There are four methods of heat transfer in most pipeline situations. Each involves significantly different methodology and hence different calculations. It’s hard to condense a book (e.g., J. P. Holman’s Heat Transfer, with 550 pages) into a thousand words. Hopefully this will enhance conceptuality by avoiding specificity.
There are usually several thermal “environments” along a pipeline. Suppose it starts in 600 feet of water at an offshore production well and follows the seabed up a continental shelf, onto land where in some cases it is buried in earth and in others runs exposed on the surface or in above-grade racks. Heat transfer takes place in each environment, but the mechanism can be quite different. Depending on the specific situation some heat transfer methods may be active or absent.
Heat transfer by advection involves moving something from one place to another. A common example is carrying a hot water bottle from the bathroom to a bed. The more common pipeline situation is pumping hot oil into a cold pipeline. The pumping activity moves some amount of heat, contained in the product, from a production well or plant into a pipeline where it fills an empty pipe or displaces existing fill.
It can be managed by controlling the mechanical means enabling the transfer. For example, when hot oil is pumped into a pipeline containing colder oil heat energy in the pipeline segment increases with the flow rate. The rate of heat transfer, the heat flux rate, is proportional to the characteristics of the fluid (specific heat and density) and the velocity at which it is being pumped.
Heat transfers by conduction when there is a thermal path between areas with different temperatures. A buried pipe, for example, has intimate contact with the backfill, setting up a thermal path from the usually warm petroleum product through the pipe wall and insulation, into the earth or water surrounding it. Sometimes heat transfer from a non-flowing (static) fluid in a pipeline becomes important for assessing its changing hydraulic conditions and from them, what may be necessary to reinstate motion after an outage.
The common methodology is to consider the fluid to be a set of concentric annuli, each containing some amount of thermal (heat) energy and also providing some resistance to heat conduction. It’s a problem of inner annuli transferring heat into outer annuli being impeded by the insulating qualities (thermal resistance) of the annuli in between.
Convection results from fluid motion over a thermally active surface. Common examples include wind on an exposed or elevated pipeline, or ocean currents (e.g., due to tidal flows) over submerged pipe, or the flow in the pipe passing over the internal surface of the pipe containing it.
Radiation is the movement of energy by electromagnetic radiation. A common example is heating of exposed piping by sunlight shining on it. A hot pipeline may also radiate energy to its environment and even out into space.
There are also some events that can occur that can affect the temperature in a pipeline. For example, suppose the physical characteristics of the flow environment changes – perhaps due to a leak decreasing the pressure or operation of some process control equipment (e.g., a pressure safety valve). Expanding the fluid’s environment produces fluid expansion which can result in Joule-Thompson cooling, essentially a cooling effect commonly used in household refrigerators. Whether this becomes a problem depends on the specifics of the fluid and situation. Freezing a valve intended for some particular function can produce process disturbances.
Mapping process flow
Mapping process flow on a pressure-enthalpy diagram can be very useful in studying and identifying regions of operation that are sensitive to various temperature related problems. A long section of exposed pipe can heat a fluid beyond the capability of a meter to accurately measure it. I did a paper years ago on an ammonia plant with a transient heat pickup problem and it was interesting stuff.
Why does any of this matter? After all it’s in the pipe so who cares about the details?
- What’s in the pipe might not change but how it presents itself to process equipment certainly can. Transporting a gas can be significantly different than a liquid yet a liquid can become a gas as environmental and operating conditions change. NGL, for example, may be liquid until some unanticipated condition occurs at which point it becomes multiphase. Meters and valves intended for liquid flow don’t produce accurate or appropriate results in multiphase of gas flow situations. The errors in measurement, for example, can impact custody transfer and leak detection.
- A pressure safety valve sized for gas might have performance issues when what is intended as a natural gas stream becomes two-phase or liquid instead. The cause of this might be the kind of conditions the monitoring or control system was intended to detect and mitigate.
- In some cases, operating conditions in the pipe, what is liquid and what is gas, can result in concentration of corrosive fluids at places where the design and pipe choice envisioned a nice, dry, gas. Purpose changes over time and with it the sensitivity to various kinds of risk.
Heat transfer issues are not all that common on well-designed pipelines operating according to the original intentions, but awareness of the issues is important in evaluating changes in fluids, operating conditions, flow rates, safety systems, and objectives. Even if you are not charged with servicing the details it is good to understand the generalities so these “demon details” can be anticipated and controlled when the need occurs.
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