Guidelines for Hot Oil (Thermal Fluid) Systems

Introduction and Application

This guide provides general engineering principles for the design and review of systems using thermal oil (hot oil) in its liquid phase as a heat transfer medium. These guidelines are applicable for basic, front-end, and detail engineering activities.

Technical processes often require heating fluids. While water and steam are effective at lower temperatures, organic heat transfer fluids are preferred for high-temperature, low-pressure applications.

Requirements for Heat Transfer Media

An ideal heat transfer medium should have:

• High boiling point and low solidification temperature.
• Good thermal stability.
• Low viscosity across the entire temperature range.
• Good heat transfer properties.
• Non-toxic, low fire risk, and low corrosion tendency.
• Economical purchase price.

Common Heat Transfer Media by Temperature

• Water: 0°C → 100°C
• Steam: 100°C → 200°C (Temperatures > 200°C require high-pressure, expensive plants)
• Organic Oils: 200°C → 350°C
• Eutectic Salt: 350°C → 500°C (Typically Sodium/Potassium Nitrite/Nitrate mixtures)

System Overview

Hot oil systems typically operate between 200°C and 350°C. The system circulates thermal oil between a heater and various consumer heat exchangers.

• Primary Circuit: The main loop where oil is pumped through the heater and supplied to the users.
• Secondary Circuit: A separate loop created when a user requires hot oil at a different (usually lower) temperature than the primary circuit. This is achieved by mixing hot primary oil with cooler return oil from the secondary user.

Key Design Documentation

Proper design requires several key documents:

1. Utility Summary: Lists all hot oil users.
   • Heat duty (e.g., in Kcal/hr) for each user.
   • Normal and peak flow rates (in $m^{3}/hr$). Peak flow determines the required circulation rate.
   • Design margins (e.g., 10-15% on flow) and heat loss estimates (e.g., 2-10% for environmental losses).
2. Piping & Instrumentation Diagram (P&ID)
3. Process Data Sheets for all major equipment.

Equipment Design Considerations

Expansion Tank

• Purpose: Accommodates the thermal expansion of the oil as it heats up. It also serves as the high point to vent any light-boiling components formed during fluid degradation.
• Location: Must be at the highest point in the circulation loop.
• Design: Often blanketed with nitrogen to prevent fluid oxidation and contact with atmospheric moisture.
• Capacity: Sized to hold the expanded volume of the total system oil when heated from ambient (or ~100°C) to the maximum operating temperature.
• Design Pressure: Low pressure (e.g., ~1000 mm WC) for the nitrogen blanket, but must also be rated for full vacuum to prevent collapse.
• Design Temperature: Must be the maximum operating temperature of the hot oil, even though it normally operates at a lower temperature.

Drain Tanks

Used to drain the entire system volume for maintenance. They must be located at the lowest elevation in the circuit.

Main Drain Tank

• Purpose: Used for the initial filling of the system, unloading old oil, and collecting oil drained by gravity from the main system.
• Capacity: Must be large enough to hold the entire system volume.
• Design Pressure: Very low (e.g., 150 mm WC), vented to the atmosphere through a flame arrestor.
• Design Temperature: Maximum operating temperature of the hot oil.
• Features: Should include a heating coil (e.g., low-pressure steam) to heat the oil to ~100°C. This helps remove moisture from new oil during initial filling or makeup.

Low Point Drain Tank

• Purpose: Collects oil from parts of the system (like the heater) that are at a lower elevation than the Main Drain Tank and cannot be drained by gravity.
• Capacity: Sized to hold the volume of the heater and any piping that cannot drain to the main tank.
• Design Temperature: Maximum operating temperature.
• Features: This tank is often located below ground. A submersible pump is required to transfer the collected oil from this tank up to the Main Drain Tank.

