I need support from the forum on Perennial issues we have on one of our unit Fr-9E related to cooling water system.

Plant is 3x 9E DLN1 simple cycle,commissioned in 2003.

In the year 2006 on GT1B - we started seeing high temperatures on Aft leg reaching upto 100+ deg C (left side) & 85 deg C (right side) with ambient of 47+ deg C in summer when unit operated at base load.

Issue was communicated to GE and from then to till date they have verified entire system - orifice / pipings / blockage in cooling jackets of support legs / any leaks or bypass / VTR-1 & 2 functional checks / cooling water contamination / flow checks - water collection per minute.

Nothing Conclusive so for and they now asked to check flow across the orifice at various points which yet to be confirmed.

Another issue we are facing now is Flame scanners getting over heated and loosing counts while unit operated at primary modes and at peak hours with ambient of 47+ deg C all 4 primary flame scanners coubnts loosing leading to trip the unit on loss of flame while all other operational parameters normal.

GE suspect there steam forming inside the system due to high temperatures which is resulting into improper cooling of aft support legs / flame scanners. Hence we made temporary arrangements to vent the system to remove any air/ steam trap in the system by connecting tubing at the vent plug on the aft leg and this tubing we have connected back to expansion / make up tank to avoid any loss of cooling water. We are crack opening this vent line connected to legs whenever high temperature alarm appears to release any air / steam trap.

In spite of repeated followups & checks carried out GE failed to resolve this issue and now being summer we are facing big challenge to operate the unit.

What i want to understand?

1. What happened in the cooling system which resulted into aft support legs high temperatures - while after commissioning units run with no issues till 2006.

2. What is the source of cooling water getting hotter in this GT alone while GT1A /1B no such issues?

3. What can be done to resolve this issue? like increase orifice size? etc.

Any feedback from the forum is appreciated.

Thanks,
Arun.

 

Arun/kumara,

Briefly, it certainly sounds like there is some obstruction in the cooling water system in the turbine compartment of that particular unit. Another problem might be that air trapped in the piping after disassembly/reassembly after a maintenance outage could be causing this problem. There are supposed to be air bleeds on the aft support legs which should be regularly and periodically opened to bleed air which can be trapped during normal operation in the aft support leg cooling assembly.

While air bleed valvess were originally supplied in the flame detector cooling water piping (tubing) they have long since been discontinued (for cost reduction purposes). So, to bleed air out of flame detector cooling water piping it's necessary to loosen the compression fitting at the high points of the system and at the outlet of the flame detector cooling coils and to bleed the air out of the tubing/coils. This is to be done when the turbine is not running so personnel can safely enter the compartment, and with pressure on the cooling water system (from the cooling water pumps). When bleeding air out of loosened compression fittings it's difficult to determine when there's no longer any air coming out of the fitting, and there's a great temptation to continue loosening the fitting until it eventually comes loose, spraying water/coolant all over the compartment and personnel. So, use caution when performing this.

Lastly, I've seen carbon steel piping used for portions of the cooling water piping inside the turbine compartment, and the piping is usually small-bore piping, and this can cause rust to collect and occlusions (obstructions) to form in the piping which restrict cooling water flow. So, sometimes it's necessary to try to "rod out" the small-bore cooling water piping in the turbine compartment to remove accumulated rust and debris which may have been left over from a poor flush of the cooling water system during installation and commissioning.

Hope this helps!

Please write back to let us know what you find!

 

Hi,

I agree with you on something is obstructing / blocking somewhere in the cooling water system and we are planning to take out all pipings flush them and also check flow across each orifice again to see if we are loosing any water flow.

However being said that, we are bleeding continuously both flame scanner & aft legs circuit still the issue is water flowing thru the circuit is very hot and our observation inlet water temperature entering into the system itself goes as high as 60 deg C during peak hours only on this unit compared to other to units which maintains at 50 to 52 Deg C which makes me think not enough heat exchange / cooling in fin fan coolers.. We need to see what best can be done here as well.

