December 2015 • www.ProcessingMagazine.com

Whether in an emergency or on a planned basis, depressuring a pro-cess plant, commonly referred to as “blowdown,” is a critical-to-safety operation for today’s industrial plants. It might be required in the event of a fire, leak, pipe rupture or other hazard, or for a planned shut-down. During depressuring, con-trol and safety valves, relief valves, restriction orifices, and rupture disks are involved in transferring the potentially dangerous contents of process equipment to a safe, lower-pressure location, or to a flare sys-tem for controlled combustion.

Software-based design simulation models the depressuring behav-ior of process equipment, thereby

supporting good design decisions. However, many tools assume phase equilibrium, which can lead to inaccurate predictions with conse-quences from potentially danger-ous under-designs to unnecessarily costly over-designs.

Recently introduced blowdown software addresses these shortcom-ings using non-equilibrium models. For example, the blowdown util-ity in Honeywell’s UniSim Design incorporates a rigorous vessel-unit operation with non-equilibrium holdup calculations; simultaneous solution of model equations; and a clear, straightforward interface to configure the model. Moreover, it is validated and tuned against both

proprietary and open-literature experimental data.

Blowdown backgroundFor modern industrial facilities,

blowdown procedures are essential for protecting plant assets against the effects of high pressures or temperatures. Pressure vessels, heat exchangers, distillation columns, compressors and other equipment operate safely within an operating window reflecting the design pres-sure and temperature, but consti-tute a significant safety hazard if run outside established boundaries.

To take a single example, pres-surized equipment is found in oil refineries where process variables are often pushed to design limits, as refiners maximize parameters such as gasoline and diesel produc-tion. Pressure vessels are employed in services ranging from feed and product storage to reactors, separa-tors and fractionation towers.

By definition, a pressure vessel holds gases or liquids at a pressure substantially different from ambient pressure. The consequences of its failure can be fatal. The pressure dif-ferential defines a point of danger, and many serious accidents occur involving high-pressure equipment operation.

A process plant is typically divided into a number of independent blow-down segments. The blowdown of the entire facility entails simultane-

Blowdown analysis delivers accurate results in non-equilibrium conditions

By J. A. Barber and V. Athanasiou

Simulate blowdown system depressurizing behavior

For modern industrial fa-cilities, blowdown proce-dures are important for protecting plant assets against excessively high pressure or temperature.

Grap

hics

Cou

rtes

y Ho

neyw

ell

ously or sequentially depressuring all pressurized gas or liquid in each facility segment.

During separator depressuring, for example, the vessel’s inlets and outlets, whether gas, liquid or water, are blocked by closing isolation valves, also called emergency shut-down valves (ESDVs).

Gas is directed to the flare or vent through an isolation valve and restriction orifice, pressure relief valve, bursting disk or manual valve.

Concerns and considerationsThe rapid vapor generation and

expansion of gas during emergency depressuring can expose pres-sure vessels and other equipment to pressurized thermal shock. This is caused by combined stresses from rapid temperature or pressure change and results in non-uniform temperature distribution in the ves-sel walls and their subsequent dif-ferential expansion and contraction. Pressurized thermal shocks can also cause embrittlement of vessel metal walls, leading to failure by fatigue.

For safe use of a pressure-relief and blowdown system, important factors include construction material selection, especially concerning low temperatures; sizing of relief valves, orifices, piping and vessels; and choosing connections and determin-ing the capacity of the flare system.

In greenfield plants, an accurate prediction of the minimum vessel-wall temperature during blowdown can impact selection of construction materials, reduce over-design and consequently lower project cost.

Similarly, accurate prediction of maximum flow rate during blow-down reduces the over-design often associated with a relief valve or net-work without compromising safety. For brownfield plants, blowdown studies can lead to operating-proce-

dure changes as well as to process equipment materials or capacities to minimize the likelihood of brittle fracture.

For accurate predictions, software simulation tools model process equipment depressuring behavior to answer the following questions:

• At what rate must gas be released from each item to meet required depressuring times?

• What is the required total-flare capacity?

• What is the lowest metal tem-perature experienced in each equipment item and in the flare system?

• Which low temperature materi-als are required?

• What size-restriction orifice or other flow-rate controlling device and flare connections will depressure each section of the plant?

Industry already has software tools for dynamic simulations of blowdown events and, in turn, for depressuring and gas-rate profiles used in analysis. These utilities can simulate vessel and pipeline depres-

surization, or the combination of several connected vessels and pipe volumes through a single valve.

However, simulation tool accu-racy varies significantly depend-ing on the fidelity of the modeling approach. Predictions are often inaccurate due to simplifications in mathematical models. Additionally, depressurization is a non-equilibrium process that has not been properly represented in most commercially available depressurizing utilities.

Companies with critical concerns about equipment blowdown need effective calculation and simulation solutions to determine:

• Minimum temperatures through-out the process and pipework metal walls

• Blowdown times and relief loads entering the flare network

• Valve sizing, including blow-down valve sizes

• Fluid temperature typically entering a flare system that might contain evaporating entrained liquids

Latest technology advancementsRecently introduced blowdown

software achieves improved fidelity by a modeling approach consis-tent with recommended practices set out in the sixth edition of the American Petroleum Institute (API) 521 Guide for Pressure Relieving and Depressuring Systems. These include the following:

• Simulates equipment emergen-cy depressuring within a flow-sheet environment

• Rigorously handles non-equilib-rium three-phase systems

• Uses a variety of rigorous ther-modynamic models

• Improves heat-transfer modeling across metal by incorporating better correlations for heat-transfer coefficients

12 Plant Maintenance & Safety

Figure 1: The latest blowdown software enhancements include an accurate holdup model for non-equilibrium three-phase systems.

