Turbine exhaust pressure

When the pressure inside a container is lower than atmospheric pressure, the part with lower pressure is called vacuum, while the pressure within the container is referred to as absolute pressure. Another way of putting it is that any container with a pressure lower than atmospheric pressure is called a vacuum. There are different degrees of vacuum: when there is no pressure inside a container, that is, when the absolute pressure is zero, it is called a perfect vacuum; all other cases are referred to as imperfect vacuums. The vacuum in a turbine condenser is an incomplete vacuum. The relationship between vacuum, absolute pressure, and atmospheric pressure is as follows: h1 + h2 = h. Here, h1 represents the height of the mercury column in vacuum inside the container, in mm; h2 represents the height of the mercury column corresponding to the absolute pressure inside the container, also in mm; and h represents the height of the mercury column corresponding to atmospheric pressure, in mm. Vacuum can also be expressed as a percentage, known as the degree of vacuum. This is calculated by dividing the measured height of the mercury column in vacuum by the height of the mercury column corresponding to atmospheric pressure, and then converting the result into a percentage. The formula for this is: Degree of vacuum = h1/h × 100%. With the absolute pressure in the condenser remaining constant, the vacuum level changes as the atmospheric pressure changes. Therefore, it is more appropriate to use absolute pressure in theoretical calculations to represent the vacuum inside the turbine condenser. After measuring the atmospheric pressure and the vacuum level in terms of mercury column height in the condenser, the absolute pressure can be calculated using the following formula: P = (h – h1)/735.6 (in engineering atmospheres). For example, if it is measured that the vacuum in the turbine condenser is 720 mm of mercury, and the atmospheric pressure as measured by a pressure gauge is 750 mm of mercury, then the absolute pressure and the degree of vacuum in the condenser are as follows: P = (h – h1)/735.6 = (750 – 720)/735.6 = 0.04 (engineering atmospheres). The degree of vacuum is given by: h1/h × 100% = (720/750) × 100% = 96%. The outlet pressure should be negative!

May I ask, is there any basis or standard for the turbine outlet pressure to be negative?

I already understand well the definition of vacuum; what I don’t know is whether the vacuum level in the condenser is equal to the exhaust pressure of the turbine!

There are two types of steam turbines: back-pressure type and condensing type. The outlet pressure of a back-pressure turbine is the pressure in the back-pressure pipeline. For example, if a turbine using 3.5 MPa steam is connected to a pipeline network at 1.0 MPa, then the outlet pressure will be 1.0 MPa. Another type is the condensing type, which uses cooling equipment to condense the steam at the turbine outlet, creating a negative pressure that improves the efficiency of steam utilization. What you are referring to is a condensing steam turbine; the outlet pressure, if the pressure losses in the pipelines are not taken into account, is equal to the vacuum level of the condenser.

As a supplementary note, the condenser is also commonly referred to as a rewatering unit; it operates under negative pressure. If its performance is poor or if there are leaks, positive pressure may occur, and in such cases the condenser needs to be addressed. Good luck. . .

What was said upstairs is correct: the steam outlet pressure of a back-pressure turbine is the pressure at which the steam can be fed into the saturated steam network. The outlet pressure of a condensing turbine is related to the operation of the reheat apparatus; the lower the vacuum level (i.e., the steam outlet pressure), the lower the energy consumption.

How can there be negative pressure at the turbine outlet? It is usually 1 MPa; the negative pressure referred to is the pressure at the outlet of the rehydrator where vacuum is created in the system! This is a vacuum pumping device; it does not mean that the interior of the unit is under negative pressure. Instead, it refers to the level of vacuum created after the vacuum-pumping airflow passes through. It extracts any leaking steam before it can condense, thereby protecting the unit!

It depends on whether it’s a vacuum condensation type or another type; in the S100 model, part of the S38 steam is extracted after work is done, while the rest goes to the condenser. A negative pressure is essential for quickly condensing the steam into a liquid. For those like S38 that are used for doing work, condensation occurs directly, without any pumping of air, but the condenser is still under negative pressure. Generally, an explosion-proof plate is installed; I won’t go into detail about its function.

For condensing units, the exhaust pressure is negative. The higher the vacuum level, the more work is done by the steam expansion. However, if the vacuum level is too high, the power required of the vacuum pump also increases; therefore, there is an optimal vacuum level. I’ve learned it; I suggest creating a steam turbine forum, so the friends upstairs can go to that forum to discuss and provide support. Thank you

Turbines are of back-pressure type and condensing type; the exhaust pressure of a back-pressure turbine is, of course, positive pressure, with this pressure being determined based on the operating conditions. In contrast, the exhaust pressure of a condensing turbine is vacuum.

Whether the last stage of the turbine, or the turbine outlet, is under negative pressure depends on the type of turbine. From what the original poster seems to mean, he is likely referring to a condensing turbine. So, the exhaust should be under negative pressure! Poster: There can be a simple way to prove this – you might want to check whether the temperature at the outlet of your turbine is below 100 degrees; if it is below 100 degrees, then there must be negative pressure! There’s no need to explain this, right? Everyone can discuss it.