I shall keep you posted as we progress in coming days.

Thanks,
Arun

 

Arun/kumara,

My presumption was that the cooling water module was working properly--which I should have stated, but didn't; sorry.

One of the things which happens quite often with cooling water modules is that the strainer upstream of the pumps gets choked (blocked) and restricts flow. And, there is no differential pressure switch or -gage across the strainer. About the only indication there is a problem is low/decreasing pump discharge pressure, and problems maintaining pressure.

If the piping between the cooling water module and the turbine wasn't flushed properly then rocks/dirt/sand can cause problems in both locations.

Looking forward to hearing back about the outcome of this problem.

 

Hi..

We inspected cooling water system on this unit -

1. Both Cooling water Pumps outlet pressure is around 4.2 bar (Other units also develop same pressure).

2. No indication of sludge/bacteria growth in the system as per Water quality test report and per GE criteria parameters like PH, Iron, Corrosion Inhibitor etc. are maintained

3. As per P&ID for cooling water system - flow of 30GMP is required for the cooling of aft support legs. We tried to measure DP across the orifice when unit was down and following observations was made on two Gt's

On GT1B -
* before orifice - 1.6 bar
* after orifice - 1.4 bar
* Dp across orifice - 0.20 bar

On GT1A -
* before orifice - 1.8 bar
* after orifice - 1.6 bar
* Dp across orifice - 0.20 bar

Other details
* Pipe size - 1.5 inch (material Carbon steel)
* orifice size - 15mm (as found in both units)

Can you help us to calculate what is the flow going to Aft support legs per above details.

Thanks,
Arun

 

Arum/kumara,

To properly calculate a differential across an orifice one would need to know the orifice diameter, and to know a lot about the design and construction of the piping (length of straight runs of piping before and after the orifice; inside diameter of piping; etc).

To my thinking, why is there an upstream pressure differential--1.8 vs. 1.6? Why aren't they closer? 0.2 barg is ~3 psig--neither of which are insignificant differential pressures at the 1.8 or 1.6 barg pressure. Are you certain there are orifices installed in the flanges? (I've seen orifice flanges installed without orifices on most GE-design heavy duty gas turbines; they're there for "convenience" in the event an orifice needs to be installed for measuring/troubleshooting.)

How are you measuring the differential across the (allegedly-installed) orifice?

Have you asked GE to calculate the cooling water flow-rate to the aft support legs with the information you provided above? If so, what was their response?

Why the concern about the aft support leg temperature? How is the aft support leg temperature being measured on each machine? Is there any possibility turbine compartment cooling air flow patterns and/or flow-rates in the area of the aft support legs of the two units are different? We're the two units installed at the same time?

Lack of cooling of the aft support legs usually results in misalignment when running which usually results in higher than normal vibration at the #3 bearing. Is the unit experiencing unusually high vibrations at the #3 bearing or the turbine-end of the generator?

Are you measuring the temperature of both aft support legs, or just one?

It's most recommended to visually inspect the inside of the carbon steel piping, which can be done with a borescope, or to clean ("rod out") the inside of the piping while monitoring the effluent of the cleaning process to check for dirt, rocks, excessive rust.

Also, bleeding air out of lines can be a tedious process, and can take quite a long time using the typically-provided bleeder valves if those doing the bleeding aren't patient and attentive. Sometimes it's not enough to take someone's word for their perception--it's necessary to get out and see for one's self.

 

Hi..

We are measuring upstream pressure & downstream pressure across orifice on the flange which has provision to install gauge.

Yes we have verified orifice is installed at all locations as per dwgs.

There is individual RTD located to measure the aft legs temperatures at the outlet pipe and this unit leg temperatures are going up to 95 deg C on both. We have not seen any raise of vibrations on no#3 GT brg its around 4 to 5 mm/sec.

