Seal Barrier Fluid Systems

Why buffering systems?

All rotating pumps have a drive shaft, which must be sealed in one way or other. The seal can be in the form of a magnetic coupling, a stuffing box, a viscoseal with reverse conveyance thread, simple or double action rotation mechanical seal, multiple lip seals or even a combination of these methods.

With certain batch processes (e.g. Polyester production), the shaft seal must be able to cope with extremely difficult conditions: on the one hand, temperature and speed of the pump and the viscosity of the medium alter during a batch and on the other hand, the shaft of the pump must be sealed against a high vacuum at the inlet side. Under such conditions, the only effective solution is a dual action reverse blocked sealing systems; either a double stuffing box or a double rotating mechanical seal in a “back to back” or “face to face” configuration.

Both systems must be provided with a buffering medium. This must be compatible to the product being conveyed (in this example Polyester) and resistant to temperature variations due to the fact that Polyester is usually run at temperatures of between 280°C and 300°C (553-572 °F).

The two methods of sealing differ as follows:

I) is a dynamic buffering system
II) is a static buffering system

Both systems include a magnetically coupled gear pump, which ensures forced circulation of the buffering medium operating temperature can be up to 350°C (662°F).

1 Manual Replenishment pump
2 Seal Pot
3 Level Switch
4 Sight Glass
5 Heating/ Cooling Element
6 Magnetically Driven Gear Pump
7 Safety Valve
8 Polymer Discharge Pump

System I (the dynamic system) is employed, when the pressure on the suction side varies over a comparatively wide range. This is measure by a pressure sensor (10) which regulates the control valve (12), at the output of the rotating mechanical seal (9), via the control electronics (11). Depending upon the position of the control valve (12) the gear pump pumps the medium against the more or less open control valve (12). A buffering pressure is built up, which is always above the suction pressure of the Polymer pump (8). This differential pressure is adjustable. With this system, the dynamic buffering pressure is generated by the gear pump (6). The pipelines under buffering pressure are marked with an “A”, whilst those under atmospheric pressure are marked with a “B”. The buffering medium tank (2) is in this case not pressurised.

9 Double Mechanical Seal
10 Pressure Sensor
11 Control Electronics
12 Control Valves
13 Nitrogen Connection
A = Process Pipe under buffering Pressure
B = Process Pipe under atmospheric Pressure

System II (the static system) is employed when the pressure on the suction side of the pump (8) remains comparatively constant. In this case, the buffering medium tan (2) is pressurized, via the connection (13), using nitrogen. The buffering pressure is generated by the nitrogen. The gear pump (6) has only to combat pipeline losses. In this case, the complete system, including the buffering medium tank (2) is under buffering pressure from the nitrogen. The manual topping-up pump (1) can be used to raise the level in the buffering medium tank (2) against the buffering pressure, when the level switch (3) indicates that the level is too low.

The WITTE metering system (WDS) enable precise volumetric metering of low to high viscosity media with gear pumps, even at high temeratures and pressures. Two series connected gear pumps, one measuring and one pressure boosting pump are precisely controlled ba a PLC control system, such that a precision of better than 0.01% is achieved, depending upon the medium.