Modeling Dynamic Heat Transfer in Piping Systems Using An Enhanced ASTM C-680 Algorithm

AIChE Spring Meeting and Global Congress on Process Safety
April 4, 2012 - 12:00am

AIChE 2012 Spring Conference

Modeling Dynamic Heat Transfer in Piping Systems using an Enhanced ASTM C-680 Algorithm

Debby Sielegar

Process Engineering Specialist

Bechtel Oil, Gas & Chemicals, Inc.

David Messersmith (co-author)

LNG Technology Manager

Bechtel Oil, Gas & Chemicals, Inc.

ASTM C-680 methodology is routinely used to model steady state heat transfer in piping systems. But not all applications can be treated as steady state. Some applications require modeling of systems with mixed phase streams in condensing or vaporizing services that are dynamically interacting with each other.

One of the common dynamic heat transfer problems is the relief of large quantities of low temperature gas to the dry gas flare system followed by the immediate relief of a much warmer vapor at its saturation temperature. The warm vapor comes in contact with the cold pipe surfaces and loses heat to the piping causing condensation on the inside surfaces of the pipe. The fluid thermally contracts which may result in air ingress into the flare header that can potentially create a flammable atmosphere. It has been typical to add displacement gas to purge the flare header and raise the temperature of the piping following the relief of low temperature gas to prevent condensation along the walls of the header. ASTM C-680 methodology is typically used to determine the required quantity of displacement gas and the duration of purging for each pipe segment at every time step. Other heat transfer applications include the determination of insulation requirement, sizing of condensed fluid containment, natural cooling as a cost saving approach, etc.

ASTM C-680 algorithm allows the user to iteratively calculate the temperature of all the piping layers (including pipe wall, insulation etc.), which in turn is used to calculate the inside and outside heat transfer coefficients and thermal conductivity. In this paper, a few case studies will be presented using an enhanced ASTM C-680 algorithm where the piping system is divided into multiple segments for accuracy, heat transfer is time-dependent, and the fluid properties are continually updated to reflect the impact of pressure drop.

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