Please need somebody to clarify the Turbine Compartment Pressure. our turbine frame 5 mark 6 . there are two protection in the Turbine Compartment Pressure and Temperature . did the system work with pres. or temp.

What is the pressure measured in this room is it for the fan with the turbine compartment or turbine compartment pressure with atmosphere. the unit i see mmh2o ?

Thank You

 

The turbine compartment pressure is "monitored" to ensure that the integrity of the compartment fire protection is intact and that there is either a positive or negative pressure (depending on whether the compartment is pressurized by the compartment ventilation fan or is under a very slight vacuum by the compartment ventilation fan), usually measured in mm H2O (or in H2O). If a compartment door is left open, then the compartment pressure (positive or negative) won't be what it should, and an open door means that if a fire were to be present and detected and the fire extinguishing agent released that it would likely be ineffective because of the open door.

Also, a failed compartment ventilation fan will result in an improper pressure in the compartment (as will a fire damper in the wrong position!).

The turbine compartment needs ventilation to ensure that combustible gases don't collect in the compartment, and for cooling.

And, that's where the temperature switch comes in. Excessive temperature can damage wiring and instruments, and could be caused by many things, including a failed compartment ventilation fan, an out-of-position damper, a hot gas leak (compressor discharge or combustor or turbine shell), or even a fire that is undetected by the fire detection system.

So, both parameters (pressure and temperature) are important for similar, but different reasons, and both parameters are important for unit protection and safety.

You need to examine the P&IDs for the unit to determine whether the compartments are pressurized by the ventilation fans or are under a slight vacuum by the ventilation fans and to determine how the compartment pressure switches (usually differential pressure switches) are installed to sense the pressure/vacuum properly and calibrate/test the switch properly.

 

Dear

Normally Frame 5 turbine are tripped because of Turbine compartment High temperature. I have never seen a unit to be tripped by low pressure. The trip SP of temp is something about 90 to 100 DegC. But pressure low alarm is about 10mmH2O or 1mbar. This pressure is delta P to atmosphere.

Regards
A.Khakpour
GE Control Systems Specialist

 

Dear CSA Thanks for usual cooperation and replay
please can i know your email please.


>Regards to Mr. A.Khakpour before commission i saw the turbine trip by both one time by Low Pressure less than 4 mmH2O else the if the turbine door open more than 15 second turine trip .
also one time i remember Turbine trip by temp high due to thermocouples close to casing turbine and relocated these thermocouples and now it is good.

Many Thanks to contribute your knowledge .

 

maint,

The reason for contributing to control.com is that many people can read the questions and responses to questions. If we take the discussion off-line, then only one person/site can benefit from the exchange(s).

 

dear csa can you explain exhaust pressure? it is the same turbine compartment positive and negative? Is the unit measure mBar?

 

Exhaust duct (back-)pressure is also measured in mbar (or in H2O or mm H2O).

If the gas turbine is running, it should be positive, never negative!

The usual problems with exhaust duct back-pressure instrumentation are the result of improper installation, improper placement of the probes, and improper elevation of the sensor (differential pressure switches and/or transmitter) with respect to the probes, even improper calibration of the sensors. It can even be some combination of any or all of the above.

Typically, GE uses a probe which protrudes through the exhaust plenum wall somewhere near the exhaust diffuser called a "pancake probe", because the face of the probe is a flat disk, like a pancake, with a beveled edge and a very small diameter hole (orifice) in the middle of the face of the disk. The disk is welded to a piece of stainless steel (usually) pipe approximately 3/4-inch in diameter, and the pipe is passed through an opening in the exhaust plenum (from the inside to the outside). The open end of the pipe is usually threaded with a Swagelok (compression) adapter screwed on the end to which tubing can be attached on the outside of the exhaust plenum.

There should usually be at least two "pancake probes" (for redundancy and for equalizing the pressure measurements) and they are usually installed on opposite sides of the exhaust plenum from each other. The two probes should be connected to each other with tubing and to the sensors (differential pressure switches and/or transmitters). The sensors should (read "must") be mounted above the elevation of the pancake probes so that any condensate--which WILL form in the lines otherwise--will drain back to the probes and into the exhaust. This is probably the biggest single mistake--locating the sensors at ground level, below the probes.

