Blowdown analysis delivers accurate results for depressurizing

Whether in an emergency or on a planned basis, depressuring a process 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 shutdown. During depressuring, control 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 system for controlled combustion.

Software-based design simulation models the depressuring behavior of process equipment, thereby supporting good design decisions. However, many tools assume phase equilibrium, which can lead to inaccurate predictions with consequences from potentially dangerous under-designs to unnecessarily costly over-designs.

Recently introduced blowdown software addresses these shortcomings using non-equilibrium models. For example, the blowdown utility 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 background

For 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 pressure and temperature, but constitute a significant safety hazard if run outside established boundaries.

To take a single example, pressurized equipment is found in oil refineries where process variables are often pushed to design limits, as refiners maximize parameters such as gasoline and diesel production. Pressure vessels are employed in services ranging from feed and product storage to reactors, separators 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 differential 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 blowdown segments. The blowdown of the entire facility entails simultaneously 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 shutdown 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 considerations

The rapid vapor generation and expansion of gas during emergency depressuring can expose pressure 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 vessel walls and their subsequent differential 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 determining 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 blowdown reduces the over-design often associated with a relief valve or network without compromising safety. For brownfield plants, blowdown studies can lead to operating-procedure 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 temperature experienced in each equipment item and in the flare system?
• Which low temperature materials 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 depressurization, or the combination of several connected vessels and pipe volumes through a single valve.

However, simulation tool accuracy varies significantly depending 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 throughout the process and pipework metal walls
• Blowdown times and relief loads entering the flare network
• Valve sizing, including blowdown valve sizes
• Fluid temperature typically entering a flare system that might contain evaporating entrained
  liquids

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