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Turbine Throttle Pressure and 1st Stage Pressure
Difference between throttle pressure and 1st stage pressure

What is the difference between throttle pressure and 1st stage pressure when talking about steam turbines?

As I understand it, throttle pressure is the pressure of the steam right before the Main Stop Valve before entering the turbine.

Is the 1st stage pressure, then, the steam pressure after the steam has passed through the nozzles and entering the High pressure stage of the turbine?

Your understanding is correct for GE steam turbines.

By Dave Amundson on 21 April, 2014 - 8:57 am

The first stage pressure on GE steam turbines is measured just downstream of the first stage buckets/blades, (just upstream of the second stage nozzle). It is used as a relative measure of steam flow for valve testing and also used as a measure of boiler heat release.

Interesting.

Is 1st stage pressure and Throttle pressure used to calculate throttle valve position?

How do other Turbine manufacturers (Say Siemens) differ when talking about throttle and 1st stage pressure?

By Dave Amundson on 23 April, 2014 - 9:23 am

With valves wide open, the 1st stage pressure/throttle pressure ratio will be a relatively constant number. This ratio can be used for as a relative valve position measurement for various purposes. LVDT's are used for precision individual valve position measurement.

1st stage pressure is a reliable and fast measurement of relative steam flow to the turbine without suffering pressure drop penalties from a flow nozzle. It is a critical measurement in load control loops, feedwater/drum level control loops, and drum pressure. 1st stage pressure responds more quickly to a valve positon change than does the MW output due to the delays in raising the pressure in the reheater.

Filtering is sometimes required in the first stage pressure measurement due to the highly turbulent conditions in the 1st stage.

In our plant, the first stage pressure is also used for load related interlocks associated with turbine drain valves, boiler desuperheater spray block valves, and others.

I can't speak for other turbine manufacturers besides GE.

While not a direct answer to your query,

I have used stage pressure as represenitive of steam flow to verify and correct the linearity of the CV camming. To farther the exactness of steam flow, a correction for deviation from rated throttle pressure and temperature should be used. I found most of the stations had such a calculated value used in the boiler control algorithms, normally called calculate effective CV position.

Plotting actual versus effective CV position should be near linear, however a lot of the CV camming (either mechanical or electronic) have loading regions where the two were not. This type of evaluation of the actual steam flow versus CV demand is referred to as incremental regulation and minimum/maximum deviations from linear are defined in some of the codes (ASME or IEEE I believe)

Hello Dave,

Do we have an online resource which briefly describes (along with figures) the power generation and energy efficiency process in power plants ?

Thank you!!

Hello,

< First stage pressure is used as a relative measure of steam flow for valve testing >

Could you please explain me this?

1 out of 1 members thought this post was helpful...

I sense that you must be new to the industry and I will take the liberty to discuss some important related issues that are very important.

Periodic valve testing at OEM recommended intervals is extremely important for overspeed protection. Periodic testing helps to limit the buildup of blue blush on valve stems and bushings that can eventually lead to valves sticking open - especially if not properly maintained per OEM recommendations. It also ensures that the valve is moving all the way to the hard closed position and that the quick slamming features work properly.

Steam turbines typically accelerate at a rate of 15% per second or more. Our 3 steam turbines accelerate at 18% per second on a full load rejection. It is really important that steam valves slam closed within 300 mSec or so. Modern turbine controls should contain features to monitor tripping times and valve closure times during trips and during valve testing.

Tripping should normally be done sequentially (except if the trip signal is due to an electrical fault where immediate electrical isolation is needed) so that full valve closure and reverse power are both verified prior to opening the generator breaker so that you are absolutely sure the turbine will slow down instead of accelerate after the breaker opens. If you don't have reverse power relays and you have a steam turbine, you should add them.

Valve tightness testing is also very important for overspeed protection. Normally there are always two sets of valves that can redundantly shut off the steam flow. So when a turbine trips, and both sets of valves slam shut, one of the sets of valves could still have a serious leakage problem that you would not be able to detect without conducting a tightness test.

So, back to your initial question. First stage pressure is typically measured just downstream of the first stage rotating blades. This pressure varies proportionally to the steam flow. This signal is also commonly used in boiler controls as a steam flow measurement. The first stage pressure transmitter needs to have a fast update and overall response time (< 50 to 100 mS). The first stage pressure does contain high frequency fluctuations at the 1st stage blade passing frequency that need to be filtered out. This is sometimes done with a mechanical pulsation dampener.

When control valves are tested, there needs to be a way to compensate for the large change in steam flow in order and avoid potentially tripping the boiler. The goal is to keep steam flow from the boiler as constant as possible during a control valve test, and this is done using the first stage pressure signal. often the turbine control is placed in a first stage pressure control mode, or else first stage pressure feedback is utilized. Typically the steam flow disturbance can be reduced by 70 to 80% with these tools and will not be enough to upset the boiler. If the valve slams shut when there is very little steam flow left in the valve, the disturbance will be less than if the valve slams when further open.

It is not uncommon for plants to avoid valve testing for fear of problems like upsets or trips. This is very irresponsible and dangerous. If valve testing cannot be done without risk of unit upsets or trips, you need to get the needed help to fix the problems and restore confidence in your system. There are plenty of disaster stories out there that can be accessed on the internet on what can happen within a couple seconds after a load rejection where valves failed to slam shut. Hydrogen cooled generators in an enclosed building is perhaps the biggest danger to plant personnel.

I hope this helps.