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#1 delphitt

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Posted 03 July 2008 - 09:48 AM

Hi all,

The vacuum pressure in our surface condenser has dropped from -0.84barg to -0.72barg in just one week. The handbook of evaporation technology has indicated several causal factors namely:
(1) inadequate cooling water to condenser or intercondensers
(2) air leak through water supply
(3) loss of steam pressure to ejector
(4) wet steam
(5) empty feed tank with resultant air leak
(6) sudden system leak
(7) fouled condenser

All of these has been verified as ok, except for the air leak. Does anyone know how to determine the presence of an air leak in the surface condenser, without taking the Plant offline?
Really appreciate your help...

sophia

#2 Dev 009

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Posted 03 July 2008 - 11:26 PM

Sophia air leak can be checked by very simple methods.

1. Burning candle method.
2. Soap bubble method.

If there at all any leak is there from atm into condenser due to which your vacuum pressure inside condenser is dropping, by putting candle close to probable leak points you can determine whether actually leak is there or not.
Similarly soap bubble method, blowing soap bubble ( like a child does) also gives you a indication of leak.


Try this.

Rohit

#3 Satyajit

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Posted 04 July 2008 - 02:44 AM

QUOTE (Rohit Namdev @ Jul 4 2008, 12:26 PM) <{POST_SNAPBACK}>
Sophia air leak can be checked by very simple methods.

1. Burning candle method.
2. Soap bubble method.

If there at all any leak is there from atm into condenser due to which your vacuum pressure inside condenser is dropping, by putting candle close to probable leak points you can determine whether actually leak is there or not.
Similarly soap bubble method, blowing soap bubble ( like a child does) also gives you a indication of leak.


Try this.

Rohit


Sophia,
You can go for smoke test or dry powder test to find out the leakage through flanges.
But I think you may try to check following things:
1. Condensate trap and its bypass( from after condenser of ejectors system).
2. Condensate pump split range recycle( level control) valve opening ( to avoid too much flushing in the condenser).
3.Check the condensate pump priming line, drains ( standby pumps) , pressure tapping etc for air ingress.
4. Please check the turbine energy balance .

Regards,

Satyajit

#4 fallah

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Posted 04 July 2008 - 04:58 AM

Increasing the rate of collection of noncondensable gases could be a cause for vacuum breaking.
Regards

#5 ankur2061

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Posted 04 July 2008 - 08:58 AM

Hello,

The best leak detection technology in high vacuum systems are Helium Leak Detectors. They are extremely reliable and the system are also available on a rent-a-system method.

"Alcatel" is a leading supplier of HLD's.

Here is a web link for you to look for further details:

http://www.adixen.com/

Regards,
Ankur.

#6 Jiten_process

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Posted 05 July 2008 - 05:19 AM

Hi,

the same problem we haf faced sometime before when i was at site for commissioning. Difference was we had there liquid ring vacuum pump in case where you have steam jet ejector.

But by virtue of it's performance characteristic, the performance curve of my liquid ring vacuum pump was a bit flat so it could handle considerable variation in air loss keeping vacuum constant.

But still we were facing vacuum drop problem and we carried out soap solution test on each joint of pipe and gaskets. We found that one of the flange gasket of steam inlet nozzle damaged and air was ingressing throught it.

so this was my experience, you simply take soap solution and try spread it on each flange joint, you will come to know if leakage is considerable.

all the best.

#7 Zauberberg

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Posted 07 July 2008 - 10:50 AM

Prior to field troubleshooting - in search for air ingress locations - a quick laboratory analysis (GC) of ejector off-gas stream should be performed, because it will save a lot of time and efforts, especially if air in-leakage is not the culprit. Just make sure you have proper sampling and analysis executed as fast as possible, in order to suppress possible reactions of O2 with off-gas contents.

By the way, I'm not convinced how can you be so sure there's no condenser fouling problem. Surface condensers (usually TEMA-X shell) are kind of tricky for troubleshooting. But anyhow, I haven't seen many fouling issues in VDU overhead ejector train. If condensate is not backed-up in the condenser (which you can easilly check by touching external surface of exchanger), then fouling is not very likely.

If all other items (1)-(7) are carefully checked, there's not much left for theoretical survey. Have you changed feed type and operating parameters (temperature, pumparoud flows and return temperature, velocity steam and stripping steam, etc.) in the last couple of weeks?

