Preliminary Pulsation Analysis for High Pressure Piping Size Evaluation for Hyper-Compressors for LDPE Plants

Abstract

The development of the market for large scale plants, driven by their greater economic return requires processing equipment with increasingly larger capacities. This includes the reciprocating compressors used in Low Density Polyethylene Plants. The design of the high pressure piping associated with hyper-compressors, particularly for reactors and intercoolers, requires careful analysis. Due to the lengthy delivery times for such components, decisions regarding the diameter of high pressure piping usually have to be made at an early stage. Because of the importance of pressure pulsations that occur during operation, an analysis has to be made based on preliminary information available. Such a simulation can be made using reasonable assumptions about the length of the piping involved, and the configuration of both the compressor and the cylinders. At this point, the piping diameter can be confirmed and later, when the plant arrangement has been finalized, a more detailed analysis can be performed which may result in minor adjustments to the piping design.

Preliminary Piping Sizing and Pressure Pulsation Evaluation

“…Depending on process and reactor type [4,5,11], secondary compressors reach reactor pressures ranging from around 100 MPa (15,000 psi) up to 350 MPa (50,000 psi). Hyper compressors ( Figure 1) are the heart of these plants and, together with the high pressure piping systems, are fundamental to reliability and availability [4,5].…”

“…It takes a very long time to deliver piping, valves and fittings to LDPE plants because of the very high pressures involved [5].…”

“…Some rules applied in the first design of the piping layout can reduce modifications arising from the acoustic study and limit the number of orifices required to manage pressure drops, leading to a smaller piping diameter or, for the same diameter, to lower pulsations [5].…”

“…The highest values generally occur near the cylinder flanges, while lower values occur on the common piping (i.e., in the part where multiple cylinder lines are connected together in a stream as the intercooler, preheater or reactor). As the common parts represent the largest portion of the plant, a lower pulsation level (half of the value at the cylinders is desirable) is essential to facilitate the supports design [5,8].…”

“…The same kind of analysis can be made of the compressor frame itself through an FE model, including the cylinders and connected piping till the ground and the relevant supports, using the following input exciting forces [5]:…”