Pumps and lines for viscous products are sized for the highest viscosity the system will face, not for the nominal operating point. For heavy oil and fuel oil that usually means the lowest product temperature: the first start on a cold morning, a restart after a long shutdown, or a fault that takes part of the heating out of service. If pump type, suction line, pipe diameter and motor power are not checked against that worst case, a system that runs smoothly day to day can fall short exactly when it matters.
This page covers the sizing logic specific to heavy oil and fuel oil transfer. For keeping the line hot, see heat tracing systems; for issues specific to filling tank trucks, see heavy oil truck loading systems.
Start with viscosity–temperature data
The kinematic viscosity of petroleum products changes quickly with temperature. Viscosity measured at two temperatures allows the value at other temperatures, within a limited range, to be estimated with standard methods, so ask the laboratory for at least two data points rather than a single figure. Residual fuel oil grades usually need preheating for handling and atomization, and a change of grade shifts both the target transfer temperature and the design viscosity.
Pour point is a supporting limit in this exercise, not a design temperature. In waxy products, flow behavior can depend on thermal history, so the same product may behave as a liquid under one set of conditions and as a solid under another. That uncertainty is a good reason to study the cold-start case separately.
Choosing the pump type
Centrifugal pumps lose efficiency markedly as viscosity rises: head and flow fall, input power grows, and the required NPSH and starting torque increase. The performance of a centrifugal pump tested on water therefore has to be corrected for the viscous liquid. In rotary positive displacement pumps, higher viscosity reduces internal slip and raises volumetric efficiency, and these pumps handle viscosities far beyond the reach of centrifugal designs. That is why rotary positive displacement types such as gear, screw and lobe pumps are a common choice for heavy oil and fuel oil transfer.
Criterion | Centrifugal pump | Rotary positive displacement pump |
Flow as viscosity rises | Drops significantly; the curve must be corrected | Largely maintained at constant speed; slip decreases |
Power demand | Rises because of lower efficiency | Rises with friction losses; the motor is sized for maximum viscosity |
Suction requirement | Required NPSH increases | Required net inlet pressure rises with viscosity; speed may be reduced for very viscous service |
Flow control | Valve or speed | Speed; overpressure protection against a closed discharge is part of the design |
Typical role | Low-viscosity, high-flow transfer | Heated heavy oil, fuel oil and similar viscous products |
The suction side is where most problems begin
Net positive suction head available (NPSHA) shows how far the absolute pressure at the pump inlet sits above the product's vapor pressure at operating temperature. With viscous products, friction in the suction line eats into that margin quickly, while the net inlet pressure a positive displacement pump requires also rises with viscosity. The result is a risk of cavitation, incomplete filling and unstable running.
· Short, generous suction line: Placing the pump close to the tank and making the suction larger than the discharge reduces losses.
· Flooded suction: Locating the pump below the product level so the tank feeds it by gravity improves inlet conditions.
· Heated suction piping: Losing just a few degrees on the suction side can raise viscosity noticeably, so tracing this section must not be overlooked.
· Suction strainer: Protects the pump from debris, but its pressure drop climbs quickly with viscous product; a differential pressure gauge and an easy-access basket help.
Line size and pressure drop
Viscous products usually flow in the laminar regime, where pressure drop rises almost in proportion to viscosity. A larger line reduces that loss, but every size step also increases the surface to be heated, the product volume left in the line and the effort needed to clear it. The right diameter comes from balancing pump pressure, tracing output and hold-up volume. Instruments belong in the calculation too: pressure drop across positive displacement and Coriolis meters increases with viscosity, and if it is ignored during pump selection the meter can become the system bottleneck. Meter selection for heavy oil is covered on the heavy oil metering skids page.
Clearing, cleaning and restarting the line
Once a transfer ends, the product left in the line cools and sets up the hardest condition for the next start. Several strategies are available, and the right one depends on how the site operates:
· Continuous circulation: The line is designed as a loop back to the tank, so product stays hot, at the cost of ongoing pumping and heating energy.
· Holding with tracing: The line stays full and heat tracing keeps the product at maintain temperature.
· Draining: The line is sloped and emptied into the tank or a drain vessel; dead legs and low points are the weak spots of this approach.
· Blowing or pigging: On long lines the product is pushed out; where a flammable atmosphere may form, inert gas is required instead of compressed air.
· Flushing with a lighter product: Used for grade changes or before long shutdowns; where the flush product ends up must be planned in advance.
Design inputs
Parameter | Why it matters in design | Effect on selection |
Viscosity at the lowest product temperature | Pump, motor and line losses are checked against this value | Pump type, speed and motor power |
Normal and maximum transfer rate | Pressure drop increases with flow | Line size and pump size |
Tank location and level range | Sets the suction condition | Pump location, suction size and flooded suction option |
Line length and elevation changes | Set total pressure demand and drainability | Pump pressure, draining and cleaning method |
Meters, strainers and valves in the line | Each adds pressure drop with viscous product | Pump pressure and equipment sizing |
Heating and insulation status | Shows whether transfer temperature can be held | Tracing needs and design viscosity |
Shutdown and restart scenario | Start-up is often the toughest operating case | Circulation, draining or cleaning decision |
Working with TLY Enerji
On viscous product transfer projects, TLY Enerji can provide engineering support covering evaluation of process data, selection and supply of pumps, valves and metering equipment, integration of pump and meter control into PLC-based automation, and site installation, testing and commissioning. Responsibility for hydraulic calculations, heat tracing design and mechanical installation is agreed on each project against the technical specification.
Data needed for sizing
· Product grade, density and viscosity measured at two or more temperatures
· Product temperature in the tank and the lowest temperature the line may reach
· Required transfer rate and daily or batch transfer quantities
· Tank and pump locations and the lowest operating level
· Line routing with lengths, elevation changes and existing diameters
· List of meters, strainers and valves in the line
· Existing heating and insulation, and available utilities
· Whether the line stays full after transfer, and how often grades change
Related pages
· Heavy oil truck loading systems: Keeping product fluid while loading trucks from heated tanks.
· Heat tracing systems: Selecting a method to keep transfer lines at maintain temperature.
· Heavy oil metering skids: How viscosity affects meter technology and pressure drop.
· High-viscosity bitumen transfer systems: Jacketed pumps and lines for even hotter service.
Frequently asked questions
Can a centrifugal pump be used for heavy oil?
It can work in some high-flow duties if the product is heated enough to keep viscosity low. However, centrifugal performance falls quickly as viscosity rises, and the pump may not deliver the required flow during a cold start. Evaluate it using viscosity-corrected performance and the worst-case temperature scenario, not the water curve.
Why does a positive displacement pump need a relief valve or pressure protection?
A positive displacement pump pushes a fixed volume into the discharge with every revolution. If a discharge valve is left closed or the line cools and blocks, pressure can keep rising until the pump or piping design limit is exceeded. Pressure relief on the pump or in the system, together with suitable pressure monitoring, is therefore treated as a standard part of the design.
Is a larger line always better?
No. A larger diameter reduces pressure drop and pump power, but it increases the surface to be heated, the volume of product left in the line and the effort needed to clear it. On sites with frequent grade changes, that hold-up volume can also become a commercial and operational problem. The right size balances all of these factors.
Why does the pump struggle on a cold start?
Product left in the line during a shutdown thickens as it cools. At start-up the pump has to draw that heavy product through the suction side and push it along the discharge line, so power and starting torque can far exceed normal operation. Preheating with heat tracing, a gradual low-speed start or draining the line before shutdown all reduce the load.