#8 Qalander (Chem)

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Posted 11 July 2008 - 01:34 AM

QUOTE (Zauberberg @ Jul 7 2008, 10:50 AM) <{POST_SNAPBACK}>
Prior to field troubleshooting - in search for air ingress locations - a quick laboratory analysis (GC) of ejector off-gas stream should be performed, because it will save a lot of time and efforts, especially if air in-leakage is not the culprit. Just make sure you have proper sampling and analysis executed as fast as possible, in order to suppress possible reactions of O2 with off-gas contents.

By the way, I'm not convinced how can you be so sure there's no condenser fouling problem. Surface condensers (usually TEMA-X shell) are kind of tricky for troubleshooting. But anyhow, I haven't seen many fouling issues in VDU overhead ejector train. If condensate is not backed-up in the condenser (which you can easilly check by touching external surface of exchanger), then fouling is not very likely.

If all other items (1)-(7) are carefully checked, there's not much left for theoretical survey. Have you changed feed type and operating parameters (temperature, pumparoud flows and return temperature, velocity steam and stripping steam, etc.) in the last couple of weeks?

Zauberberg, I appreciate the excellent advice regarding GC(Gas Chromatography Analysis of) outflowing gases/No-condensibles and also the Hotwell Vessels/Tanks Liquids Contents analysis on regular basis to develop/generate the useful info record for problem solving and trouble shooting.
As the physical check-up of huge(most usually complex system)containing numerous pumps Pipefittings/pipelines and process equipments etc. is extremely cumbersome.
Although finally it will have to be carried-out in a phased-out manner.
Best Regards
Qalander

#9 delphitt

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Posted 12 July 2008 - 06:21 AM

Thank you all so very much!
I have listed out all these and are making preparations to execute.
I also checked the website and found it really useful....
I'll keep all all posted on the progress.
Thanks again!

#10 delphitt

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Posted 12 July 2008 - 07:00 AM

Hi all,

There has been no variation in the Operating Parameters beyond the normal range and I had ruled out fouling since the condenser was pressure washed and cleaned in a Plant shutdown just a few weeks ago. Also, the differential pressure on the Cooling Water side was marginally below 1 barg, and the design for the unit is 1 barg.

I measured the hotwell temperature and it is actually 6 deg C higher than usual. The bypasses/traps were checked and were all ok. What else should i look for?

The condensate pump settings were ok in the field, however, I have asked that the gauges be calibrated and and a Maintenance check be done. I'm still running an efficiency calculation as recommended and our Lab will be doing the GC as recommended.

thanks again,
Sophia

#11 Zauberberg

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Posted 12 July 2008 - 07:40 AM

Increased hotwell temperature is a consequence of higher condensing pressure in vacuum condensers, indicating lower coolant (CW) flow or increased overhead load. If this is a system with pre-condenser upstream of 1st stage ejector, try to increase or decrease column top temperature in order to establish possible cause-and-effect between these two variables.

Also, since you are measuring vacuum and not the absolute pressure in the system, be aware of the fact that changes in barometric pressure affect absolute pressure in the tower (Pa = Pb - Pv). In such case, decrease in barometric (air) pressure will correspond to the same absolute pressure inside the vacuum system, if decrease in measured vacuum is the same as the drop in ambient air pressure.

#12 benoyjohn

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Posted 12 July 2008 - 11:45 AM

Sophia,

Please rule out if you can,

1. Tube leak. (CW leaks into the condensate)
2. Any incorrect installation of condensor after the shutdown. e.g. Missing out channel partition plate.

Caution : Trying out the "burning candle" method may not be always safe in a hydrocarbon processing plant.

Regards
Benoy

#13 ASH25320

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Posted 24 July 2008 - 10:49 AM

Pls control any air ingress by applying johnson or some sealing tape at all flange joints, this has helped us a lot in improving vacuum in condenser. Also try to apply grease or something on other pumps flange to avoid air sucking. During shut down we can carry out water fill test to test leakages

#14 Mehrdad

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Posted 25 July 2008 - 11:15 PM

sophia

as you attend,one of the most effective factors on vaccum , is cooling water temperature.

you can check cooling tower accuracy and wet bulb temperature .

in this time i have the same problem for high wet bulb temperature in my plant.
sincerely
mehrdad

#15 pawan

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Posted 26 July 2008 - 01:19 AM

After so many points I will suggest checking pressure
at each stage of ejectors (I assume there must be 2-3 stages of ejectors
Just check the inlet & throat pressure and you will be able to find out
problematic ejector.