About water quality - we are maintaining as per GE recommendations and being tested every quarter. We have done borescope of legs itself with GE TA onsite and found nothing abnormal as blockage perhaps individual legs were flushed with all pipings removed.

About bleeding - we doing continuously and vents are connected back to make up tank to avoid loss of water and our observation cooling water is really hot what we can feel from these vent lines taken from individual legs.

GE wants to know flow across orifice and they advised to measure pressure drop across the orifice to calculate the flow and data has been sent to GE for their analysis.-

Data sent to GE -

1. before VTR1 - water going to lube oil cooler
installed Orifice (circular) size - 66mm
Pipe size 6 inch
upstream pressure as measured - 3.2bar
downstream pressure as measured - 2.3bar
Dp across oricie - 0.90bar

2. before VTR2 - water going to atomizing cooler, legs / flame scanner / purge air cooler
installed Orifice (circular) size - 43.5mm
Pipe size 6 inch
upstream pressure as measured - 2.2bar
downstream pressure as measured - 1.4bar
Dp across oricie - 0.80bar

3. Water flowing to aft legs
installed Orifice (circular) size - 14.9mm
Pipe size 1.5 inch
upstream pressure as measured - 1.8bar
downstream pressure as measured - 1.6bar
Dp across oricie - 0.20bar

So i was looking for some information from forum on calculation for flow across the orifice while we await for GE to come back with their recommendations.

Regards,
Arun

 

Arun/kumara,

Are you also continuously bleeding air/water from the lines of the other unit back to the cooling water tank?

Have you measured the temperature of the water being bled from the lines back to the cooling water tank?

I would think the formulae for calculating flow using the differential pressure across an orifice of know size would be suitable for the purposes. I don't think there's any rocket science behind this just because it's the cooling water system (thought depending on the amount of rust inhibitor and any other treatments added to the water perhaps the specific gravity is different than that of water). The formulae should be found in most engineering handbooks and probably even exists in spreadsheet format where you just plug in the values and it will calculate flows for you.

I am kind of questioning the upstream pressure readings as I would expect them to be near pump discharge pressure, at least ahead of VTR1-1. And, I would also wonder if the two VTR2-1 valves are adjusted to maintain the same atomizing air temperature, or if maybe there is some flow restriction through the AA Pre-cooler. May even be some air in the water side of the AA Pre-cooler and/or the L.O. Coolers of the unit with aft support leg cooling issues.

I also have to wonder if there isn't something different about the cooling air flows in the compartments, or an exhaust shell leak or flex-seal leak which is allowing hot air to impinge on the RTDs on the aft support legs of the unit with cooling water issues.

My best guess based on the information provided is still some kind of blockage/occlusion inside the carbon steel cooling water piping in the turbine compartment. No external inspection is going to prove or disprove that. I've seen that on more than one occasion, in the same piping--once it was just rust and debris collected in an elbow of the piping, and once it was a very bad weld that had also allowed the collection of rust and debris. And, once it was just debris from a very poor flush of the cooling water piping during commissioning.

Please keep us informed as you progress!

 

Hi further investigation..

If we look at the schematic dwg Cooling water system before VTR2 tapping taken to common header where it splits to flame scanner, legs cooling, purge air cooler which is supply line for all 3 system, and outlet of all system is again common header connected back to system afetr VTR2.

Our observation here at site on all 3 units - for flame scanner/legs piping routing is respected as per dwg but for the purge air cooler.

* Supply line of common header is connected at outlet of purge air cooler where we have an orifice of 0.450 inch installed which should have been at inlet.

* Return line of common header is connected to inlet of purge air cooler which should have been at outlet.

With this wrong pipe routing - we have hot water going into purge air cooler and outlet, which is further hot going to flame scanner header/legs piping getting mixed, raising temperatures further.

But issue of this high temperatures is only noted on GT1B and not in GT1A/1C which is strange or in question?

Checking with other customers 9E DLN 1 units for clarification and will take up with GE.

What do you think? can this be an issue?

thanks,
arun