Due to the malfunction of trip oil pressure switch the steam turbine got tripped. The trip oil is used to overcome the spring action of emergency stop valve (ESV) by which the steam goes inside the turbine. Process people are saying to remove the tripping of turbine from trip pressure low. They are saying that due to the less trip oil the ESV will close and it itself will stop the turbine? Pl suggest should I remove the tripping from Trip oil?

 

Most of the time, same oil is used for actuating governing valves also. Operating with low control oil may cause malfunction in governing too.

Easy and more reliable solution may be to migrate to pressure transmitters and preferably redundant ones. Again it depends on criticality of the turbine.

 

You really need to KNOW the details of how your system works, electronics as well as the hydraulics. Case in point, For LSTG MI

the trip header pressure (ETS) switch is not a trip, however it does select "valves closed" to return the electronic "governor" to a restart condition.

the trip header pressure is not used to open the TV, but to signal a hydraulic relay internal to the actuator to quickly close it. the same trip headed is also used by the CV actuators. so a loss of trip header pressure resulted in all valves quickly close.

i was involved with an incident where the trip header pressure switch failed. It was removed from the logic with standing instructions for the operator to press "valves closed" following a turbine trip. ran that way for couple years till switch could be repaired

Is your control system capable of the same? I don't know and would advise you not to make the decision, but rely on someone familiar

 

Thanks for your reply as u have mentioned the same oil is used to lift the control valve. but in our case the valve is lift by the oil which comes from current to hydraulic converter (I/H).

The trip oil is generated by the tripping (Hand trip )device. there is a separate oil lines for the trip oil which overcomes the spring action of ESV. so the control valve will always run with the amount of oil required to lift valve by I/H converter.

The fluctuation in trip oil will never fluctuate the control valve operation.

 

If the trip oil pressure is not used by the control/governor valves, then the trip oil pressure switch may the the electronic signal used to close the CVs following a trip.

given most trips may be electrical and hit both the dump for the trip oil pressure and the electronics for the CVs. If there is a pure mechanical device that dumps the trip oil pressure, the trip oil pressure switch may be the method to relay the trip to the CV electronics.

 

This thread is getting off track. Trip Oil is used on STOP valves, not generally on control valves.

Trip Oil does what its name implies: Trips (very quickly shuts off) the Stop valve, thereby preventing steam flowing into the turbine.

Control valves are not generally used as stop valves, and therefore aren't usually included in the Trip Oil system configuration. The Stop valve should shut off the flow of steam before the control valve, and when the turbine is tripped the control valve reference(s) usually go to zero (or sometimes negative to ensure the control goes to full mechanical stop). If hydraulic oil pressure is lost when the turbine is tripped, the control valves will go closed because they are usually spring-operated to fail in the close direction. (If we are truly talking about steam turbine inlet control valves, and not some extraction control valves.)

Stop valves usually have Trip Oil; control valves usually do not.

 

Thanks CSA for the reply,

what u have mentioned is the oil scheme at my unit. plz suggest can I remove the turbine tripping from trip oil switch as this oil is used to overcome the spring action which stops the emergency stop valve. The ESV will automatically close if the trip oil pressure gets reduced due to the spring.

Pl suggest

 

GM,

If the unit is not under some kind of warranty, you are free to do whatever you want to do--provided you or the person(s) you work for want to take responsibility for any consequences.

I'm not clear exactly what the problem is that makes you (or the person(s) you work for) want to "...remove the turbine tripping from trip oil switch...." Perhaps if you explained what you believe is happening someone here might be able to suggest another--better--solution.

If I understand correctly (and, again, I'm not clear on this) you want to remove the Low-Low Trip Oil pressure switch "permissive" from the trip logic. The reason the Low-Low Trip Oil pressure switch is in the trip logic is because the manufacturer didn't want the turbine to continue to run with insufficient Trip Oil pressure. Most Trip Oil systems I have seen used what's sometimes called a "dump valve" which is operated by Trip Oil pressure and when actuated by Trip Oil pressure allows high pressure (or sometimes low pressure) hydraulic oil to get to the stop valve actuator. So, it's like a "second line" of defense, and some turbine with mechanical overspeed bolts dump trip oil to shut off the flow of hydraulic oil to the stop valve actuator--thereby allowing the stop valve to close by spring pressure.

