TURBINE VALVES WIDE OPEN 1

[marada93]

Some time ago I had posted concerning the potential benefit to operation our steam turbine with valves wide open. As it turns out, I have had the opportunity to run in this mode, so I need to bounce some numbers back to you guys who pushed me in the right direction before...so heres what I've got.

Just as a reminder, my turbine has four control valves that operate sequentially with an inlet steam condition. The turbine is rated for 68.9 MW, with design inlet steam conditions of 1450 psig and 950 F. Our dispatched full load output is only 58 MW. At 58 MW, CVs 1&2 would be approximately 60%, with CV 3 at about 15%.

Recently (last week) I was given the opportunity to go valves wide open on the turbine to evaluate performance and efficiency. Now, I'm able to make the 58 MW output with inlet steam conditions at 1325 psig / 950 F with all four control valves wide open. MW load is being controleed by the Plant Master, which directly controls the firing rate on two boilers. Overall plant operation has been exceptional, although it was exceptional before as well.

That being said, it seems to me that if I'm making full load with 1325 psig, whereas before I was using 1450 psig, then I was taking a 125 psig pressure drop across the three throttled CVs.

It seems like since this is the case, we're better off running VWO for a number of reasons, such as:

1- Lower pressure on the boilers and related piping by the same 125 psig.

2- Less wear on the turbine valve plugs and seats.

3- Feed pumps deliver water to the boilers much easier at reduced pressure (lower auxiliary power requirements

4- Silica carryover is less of a factor at reduced pressure.

5-Enthalpy of the steam is slightly increased at the lower pressure (1465.6 BTU/lb vs. 1461.2 BTU/lb at 1450 psig)

Does the increased BTUs correspond directly to an equvalent BTU input to the boilers? Seems like it should, even though it's slight.

What else am I missing? Any and all input is greatly appreciated byrdj and poetix gave me alot of useful iinfo last time. You guys still out there?

Thanks in advance!

 

[byrdj]

Still here.

does your plant perform a "heat rate" to compare fuel burnt to MWs made. This will be the answer if the HR is better at CVWO.

 

[marada93]

Hey byrdj...

Yep, they do run the plant heat rate, but they usually only do it after the month is over, then look at the stats for an entire month at a time.

So, I'm eager to see the results. I'm not looking for some big blue-light special. What I'm hoping to see is an otherwise unexplained slight reduction in the heat rate.

From what I've seen so far after a week of running like this, it's clearly not going to be something dramatic that you could see right off....but you can bet I'm looking close.

 

[byrdj]

I'm not that familar with daily operations, but thought hourly readings were logged that are used.

Performance and efficency aren't my normal area so I don't recall what increase was expected(and I don't have time this fortnight to review Kotton's notes). I would think 1% would be significant.

Did't we discussed the best valve position would be if only 3 valves were full open, thus only sligtly reducing the maximun velocity in those arcs? By going CVWO will be the best from a maintenace veiw (less wear). There will be a slight reduction in efficeincy of the nozzle quadrants due to reduce pressure (but the sum for the stage may be better)

If you get a chance, can you try setting CVs till CV#4 is at crack point (1 2 & 3 as far open as camming will allow) and checking HR. If this condition is even better, misadjusting 4's cp to open 3 more. could be done

Sorry, It will be a couple days before I'll get a chance to visit here again.

Keeping fingers cross for a "documented" HR improvement.

 

[byrdj]

I did a quick scan of my Kotton notes and review the past posts. I'll repeat a comments

The changes in CV will result in a change in stage effiecency. The recomended would be to measure the HP turbine efficieny to document results and calculate HR. (from quickly looking at expected HP ef vers CV position, a 1% nominal to 5 % in HP efficenecy at "best valve point") a 1% HP efficint change results in expected HR change of 0.16% (which falls into the uncerteranty of the method of measurements

 

[rmw]

The reason for the loss of stage efficiency is because of the specific volume difference of the two steam states.

Since you are at 950 F in both cases, you may be spraying more for final SH temperature control. Higher specific volume steam at lower boiler outlet pressure would result in higher velocity in the superheater tubes, resulting in higher reynolds numbers and possibly a higher final outlet temperature before the sprays.

Look at the two throttle conditions on a mollier diagram, and plot the outlet temperature/pressure where ever your turbine outlet is (cold RH, or condenser, depending on your plant). Compare the slope of the expansion line of the CVWO at reduced inlet pressure to the slope of the expansion line of the original heat balance to get a quick glance at the change in overall efficiency, if any.

You are right about lower BFP horsepower, and if you have extraction steam driven BFP's then less steam is extracted and goes through the lower stages of the turbine to produce power. If it is electric drive, then just less aux power costs, although if fixed speed electric, you could be taking a larger pressure drop across your BFW control valve.

