Cost-Effective Solution to a Tar Pump Perennial Performance Problem

Abstract Two identical gear pumps, designed for a maximum speed of 800 RPM, are each rated for a flow of 2.5 m3/hr, 1.5 break horsepower, 160 RPM speed, and a discharge pressure of 750 kPa a. The electric motor driving the pumps through a gear reducer has an output speed of 1800 RPM. With a gear reducer ratio of 5.6:1, the maximum speed of the gear pump is 320 RPM. The pumps are designed to recycle residual styrene tar back to the process as inhibitor in a Styrene plant. Since commissioning, the pumps discharge pressure is insufficient to return the tar back to the process per design. Instead, it is delivered to low pressure utilities area to be flared. Flaring of the styrene tar, due to insufficient pump head, resulted in the loss of inhibitor and styrene monomer. An estimated sum of $ 0.7 MM is lost per year to procure the required inhibitor due to this problem. The investigation commenced with the review of the gear pump as-built datasheet and characteristic curve. A field sample of the tar was taken for lab analysis to determine, among others, its viscosity and specific gravity. The viscosity of the actual tar in the field is multiple times the viscosity of the tar in the as-built datasheet. Parametric studies were conducted that confirmed that the pump performance is in line with the characteristic curve but limited in the discharge pressure due to the limiting pump speed. An appropriate limiting pump speed was calculated, based on the actual fluid properties in the field, to be 600 RPM. As such, the gear reducer ratio was proposed to be changed to 3:1 from the current ratio of 5.6:1. The proposal was implemented first on the installed spare pump and tested with astounding result – the perennial gear pump problem was resolved. The gear reducer was subsequently changed on the main pump as well. The low pump head problem was resolved with negligible CAPEX, no downtime nor major redesign of the piping system.