Modelling and Control of Dry and Wet Gas Compressors

Over the past years, the oil and gas industry has directed its attention towards subsea wet gas compression. Previous research has demonstrated that the operation of subsea wet gas compressors represents challenges related to transient response at various inlet conditions. Experimental work has been conducted to study the effects of wet gas on compressor aerodynamic and mechanical performance, which has presented many challenges in quantifying the effect of liquid presence on the compressor performance. To date, the wet gas research has focused on the compressor and little attention has been given to the control valves surrounding the compressor. Control valves are an important part of the compression process, increasing the operational range and making the operation more flexible. Consequently, it is important to consider the effects of wet gas on the control valves. This thesis evaluates the performance of a centrifugal compressor and its discharge control valve at both wet and dry condition. The main objective of this thesis is to document the control valve performance at different gas mass fractions. Previous research on the wet gas compressor is presented, but the experimental campaign focuses on investigating valve wet gas performance. No standard performance analysis for wet gas throttling through control valves exists. Thus, various methods are used in the industry. The valve performance calculation procedure used in this thesis is the Addition method. An experimental campaign was conducted in the NTNU wet gas compression test facility. Two different experimental set-ups were used to document the control valve performance. Water was injected either upstream or downstream of the compressor. The volume flow through the system was varied by altering the control valve opening. The gas mass fraction was varied from 1 to 0.8. Dry gas experiments were conducted to compare data from the valve manufacturer and the wet gas tests. The results demonstrated that liquid properties influenced the control valve and compressor performance. A reduced volume flow and an increased pressure differential were observed for each valve opening when liquid was introduced. This indicated an increased throttling effect due to liquid blockage. The valve flow coefficient was reduced and a shift towards a linear valve characteristic was observed when the gas mass fraction was reduced. Additionally, a shift in operational point and system resistance curve was observed when liquid was introduced. The results in this thesis confirm the need for improved understanding of wet gas control valve throttling.

Wet Gas Compressor Transient Operation

Modeling alternatives for multiphase boosters, for field performance studies

Gas turbine inlet fog / overspray cooling is considered as a simple and effective method to increase power output. To help understand the water mist transport in the compressor flow passage, this study conducts a computational simulation of wet compression in a single rotor-stator compressor stage using the commercial code, Fluent. A sliding mesh scheme is used to simulate the stator-rotor interaction in a rotating frame. Previous researchers have modeled wet compression using DPM (Discrete Phase Model), where spray amount is very small (1–2%). Compressor washing is also becoming a new interest in the wet compression technology, which involves much higher amount of water (10–15%), which is not easy to handle with DPM. It can be done by multiphase model. To start compressor washing it is important to validate wet compression (1–2% spray) using DPM and the same using multiphase. This study takes the initial step to compare these two models, however multiphase model needs further development to perform an apple to apple comparison with DPM.Copyright © 2013 by ASME

Comparison Between Discrete Phase Model and Multiphase Model for Wet Compression

Finalizing the Wet Gas Laboratory

During upstream production of natural gas fields, it is common that a gas-liquid mixture of product is brought to the surface. The mixture, termed wet gas, is generally made up of mostly gas with a small amount of liquid, typically up to 5% by volume of the mixture. Because of the difficulties of compressing wet gas, the practical approach has been to separate the liquid and gas phases before compression. However, large separation equipment is unfavorable for subsea installations because of the cost to place machinery on the sea floor. Instead, a compressor designed for wet gas operation is preferred because it eliminates the need for large separation equipment leading to plant simplification and cost reduction. To address this design need, researchers have been active in addressing the challenges with wet gas compression. As result, experimental work has been conducted to study the effects of wet gas on compressor aerodynamic and mechanical performance. This experimental research has presented many challenges in recreating wet gas conditions and quantifying the effect of the liquid on the compressor performance. The results from this testing have helped to characterize the performance effects. But so far each work has focused on a range of test variables without identifying those that have the largest effect on compressor performance. This paper aims to provide the reader with an overview of the completed wet gas research, the challenges associated with doing the experimental work, and a discussion of the resulting trends observed in most of the wet gas research. This will include an in-depth review of relevant literature on wet gas compression testing and performance, a discussion of the important

Massimiliano Cirri

Papers

Overview of Important Considerations in Wet Gas Compression Testing and Analysis