(Daily Question) Coal-to-oil and olefins forum, 2018.1.29

How is the turbine steam supply to the (DMTO) olefin separation unit regulated? Answer: The common methods for regulating the steam inflow to a turbine include throttling control, nozzle control, bypass control, and sliding pressure control. (The DMTO) Olefin separation unit employs throttling control; it regulates the steam by controlling the opening degree of the control valve, thereby changing the steam flow rate and enthalpy drop entering the turbine, and thus adjusting the turbine’s power to adapt to changes in external load.

Answer: The common methods for regulating the steam inflow to a turbine include throttling control, nozzle control, bypass control, and sliding pressure control. (The DMTO) Olefin separation unit employs throttling control; it regulates the steam by controlling the opening degree of the control valve, thereby changing the steam flow rate and enthalpy drop entering the turbine, and thus adjusting the turbine’s power to adapt to changes in external load.

Throttling control, nozzle control, bypass control

1. Throttling regulation method: The throttling regulation method is also known as the mass regulation method. The steam supplied to the turbine passes through one or several control valves that are opened simultaneously before reaching all the nozzles. This type of regulation results in minimal throttling losses only when the control valves are fully open under rated load conditions; at this point, the turbine operates at its highest efficiency. When the load decreases, the control valve is closed to a smaller degree, causing throttling of the steam within that valve and thereby reducing the steam pressure. The steam then enters the turbine; due to this throttling effect, losses occur and the efficiency of the turbine decreases as well. 2. Nozzle adjustment method: Also known as the flow interruption adjustment method, the amount of steam entering the turbine is controlled by several control valves that are opened and closed in sequence; the nozzles at the first stage of the turbine are used to adjust the load on the turbine. Each control valve operates a set of nozzles; the number of control valves that are opened is determined based on the level of load. Even when a control valve is not open, there is still some throttling loss, but this represents only a portion of the total fresh steam. Therefore, at low loads, this type of regulation results in lower throttling losses compared to throttling control, making it more economical. 3. Bypass regulation method: Usually, at the turbine’s economic load, the main control valve is fully open; when the load exceeds this economic level, the bypass valve is opened to direct the fresh steam to the subsequent stages of the turbine blades.

There are three methods for regulating the steam inlet to a turbine: (1) Throttling regulation method: The throttling regulation method is also known as the mass regulation method. The steam supply to the turbine enters all nozzles through one or several control valves that are switched on simultaneously. This regulation method only works when the rated load is applied, with the control valve fully open to minimize throttling losses; this is when the turbine achieves its highest efficiency. When the load decreases, the control valve is closed to a smaller position, allowing throttling of the steam within that valve and thereby reducing its pressure before it enters the turbine. Due to throttling, there is throttling loss, which also reduces the efficiency of the turbine. 　　(2) Nozzle adjustment method: also known as flow interruption adjustment method. The amount of steam entering the turbine is controlled by several control valves that are opened and closed in sequence, thereby adjusting the load on the first stage nozzles of the turbine. Each control valve operates a set of nozzles, and the number of control valves that are opened is determined based on the level of load. When each control valve is not fully open, there is also throttling loss, but this represents only a portion of the total fresh steam; therefore, at low loads, this loss is smaller than that in throttling control, resulting in better economic efficiency. The downside is that adjustment during maintenance and installation is rather complex ； The temperature of the steam chamber changes significantly under variable operating conditions; therefore, the rate of load variation should not be too fast. 　　　　(3) Bypass regulation method: This method is widely used in turbines that employ throttling regulation, especially reaction turbines. Typically, at the economic load of the turbine, the main control valve is fully open. When the economic load is exceeded, the bypass valve is opened to direct the new steam to the subsequent stages of blades. Its advantage is that it achieves the highest operating efficiency at economic (design) load conditions, with the least throttling loss. Its drawback is that when the economic load is exceeded, bypass steam is introduced, the proportion of high-quality metal materials increases accordingly, and its efficiency declines due to the throttling losses of the bypass valve and the rise in pressure in the bypass chamber.