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Back Pressure Turbines

By Rutvik Solanki
Published in Engineering
2 min read

As we discussed in the types of Turbines, there might or might not be a drop in the pressure of the steam as it passes through the turbine. Simply put, the back-pressure turbine is a turbine design wherein the steam released from the turbine has a considerably higher pressure compared to the atmospheric pressure. The Steam released contains a lot of energy and has a high temperature, thus, an efficient way to utilize the steam is to use it for other purposes. That concept of multi-purpose turbines is used in back-pressure turbines. The steam is directed for other industrial purposes after the main work of electricity generation or compressor operation is complete.

Thus, we have a boiler in the circuit diagram, a single-stage turbine, a process drain, and a water compensator(No system can be 100% leak-proof). The final setup is then passed through the pump and then to the boiler, the cycle repeats.

Now, we can either have a straight back-pressure turbine, an Extraction Back-pressure turbine, or a Bleed/Uncontrolled Back-Pressure turbine.

Straight Back-Pressure Turbine

This is the most widely used back-pressure type turbine. Its aim is to expand the available steam through the turbine stages. The exhaust steam is connected to the process header.

Controlled Extraction Back-Pressure Turbine

A controlled extraction at various loading conditions of the turbine based on seasonal variations. In a controlled extraction type, a control valve integrated with the steam path is provided at a predetermined stage and the steam is allowed to escape the turbine at desired constant pressure based on the process requirement. There are basically two types of control valves a) A Grid Valve, b) a Bypass Valve, which controls the flow of steam at preset pressure accommodating the seasonal variation of turbine load. The control system works on a closed-loop feedback mechanism. The pressure at the user end is measured and feedback is provided to the control system. Based on this error signal the feedback control system actuates a control valve designed to vary the area of the valve aperture and thereby regulate the pressure. This setup is applied for processes; where the steam pressure is a critical parameter. In some cases, a nozzle/port is provided at a predetermined stage and an un-controlled high-pressure steam flow/bleed is drawn out of the turbine. This tap is provided before the controlled extraction in the steam path of the turbine. Its utility is found mainly in LP heating requirements like de-aerator, LP/HP Heaters, or some processes.

Uncontrolled Extraction Back-Pressure Turbine

In an un-controlled extraction type, a tap is provided at a predetermined stage for drawing out the steam for heating solutions. The un-controlled extraction steam is used for process steam applications (with a smaller quantity of steam flow and set temperature requirements) and LP heating applications like de-aerator for heating solutions. These turbines can also be deployed in District heating applications.


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