The disk faces of the probes should be 10-15 cm (2-3 inches) from the inside wall of the exhaust plenum, and the bevel on the edges of the disks should be sharp and smooth, usually angled from the face towards the back edge.
<pre>
__________ __________
/__________ __________\ Pancake Probe Disk Face
| |
| |
| |
-------------| |----------------- Exhaust Plenum Interior Wall
| |
| |
-------------| |----------------- Exhaust Plenum Exterior Wall
| |
| |
\ /
| | Sensor Tubing Connection
</pre>
The disk face is usually approximately 15-20 cm (3-4 inches) in diameter. I don't think the exact dimensions are critical, but the placement with respect to the diffuser and walls and any internal duct/supports is important, and there needs to be some space between the inside wall and the back of the probe face and the edges need to be smoothly beveled. The diameter of the hole in the disk face is also not critical, but should not be more than approximately 1/16th-inch or so (what is that, 1.6mm or so?), and frequently they are smaller than this. If the diameter of the hole is too large it can result in large and frequent fluctuations. If it's too small, then it can't measure the pressure very well.

Pancake probes occasionally require inspection and cleaning, and the tubing should also be periodically inspected and blown out if necessary, especially if the unit runs on liquid fuel, even only occasionally. It's important that the hole in the disk face remain clear and unobstructed.

If the sensors at your site were mounted at ground level (and they were at many sites, in spite of notes indicating otherwise on the drawings!), then if there is no "drain" valve below the sensors, it's a very good idea to add one. In fact, the best idea is to add a quarter meter of tubing (10-12 inches) or so below the sensor taps and put a drain valve at the bottom of the tubing. This way, there is a point for the condensate to collect below the sensor taps and it can be easily drained. But, it's important this be done frequently and periodically, and not forgotten.
<pre>
Tubing
From Pancake Probes
|
|
|
|---- Tubing Tee Connector (branch to sensors)
|
| (Tubing for Condensate to Collect, Below Sensors)
|
|
-
X Drain Valve (Normally Closed) To Drain Condensate
-
|

</pre>
Exhaust duct pressure should never be negative when the turbine is running. And, when the turbine is not running and the exhaust is cool to ambient conditions and there is little or no draft through the exhaust (and HRSG if present), the exhaust duct pressure should be zero. If the exhaust (and HRSG) is warm or hot and there is a draft through the inlet, turbine and exhaust (called the "chimney effect") the exhaust duct pressure can be negative, and another common mistake is to adjust/calibrate the sensors to read zero when the exhaust is hot and there is a draft. Even if the turbine shaft is at rest (zero speed) immediately after a shutdown and for several hours after, there can be a very strong flow through the unit and exhaust, creating a slight negative pressure at the exhaust (remember: we're talking mbar!).

So, installation, placement, and calibration are all possible problems with a stable exhaust duct (back-)pressure reading.

Hope this helps!

 

It's easy to ignore combustion turbine compartment integrity because CTs will run whether the doors are open or closed. Keeping it online is the prime and sometimes only objective.

Turbine and auxiliary compartments are a functional part of the CT's systems, not just noise suppressors and rain guards.

One often overlooked aspect of GE's turbines that there is no insulation on their outer casing, making them susceptible to compressor blade tip rubs, especially if you're in a hurry and open the doors while force cooling the machine. We know from experience that this practice causes the compressor casing to shrink too rapidly, eliminating the clearance between itself and compressor blade tips. The result has been blade "tip liberation", a GE term for tips breaking off.

Once liberated, tip pieces are propelled downstream wreaking havoc along the way and liberating large sums of money from maintenance budgets. Rotors have been "corncobbed" from this sort of damage. The moral of the story is to keep the doors closed for a variety of reasons, and to let everyone know what the rules are.

 

Well, we have observing a strange phenomenon at our facility. If we close the door of Turbine compartment, the seismic vibration of the Turbine goes up, and when we open the door of the compartment, the vibration goes down.

