In midstream oil and gas operations, hot oil systems (also called heat medium systems) play a critical role in transferring heat to process equipment. These systems operate as closed-loop heating systems, circulating thermal fluid continuously through heaters and process users.
One component that often gets overlooked, but is essential, is the expansion drum.
Properly designed expansion drums protect the entire heat medium system from pressure issues caused by thermal expansion of hot oil. When expansion drums are undersized or improperly designed, the result can be system pressure problems, equipment damage, and operational safety risks.
At Tulsa Heaters Midstream, we frequently design and troubleshoot hot oil heater systems across the midstream sector.
An expansion drum is a pressure vessel used in hot oil systems and heat medium systems to accommodate the expansion and contraction of thermal fluid as temperatures change.
In a typical midstream heating system, hot oil circulates continuously through the loop:
Because the system is closed-loop, the fluid remains inside the piping and equipment at all times.
However, when thermal fluid heats up, its density decreases and its volume expands. Without a place for that expansion to go, pressure would increase dramatically inside the system.
This is exactly why an expansion drum is required.
Consider a typical midstream heat medium system:
When thermal oil heats from 40°F to 400°F, its density decreases significantly and the total fluid volume increases.
Since the system piping, heater coils, and exchangers are rigid, the fluid must expand somewhere.
The expansion drum provides that extra space, acting almost like a spring for the system.
An expansion drum sits above the circulating loop and is partially filled with thermal fluid.
The remaining space above the fluid contains blanket gas, which compresses as the liquid level rises.
When the system heats up:
When the system cools:
This constant movement allows the system to operate safely while maintaining controlled pressure.
Expansion drums typically include pressure control systems to maintain stable operation.
Common components include:
Incoming gas pressure may be reduced from supply pressure (for example 100 psig) to a lower operating pressure, such as 15 psig.
This pressure sits above the thermal fluid.
As fluid expands and compresses the gas, pressure increases.
A back pressure regulator opens at a preset pressure (for example 30 psig) and vents excess gas to maintain stable pressure.
Expansion drums are ASME pressure vessels, so they must include a pressure safety valve.
Typical design parameters:
The PSV acts as the final safety protection if the pressure regulation system fails.
Elevation of the expansion drum is another key design consideration.
The safest configuration is to install the drum at the highest point in the system.
Advantages include:
However, this is not always required.
Many installations place the expansion drum 10–15 feet above the pump suction to provide adequate NPSH (Net Positive Suction Head) while still allowing proper expansion.
Some facilities prefer higher installations for additional safety, sometimes 30 feet or more above grade, though this requires additional structural support.
Several common issues appear in poorly designed thermal oil systems:
This causes excessive pressure fluctuations and potential system shutdowns.
Using air instead of inert gas can create combustion hazards.
Poor regulator setup can cause unstable system pressure.
Failure to remove oxygen before startup can create safety risks.
The expansion drum must be rated for the same pressure as the heater coil.
To ensure reliable operation in midstream heater systems, engineers should follow these best practices:
Following these guidelines prevents system instability and ensures long-term reliability.
Expansion drums may seem like a simple piece of equipment, but they are essential to the safe operation of midstream hot oil heating systems.
A properly designed expansion drum allows the system to safely handle temperature changes while protecting pumps, heaters, piping, and process equipment.
At Tulsa Heaters Midstream, our engineering team designs process heaters, hot oil systems, and heat medium systems specifically for the demands of midstream gas processing operations.
Our focus is always on:
If you're designing or upgrading a midstream heater or hot oil system, our team can help ensure every component, from the heater coil to the expansion drum, is engineered correctly.
Contact our team to discuss custom heater design, thermal fluid systems, and midstream heating solutions.