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Crude oil treatment

Crude oil heaters

A crude oil heater raises crude to the temperature at which water can be separated from it. In oilfield treatment this is usually an indirect fired heater: a burner heats a liquid bath, and the crude flows through a coil immersed in the bath, so no flame touches it. Axsia Howmar designs heaters as part of the treatment train.

Typical process outlet temperature for upstream treating
40 to 90 °C
Burner heats a bath; the bath heats the process coil. No flame touches the crude
Indirect fired
Combustion air by stack draught, or by fan for higher duty and tighter control
Natural or forced draught

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How it works: Crude oil heaters

  1. A burner fires into a fire tube

    Fuel gas burns in a U-shaped fire tube immersed in the lower part of a bath vessel. Combustion gases leave through a stack.

  2. The fire tube heats the bath

    The bath, usually water or a water-glycol mixture at atmospheric pressure, circulates by natural convection and holds a uniform temperature.

  3. The bath heats the process coil

    Crude flows through a pressure-rated coil in the upper part of the bath and is heated through the coil wall.

  4. Controls hold the outlet temperature

    Burner firing follows bath or process outlet temperature. A burner management system supervises ignition, flame and shutdown.

Designed in 3D

Every heater is modelled before it is built

Each heater package is designed as a complete 3D model: bath vessel, fire tubes, stacks, process coil, burners, fuel gas train, instrumentation, piping and skid. The model is the single reference for the whole package, so the process, mechanical, piping and electrical designs are checked against each other before anything is fabricated.

The model is used to confirm access and clearances for burner and fire tube withdrawal, to route piping and cable trays, to check the lifting arrangement and skid weights, and to produce the fabrication drawings and the material take-off. Clients review the model at the design review, so what arrives on site is what was agreed.

3D model of an indirect fired heater package: bath vessel with twin fire tubes and stacks, forced draught burner fans, expansion tank, fuel gas train and interconnecting piping on one skid.
3D model of an indirect fired heater package: bath vessel with twin fire tubes and stacks, forced draught burner fans, expansion tank, fuel gas train and interconnecting piping on one skid.

Natural or forced draught

Choosing the draught: natural or forced

A natural draught heater draws its combustion air by the buoyancy of the hot flue gas in the stack. It has no fan, no motor and no air-side controls, so it is simple, quiet and suited to remote sites with limited power. Its limits are the draught available, which sets the fire tube heat release, and its sensitivity to wind and ambient temperature.

A forced draught heater supplies combustion air with a fan and regulates it against fuel gas flow. That allows a higher heat release in a given fire tube, tighter control of excess air and so better fuel efficiency, stable combustion in high wind, and premixed low-NOx burners where emissions limits apply. It costs a fan, a motor and a more involved burner management system.

Axsia Howmar designs both. The choice is made in process design from the duty, the fuel, the site power available, the emissions requirement and the turndown the operation needs.

Installed and operating

Proven in service: heaters we have designed and commissioned

Heaters are delivered as skid-mounted packages with the burner, fuel gas train, burner management system, instrumentation and local junction boxes fitted and tested before despatch, so site work is limited to setting, tie-in and commissioning.

Axsia Howmar engineers have designed and commissioned indirect fired heaters for oilfield treatment in the Middle East and other regions, including packages that operate at high ambient temperature, in dust and on limited site power. We support commissioning, start-up and operator training, and we review and troubleshoot heaters built by others.

Skid-mounted indirect fired heater package installed and operating: bath vessel with stacks, forced draught fans, fuel gas train and local junction boxes.
Skid-mounted indirect fired heater package installed and operating: bath vessel with stacks, forced draught fans, fuel gas train and local junction boxes.

Where a crude oil heater sits in the process

A crude oil heater sits between the last separator and the first electrostatic coalescer. Where heat can be recovered from treated crude or from another stream, a heat exchanger ahead of the heater reduces the fired duty. The crude oil treatment overview shows the heater in the train.

Design and selection considerations for crude oil heaters

The design of a crude oil heater is governed by duty, by the pressure drop available through the coil, and by the fuel. Points that decide the design are:

  • Duty over field life. Water cut raises duty sharply, so the design case should be the worst combination of flow, water cut and inlet temperature, not the first-year case.
  • Coil velocity and pressure drop. Velocity must be high enough to limit fouling and low enough for the pressure drop available. Multiphase flow in the coil changes both.
  • Fire tube heat flux. Flux is limited to protect the fire tube and the bath. It sets fire tube area and therefore vessel size.
  • Bath liquid. Water suits most duties. Glycol mixtures give freeze protection.
  • Fuel gas quality. Wet or sour fuel gas needs conditioning ahead of the burner.
  • Hazardous area and emissions requirements. These decide the burner type and the burner management system.

Materials used in crude oil heaters

Crude oil heater bath vessels and fire tubes are normally carbon steel. Process coils are carbon steel or low-alloy steel to the line specification, with sour service materials where hydrogen sulphide is present.

Common operating problems with crude oil heaters

The common operating problems with crude oil heaters are failure to reach outlet temperature, fire tube failure, and burner trips. Failure to reach temperature is usually a duty problem: water cut or flow above the design case, or a fouled coil. Fire tube failures follow scale on the bath side, low bath level or flame impingement. Burner trips usually trace to fuel gas pressure or liquids in the fuel gas. A heater that cannot hold temperature shows up downstream as rising BS&W, which is why we ask for temperature trends when troubleshooting a coalescer.