December 2015 • www.ProcessingMagazine.com

• Incorporates rigorous formulas for volumes, and for surface and interfacial areas

• Evaluates different configura-tions of vessel orientations

The latest blowdown technol-ogy, compared with experimen-tal data, is a strong match for fluid- and wall-temperature pre-dictions, especially on horizontal vessels. The software is intuitive and easy-to-use, clarifying and simplifying required input while remaining numerically robust, and with no reported cases of unex-plained trends. It features detailed descriptions of vessel dished heads and integrates two wall material layers and two insulation layers (see Figure 1).

Typical user benefitsWith blowdown design simula-

tion technology, vessel depressur-ing behavior can now be accurately

predicted, improving pressure ves-sels, relief devices and relief/flare systems. This increased accuracy improves intrinsic safety.

A state-of-the-art blowdown tool enables improved prediction of the maximum flow rate through the relief device. Coupled with knowl-edge of the lowest metal tempera-ture experienced, more reliable and accurate sizing of pressure vessel, relief devices and relief/flare sys-tems is possible.

Finally, blowdown simulation tools are simple and user-friendly, reducing the effort needed for engi-neers to run case studies. Ongoing blowdown utility enhancements will support multiple-vessel model-ing with interconnecting piping in an equation-based modeling (EO) environment. Users can more accu-rately predict temperature profiles in vessels and surrounding piping, integrate blowdown scenarios with

the process and configure the flow-sheet to achieve optimization with a single utility.

Plant personnel can make accu-rate predictions, despite the non-equilibrium nature of a pressure ves-sel, as regards different vapor and liquid temperatures, which legacy tools do not distinguish. By eliminat-ing the need for adjustable parame-ters in the model, fewer user inputs are needed. While this solution does not account for the solid phase in the model, it can accurately predict the incipient solid CO2 and hydrate formation temperature.

J. A. Barber is a principal engineer with UniSim Design After Market Services at Honeywell Process Solutions, and V. Athanasiou is the global product manager for process design at Honeywell.

Honeywell Process Solutionswww.honeywellprocess.com

UniSim Design R442 Blowdown Utility flowsheeting environment.

14 Plant Maintenance & Safety

December 2015 • www.ProcessingMagazine.com

hpsmarketing@honeywell.com

Periodical Class December 2015

www.ProcessingMagazine.com

Plant Maintenance & Safety

Flow Computer Platform for Oil & GasEmerson Process Management’s next-generation flow computer family con-sists of single- and dual-meter run gas flow computers. These field-mounted flow computers provide a new solution to flow metering and flow computing where challenges with power, safety and measure-ment accuracy are addressed, delivering a more convenient approach to remote oil and gas production sites. The flow computer family FB1100 and FB1200 has an ultra-compact design including integral sensor, battery and I/O options, for fiscal metering with many different flow meter technologies. Read more about flow measurement on page 26.Emerson Process Managementwww.emersonprocess.com/FBFlowComputersWrite In 502

Non-Invasive Signaling UnitREMBE’s new addition to its signaling devices for process safety is the

NIMU (Non-Invasive Monitoring Unit), which provides information about the way a bursting disc responds to positive and negative

pressure. NIMU is installed in a blind tapping in the outlet section of the rupture disc holder, iso-

lating it from the process medium. This signaling method is particu-larly suited for critical processes and applications, which require the

entire system to be completely impermeable. As a reusable monitor-ing system, only the rupture disc itself must be replaced after a rupture

disc has opened. It can also be serviced or replacede without opening the pipeline. For more information on plant maintenance and safety, turn to page 10.REMBE Inc.www.rembe.usWrite In 500

Special Feature2015 Breakthrough Products Processing’s Breakthrough Products awards recognize prod-ucts, technologies and services that made significant contribu-tions in the process industries in the last year and are expected to impact the industry for years to come. Many of this year’s 11 recognized companies are mid-sized, U.S.-headquartered makers of equipment and instrumentation that increasingly distribute their solutions worldwide. To read about this year’s standout performers, turn to page 6.

Pneumatic Series Direct-Mount Ball ValvesClark Solutions’ 8E Electric and 8P Pneumatic Series Direct Mount Ball Valves feature the lowest possible operating torque compared to all other ball valves used in industry today, according to the manufacturer. The 8P Series Ball Valves require a minimum of 80 psi air supply. The 8E and 8P are offered in a variety of models: from ¼ inch to 4 inches; brass or stainless steel; in two-way or three-way

configurations; with T port, L port, full port or standard port options; and with double acting actuators or spring return actuators. For more information on valves and actuators, turn to page 22.

Clark Solutionswww.clarksol.comWrite In 501

Reducing danger in a petrochemical facilityRob van Oostveen has one primary goal for his polymer emulsions plant: no loss of production. As the maintenance manager at Organik Kimya’s facility in Rotterdam, the Netherlands, he knows pumps play an essen-tial role in ensuring safety as they produce

polymer for the paint and coatings industry and for adhesive tapes. The plant has found its current choice in pump to be reliable, depend-able and, most importantly, safe for site personnel and the environ-ment. Read more on page 24.

Case Study

White Paper Use of Epoxies as Protective Coatings Against Corrosion for Metals

Explore the types and sources of corrosion of metal substrates in this white paper by Master Bond. Corrosion can be prevented by pro-tecting your design

with the proper selection of epoxy-based systems. Learn about the trade-offs of different curing agents used in epoxy coatings. Visit www.pro cessingmagazine.com to download this white paper.Master Bond Inc.www.masterbond.comWrite In 503

PR1215_Cover.indd 1 11/23/15 3:51 PM

As seen in the December 2015 issue