Without being able to see the P&IDs for the turbine at your site, and without fully understanding what the problem with the Low-Low Trip Oil pressure switch is or is causing (I have an idea what it might be!), we can't really say for sure what to do--except that when anyone modifies the manufacturers trip logic scheme that person--or the person(s) he works for--have to be willing to accept the responsibility for any consequences. Which means they should have a full understanding of the purpose of the trip "permissive" and come to some reasonable understanding amongst logical, reasoning adults as to what the consequences might be and if someone is willing to accept them if something unconsidered happens which cause damage to the machine or personnel in the vicinity of the machine.

Because when steam turbines overspeed, the results are not usually very pretty, and can even be deadly.

Please write back to let us know how you or the person(s) you work for decided to proceed.

Better yet. explain (better) what the problem(s) is(are) that lead you to suspect the Low-Low Trip Oil pressure switch. Have you verified every terminal connection in the string to and from the pressure switch terminals? When you write for help from a forum like this, you should tell us as much as possible about the problem(s) you are experiencing, what troubleshooting you have done, WHEN THE PROBLEM STARTED, and what the results (actionable data) of the troubleshooting were that you have done. You will get a better, more concise reply sooner by doing so.

 

We need to know what type of control system your steam turbine has. Also, if you could state the manufacturer that would help. Different OEM's use slightly different control strategies. The responses here look to be mostly for GE units.

ALSO: We need to know if you unit is (MHC) mechanical hydraulic control or (EHC)electrical hydraulic control. This has a big difference in things as well.

LASTLY: The entire turbine speed/overspeed control and hydraulic system should be given in nice schematic form in your O&M manuals.

Most steam turbines will have some type of "trip header" that is pressurized when the unit is reset. If the unit is "tripped", the fluid is dumped from this header and this signals all the valves and other protection devices to close.

One comment above I found an error in. Many control valves do receive a signal from this "trip header". The control valves usually have a fast acting valve and a dump valve that causes them to slam shut when the unit trips, just like a stop valve. Of course, not all units are like this, but on the units I worked on all valves will slam shut when the trip header loses pressure.

 

CSA

Thanks once again what we have done is provided a 2 second delay of turbine from the trip oil pressure low switch. As I have already mentioned the trip oil is used to overcome the spring action of emergency stop valve. At our unit when the trip oil pr. is less than 4 kg it will trip the turbine. normally the trip oil pressure is 8 kg when TG is running.

for the oil supply to turbine we have one pump. there is one standby pump in case previous one fails.

On that day the main oil pump tripped, though the standby pump started but the pressure switch of trip oil sensed the low oil pressure and caused the turbine tripping.

Pl suggest whether giving 2 sec. delay is right. though my advice was to reduce the setting of switch by .5 kg.

 

Two seconds is more than enough time for a steam turbine to overspeed.

If this only happened once, then it might seem there was some air in the hydraulic line or some problem with the "standby" pump coming up to speed. I think the more prudent course of action would be to understand why the "standby" pump took so long to come up to speed/pressure.

And, if you can't do that, then the prudent course of action would be to slightly lower the pressure setpoint of the switch. But to introduce a time delay on tripping is asking for trouble. If the unit is insured, and there is a catastrophic failure the insurance company may not pay for damages/repairs if it learns the tripping was changed--even if that wasn't the actual cause of the overspeed failure.

Best not to introduce a time delay on tripping--of any turbine, but a stem turbine, especially. If there is no load on the turbine it will overspeed VERY quickly.

With dire consequences.

 

The nominal acceleration from loss of load is about 10% per second. so a 2 second time delay in overspeed tripping would destroy a Steam turbine generator.

You have now provided pertinent information in that the event occurred during the auto transfer of oil pumps. Is it possible that during the pump transfer that the pressure dipped enough to cause the SV hydraulic actuator to actually trip and the pressure switch only provided indication of the trip?

Not knowing what type of system you have I can only relate to systems I have seen. Those utilize a hydraulic relay (dump valve) that would cause a SV actuator to trip if the trip header pressure did fall to less than 50% of rated.

