What is the function of a steam turbine shaft seal?
3 Answers
The function of the low-pressure end shaft seal in a condensing steam turbine is to prevent external air from leaking into the cylinder, which would otherwise damage the vacuum in the condenser, increase the exhaust pressure of the turbine, and reduce the unit's efficiency. Below is relevant information about shaft seals: 1. Packing seal: Packing seals have a simple structure, are inexpensive, and easy to maintain. However, they have significant leakage and power loss. Therefore, packing seals are used to transport general media, such as water, and are generally not suitable for petroleum and chemical media, especially not for valuable, explosive, or toxic substances. 2. Mechanical seal: Mechanical seals (also known as face seals) offer excellent sealing performance with minimal leakage and a long lifespan, but they are expensive and require higher precision in processing, installation, and maintenance compared to general seals. Mechanical seals are suitable for transporting petroleum and chemical media and can be used with various viscosities, highly corrosive, and particle-containing media. The American Petroleum Institute standard API610 (8th edition) stipulates that, unless otherwise specified by the user, packaged mechanical seals should be equipped. 3. Dynamic seal: Dynamic seals can be divided into two categories: back vane seals and auxiliary impeller seals. During pump operation, the centrifugal force of the back vane (or auxiliary impeller) reduces the medium pressure at the shaft seal to atmospheric or negative pressure, preventing leakage during use. When the pump stops, the centrifugal force disappears, and the sealing effect of the back vane (or auxiliary impeller) fails. At this time, the stationary seal device takes over the sealing role. Packing seals are commonly used in the stationary seal devices paired with back vanes (or auxiliary impellers).
I work with steam turbines quite often, and the shaft seal is essentially a sealing system installed at both ends of the cylinder. You see, the turbine rotor spins extremely fast, and we can't let it directly grind against the cylinder, so some clearance must be maintained. The shaft seal handles this sealing job—it prevents the high-temperature, high-pressure steam inside the cylinder from leaking out while also stopping outside air from being drawn in. Especially in the exhaust section, where high vacuum is critical, any air leakage would disrupt the vacuum, causing efficiency to plummet immediately. Plus, high-temperature steam spraying out is extremely dangerous and would corrode nearby piping. The shaft seal also blocks bearing lubricating oil from entering the cylinder and contaminating the steam—oil mixing with high-temperature steam is no joke. That's why during every maintenance check, I always emphasize ensuring the sealing surfaces of the shaft seal are smooth and the clearance is properly adjusted.
In our plant, the shaft sealing system is something we need to pay attention to every day when operating the steam turbine. At the start of the morning shift, we first check for any leakage from the shaft seals. If there's white smoke, we need to be alert. Steam leakage not only wastes thermal energy but also increases humidity in the engine room, which can easily cause motor short circuits. If the vacuum at the exhaust end is lower than the normal value, it's mostly due to air leakage from the shaft seals not being tight enough, and the power output will drop immediately. The steam pressure of the shaft seals also needs constant adjustment—too high pressure wastes steam, while too low pressure can't seal effectively. During shutdown maintenance, the wear of each shaft seal tooth must be measured. If it exceeds half a millimeter, it must be replaced. During the last major overhaul, it was the wear and leakage of the shaft seals that caused the vacuum to drop and the exhaust temperature to soar, nearly triggering a protective shutdown. This small component must be taken seriously.
