CST, Blackstone collaborate on API 618 recip compressor retrofit

Many considerations necessary to transition from a gas engine to an electric motor

Author’s Corner: Cosimo Carcasci is a consultative service leader with Compressor Service Technology (CST) S.r.l. Alex Fulthorpe is the executive vice president of Blackstone Industrial Services.

Introduction
In 2015, CST supported Blackstone for the replacement of an engine driver with an electric motor with variable frequency drive (VFD) on a Dresser-Rand 5RDS-4 (Southwest Industries – Ingersoll Rand) reciprocating compressor. The machine was a four-stage reciprocating compressor with five cylinders that had been operational since 1954 in boil-off service in a natural gas plant located in Tulsa, Oklahoma.

The customer wanted to transition to an electric motor drive with VFD to expand the capacity control and increase the reliability of the unit. When a new motor is installed, especially when switching from a gas engine to an electric motor, the required brake horsepower and shaft diameter must be respected, along with the mass and torsional behavior.
The new train arrangement had to be designed and CST carried out the following engineering activities:

• starting and brake torque diagram for selection of the new electric drive
• torsional study of the complete train
• selection of the coupling and flywheel
• outline drawing of the modified flywheel and the assembly

Torsional analysis
The analysis of the torsional behavior of the complete driver-compressor system, composed of an induction motor coupled to the compressor through a soft coupling (Figure 2), was carried out according to API618 requirements to check coupling and compressor flywheel selection and to ensure the torsional compatibility of the system.

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This article was originally published in the April 2020 issue of COMPRESSORTECH². We only publish a fraction of our magazine content online, so for more great content, get every issue in your inbox/mailbox and access to our digital archives with a free subscription. Click here for a new subscription or here to renew your current subscription.