Steam leakage is limited even with an open orifice.

Following are the 3 main reasons.

There is an enormous difference in specific volume between steam and condensate.

Steam flows more than 30 times faster than condensate.

When condensate diminishes, steam pressure diminishes as well.

 There is an enormous difference in specific volume between steam and condensate.

The volume increases by 1600 times when water transforms into steam at an atmospheric pressure.

Let's take an example of an orifice with an inlet pressure of 0.1Mpa.G.

The specific volumes at 0.1Mpa.G are:

　Steam           ：　0.9018　  /kg
　Condensate：　0.00106　/kg　
 

The volume of steam is approximately 850 times that of condensate.

Therefore, if the flow speed of steam were 850 times faster, the flow rate would be the same for steam and condensate. However, the flow speed of steam is only 32 times that of condensate. So, the flow rate of steam is only 3.7 % of the flow rate of condensate.
　
　 32 ÷ 850× 100 ＝ 3.7 ％

This figure is confirmed by the calculation formula and laboratory tests.　 

 Steam flows more than 30 times faster than condensate.

Normally,

   Flow speed of steam           : 30～40 m/sec
    Flow speed of condensate : 1～1.5 m/sec

Please imagine a typhoon on the coast with a wind of more than 30 m/sec.
The friction between the wind and the surface of the sea produces waves, which go beyond the break water.
　
In much the same way, when steam and condensate flow together, waves are produced upstream of the orifice, and these waves seal the orifice, preventing steam to pass through. As a result, even when the condensate load diminishes, there is almost no steam leakage.

Here are the images of this phenomenon with different condensate loads.

　 1.When the condensate load is 100 %

　 2.When the condensate load is 50 %

　 3.When the condensate load is lower than 50 %

 When the condensate flow rate diminishes, the steam pressure diminishes as well.

With steam pipes, the quantity of condensate changes depending upon the outside temperature, while the steam pressure remains constant. However, with most heat exchangers, when the condensate flow rate changes, the steam pressure changes as well.

The condensate flow rate diminishes when the heat requirement is smaller, and when the heat requirement is smaller, the steam valve is closed, manually or automatically, to reduce the steam flow, resulting in a lower steam pressure in the heat exchanger..

As the pressure downstream remains constant, the differential pressure becomes smaller, and the evacuation capacity of the orifice becomes smaller as well.

In other words, even when the condensate flow rate diminishes, the condensate load in relation to the capacity of the orifice does not change much.

In practice, with normal heat exchangers, even when the condensate flow rate diminishes, the steam leakage does not increase.

This is an explanation based on a Lotus Z used for the reboiler of a column of distillation.

The steam pressure (P) and the counter-pressure (P2) are constant.

When the load of the column increases, requiring more heat, the control valve (CV) opens to feed more steam. The pressure (P1) increases and also the generated condensate increases.

The differential pressure, which applies to the orifice （ΔP）=（P1）－（P2） increases as well, and the orifice evacuates more condensate.

When the load decreases, the opposite happens, and the orifice evacuates condensate accordingly without steam leakage, allowing a stable operation.

Lotus Z adapts instantly to the load variations. The start-up is smoother and the temperature control becomes more precise.