Previously, this wasn't the case. For few months, we are observing this. There must be some negative change in the integrity of the turbine that causes this.

 

Well, you have provided virtually no actionable information. We don't what size machine you are working on, which bearings the vibrations increase on, how much the vibrations increase, and what happened before the problem started (like a maintenance outage, the replacement of a vibration sensor, etc.), or which door on which side of the turbine causes this problem. What have you done to try to resolve the problem, and what were the results of your efforts?

So, given the information you <b>have</b> provided you must be correct. The integrity of the turbine must be negatively impacted when the door is closed. Have you tried opening the other door?

By the way, what is the integrity of the turbine?

 

> Well, we have observing a strange phenomenon at our facility. If we close
> the door of Turbine compartment, the seismic vibration of the Turbine goes
> up, and when we open the door of the compartment, the vibration goes down.

> Previously, this wasn't the case. For few months, we are observing this. There
> must be some negative change in theintegrity of the turbine that causes this.

well checkout the turbine compartment cooling air fan and auxillary fan if available,and note that fan is running properly or not vacuum can also create a problem sometime when fan is not working properly

 

I'm sorry for not giving the details.

Turbine: Vericor ASE-40

Vibration monitoring: No Radial probes on Bearings. Only seismic vibration probe placed on the HP side of the gas producer.

Turbine Compartment: There is a vacuum inside the turbine compartment. Turbine vent fan is axial, and there is only one fan. The other fan is Turbine scavenger fan. If the Doors co-axial to the turbine are open (i.e. Front and Back), there is no effect on the vibration. Opening Right and Left Doors (perpendicular to the turbine axis) impart effect on the Vibration.

Vibration probing wiring route: No wire is near to the door area.

Turbine EGT: Turbine EGT is also bit low compare to the turbines in operation.

Thanks

 

What is meant by Positive pressure ventilation and negative pressure ventilation.

How to identify the Positive pressure or negative pressure ventilation requirement for Gas turbine Enclosure.

Can some one help?

Thanks.

 

I realise this is an old thread, but I wonder if anyone can help. I am looking for stats on the flow rate of a Frame 5 ventilation exhaust fan. Is there a specific rate prescribed or is there a range within which it should be?

Many thanks,

 

Hi,

If you check through the applicable standards such as NFPA, API, ISO etc. you will see that there is no specific value for ventilation air flow. Usually when gas turbine enclosure is designed, a CFD analysis is performed for effective ventilation. Basically design flow is based on CFD predictions.

As known gas turbine compartment ventilation is an important requirement because in the event of a gas leak inside the compartment gas pockets may accumulate in stagnant areas and easily reach its explosion limit. A proper ventilation system dilutes this leak and removes from the compartment. Secondly, compartment ventilation is designed to cool the gas turbine casing as gas turbine casing temperature has strong influence on bucket tip clearances, consequently on turbine performance.

The best document you can find more details is;

"HSE Guidance Note PM84 Control of Safety Risks at Gas Turbines Used for Power Generation".

EC initiated directives known as ATEX Directives have also set rules for proper ventilation, but none of those gives an exact figure of flow. These directives recommend a design where the ventilation is used to dilute any potential gas leak. Note that ventilation is considered as dilution ventilation and appropriate distribution of air is more important than its quantity. High flows may mask the small leaks.

I remember at one Frame 6B application, ventilation system provides approximately 70 times air changes per hour through the enclosure.

Just for information to explain the significance of ventilation system; in recent advanced technology turbines such as at 9FB,ventilation system comprises of three VFD fans and ventilation control has five modes of operation, these are; dilution control, T-exit control, buoyancy control, T1 control and cooldown mode. Ventilation fans run at whichever of these modes calls for the highest.

Just to answer the previous post regarding positive and negative pressure ventilation, positive pressure ventilation is prefered where gas turbine is installed in hazardous environment, such as a gas compressor driven by a gas turbine. Negative presure is widely used in power generation applications.

Regards

 

TOral,

Thank you very much for this info. Very helpful

Regards