Before thinking about adding a time delay for the trip header pressure switch. You need to review the drawings and using a though knowledge of your system make note of exactly what this switch does for your system and the consequences if a time delay is added.

for example the GE legacy EHC systems do have a 2 second time delay for this pressure switch (PS100a&b), the MI being hydraulic orifice/accumulator and the MII being a TDDO relay. But as stated earlier, this pressure switch is not an input to the trip logic, but an acknowledgment that a trip has occurred to the governor logic.

IF, your hydraulic system does not have some type of trip latching device, a time delay would not be recommended. for example if an electrical trip initiates, but clears before the 2 second delay, the trip could reset and cause an overspeed event

Here is a link to an event where a change was made to control system.
http://www.scribd.com/doc/61342541/Duvha-2011-Overspeed-Testing-Failure-V2

even if you provided drawings, by not being familiar with your system, no recommendations could be made.

As for the pressure dip during the auto pump transfer. Most control oil systems I have seen utilize accumulators to mitigate such transients in the supply pressure. If your system has accumulators, you should verify their operbility

 

could you clairify your pressure kg.

I am not familiar with metric so I was not initially concern with actual values but in an attempt to better guess what type of system you have I was interested in the equivalent to PSIg.

8 kg/cm*cm would only be 100 psi, thus a very low pressure system.

> At our unit when the trip oil pr. is less than 4 kg it will trip the turbine. normally the trip
> oil pressure is 8 kg when TG is running.

 

The logic of the process people is correct. however i am not able to understand why only this pressure switch is mal functioning (I hope this tripping also is with 2/3 logic if not better provide for more reliability), as there are various pressure switches installed in trip logic. First we need to find out the root cause of this mal functioning and take corrective action.

Other argument from my side is,
As this switch, most probably is provided in the line after the trip block and which is only connected to ESV for tripping. When the trip oil pressure really goes below the set value( less than the ESV spring force) then automatically the ESV closes, even without trip oil low low switch acting.

Hence my experience says even if it is removed from the logic there is no harm as for as turbine safety is concerned. If there is no other tapping taken for any other part of the turbine.

 

Thanks to all for the recommendations. What MR.Rangacharya has said that there will be no problem as the ESV will close by the spring action. this is what I was also thinking as the pressure required for opposing the spring action is 3.5 Kg/cm2 and the trip oil pressure switch is set at 4 kg/cm2. The ESV will close once the trip oil pressure reduces less than 3.5 kg/cm2

Also as one of the member has said the accumulator may not be working properly or some air bubble may have come in the line.

Tomorrow we have taken the shut down of our unit to take the trial for the stand by pumps and main oil pumps.I will communicate what has happened and what was the cause of tripping.

 

Hi Just wanted to join, maybe it is also wise for you to check your hydraulic pressure accumulator. They usually have bladder with nitrogen gas pressurised to 50% of your system oil pressure. This accumulator will prevent your oil pressure dipping rapidly as the standby cuts-in. We have had ruptured accumulator bladders in our plant.

 

GM

Please do tell what you ended up doing to solve the pump changeover problem.

 

Mr.GM

I would suggest that instead of removing the switch they should convert the wires to a separate low oil pressure indication. It will help out to reach the actual problem and at the time when low oil indication glow they can see the actual oil pressure by fitted pressure gauge. It the actual pressure is not low it means you can run the turbine safely in the mean time arrange new pressure switch. If the actual pressure on gauge is also low it means switch is correctly working and needs to be find out the reason of low oil pressure and not suggested to run turbine.

Regards
Shaheed

 

Dear Friends,
We have a steam turbine of 8MW,Make BHEL,Model no-K800-2,year manufacturing 2001.After three years of Non-operation,this turbine has been put in operation.Many problems were faced.Those were removed one by one.Now turbine load does not increase above 2.2 MW and trips on "Low trip oil pressure".
Its Servo Cylinder has scoring mark in inside surface.Seal kit has been replaced by new ones.
Kindly suggest remedy.
Regards,
D.S.Sahu

 

Hi



Can you tell us

-Is that control system (ww 505) doing the work properly..

-What problems did you find as you mentioned...

-How hydraulic trip oil pressure is monitored... Is that a 2003 voting system used..

At the moment I can add any statements by now..
We need datas like trends.. For better support..

Any time
James.