I suspect leakage in ejector nozzle.

#16 man2k

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Posted 26 July 2008 - 08:24 AM

HI ,
we use to have such a problem in our condensers.
1. check for any blockage in suction line of ur ejector/water ring pump.(water accumulation in this line or any valve element in the suction may got stuck in partially open condition will also lead to such problem)
2. leak check online can be done with smoke (agarbhathi) near joints & leak prone points.
3.chemistry control of the system can also important dissolved oxygen if water is the condensate as in the case of turbine exhaust steam condensers.
best wishes

#17 C.Cesar

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Posted 28 July 2008 - 10:49 PM

Delphitt,

Focusing in a possible vacuum system problem :


- First of all, could you post a diagram of your system ?

- I will assume that your vacuum system is in good physical condition (no excessive wear, no leaks in gaskets, etc...) and that utilities being provided to the system are OK (motive steam pressure, temperature and quality are OK, and water in the inter-condenser and post-condenser are being supplied with the proper pressure, initial temperature and volume ). I´ll also assume that you have the performance curve of your system, in wich the absolute pressure each stage has to mantain is clearly defined. Every jet ejector manufacturer must supply this information to their clients.

- Given the vacuum you informed before ( -0.84 Bar gage ), and assuming your barometric pressure is near 760 torr A, this imply that your vacuum system is (or should be) providing an absolute pressure of about 130 torr A ( = 760 - (0,84 x 750) torr A).
This pressure can be provided by even a single stage (actually it depend, for example, on the amount of non-condensable gases you are handling) - but I will assume a 2-stages system, separated by an inter-condenser. A common figure is : Psuction of first stage = 130 torr A, Psuction of 2nd stage = [250-300] torr A. This kind of system may have also an after-condenser, discharging to the atmosphere.

- You can check the absolute pressure each stage is maintaining installing a vacuummeter in the suction head of each stage. Every ejector stage must have a 1/2" or 1/4" tap, where you can put the instrument. Compare the values with the original performance curve.

- "Shortly" speaking :

A. if the absolute pressure in the 2nd. stage is above the pressure informed in the performance curve (I am assuming - as you have already said - that utilities are OK, and its physical conditions are fine), than we have 02 alternatives :
case a. this stage is handling a load above the project ;
case b. its discharge pressure is above the project.

Considerations :

case b : check if there is no obstruction in the dischage tubulation of the ejector (or of the after-condenser, if it exists), like condensate, for example.

case a : This extra load may be coming from air leakage, extra non-condensable loads coming from your process, or even an extra steam load coming from the inter-condenser. This extra steam load may be coming from basically 2 sources :
a. steam leakage in the motive steam nozzle of the first stage (as Pawan suggested), that may be a load above the inter-condenser design ;
b. steam that saturate the extra non-condensable load coming from the process, or from the leakage source.
Steam can also be coming from a fouling condition in the condenser (assuming this is a surface condenser).
If you have a direct-contact condenser, you should check the water distribution inside it, for example, if spray nozzles (if exist) are working properly. Poor water distribuition may cause improper condensation.

B. if the absolute pressure in the 2nd. stage is OK, check the Psuction of the first stage. If this suction pressure is above the design pressure, we have again the same options discussed above :
case a. this stage is handling a load above the project ;
case b. its discharge pressure is above the project.

Considerations :

case b : Even if the suction pressure of the 2nd. stage is OK, there may be an extra pressure loss in the inter-condenser, causing the pressure at the discharge of the first stage to be above its design discharge pressure. This may be caused by an excessive load reaching the inter-condenser (extra non-condensable, or even extra steam load). The inter-condenser may be condensing the extra steam load due to design considerations (extra condensing area) , but the price to pay is an additional pressure loss. This could be verified checking the initial and final cooling water temperature. Assuming that the water volume being supplied to the condenser is OK, you should verify a bigger delta T. The problem with this approach is that is very uncommon to have termometers accurate enough in the field to detect small variations (small variations in this delta T may signify big difference in steam load).


case a : This extra load may, again, be coming from air leakage, extra non-condensable loads coming from your process, or fouling condition in your process condenser. Don´t forget that any extra non-condensable load coming from your process will impact the performance of the condenser. The condenser will be operating with a "U" smaller than the projected "U", and, depending on the gases you are handling, the amount of gases that are saturating the
non-condensable gases will be bigger, causing the extra load in the first stage.


