Petroleum bitumen, normally called ‘bitumen’ or ‘asphalt’, is a very viscous, black or dark brown material that is widely known for its use as a binder in road-building products. Less well-known is how bitumen is produced from crude oil. During the refining process, the crude oil is pumped from storage tanks, where it is kept at about +60°C, through a heat exchanger system where its temperature is increased to typically +200°C.
Throughout the world, the use of thermal oil is becoming an increasingly popular way to heat up the crude oil. This thermal oil passes through a system of coils integrated into the tank. Double eccentric valves with PTFE sealing have a good track record in such applications.
The crude oil is then further heated in a furnace to typically +300°C where it is partly vaporized into an Atmospheric Distillation Column. Given the more demanding conditions, many companies select triple offset butterfly valves with metalgraphite seals for this part of the process.
It is here that the physical separation of the components occurs. The lighter components rise to the top and the heaviest components (the atmospheric residue) fall to the bottom of the column and pass through a second heat exchanger prior to treatment in a vacuum distillation column. Finally, bitumen is obtained by vacuum distillation or vacuum flashing of atmospheric residue from the vacuum distillation column. This is called ‘straight run bitumen’. This process is called bitumen production by straight run vacuum distillation. On these lines, double eccentric valves and triple offset valves have demonstrated their worth.
An important point to note, however, is the high viscosity of this medium in combination with pipeline diameter limitations (usually DN 80, 100, 125). Hence valves are often selected that allow higher Kv and Cv flow values. This can be achieved by selecting valves that have a two piece shaft design for bigger cross-sectional area.
Further valve selection considerations
There are some other points that can be taken into consideration when choosing the right valves with the best properties. These include:
- floating seat design, which allows the thermal dilatation of components (expansion or shrinkage) as the temperature rises or falls. This helps to assure the right seat positioning
- Fire-safe design (R-PTFE + Inconel seat) optional
- Two piece shaft over single piece shaft with 2 pin design produces a better torque transfer and firm support between the stem and the disc
- Flexible seat design which can provide bidirectional tightness (optional)
- TA-Luft Certified stuffing box
Sources: ABO valve and bitumina.co.uk