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How Venturi Orifice Works​

The ECOFLOW Venturi orifice steam trap, from EBE Engineering, is the most advanced Venturi orifice steam trap on the market today.

The fully flanged cast housing was designed, developed and refined using 3D modelling and casting simulation software. From these designs we have produced a Venturi steam trap to meet the rigorous requirements of today’s steam plant users.

Developed on a modular concept, the ECOFLOW steam trap encompasses a wide range of capacities, from minimal condensate flow requirements on line drainage and trace heating systems, through to the high flow volumes and variable loads of process heating applications.

Prior to explaining the principle of operation of the Venturi orifice steam trap, it is important to understand two fundamental differences compared to conventional mechanical traps.

  • Condensate from the Venturi steam trap is discharged continuously.
  • There is no physical seal in the trap as it has no mechanical action.

Start Up 1

When a valve is opened in a steam system, steam flows into the pipework or heat exchanging equipment. The velocity of steam in the pipework varies based on the steam pressure and saturation level. Higher pressure and temperature, such as in the case of superheated steam, result in increased steam velocity. Typically, steam velocities in medium-pressure applications range between 25 to 40 m/s.

During the initial start-up phase, air and non-condensable gases are efficiently vented through the Venturi orifice trap nozzle. It’s important to note that, as there is no seal in the trap, these gases are easily released.

Start Up 2

As the Venturi orifice steam trap expels all air and non-condensable gases, saturated steam moves towards the nozzle. In a steam system, energy losses manifest as condensate. When the valve is opened, hot steam swiftly enters the pipework or heat-exchanging equipment. The temperature disparity between the hot steam and the cooler walls of the pipework or heat exchanger results in the steam losing heat energy. Consequently, this diminishes the steam’s energy content (enthalpy), causing it to transition towards a more saturated (wet) state.

Start Up 3

As the steam progresses toward the nozzle, it gradually becomes more saturated, causing condensate to form on all exposed cooler surfaces. Typically, a steam trap is positioned at the lowest point in the steam system, facilitating the drainage of condensate towards this location. Once the steam reaches the nozzle, it initiates the ejection of condensate through the orifice.

At both the entry and exit of the orifice, there exists a pressure differential. This is the juncture where the energy system (represented by the steam) intersects with the waste system (represented by the condensate). Notably, there is an approximately 1000-fold difference in density between steam and condensate. This substantial density contrast effectively prevents the steam phase from entering the orifice. Additionally, the high-density condensate moves at a slower pace, typically around 10 – 15 m/s.

Running Load 4

The ‘start-up’ condition persists until the system achieves a steady state. At this juncture, all components of the pipework or heat-exchanging equipment have stabilized at a consistent temperature. No additional energy is needed to elevate their temperature beyond the ambient level. Consequently, heat losses in the system are now solely attributed to the heat required by the process.

In practical terms, this signifies that the quantity of condensate produced significantly diminishes compared to the start-up phase. The orifice remains saturated with condensate, but the steam phase begins to approach it. As the steam approaches the orifice, the temperature of the condensate increases.

Running Load 5

A closer examination of the nozzle reveals the flow of condensate through the orifice into the Venturi sector. In the operational state, the condensate passing through the orifice closely interfaces with the steam phase. Consequently, the temperature of the condensate is nearly identical to that of the steam. Given its high temperature, the condensate harbors significant energy content, attempting to dissipate. However, being under pressure from the steam and constrained in the orifice, it is unable to do so.

Upon exiting the orifice, the energy contained in the condensate is discharged, manifesting as ‘flash steam.’ The quantity of ‘flash steam’ generated is directly proportional to the pressure differential across the orifice. With a 1-bar differential pressure, approximately four percent of the condensate converts to ‘flash steam,’ while at 10 bar, this percentage increases to sixteen percent.

Running Load 6

The orifice expanded in detail.

  1. Condensate flowing through orifice at 100% load. At start up when the condensate is cool, the orifice can accommodate two to three times the running load. 
  2. As the condensate load reduces, the hot steam mixes with the hot condensate.
  3. The steam element of the condensate increases as the load decreases. This bi-phase mixing is turbulent but contained within the orifice.
  4. A further reduction in the operating load introduces more steam and the condensate begins to laminate on the orifice walls. It is still contained within the orifice, as condensate blocks the entire diameter before the discharge in the condensate line.
  5. At this stage there is more turbulent bi-phase activity in the orifice than laminar flowing condensate. The condensate at the discharge is now at the hottest temperature and contains the most energy. This is where ‘flash steam’ pressure is at its maximum and comparable to the main steam pressure. Due to the geometry of the orifice, a localised back pressure is generated which reduces the volume of condensate that discharges. The condensate volumes increase and the orifice ‘reverses’ through steps 4, 3 and 2. Somewhere between steps 2 and 3 the temperature gradient in the orifice results in the ‘flash steam’ pressure at the discharge reduction. In practice the Venturi orifice continuously cycles between conditions 1 to 5 and back again.
  6. *If the orifice is oversized, the volume of condensate at the discharge will not block the entire diameter and system steam will begin to leak into the condensate line.
  7. *The leakage of system steam through the orifice will increase as the condensate volumes decrease.

*These conditions are typical for a simple orifice plate device but not for venturi orifice steam traps.


Continuous Operations 7

As mentioned earlier, Venturi orifice steam traps maintain a continuous discharge of condensate. The discharge from the orifice oscillates between steady condensate drainage, akin to water from a tap, and ‘flash steam,’ which diminishes the velocity of the discharging condensate, essentially ‘blocking’ its release. This self-regulating process is responsive, and if the system is controlled by a modulating valve, the ensuing ‘flash steam’ pressure will align with the modulated main steam pressure.

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