Jun-2016

Multiphase flow analysis in slug catcher design

Computational fluid dynamic simulation extends the range and flexibility of designing slug catchers

S SATHISH KUMARAN, DATTESH KONDEKAR and M K E PRASAD Technip India
CEDRIC LEBER Technip Paris

Article Summary

The United Nations-led conference on climate change in Paris (COP21) set the trend of fuel mix in the coming decades by shifting fossil fuel based energy to renewable energy to limit global warming to 2°C. Though the oil and gas industry paves the way to make the transition to a low carbon economy, it is seen that oil and gas is part of this transition, as developing countries need to alleviate the poverty of huge populations. Hence for some time to come, oil and gas will be one of the important drivers of energy demand forecast. However, of late, the oil and gas industry has been shaken by the oil price falling to low levels as a result of higher global supplies on the advent of US shale oil, when demand for oil is not growing very fast. Investment has drastically reduced all over the world and operators are looking for the most economical ways to improve plant performance. In this context, the oil and gas industry is focusing on capex optimisation and reduction in energy consumption to sustain production and market share.

Oil and gas companies are consequently looking to engineering companies to provide simple, cost effective solutions and reliable designs. Some of the key issues to be addressed in the design phase include minimising overall capital cost and footprint, design provisions for flexibility in operation and maintenance, and ease of fabrication and installation like modularisation and stringent separation requirements and flow assurance. Hence it becomes pertinent for engineering companies to adopt innovative methods and tools to design critical equipment. Computational fluid dynamics (CFD) is a key tool to achieve the design improvement of a slug catcher in the oil and gas industry.

Slug catcher

A slug catcher, which is a part of the gas pipeline system, is essential equipment at the onshore gas receiving terminal. The specific function of a slug catcher is to hold slugs of liquid coming from pipelines and to separate the gas and liquid phases. The separated gas is sent for further treatment in the gas treating facilities. Hence the design of the slug catcher is critical to prevent slugs of liquid going to the downstream gas treating facility.

Of the three types of slug catcher prevalent in the industry – vessel type, stored loop type and finger type – the last of these is most appropriate for handling large slugs from pipelines. A finger type slug catcher uses pieces of large diameter pipe in contrast to a conventional vessel type slug catcher that provides a buffer volume to hold slugs of liquid (see Figure 1). Since pipes can be more easily designed to withstand high pressures, compared to vessels, the finger type slug catcher is better suited for large capacities.

Limitations of conventional design methods

A slug catcher has to perform phase separation in the following extreme process conditions:

• Gas/liquid arriving under steady flow conditions: predominantly liquid droplets entrained in the gas flow
• Liquid arriving with a sphere or pig during pigging: predominantly slug or bubble flow
• The scenario of liquid produced as a result of an increase in gas flow rate (ramp up): the flow pattern falls in between the above two extreme scenarios

The first two process conditions typically govern the slug catcher design. The steady flow condition determines the design from the droplet entrainment point of view, and the pigging condition from the liquid distribution point of view. The separation desired in the slug catcher invariably involves gas-liquid separation as well as liquid-liquid separation. A greater insight into the hydrodynamic multiphase flow pattern during the design of the finger type slug catcher is crucial in addressing most of the key design issues. For a given scenario, slug catcher design involves detailed analysis and sizing of the following sections of the equipment (see Figure 2):

a) Inlet header/manifold This section is to be designed so that gas-liquid multiphase flow is evenly distributed in fingers

Conventionally, slug catcher design methods rely on simplified empirical correlations, with design margins based on the designer’s experience. However, these conventional methods do not take into account actual inlet flow maldistribution and localised velocity variations within the fingers. Such an approach with design margins may be an adequate approximation for small size slug catchers with low numbers of fingers. However, for large slug catchers with higher numbers of fingers, this conventional method may not give an adequate design due to flow maldistribution issues among the fingers, which ultimately affects performance. Hence, it is required to develop more rigorous methods to address the shortcomings of conventional design methods to enhance design reliability.

Role of computational fluid dynamics

The term computational fluid dynamics (CFD) represents the use of computer based simulations to analyse fluid flow, heat and mass transfer and associated phenomena.¹ For most engineering problems, it is impossible to obtain analytical solutions of the non-linear partial differential equations that govern fluid flow. Hence, CFD applies a discretisation method that approximates differential equations to algebraic equations to solve them on computers.² Compared to experimental and theoretical fluid dynamics, CFD has unique advantages,³ such as speed, cost effectiveness, ability to simulate ideal and real conditions, and comprehensive and detailed information about the flow domain.

Unlike the traditional approach, in CFD analysis the flow domain is modelled and discretised into small cells called mesh. The model and mesh takes into account several geometrical configurations of a slug catcher, including pipes and pipe fittings. The governing equations of flow, turbulence and multiphase interaction are solved in these small discretised domains. Hence the velocity and flow distribution resulting from CFD better represents the effects of instantaneous phenomena, such as phase separation, interaction and redistribution in a slug catcher.

Application of CFD in multiphase flow analysis

The following case studies demonstrate the successful application of CFD to meet the design objectives in a typical slug catcher design in an onshore gas terminal project. The slug catcher design performed using the conventional method is to be analysed 3using computational studies.

The key objectives under focus in the CFD analysis are:

1. Case study 1 (normal operation) Analysis of the design of the primary bottle to achieve stringent separation efficiency with a maximum particle size of 150 microns at gas outlet during normal operation (steady state analysis)
2. Case study 2 (pigging operation) Analysis of even distribution of the condensate expected during a pigging operation in fingers (transient analysis).

The design of the inlet manifold plays a vital role in meeting design objectives as it plays a critical role in flow distribution in individual fingers and hence flow separation. A common inlet distribution header is required because of future requirements for expansion flexibility in this project. Hence a detailed CFD analysis of various options for the inlet manifold design is not performed. The design of the inlet manifold adopts symmetrical piping, which is taken into account in the complete slug catcher CFD model.

The CFD model considered for these case studies mainly consists of the inlet distribution manifold, distribution header, downcomers, gas risers, gas collection header and fingers (see Figure 3). The complete length of the fingers’ secondary bottle is not considered in the CFD model to minimise the computational effort. A hexahedron dominant mesh was generated to perform the study (see Figure 4).


The deviations of mass flow distribution in individual gas risers are not within the over-design limits built into the conventional design method. Further, the multiphase liquid particle entrainment study revealed that the desired gas-liquid separation efficiency of a 150-micron droplet could not be achieved even in fingers and gas risers having flow maldistribution within acceptable limits (see Figure 6).

The results of detailed CFD analysis play a significant role here to reveal the reason for poor liquid particle separation. The reason can be attributed to the combined effect of both the unbalance in mass distribution as well as the velocity maldistribution at the inlet to the primary bottle of the individual fingers. It is explained by detailed analysis of velocity distribution in individual fingers and within its cross-section (see Figure 7).

The mass flow distributions obtained by CFD and other 1D software are qualitatively compliant with each other. Hence various possible modifications to improve flow distributions were studied using conventional empirical correlation based 1D software. The best suited modification, selected from the above study results, is then analysed in detail in a ‘single finger’ CFD model prior to a ‘complete model’ CFD study of the modification. This helps to optimise time and computational effort required for narrowing down the required modification.

The final modified design of the slug catcher is reviewed again with a complete model CFD study. The analysis showed a positive response of velocity magnitude mass flow distribution at the inlets of individual fingers (see Figure 8). The desired separation efficiency with a maximum particle size of 150 microns at the gas outlet could also be achieved (see Figure 9).