Industry Insights
Turbine Bypass Operation Unstable…
Conventional Thermal Power Plant – Unit Operations – Turbine Bypass Operation Unstable
Plant: 370 MW PF-fired Power Plant with HP&LP Turbine Bypass Systems.
During unit start-up operations, I am experiencing great difficulty with establishing and stabilising the Turbine HP and LP Bypass systems. When I set the initial opening position of the HP Bypass Control Valve, then put it on to Auto, the LP Bypass Control Valves (already on Auto), then ramp open and the HP Bypass Control Valve closes again.
Usually, this cycle of opening and closing repeats itself, numerous times, until it eventually settles down and starts to work normally. As a consequence of these cycles, everything else also becomes unstable (especially the Boiler Drum, Feedwater Tank and Condenser Hotwell levels, along with the Boiler Feed Pump operation).
Sometimes, it can get so bad that we suffer an MFT, and have to re-start all over again. I have tried initially leaving the Turbine Bypass Control Valves in manual (both HP and LP sets), until everything looks stable. But, as soon as I then place them into Auto, then it all happens again (and sometimes is worse). This is very stressful, and embarrassing, and I don’t understand why it is happening. Hugo.
Hello Hugo, yes, this would certainly be a very stressful and embarrassing event, for anyone. Mastering the operation of the Turbine Bypass systems is the key to success, for every operator. When you do master them, you will quickly realise that they are “God’s gift to the Operator”.
It’s important to remember that these are large valves and, accordingly, they require a good deal of “oomph” to drive them properly. Planning and setting up the right conditions is also very important, especially to be confident that firstly there is adequate steam production available to start with, and secondly that there will be plenty of “increasing” steam production to follow.
So, you need to be positive, not tentative and unprepared, as can quite easily be the case. Of course, and underpinning all of this, is also having a sound knowledge and understanding of the Turbine Bypass CV control loops, and this is what will enable you to use them with finesse and confidence.
You are correct in first placing the two LP Bypass CV’s in Auto, then in first manually setting an initial opening of the HP Bypass CV. But there are two key things to note here:
Firstly, don’t operate blindly ‘by the book”. Whatever initial opening value is stated in the procedure, it is only a guide, as it was written for a particular set of circumstances. You need to take into account what the present circumstances are and, guided by your knowledge of the control logic, you must decide what the best value for the initial opening should be. Too much or too little will not lead to a smooth operation.
Secondly, leave the HP Bypass CV at it’s initial opening position for a time (in Manual), while you observe the initial operation of the LP Bypass CV’s. Remember that the Reheat steam pipes have just been pressurised by the HP Bypass CV opening, and that the LP Bypass CV’s will try to reduce that pressure to the minimum setpoint value (usually about 700 kPa).
During this period, don’t forget that there is now a larger steam flow, and this will necessitate increased feedwater flow to maintain Drum Level. So, it might be time to set up the BFP and FWCV properly, either in BFP Pump Speed mode, or perhaps Auto (if steam flow warrants it).
And be aware that it can be a trap to leave the BFP in Pump Speed Drum Level Control, especially if Drum Level increases appreciably (and while you are distracted with other things).
When the LP Bypass CV’s have stabilised, check that there is sufficient steam supply via the HP Bypass CV to maintain (and preferably to steadily increase) opening of the LP Bypass CV’s.
Then check the Demand Deviation on the HP Bypass CV and, if it is just slightly positive (ie, calling for the valve to open), you can set it to Auto. If the deviation is too much or too little, then manually and incrementally adjust the valve opening to achieve the desired setting.
As soon as you have placed the HP Bypass CV on Auto, then proceed to sequentially close all Boiler Drain valves (finishing with the Final Boiler Steam Drain). This will give the extra “oomph” to keep the Turbine Bypass CV’s in positive territory, and operating smoothly (along with the rest of the plant).
Don’t forget that, with good steam flow through the Reheat system, you can now change from “imported” auxiliary steam, to the unit’s own supply from the Cold Reheat Pipe – provided that you maintain Boiler firing and increasing steam production.
I am confident that this approach will work well for you, and we look forward to hearing that you are enjoying “happy days”. Cheers, Graeme (Mod101)