend to be higher than anticipated. Higher friction losses would occur and pump performance would be adversely affected.
In the case of a liquid source above the pump suction, and particularly where the suction line consists of an eccentric reducer followed by an elbow turned vertically upward and a vertical pipe lengthall assembled in that sequence from the pump suction flange upstreamit will be mandatory for the eccentric reducer flat side to be at the bottom. That said, Fig. 2 should clarify what reliability-focused users need to implement.
Fig. 2. Suggested modifications for eccentric reducer
Also, whenever vapors must be vented against the flow direction, the line size upstream of any low point must be governed by an important criterion. The line must be a diameter that will limit the pumpage velocity to values below those where bubbles will rise through the liquid.
In general, it can be stated that wherever a low point exists in a suction line, the horizontal piping run at that point should be kept as short as possible. In a proper installation, the reducer flange will thus be located at the pump suction nozzle and there is usually no straight piping between reducer outlet and pump nozzle. Straight pipe lengths are, however, connected to the eccentric reducer inlet flange. On most pumps, one usually gets away with five diameters of straight length next to the reducer. In the case of certain unspecified velocities and other interacting variables (e.g., viscosity, NPSH margin, pump style, etc.), it might be wise to install as many as 10 diameters of straight length next to the reducer inlet flange. The two different rules-of-thumb explain seeming inconsistencies in the literature, where both the 5 and 10-D rules can be found. HP
1 Karassik, Igor J., Centrifugal Pump Clinic, 2nd Ed., Marcel Dekker, Inc., 1989.
|The author |
|Heinz P. Bloch is HPs Equipment/Reliability Editor. The author of 17 textbooks and over 470 papers or articles, he advises process plants worldwide on reliability improvement and maintenance cost reduction opportunities. His coauthored Bloch/Budris text, Pump Users Handbook, is comprehensive and very widely used. Find the 2nd edition under ISBN 0-88173-517-5. He can be contacted at HB@HydrocarbonProcessing.com. |