Design and Development of a Specially Modified Positive Displacement Rotary Screw Pump and Relevant Hydraulic Circuit to Enhance “Entrained Air Handling” Capability in a Closed Loop Lube Oil System

Abstract

Rotary screw type positive displacement (RSPD) pumps are commonly used in Oil and Gas Industry for pumping of mineral lube oil in services where they can be mechanically driven by gears coupled to a train driver. Installation of these pumps is critical and should be designed jointly by vendors and users according to project specific restrictions (i.e. the arrangement of the entire oil circulation system). This paper describes a real case in which restrictions due to lube oil system arrangement have produced low pump suction head and have amplified the influence of air bubbles that remained entrained in oil despite lube oil tank degassing. The investigations have been directed toward the mathematical modeling of the aeration phenomenon coupled with experimental measurements of critical parameters taken on the shop plant. Among corrective actions identified and considered there are reduction of quantity of air entering the lube oil system and revamping of the entire lube oil system with changes in piping, tank and also in pump model together with special modifications of internal path to enhance air handling capabilities. In order to validate pump behavior with reference to resistance to aeration (monitoring noise and vibration) a special simulation set-up was jointly developed by end user and manufacturer on a pilot test bench to carry out the various performance tests. The numerical data collected during shop aeration test have confirmed that the pump was able to handle the expected amount of entrained air with noise and vibrations within industrial limits. The pumps tested in the pilot bench were installed at user’s site and the effectiveness of the synergic corrective actions listed above was successfully verified. The study concludes that an early estimation of entrained air in the lube oil system is critical for design and development of either the RSPD pump or the entire lube oil circuit of a motor compressor train. When a critical quantity of entrained air is likely to be reached at pump suction (near 10% in volume), pump manufacturers and end users should apply some basic rules related to “design for aeration” of the pump and agree on a non-routine test to be performed at manufacturer’s shop before pump installation at site. This will serve as a reliable prediction of pump air handling capabilities, without which effective operation, reliability and durability of the pump could be jeopardized.