If you have a reheater, look at the cold reheat temperature and pressure, because with the change in stage efficiency, possibly affecting each stage of the HP turbine, you could also have an increased efficiency loss through the RH circiut if the pressure/temp is such that the specific volume is higher than design. Depending upon your turbine OEM, some of them publish a RH efficiency loss curve due to increased pressure loss in the RH circuit.

You are going to have to get some performance data at all the points along your expansion line and compare them to your original heat balance.

I said most of the above to give areas to look at to help you determine where you are.

Bottom line, I think you are ahead, simply because figuring an adiabatic expansion from 1450/950 to 1325 results in a steam temperature of 942F with the same enthalpy, but with a slightly lower specific volume of the steam than your 1325/950 condition. I think the extra BTU's in the steam at the inlet to the nozzle block as compared to the CV throttled steam is available work for your machine offsetting the miniscule difference in the stage efficiency loss due to the miniscually higher specific volume.

Do some more checking and post back. Good discussion.

rmw

 

[ilan7]

Think that by reducing the pressure y'r entering less energy to the boiler and potentially saving mony to the company.
The only problem is that if the turbine need to supply more power (usually to help the utility dispacher overcome a temporry shortage) the plant will not have this capacity, and with 60 MW it might cause a major power failure in the area. As the dispatcher know you have an extra 10 MW in the plant (and probebly the utility is actually paying for that extra 10 MW to be available) when they will ask for it and the plant will be able to supply it only after so and so minuts this might result in very high penalties. Check this point before changing the plant name plate from 69 to 58 MW.
Other option is to try and talk to the utility to buy the extra 10 MW.

 

[byrdj]

Dispatch demand would be to change firing rate, which would increase pressure, feed water flow, and then MWs. If dispatch only demanded CVs to open, the pressure would drop and MWs stay same. You get more MWs by burning more fuel.

 

[marada93]

Hello to all...!

I was gone for a few days, but eagerly read all comments, and will attempt to respond to all.

First off the heat rate performed at the plant is performed monthly with the fuel input being determined by fuel delivered verses the fuel on-hand. The fuel feeders are nothing more than a variable speed chain with an assumed amount of fuel being delivered based on the speed only. Not a gravimetric, which would make things alot easier to calculate on a day to day, or hour to hour basis.

The HP efficiency does indicate a slight improvement. Something in the neighborhood of .3%, which as you correctly pointed out would result in a minor improvement in heat rate..

As far as running with #4 CV at crack point...Good point! I'll give this a try first opportunity and see if the change on the HP is noticable.

As far as spraying more with desuperheat sprays, it we're spraying more...I can't tell, but this i something I'll watch closer since you mentioned it.

No reheater in this configuration, but if I did have one I doubt there would be any change in the steam conditions at that point. Cold reheat temperature (in this case) would be the same as the HPT exhaust. The steam pressure and temperature with VWO remains unchanged, as is the case at all extraction points.

The "dispatched" full load is 50 MWN, which equates to approx 58 MWG, and we are rarely dispatched. While I agree the the rate of change would likely be a bit slower, it seems to be a non factor in this case.

The turbine controller is set as an Inlet Pressure control, so that by adjusting the pressure set point progressively lower, I eventually came to VWO. Currently I've set it about 10 psig lower than actual conditions so that the valves would remain open through sootblows, and other minor disturbances, but would respond to maintain in the event of loss of a boiler (we have two) or in the off chance that we would get dispatched down.

At any rate, other than the maintenance aspect, I'm banking on the increased BTUs availlable to the HPT at the lower pressure. It's admittedly a small increase, but an increase with no capital expenditure. Just a change in our operating philosophy.

I'll keep you posted if I see something either positive OR negative. I'm planning on operating in this manner through the month of August.

 

[marada93]

One more thing...

Going back to the original inlet steam conditions of 1450 psig and 950 F. Since this was being throttled by the 1st three governing valves (to the tuune of a 125 psig pressure drop across the valves), under these conditions does the steam temperature also decrease as the pressure is reduced?

By how much? By looking at the Mollier diagram it seems so, but the diagram isn't specific at the steam conditions I'm dealing with. I'm researching this right now, but wish to confirm my findings with those who've helped me so far.

What I'm getting at is, you can see a slight increase in the enthalpy of the steam as it is, but I have to consider that when I throttled the steam across the valves, then the actual conditions entering the turbine were also lower in temperature as well. You follow what I'm saying? I'm not quite sure I'm explaining myself clearly when I re-read this. In order to make an accurate comparison, I need the conditions entering the turbine, not entering the CVs.

Thanks in advance.

 

[rmw]

Look at the second to last paragraph of my previous post beginning with "Bottom line..." for the temperature after the 125 psi drop based on adiabatic expansion, which is a safe assumption in this case.

If you have contact with a Graham rep. you can get a freeware called Vacworks II which has a wonderful mollier program in it which I used in determining the temp. Or go to

http://www.graham-mfg.com

and I think you can get one sent to you from there.

rmw

 

[marada93]

RMW,

Copy that. I must have read past a bit too fast. Thanks for the info!