Rational Engineer

Introduction

In a process plant, there are a variety of equipment in use, which may include pressure vessel, piping, heat exchangers, distillation column, compressors, etc. During operation of the plant, there is possibility of an upset or abnormal situation. This may be classified as Fire scenario or Non-Fire scenario (e.g., blocked outlet, control valve failure, etc). In addition, there may be instances when the equipment may need to be vented for operational or maintenance reasons. In order to cater to these scenarios various relief devices such as relief valve, blowdown valve, manual vents, etc are provided on the equipment.

The outlets from these equipment have to be released in a controlled and safe manner. The released fluids may need to separation or burning prior to release into the atmosphere. The outlets from relief devices are commonly connected into a flare network.  

This article will provide insights into following questions:

• What are the components of a flare network
• What are the calculations involved in a flare network study
• What are the capabilities and limitations of commonly used software for conducting flare studies
• What are the criteria in designing or rating the flare network
• What are the steps in modelling flare network hydraulics using simulation software such as Flarenet / Aspen Flare System Analyzer / Unisim Flare

At the end of this article, you will find a checklist which summarises the important points for carrying out flare network hydraulics.  

Anatomy of a Flare network in a Process industry

The flare network would generally comprise of following items:

• Overpressure protection device (Relief Valves, Flare PCVs, Blowdown valves, etc)
• Tail Pipe
• Flare sub-headers and flare header
• Knock-out drum (with or without liquid seal at its gas outlet)
• Flare (Stack, Tip)
• Auxiliary systems (purge gas, purge gas reduction seal, pilot gas, ignition & monitoring system, assisted or non-assisted flares)

Figure 1: Flare Network Components Sketch

Studies in Design of Flare System

A study of design or rating of a flare system may involve various aspects of flare network components. An overview of various studies involve in the design of flare system are given below:

Figure 2: Studies in Flare System Design

As you can notice that various studies are involved in the design of flare system. Various considerations and calculations involved are indicated below:

• Segregation of flare systems based on factors such as atmospheric/LP/HP systems, toxic components and cold releases
• Relieving scenario and relief rate estimation as per API 520/521 [1] [2]. Flare loads from flare PCVs, which divert the gas during downstream plant shutdown
• Flare network hydraulics and backpressure calculations
  • Line sizing based on criteria of velocity/Mach number, momentum, noise and backpressure on relief device
  • Selection on type of relief valve (conventional/balanced/pilot) based on allowable backpressure as % of set pressure
  • Low temperature estimation for non-fire cases to provide input on lower design temperature required for flare network components
• Knock-out drum for LP/HP flare system and it’s sizing as per API 521 [1]. Blowcases to collect liquids for atmospheric flare systems where no knock-out drums provided.
• Liquid seal (drum) to prevent air ingress / reduce purge rate / provide staging between multiple flares. Sizing as per API 521 [1]. An alternative to liquid seals could be the use of buckling pin with bypass PCV.
• Purge gas to prevent air ingress into the elevated flare during low flow cases and estimation as per HUSA correlation. In case of use of buoyancy/velocity seals then purge gas required is reduced and estimated based on velocity criteria  
• Flare stack height estimation as per radiation, toxic dispersion and hydrocarbon LFL limits
• Pilot gas to ensure flare is burning even during normal operation when there is no emergency or maintenance release of process gas. Pilot gas rate as per input from flare vendor.
• Selection on type of flare [4] such as elevated flare or enclosed ground flare, (steam/air) assisted flare or non-assisted flare and sonic flare tip or non-sonic pipe flare tips.

This article will focus on flare network hydraulics studies. It will elaborate on inputs required, scenarios considered, criteria used, steps required to build Flarenet model and analysis of results.

Flare Network Hydraulics

Inputs required

Inputs for flare network hydraulics are summarised below:

• relief and blowdown calculations – flow rates, relief device upstream/set pressure, fluid properties at relieving conditions, fluid composition, fluid phase – vapour/liquid/two-phase/supercritical, type of relief device – conventional/balanced/pilot
• segregation of flare systems – to Atmospheric/LP/HP flare systems
• pipe routing details – pipe fitting details, pipe lengths and elevation profile, location of knock out drum in the flare network
• flare tip – pressure drop vs flow rate curve – input from flare vendor
• criteria for the design of flare system – velocity/Mach number, momentum/sound power level, noise limit, backpressure limits, pipe roughness, design pressure of flare system
• environmental conditions – minimum/maximum ambient temperatures, solar radiation, minimum/maximum wind speed

FLARENET model is built using the relief device details, pipe routing details, preliminary sizes, heat transfer inputs, flow correlation, pipe roughness, etc.

Before we start running the FLARENET model, we need to determine the scenarios and governing loads for the flare system. In order to do this, we will first start listing the relief devices and the relief scenarios as indicated in the table below.

