**Impulse Steam Turbine**

Impulse steam turbines are a type of steam turbine that operate on the principle of impulse and reaction. In these turbines, the steam expands through a series of nozzles, where it gains high velocity, and then impacts the moving blades of the rotor. The high velocity of the steam imparts a high impulse force on the blades, causing them to rotate.

**Blade Passages**

1. **Constant Cross-Section:** In impulse steam turbines, the blade passages are designed to have a constant cross-section. This means that the shape and size of the passages remain the same throughout the turbine. The constant cross-section is important to ensure a uniform flow of steam through the turbine and to maintain the efficiency of the turbine. It also allows for easier manufacturing and maintenance of the blades.

**Partial Admission of Steam**

2. **Permissible Partial Admission:** In impulse steam turbines, partial admission of steam is permissible. This means that only a fraction of the available steam flow is admitted into the turbine. The rest of the steam is either bypassed or used for other purposes. Partial admission allows for better control of the turbine operation and enables the turbine to operate efficiently over a wide range of load conditions. It also helps in reducing the losses associated with the expansion of steam through the turbine.

**Axial Thrust**

Axial thrust in impulse steam turbines is primarily due to the change in flow velocity of steam at the inlet and outlet of the moving blades. This change in flow velocity creates a net force in the axial direction, which is known as axial thrust. The magnitude and direction of the axial thrust depend on various factors such as the design of the blades, the steam flow rate, and the operating conditions of the turbine.

To counteract the axial thrust, steam turbines are equipped with thrust bearings. These bearings are designed to withstand the axial thrust and provide support to the rotor. Proper design and maintenance of the thrust bearings are crucial to ensure the smooth and reliable operation of the turbine.

In conclusion, impulse steam turbines have blade passages of constant cross-section to ensure a uniform flow of steam. Partial admission of steam is permissible, allowing for better control and efficiency. Axial thrust in these turbines is primarily due to the change in flow velocity of steam at the inlet and outlet of the moving blades, which is countered by thrust bearings.

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