Capacity enhancement and revamp options for crude oil heaters

A crude oil heater that has become the limit on a treatment train can be relieved by recovering heat from treated crude, cleaning or replacing the process coil, re-rating the burner within fire tube flux limits, or adding a parallel unit. A design review checks the heater together with the coalescer, since lowering the required treating temperature is sometimes cheaper than adding duty.

Design parameters: Crude oil heaters

Design parameters: Crude oil heaters
ParameterTypical rangeUnitBasis / note
Process outlet temperature40 to 90°CTypical published range for upstream treating; set by the downstream coalescer duty
Bath temperatureBelow the boiling point of the bath liquid°CAtmospheric water or water-glycol bath

Typical ranges for orientation only. The design basis for each unit is set from the feed analysis and the required product specification.

Codes and standards applied to Crude oil heaters

Typically applied; the governing codes are confirmed per project with the client and the jurisdiction.

API 560
Fired heaters for general refinery service Where a direct fired heater is specified
ASME BPVC Section VIII Division 1
Pressure vessels Process coil and pressure parts, where applicable
IEC 60079 / ATEX / IECEx
Equipment for explosive atmospheres Burner management system, instruments
NACE MR0175 / ISO 15156
Sour service materials Process coil where hydrogen sulphide is present

Applications: Crude oil heaters

  • Upstream of a dehydrator or desalter: Bringing crude to treating temperature ahead of an electrostatic coalescer.
  • Heavy and waxy crude: Lowering viscosity or keeping crude above its wax appearance temperature for treating and pumping.
  • Cold climates and winter operation: Restoring treating temperature where ambient conditions pull the train below design.

Scope of supply: Crude oil heaters

  • Thermal and hydraulic design (Duty, coil sizing, pressure drop, fire tube heat flux)
  • Bath vessel, fire tube, process coil, stack
  • Burner, fuel gas train and burner management system
  • Instrumentation, control panel and skid
  • Commissioning and operator training

  • Electrostatic coalescers

    An electrostatic coalescer is a pressure vessel that uses a high-voltage electric field to merge small water droplets in crude oil into drops large enough to…

  • Crude oil dehydrators

    A crude oil dehydrator is an electrostatic coalescer used to remove produced water from crude so that it meets its basic sediment and water (BS&W) specification.…

  • Crude oil treatment

    A crude oil treatment system removes water, salt and sediment so that crude meets its sales or refinery specification. It works as a train: separation, heating,…

  • Troubleshooting and performance diagnostics

    Troubleshooting a desalter, dehydrator or hydrogen pressure swing adsorption (PSA) unit starts with the symptom, because each symptom has a short list of probable causes and…

  • Process and detailed design

    Process and detailed design turns a requirement, often no more than a datasheet, into a complete package that a fabricator can build and an operator can…

  • Design review for capacity enhancement

    A design review for capacity enhancement establishes what actually limits an existing crude oil treatment train or hydrogen purification unit, and which changes remove that limit…

Indirect fired heater
An indirect fired heater warms a process fluid without exposing it to flame. A burner heats a bath of water or other liquid through a fire tube, and the process fluid flows through a coil immersed in the same bath.
Emulsion (water-in-oil)
A water-in-oil emulsion is a dispersion of small water droplets in crude oil, held apart by natural surface-active films of asphaltenes, resins, waxes and fine solids. The droplets will not settle until those films are broken and the droplets merge.
Electrostatic coalescer
An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.
Crude oil dehydrator
A crude oil dehydrator is an electrostatic coalescer that removes produced water from crude oil to meet a water specification, usually stated as basic sediment and water. No wash water is added. It is the same equipment as a desalter.
Demulsifier
A demulsifier is a surface-active chemical injected into crude oil at parts-per-million levels to displace the natural films that stabilise water droplets. With the films weakened, droplets can merge and settle. It works together with heat, time and electrostatic fields.
Interface level
Interface level is the height of the boundary between settled water and the oil above it in a separator, dehydrator or desalter. Controlled by the water outlet valve, it must sit below the electrodes and above the water outlet.

Frequently asked questions: Crude oil heaters

Why heat crude oil before dehydration or desalting?

Heating crude oil lowers its viscosity, which is the main resistance to water droplets settling. For most crudes it also widens the density difference between oil and water, and it weakens the surfactant film that stops droplets merging. The result is faster, more complete separation in the electrostatic coalescer downstream.

What is the difference between a direct and an indirect fired heater?

In a direct fired heater, flame and hot combustion gas heat the process tubes directly. In an indirect fired heater the burner heats an intermediate bath, and the bath heats the process coil. The indirect design limits the coil wall temperature to the bath temperature, which reduces coking and the consequence of a coil leak.

How is the duty of a crude oil heater calculated?

The duty of a crude oil heater is the mass flow of each phase multiplied by its specific heat and the temperature rise, summed for oil and water. Water has about twice the specific heat of crude, so the water cut has a large effect: a heater sized for dry crude falls short as water cut rises.

What treating temperature should a crude oil heater deliver?

A crude oil heater should deliver the lowest temperature at which the downstream coalescer meets specification with a sensible demulsifier dose. Going hotter costs fuel, drives light ends into the gas phase and lowers crude volume and API gravity. The temperature is fixed during process design from viscosity data and bottle tests.

Talk to an engineer about your unit

Tell us the equipment, the duty and what you are seeing. Partial data is normal: send what you have and we will tell you what else matters. We acknowledge every enquiry within 1 working day.

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