Ventilator Lines for Steam Turbines 1

[SOROURA]

In certain Steam Turbine designs, some manufactures use the so-called "Ventilator Line" on the Cold Reheat (HP Exhaust) Line to vent the exhaust steam during low load, start-up, and turbine-trip operational modes. Due to a decelerated flow the HP exhaust steam is at a relatively high temperature compared with the expected exhaust temperature during normal operation.

I would really appreciate it if an expert can give me a little bit more in-depth detail of how the process really works (from a thermodynamic and fluid mechanics perspective), because I feel like I didnt grasp it quite well yet.

Thanks in advance

 

[FredRosse]

The “Ventilator Line” provides flow to avoid overheating the HP turbine and Cold Reheat Piping.

Turbines are generally designed for a specific volumetric exhaust flow, and under these conditions the turbine extracts energy from the flowing steam: steam enthalpy, pressure, and temperature go down as the steam flows through the circuit.

At very low flows, (startup, trip, very low loads) the low velocity of steam through the turbine blade passages is completely incorrect for extraction of energy from the steam flow. In fact, the rotating turbine blades churn the steam, and actually add energy to the steam, increasing its enthalpy and temperature.

If the flow is close to zero, then the temperature rise in the steam is excessive, and would exceed the allowable temperatures for the piping and equipment. The “Ventilator Line” provides a flow path that allows enough steam flow to keep this temperature rise below the maximum allowable values.

The “Ventilator Line” is connected to the condenser (vacuum condition), and thus lowers the exhaust pressure of the HP turbine. This significantly lowers the density of steam within the HP turbine casing, which tends to reduce the energy added to the steam by the churning action mentioned in the second paragraph of this posting.

 

[SOROURA]

Thanks for your reply FredRosse.

Just one question: does the churning action tend to increase the energy of the steam or decrease it, because you mentioned that it increases the steam's energy in the second para., and then mentioned that the steam's energy decreases in the last paragraph.

Also, how long (average) is the duration of this ventilation process?

Thanks again

 

[stgrme]

Churning (frictional heating) tends to add energy to (does work on) the steam causing the temperature to rise. Churning in steam at a positive pressure will add more heat than churning in a vacuum. In other words, more frictional heating occurs in a dense medium.

The duration of the ventilation process varies depending on a number of factors. Some of these factors are: turbine OEM's philosophy, presence of a turbine bypass system, presence of a turbine prewarming system, turbine starting method (initial steam admission into the HP or IP section for reheat machines). However, as a general rule, ventilation is applied for operation above about two-thirds of rated speed and some minimum load (typically, 5%), or for a reheat unit with initial steam admission in the IP section, minimum flow through the HP section. In addition, the ventilator valve is opened immediately when a turbine trip occurs.

 

[FredRosse]

SOUROURA asked: “Just one question: does the churning action tend to increase the energy of the steam or decrease it, because you mentioned that it increases the steam's energy in the second para., and then mentioned that the steam's energy decreases in the last paragraph.”

Sorry for the lack of clarity in the first reply.

The churning of the steam at these low flowrates always results in energy added to the steam, as stated in paragraph 2.

Paragraph 4 states that the vacuum condition “tends to reduce the energy added to the steam by the churning action”. This did not mean to imply that the steam’s energy decreased. Energy is still added to the steam, but at a lower rate than would occur if there was higher density fluid (higher pressure steam) within the machine.

A good way to imagine this is to consider stirring of various fluids in an ordinary drinking cup. Stir a spoon in a cup filled with air only, you notice there is very little resistance to the stirring, so very little energy is given to the fluid, although you are still giving some energy to the fluid. Then fill the cup with water, which is several times more dense than the air, and stir again. You notice that the energy required for stirring the water is several times more than was required to stir the air.

As to the duration of the ventilation process, it stgme’s answer is excellent.

 

[davefitz]

refer to tech papers written in the period 1980-1995 by Heinz Termuehlen on this subject- papers published at ASME JPGC conferences and powergen conferences. Subject area is sizing and operation of steam turbine bypass systems and windage heating of the HP turbine during startup ops with a ventilator line.

Each STG vendor has a different system and/or control philosphy , for patent reasons. GE has a ventilator line sourced upstream of the CRH check valve dumping to the condenser, and this operates in conjunction with a reverse flow bypass around the CRH check valve- so the reverse flow enthalpy is related to the HP bypass discharge enthalpy ( CRH steam from HP bypass).

The max permitted overheat of the HP last stage blading is defined by the STG OEM, and this startup operating temperature will be a function of HP inlet enthalpy, HP bleed steam flow, HP exhaust pressure ( CRH pressure) , and also HP bypass enthalpy( for reverse flow systems).

some systems use reverse flow cooling, some don't. Some units startup thru the IP turbine , some thru the HP turbine. Itmight be useful fo someone to publish a paper that compares all commercially available STG's and their startup mode and configuration.

 

[SOROURA]

Thanks a lot for your reply guys!

I got another question for you (which I was curious to know the answer to but never figured it out):

Why is it that some Turbine Manufacturers don't employ this Ventilator Line while others do? I understand that this is not a simple question to answer and that it depends on the design of the Turbine, but is there some kind of critical design criteria that sets forth the need to have it or not?

 

[stgrme]

Unfortunately, I can only reply in general terms. For some OEMs, in smaller turbines with relatively short blades in the HP section, the friction heating effects are small. Therefore, those OEMs do not consider ventilator valves necessary.

 

[rmw]

The ventilator valve discharge is VERY HOT as compared to LP turbine exhaust temperatures and it plays heck on condensers both on the shell walls at the nozzle attachment points (yes, even with thermal sleeves) and to internal parts (yes, even with spargers). The less they are used, the better the condenser likes it.

rmw

 

[SOROURA]

I would really appreciate it if somebody can share something with us (like a document that speaks about Ventilator Valves and their typical installations/operation), because I think it would be very useful, especially for system and pipe designers to be aware of.

 

[stgrme]

Ventilation is specific to each installation, as noted in my earlier post on the duration of ventilation. For this reason, it is not practical to provide a document, which would be helpful for all installations.

 

[davefitz]

soroura:
The steam turbine OEM will provide design requirements and the control and operating requirements for the ventilation piping ( if required), or the minimum bypass system size, for each specific installation of a large steam turbine. In no way would they accept a warranty on a steam turbine that has a ventilator or bypass system that contradicts their specific requirements.

The performance of the ventilator or startup bypass system is essential to retaining the integrity of the HP turbine last rows of blades, and other issues.

For basic background on the need for such systems and the thermodynamic theory, refer to the ASME tech papers I suggested above by Heinz Terhmuelen.