Send me a diagram of your process, an I´ll send you a worksheet indicating all the field data you should collect. All the talk above can be greatly reduced with the worksheet in hands.


By the way, I have worked for about 10 years in a jet ejector company, and I may be of some help in your case.

#18 delphitt

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Posted 30 July 2008 - 06:09 PM

Thank you all for your highly informative input.
Based on all your recommendations, this is what I found so far...

We had noticed a spike in the Silica concentrations of the turbine condensate a few days after the drop in vacuum pressure. We came up with two possible root causes:
1. Silica carry-over from the boilers
2. Leaks in the vacuum condenser – the post by Benoy got me thinking about the jobs that were done during the outage.

Several lab tests over a few days showed that the silica concentrations in the boiler feed water and saturated steam samples were within the ASME limits of <20ppb. This indicated no significant boiler carry over.

We ran traced dye tests on the turbine condensate and picked up traces of a cooling water treatment chemical dye - indicating cooling water leakage in the condenser. This was initially ruled out since we had recently cleaned and pressure tested the condenser. It is apparent that this was inadequately done. Thanks Benoy...

I also reviewed the reports from the outage and they have all indicated that both the inter and after condensers were washed and cleaned, valves were serviced and pipelines were cleaned from debris. All the flanges etc. are, however, to be sealed off to prevent any air ingress.

The GC on the off gas was inconclusive – all the results were below the detectable limits of the instrumentation so we will be running the test a second time.

Cesar, I have tried to locate the performance curve for the ejector package but there was none in any of our data books. The ejector package is a GRAHAM design, dated back to 1994. I attached the only flow diagram we have - I can have a more detailed one drawn soon. I have arranged to have the vacuum meter tomorrow so I can run the pressure checks you suggested, but without the performance curve, I’m not sure how to interpret the data. I'm looking forward to see the calculations...

thanks a lot!!
Sophia

Attached Files



#19 C.Cesar

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Posted 30 July 2008 - 08:30 PM

Sophia,


Thanks for the diagram.

This setup is a very common design for condensing turbines. It is actually a dual system. Since this is a critical process, and it cannot be stopped, you have two systems, one being the default, and the other being the backup.

Let´s get into some detail :

A. Vacuum System :

a. The most common back pressure for a condensing turbine (working in a tropical country, with
cooling water at about 30ºC) is about 3"Hg A ( 76,2 torr A ,or about 75 torr A ). We have sold these type of systems all over Latin America.

b. In normal conditions, you utilize only one of the first stages, and one of the second stages.
Both condensers (inter and after) are designed to condensate steam coming from all the stages
simultaneously.

c. If the ejector system load increase (whatever the reason), you have the option to turn on both first stages (and only one second stage), or even all 4 ejectors.

d. Of course, this alternative has its limitations (in relation to the amount of extra load, and if this extra load is non-condensable or condensable.).


B. Water Leakage and its effects in the performance of both the condenser and the vacuum system :

So far, you are not able to precise the amount of water leakage, so, it is not possible to predict its effects. But, if water leakage is really the only modification from the standard parameters, we have basically two options :
- first : this water is being evaporated inside the condenser, and, if the condenser is not being able to condensate its normal steam load plus the extra steam (from the evaporated water), this extra steam (or part of it) is going to the vacuum system. This increase in load cause the decrease in vacuum.
- second : the water leakage means less water inside the tubes, decreasing its capacity to condense steam, causing more steam going to the vacuum system, decreasing its vacuum.

---------------------------------------------------------------------------------------------------------
So, if you have not already done this, try to turn on both first stages, or even all four stages. I don´t know the size of your turbine, but this decrease in delta H is surely of some concern...
---------------------------------------------------------------------------------------------------------



Just for the sake of curiosity, please, clarify : what is the altitude of your plant ?
I thought that it was near sea level ( 0 meter), with a barometric pressure of 760 torr A. I am
addressing this issue because given that your nominal vacuum is -0,84 Bar A (630 torr vacuum = 750*0,84), this imply that your barometric pressure is about 706 torr A (76,2 torr A + 630 torr vacuum), with a altitude around 700 meters. Of course, I am assuming the the design pressure of your condenser is about 3" Hg A.

