The purpose of pump piping is to provide a conduit for the flow of liquid to and from a pump, without adversely affecting the performance or reliability of the pump. However, many pump performance and reliability problems are caused or aggravated by inadequate system piping.
This article, largely based on the latest International Standards Organization/ Hydraulic Institute (ISO/HI) draft standard, "Pump Piping for Rotodynamic (Centrifugal) Pumps," outlines the methodology required to ensure optimum piping leading to and from a centrifugal pump.
The function of suction piping is to supply an evenly distributed flow of liquid to the pump suction, with sufficient pressure to the pump to avoid cavitation ," and related damage ," in the pump impeller. An uneven flow distribution is characterized by strong local currents, swirls and/or excessive entrained air.
The ideal approach is a straight pipe, coming directly to the pump, with no turns or flow disturbing fittings close to the pump. If the suction piping fails to deliver the liquid to the pump in this condition, a number of pump problems can result, including:
Random axial load oscillations.
Premature bearing or seal failure.
Cavitation damage to the impeller and inlet portions of the casing.
Occasional damage from liquid separation on the discharge side.
Discharge piping flow characteristics normally will not affect the performance and reliability of a rotodynamic pump, but a few exceptions exist in situations in which:
High-energy pumps might be sensitive to flow-disturbing fittings mounted close to the pump discharge flange.
Sudden valve closures might cause excessively high water-hammer-generated pressure spikes to be reflected back to the pump, possibly causing damage to the pump.
Discharge piping might affect pump starting, stopping and priming.
The discharge piping configuration also might alter any discharge flow recirculation that might extend into the discharge piping at very low flow rates. This could have a small effect on the head developed by the pump.
Two of the more common detrimental effects from pump piping are the excessive nozzle loads the piping can place on a pump and the excessive nozzle loads that unsupported equipment such as valves or vertical inline pumps can place on the piping. Excessive nozzle loads can be caused by thermal expansion of the pipe, unsupported piping and equipment weight, and misaligned piping.
Proper piping and pump layout design and analysis prior to installation of a system are absolutely essential to the life and reliability of a pump. These steps can help ensure nozzle loads remain below acceptable limits for installed pumps. A properly supported piping system minimizes field adjustments during installation, saving both time and money.
Nozzle loads affect pump operation in various ways. At low levels, the effects might be insignificant. At high levels, nozzle loads can contribute to:
Coupling misalignment, which can lead to heat buildup in bearings and decrease bearing life; in severe instances, fractures within the pump case; decreased coupling life; increased noise and vibration levels; and breakage of the pump shaft.
Shaft movement, which can lead to reduced mechanical seal life.
Fatigue or failure of the shaft.
Catastrophic structural failure of pump hold-down bolts/supports and pump nozzles.
Pump casing gasket leaks.
Pipe-to-pump flange leaks.
Decreased mean time between repair or failure.
The ISO/HI Pump Piping Work Group developed the flow chart shown as Fig. 1, which gives the methodology that should be followed to avoid these types of potential mechanical pump problems.
Figure 1. Flowchart for use of ISO/HI Pump
Piping Standard (Mechanical Considerations)
By following the flowchart methodology, chemical plants can avoid many piping-related mechanical pump problems.
Suction piping. In general, pumps should have an uninterrupted and unthrottled flow into the inlet (suction) nozzle. Flow-disturbing fittings should not be present for some minimum length. Flow disturbances on the inlet side of the pump can lead to
Deterioration in performance.
Damage to the impeller and shortened impeller life (especially with high-suction-energy pumps).
Shortened mechanical seal life.
Shortened bearing life.
Isolation valves, strainers and other devices used on the inlet (suction) side of the pump should be sized and located to minimize disturbance of the flow into the pump.