Note that rated flow is considered for sizing of tail pipes of pop acting/spring loaded relief valves [1]. Rationale: When pop acting type RV reaches its set pressure, it tends to open 50% or more. This may lead to initial flow from RV at a relief load higher than required relief rate. Thus, rated flow is used for sizing/backpressure calculations.

Design Criteria

Criteria to be considered for the design or rating of flare network hydraulics are indicated below:

• Backpressure limit
• Velocity or Mach number limit
• Flow correlation used
• Pipe roughness
• Noise limit and Acoustic fatigue limit
• Momentum limit
• Assessment of slug flow

Refer to the checklist given at the end of this article for further details.

Modelling

Simulation software – Capabilities and Limitations

The simulation model to run steady-state flare network hydraulics was first developed by HYPROTECH as FLARENET model. HYPROTECH also managed HYSYS process simulation software. Later HYPROTECH was acquired by AspenTech and intellectual property rights of HYSYS was obtained by Honeywell. AspenTech renamed HYSYS as Aspen HYSYS and Honeywell renamed HYSYS as UNISIM Design.

AspenTech renamed the FLARENET software as Aspen Flare System Analyzer, while Honeywell renamed FLARENET as Unisim Flare. The use and functioning of these flare hydraulics simulation software remain similar. In this article, the term FLARENET is used for simplicity.

Figure 3: FLARENET PFD with description

Steps of FLARENET modelling

1. Start a new case

File > New

Enter Case description in the ‘Description Editor’

Enter components and Thermodynamic package in the Component Manager

2. Draw your flare network

Add components of the network  - Relief valve / Control valve, tail pipe, header pipes, connection with connectors and tees, knock-out drum (if required), flare stack & tip

Provide process inputs for the network component

3. Define the design criteria

Build > Scenarios > Scenario Manager > Edit

This will enable Flarenet to know what are your back pressure, velocity/mach no. and noise limits, and thus report violations as applicable during your calculations

4. Define calculation criteria

Calculations > Options > Calculations Options Editor

5. Select Rating or Design or Debottlenecking mode

Note final calculations to be done in rating. Design or debottlnecking mode is used to estimate initial line sizes if you are note sure.

6. Ensure that you are working in the relevant scenario

since multiple scenarios can be developed with respect to your sources (relief valve / control valve)

7. Check your software preferences

File > Preferences > Preferences Editor

8. Start your calculations by pressing GO button

9. View your results

Once your calculations run is over, Flarenet will show violations (if any) in red colour.

Further the violation messages can also be seen from > View > Results > Messages

Review Flow Map for slug flow regime from View > Results > Flow Map

Low Temperature Studies

When hydrocarbon vapours flash across a relief device it is accompanied by a pressure drop. During non-fire cases, this would result in low fluid temperature at the throat of relief device and in the downstream/outlet flare network. During a prolong relief event, the metal temperature of flare network may come in steady-state with low fluid temperature. As the fluid travels further in the flare network, the fluid would exchange heat with the surroundings and may warm up depending on the ambient conditions.

If the low temperature has not been accounted in the design of flare network components, then there is potential of brittle fracture and loss of containment. Hence, apart from carrying out flare network hydraulics, it is important to consider the low temperature studies.

Analysis of Hydraulic Results for Design of Flare Network

The end result from flare network hydraulic study and low temperature study is to determine the following:

• Tail pipe sizes at the discharge of various relief devices for governing cases based on backpressure, mach number, momentum and noise criteria
• Flare sub-header, flare header and flare stack sizes for governing flare loads based on backpressure, mach number, momentum and noise criteria
• Minimum metal temperature (MMT) of tail pipe, flare sub-headers and flare header for governing low temperature cases

The conclusions may be presented in following tabular format.

Individual relief valves:

Individual blowdown valves:

Flare header:

Inputs for other flare system studies

The outcome of flare network hydraulics would provide conclusion to arrive at required sizes of tail pipe, flare sub-headers, flare headers and flare stack.

Further to this, it also provides inputs for other studies for flare system design such as:

• Selection of relief valve type based on maximum estimated backpressure
• Sizing of KOD based on fluid composition and backpressure estimate
• Input for radiation and dispersion analysis based on fluid composition and rate at flare tip
• Selection of flare network component materials based on low temperature studies
• Purge rate estimation based on size of flare header

Checklist for Flare Network Hydraulic Study

A checklist is prepared to summarise various aspects in conducting flare network hydraulics including inputs required and design criteria to be considered. In order to obtain a copy of this checklist, follow the instructions given below.

Do let us know your feedback on using this checklist. Where and how did you use the checklist? We would also like to hear from you on what other topics you want us to cover.

References

[1]          “API Standard 521, Pressure-Relieving and Depressurizing Systems”.

[2]          “API Standard 520 Part I, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries – Sizing and Selection”.

[3]          “API Standard 520 Part II, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries – Installation”.

[4]          “API Standard 537, Flare Details for Petroleum, Petrochemical, and Natural Gas Industries”.

[5]          “API Standard 2000, Venting Atmospheric and Low-pressure Storage Tanks”.