Optimization of the pumping of viscous liquids from railway tank cars

There is nothing unusual about the pumps used for unloading railway tank cars. Unloading wagons loaded with liquid tar, on the other hand, is less common. Due to its high viscosity, the raw material is difficult to handle. To optimize the unloading process, an end user who needed to unload raw tar from railcars was looking for a pump that could meet the following criteria:

  • complete emptying of residues without pressurizing the wagons
  • meeting the highest security levels
  • the unloading process carried out with a single pump for economic reasons

IMAGE 1: Self-regulating pump pressure flow (Images courtesy of Bungartz)

In practice, this meant that four wagons containing raw tar had to be emptied with a single pump. Typically, such requirements are difficult to implement. An important criterion was the safety of dry running, which is usually not ensured by standard pumps.

Standard pumps are vented through an empty discharge line when stopped. The level of the liquid decreases as the pumping process progresses, more and more gas being entrained. Gas bubbles form, reducing performance and causing erratic operation. If the gas content is too high while the pump is running, the flow may stop completely. This stops the conveying process and the pump must be purged, resulting in downtime. Today, unplanned downtime is usually flagged by diagnostics and early warning systems.

The solution

For a manufacturer of centrifugal pumps, this was one of many challenges. The engineering team and the pump manufacturer provided advice and recommendations (image 1), from pump design to commissioning and beyond. Dry-running pumps with and without fluid-matched self-regulating pumping and sealing systems were available, and in this solution the vertical pump was fitted with a magnetic coupling and liquid-lubricated plain bearings.

This design combined the properties of vertical, self-regulating pumps with those of the horizontal, dry-running, hermetically sealed pump. The pump runs permanently dry and operates continuously in the pumped liquid. Grease lubricated roller bearings are protected against product vapors by a gas barrier.

The shaft air gap sealing concept of this type of permanently dry-running pump consists of several components. This is achieved by rear vanes and the gas barrier, which prevent product vapors from entering the bearing unit. The eddy current-free magnetic coupling hermetically seals the pump from the environment. Due to the vertical orientation of the pump, the bearing and sealing unit operates completely without contact with the product, even in the event of a sealing gas failure. Roller bearings are typically in service for more than five years of continuous operation.

pressure flow
IMAGE 2: Normal priming pump pressure flow

In addition to economic and technical benefits, the solution offered maximum safety in the event of leakage, ease of use and savings in construction costs. Intrinsically Safe Centrifugal Pumps—pumps that, based on specific design principles that prevent hazardous conditions in the event of a malfunction—automatically adjust to varying feed rates.

The principle is based on balancing the pressure between the pump impeller and the supply container: when a fluid flows into a container, the liquid level rises until the inlet and the outlet of the container are in equilibrium. These control features operate without additional mechanical or electrical devices. These pumps deliver according to the flow and automatically reduce the flow without any pressure reduction at the impeller.

As a result, the net positive suction head (NPSH) is close to zero. This type of pump also has a self-venting function, which makes it insensitive to gas bubbles. The pumps generally operate without cavitation, can be run dry safely, and operate reliably.

unloading a tank car
IMAGE 3: Functional model of a tank car unloading: The glass tank car is unloaded quickly and completely with a self-regulating pump.

The low pulsation pump keeps the product and bearing area separate, making it suitable for almost any fluid. Application areas range from gas-laden, viscous, hot or boiling liquids to liquids containing solids. Typical fluids include toxic liquids with special shaft seal requirements: titanium tetrachloride, liquid tar, or liquid gas. The pump can withstand fluid temperatures up to 400 C (752 F) without coolant. Self-regulating transport behavior does not require a minimum flow. Cavitation problems do not occur; the pump has a required NPSH value (NPSHr) < 0.1 meter (m). Lifespans can exceed 15 years.

In addition, costly planning and construction measures such as the construction of pits are not necessary. This makes this type of pump, which is installed at ground level, easier to install, operate and monitor. All four cars are emptied from below and the supply lines to the cars must be open. On start-up, the pump descends to the level of the pipe gauge. The liquid flows from the tank cars under the effect of gravity. The pump is only stopped when the minimum level in the pipe gauge is reached and therefore all wagons have been emptied.

Definition of dry-running safety

Many pumps use the fluid to be pumped to sufficiently lubricate the plain bearings and cool the seal used. Dry-running safety is the ability of a pump to operate permanently without fluid. This is achieved by decoupling the bearing and sealing unit from the pumped liquid.

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