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Re-start of Compression Strings in a Parallel Compressor Configuration

IP.com Disclosure Number: IPCOM000242054D
Publication Date: 2015-Jun-16
Document File: 7 page(s) / 260K

Publishing Venue

The IP.com Prior Art Database

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Re-start of Compression Strings in a Parallel Compressor Configuration

Introduction

The LNG industry has a need for increased unit capacity and greater operating flexibility while using proven machinery for the compression of refrigerant. This need can be met using parallel compression strings. The refrigerant used for pre-cooling is typically propane, possibly with some ethane and/or butane and for natural gas liquefaction typically a mixed refrigerant comprising nitrogen, methane and other hydrocarbons.  

This paper provides methodologies to re-start an offline compression string in a parallel compression system.  Although the figures and description refer to a system with two parallel strings, the ideas disclosed herein are applicable to systems with any number of parallel compression strings, any number of compression stages on each string, with or without cooling and for any compression service.  The methodologies presented here may be used with any refrigerants suitable for liquefaction of natural gas and various liquefaction cycles such as pure component cascade, pre-cooled mixed refrigerant, single mixed refrigerant, methane and/or nitrogen based gas phase cycles, etc. 

Figure 1 shows a simplified schematic of a parallel compressor configuration with two parallel compression strings.  The refrigerant flows into a compression system with two parallel compression strings.  Multiple compressor stages and/or cooling steps may be present on each string.  After compression and possibly inter-cooling and after-cooling, the refrigerant from the parallel strings combines and the combined stream is sent back to the process.  In a natural gas liquefaction process, warm low pressure refrigerant obtained from the main exchanger or pre-cooler is split into the parallel compression strings and combined after compression to be sent back to the main exchanger or precooler. 

Benefits of this configuration include (1) higher plant availability, (2) higher turndown efficiency and (3) higher LNG train capacity.  In the scenario that there is a trip in one compression string, the other string(s) can continue to operate, such that the plant continues to operate at reduced production.  

Figure 1: Parallel Compression Configuration

Cross Connection

Description

A cross connection line connects the offline string to the online one. 

Benefits

Reduced Flaring

Refrigerant inventory between the suction and discharge block valves of the offline compressor string must be de-inventoried before the compressor is brought back on-line.  A cross connection line permits the refrigerant inventory from the offline compressor string to be transferred to the online compressor string.  This action avoids sending refrigerants to flare and reduces import requirements.

Shorter duration for compressor tie-in

A cross connection line allows more flow to be pulled though the offline compressor string, cooling the suction drum down to operating temperatures and establish...