Pumps

Hot Oil Circulation Pump

• Type: Centrifugal.
• Capacity: Based on the total heat duty from the utility summary. The flow rate is calculated based on an allowable temperature drop ($\Delta T$) across the system, typically limited to 20°C to 30°C.
• Head: Based on the pressure drop of the heater, all piping, and the pressure required by the users.
• Important: Static head is not considered in the pump head calculation for a closed loop.
• Design Temperature: Max operating temperature + $30°C$ margin.
• Sealing: Typically single mechanical seal with self-flushing.

Hot Oil Feed Pump

• Purpose: Used for the initial filling of the circuit from the Main Drain Tank to the Expansion Tank.
• Capacity: Low (e.g., $1-5~m^{3}/hr$).
• Head: Must overcome the static height to the top of the expansion tank, plus line friction.

Hot Oil Unloading Pump

• Purpose: To unload new oil (often from barrels) into the Main Drain Tank.
• Capacity: Very low (e.g., $1~m^{3}/hr$).

Submersible Pump

• Purpose: To transfer oil from the Low Point Drain Tank up to the Main Drain Tank.
• Design Temperature: Max operating temperature + $30°C$ margin.

Hot Oil Selection Criteria

The first step is to identify the temperature requirements of the process. The hot oil's maximum operating temperature must be higher than the highest temperature required by any user.

Hot oil fluids generally fall into three categories:

1. Mineral Oils: Derived from natural products; suitable for liquid phase only.
2. Synthetic Aromatics: Synthetically produced; suitable for liquid phase only.
3. Uniform Mixtures: (e.g., Diphenyl/Diphenyl Oxide) Can be used in both liquid and vapour phases.

Key Fluid Properties

• Thermal Stability: The fluid's resistance to breaking down at high temperatures. Decomposition creates "low boilers" (which must be vented) and "high boilers" (which increase viscosity).
• Vapour Pressure: This property determines the operating pressure of the system. If the fluid's vapour pressure at the operating temperature is above atmospheric, a pressurized system is required.
• Oxidation Resistance: Oxidation (from contact with air) can form organic acids and increase viscosity. This is why nitrogen blanketing on the expansion tank is critical.
• Pumpability: The fluid's viscosity at ambient temperature must be low enough to be pumpable at startup. A highly viscous fluid may require expensive heat tracing on all piping.
• Corrosion: The fluid and its decomposition products should be non-corrosive to common construction materials like carbon steel. Fluids containing chlorine should be avoided.
• Cost: The final selection is often a trade-off between thermal properties and the economic cost of the fluid.

Instrumentation & Control

Flow Control (Primary Circuit)

Temperature Control (Secondary Circuit)

For users requiring a lower temperature, a secondary loop is created:

1. A secondary pump circulates oil for that user.
2. A temperature controller on the secondary pump's discharge measures the oil temperature.
3. This controller operates a control valve that injects hot oil from the primary loop into the suction of the secondary pump, mixing it with the cooler return oil to achieve the desired temperature setpoint.

Venting & Relief

• Expansion Tank Breather Valve: The vent (out-breathing) should be routed to a drain tank or, if not possible, locally to a safe location via a flame arrestor.
• Low Point Drain Tank Venting: This tank should either be vented locally (via a flame arrestor) or equalized with the Main Drain Tank's vent system.
• Liquid Safety Valves: All thermal relief valves on liquid lines must have their discharge routed to one of the drain tanks.

System Design Features

Hazard Management

Piping System Design

• Thermal Expansion: Piping design must account for significant thermal expansion.
• Vent Lines: Must be designed with no pockets and minimal bends.
• Materials: Carbon steel is acceptable (e.g., A106 Grade B).
• Valves: Globe valves are preferred for control. Bellow-sealed valves are recommended to prevent leaks.

Insulation

• Hot Insulation (for energy conservation):
  • All hot oil circulation lines.
  • Both Main and Low Point Drain Tanks.
  • Hot oil pumps.
• Personal Protection Insulation (for safety):
  • The Expansion Tank (as it only sees high temperatures for short periods).
  • Discharge lines from safety valves (as they are not normally hot).