Dynamic Analysis in the Petroleum Market & Piping Installation for Rotary Pumps

Q. What dynamic analysis considerations are recommended for the petroleum market? A. End users should evaluate their need for dynamic analysis by considering the level of proven field experience available for any given configuration. The vendor and user should agree on which types of analysis should be performed at any level. Lateral, torsional and structural analyses are three identifiable and normally separable deliverables. In all cases, it is the user's prerogative to specify additional tests, validations and/or analyses to further mitigate risk. Historically, dynamic analysis trends have developed within the various pump application markets because of the types and characteristics of equipment typically used and as a result of past experiences. In the oil and gas industry, single-stage overhung horizontal pumps and between-bearings, one- and two-stage pumps must be designed to be classically rigid, which can eliminate the need for lateral dynamic analysis. Multistage pumps identical to pumps proven in-field are also not subject to lateral analysis. Vertically suspended pumps are required to be designed with established limits on bearing spacing to ensure suitable lateral rotodynamic performance. Drive system configuration and power levels determine the need for torsional dynamic analysis. High-energy, high-speed, critical-service and unspared machines are subject to high levels of customer intervention and scrutiny, with the user having varying definitions of these terms. For more information on dynamic analysis, refer to ANSI/ HI 9.6.8: Rotodynamic Pumps Guideline for Dynamics of Pumping Machinery. Q. What piping installation recommendations are important to consider for rotary pumps? A. Because rotary pumps are designed with close running clearances, clean piping is a must. Dirt, grit, weld bead or scale, later flushed from an unclean piping system, will damage and may seize the pump. Figure 3.4.3.11 illustrates pipe-to-pump alignment considerations.

Piping should be installed on supports independent of the pump. Supports must be capable of carrying the mass of the pipe, insulation and the pumped fluid. Supports may be hangers, which carry the mass from above, or stands, which carry the mass from below. Clamps or brackets may be used to secure piping to existing columns. Supports must allow free movement of the piping caused by thermal expansion or contraction. Supports should be installed at intervals that uniformly and amply support the piping load, precluding contact with piping and equipment. Pipe strains or stresses transmitted to the pump by improper piping support systems may cause distortion, wear or binding of the rotary members and excessive power requirements. Piping systems that contain expansion joints must be designed so the expansion joint is not exposed to more motion than accounted for in its design. Expansion joints or flexible connectors should not be used to compensate for misaligned piping. Threaded joints should be coated with compounds compatible with, but not soluble in, the pumped liquid. End users working with Teflon-taped joints should be careful to prevent shredded pieces of Teflon from entering the piping system. Piping should start at the pump and work toward the source of supply and the point of discharge. Shutoff valves and unions are recommended to facilitate future inspection and repair. Reducers are preferred to bushings when a change in pipe size is necessary. Avoid unnecessary restrictions in the pipeline, such as elbows, sharp bends, globe or angle valves, and restricted-type plug valves. Users should predetermine pipe size by taking into account the required flow rate; minimum or maximum velocities; the fluid viscosity at the lowest pumping temperature; the length of the piping system, including valves, strainers and other restrictions; and the elevation of the pump with reference to supply and discharge points.