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Oct-2013

Minimising transfer line vibrations

Computational fluid dynamics was used effectively to solve start-up problems in a transfer line

RAHUL PATIL, RANDHEER YADAV, AJAY GUPTA and ASIT DAS
Reliance Industries Ltd, Jamnagar
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Article Summary
Delayed coking is an important process in a petroleum refinery and adds significantly to the gross refining margin. The delayed coking unit (DCU) takes vacuum residue as feed and thermally cracks it into useful lighter products such as liquefied petroleum gas (LPG), naphtha, gas oils and residual coke. Vacuum residue is first heated in a furnace to about 500°C for a very short time, to avoid coking inside the furnace tubes, and then cracked inside the coke drum, where sufficient reaction time is provided. Steam is added along with vacuum residue in the furnace to create additional turbulence for better heat transfer and to avoid coking inside the heater tubes.

Figure 1 depicts the flow scheme for a vacuum residue and steam mixture during start-up and normal operation of the DCU. The vacuum residue and steam mixture at the furnace outlet enters a transfer line connected to a header, which is connected to the fractionator during start-up for closed loop circulation at a temperature of 300-350°C and a pressure of 1.5-3.0 kg/cm2(g). After establishing the desired flow circulation, the header is disconnected from the fractionator and connected to the coker drum. During normal operation of the coker unit, the furnace outlet temperature is raised to about 500°C and therefore the vapour flow rate at the outlet increases significantly due to vaporisation and mild cracking of vacuum residue even at higher pressures of 3.5-6.0 kg/cm2(g).

During a start-up operation of the DCU, the transfer line experienced vibrations, giving rise to safety and reliability concerns. This article presents an analysis carried out using both conventional techniques and computational fluid dynamics (CFD), for determining the causes of vibrations in the transfer line exhibiting two-phase flow of vapour and liquid.

What could cause vibrations in the pipeline?
Large bubbles or slugs are the usual suspects in the case of vibration of a pipeline carrying a gas-liquid mixture. Therefore, establishing the flow regime in the pipeline is of prime importance. The presence of a particular flow regime (bubble, stratified, slug, annular) within the pipe is a function of the volume fraction of the phases and their properties. The existence of a particular regime may provide the desired results or it can deteriorate performance or cause reliability issues. For example, mist flow can provide efficient heat and mass transfer between two phases; however, the presence of a slug flow regime may cause severe vibrations in equipment because of the impact of high-velocity slugs against bends or other fittings. The flow regimes existing in a pipe for a two-phase system are complex and yet important for better design and for deciding the operating conditions to achieve the desired outcome.

Determine flow regime in apipeline: conventional analysis vs CFD
Approximate prediction of flow patterns can always be done using flow pattern maps in the literature.1,2 For a horizontal pipe, a Baker chart is widely used for the prediction of flow regime in a co-current flow of gas and liquid. For a co-current vertical upflow of gas-liquid mixture, the correlations by Govier, et al, may be used for quick estimates of flow regime. Most of the flow maps or correlations available in the literature are generated using an air–water system and may or may not be applicable to the gas-liquid system of interest. Studies correlating results of CFD simulations with Baker charts for gas-liquid systems other than air-water, although reported,2 are very few. The effect of bends on flow regime cannot be studied by conventional analysis. Therefore, for a gas-liquid system other than an air-water system, it is advisable to study the complex flow patterns in a pipeline using computational models, which solve constitutive equations for two- phase flows.

Commercial CFD software, Fluent 6.3 from Ansys, has been used to solve the equations for two-phase flow in a transfer line. The volume of fluid (VOF) method3 has been used to track the vapour-liquid interface. The flow regimes from conventional analysis are also compared with CFD model predictions.

Role of process conditions
As discussed above, the flow regime in the pipe is dependent on phase volume fractions and phase properties. The process conditions present during start-up and normal operation thus play an important role in deciding the flow regime that exists in a transfer line. The fluid properties at the heater outlet based on process conditions at start-up (start-up operation is referred to as Case 1) and normal operation (normal operation is referred to as Case 2) are shown in Table 1. Vaporisation is higher during normal operation as compared to start-up operation due to the fact that, while temperature and pressure are in the ranges of 300-350°C and 1.5-3.0 kg/cm2(g) respectively during start-up, in normal operation — although pressure is higher (3.5-6.0 kg/cm2(g)) — some part of the hydrocarbon liquid feed is cracked and vaporised due to a higher temperature of 480-520°C. This vaporised gas adds up to steam flow and increases the volume fraction of the vapour phase in normal operation.

The process conditions mentioned in Table 1 are used in conventional as well as CFD analysis to determine flow regime in the transfer line, and subsequently to find the root cause of vibrations.

Conventional analysis from flow regime maps
Widely used Baker and Govier charts1 were utilised to determine the flow regimes for the co-current flow of a vapour-liquid mixture in horizontal and vertical transfer lines, respectively.

Regime analysis for two-phase flow through horizontal pipes
A Baker chart was used for predicting the flow regime in co-current liquid-gas flow in horizontal pipes. Figure 2 shows the points marked for regimes for Cases 1 and 2 for flow through horizontal pipes. The reference properties of air and water are at 20°C and atmospheric pressure. Equations 1 and 2 are used for calculating λ and ψ:

                     (1)
                 
                     (2)

The process conditions during start-up (Case 1) lead to a slug flow regime in the transfer line. As discussed, this may be the cause of vibrations observed in the transfer line. During normal operation (Case 2), the process conditions are such that the flow regime prevailing in the transfer line is a dispersed/mist flow. In this regime, the liquid phase is dispersed completely in the gas phase. This will lead to smooth operation during normal conditions.
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