Best regards.

Cesar

PS: Try talking to Graham. They must have your site information in their databases.

#20 delphitt

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Posted 01 August 2008 - 06:39 AM

Hi all...
As per your recommendations, my investigations have brought me here:

As you rightly said, we have a first stage ejector and a second stage ejector online at the moment. My concern is that the motive steam (LP steam) is on a common header to all four. If i put more ejectors online, won't i effectively drop the steam flow to each one?

The condenser is designed to handle turbine exhaust from three compressors at a flow of roughly 1576 Metric Ton/day. Two of the three compressors are operating optimally, the third, however, is not - we are having trouble building the speed on the machine, and as a result, we have to run the machine at a lower discharge pressure ( a whole other can of worms). And with the lower vacuum, this is becoming increasingly difficult.

The design specs on the condenser are as follows:
Inlet steam pressure = 0.145 bar A
DP allowable = 1.0 bar A
We are at sea level....
Pabs = Pgauge + Patm
Barometric (atmospheric) pressure = 1.013 bar A
vacuum pressure = -0.84 barg
absolute pressure = 0.173 bar A = 130 torr

Many thanks,
Sophia

#21 Qalander (Chem)

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Posted 01 August 2008 - 02:45 PM

Dear Sophia Hello,

May I just recapitulate on whole of thread and the various stages of discussions, exchanges of advices by really supportive and guiding forum colleagues!
The jist of matter 'although slightly bitter' is that

•your initial posting and subsequent earlier posts somewhat lacked specific explanation of the problem having
•Multiple inter dependent and co-related systems. That typically resulted in such long discussion.
•We all ought to be more explicitly communicative with precise and as complete as possible details in the first instance to get better/prompt reply to our queries accurately.

In the end the velocity or the flow rate of carrier fluid is very important as the main driving force therefore this alone can affect very heavily.

As you lastly mentioned that this stream is dependent on incoming flow exited by condensing in-service turbines; thus the efficiency of vacuum pulling will greatly vary.

Hope this helps.
Regards
Qalander

Best Regards

#22 C.Cesar

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Posted 06 August 2008 - 06:32 PM

Sophie,

I spent the last 3 days in a mining company, with little access to the internet, so I could not post any additional suggestion.

1. My suggestion to turn on both first stages, or even the four stages, is just a temporary measure, until you can solve the leak problem. Turning on the additional ejectors will increase your vacuum level, since they can handle an additional load - and your turbine could work with a more favorable delta H.

2. To check if your manifold can handle the extra steam load (extra ejector motive steam load), since you do not have any technical data about the ejector system, take a look at the data attached to this message. This is from a German jet ejector company (Wiegand), and give the amount of steam that can flow in different tubes, at different pressures (with v=30 m/s). Assuming the same pressure in the incoming tube (from your boiler) and in the tubes going to the ejectors, check if the tube arriving at your manifold can handle the load corresponding at the sum of all the tubes going to the ejectors.

3. Suposing you have manometers between the valves and the ejectors (all the ejectors), and keeping the default ejectors working, try to open the valve of the other first stage, slowly, and check if the motive pressure of the default first stage is being kept constant. If this is the case, open it until the pressure of the extra ejector is the same as the default first ejector stage.

4. If the vacuum increases, OK. If not, try to work the same way with the other second stage. If, after these procedures, the results are still the same (same bad vacuum), you do not have an alternative: solve the leak problem.

5. Again: I´m only trying to give you some alternative to keep your turbine working properly. It is much more important (and cost effective) to keep your turbine working in good conditions than to spend extra motive steam to turn on all the ejectors on your vacuum system.

6. Have you talked to Graham (your ejector system manufacturer)?

Finally, sorry for the prolixity; I´m not exactly fluent in English, and I tend to detail the procedures a lot.

My e-mail is: ccesar@estadao.com.br

#23 C.Cesar

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Posted 06 August 2008 - 06:37 PM

Qalander,

Please, check my last message to Sophie. I hope it has became clear that my point is to keep the turbine working in the best possible conditions, using the jet ejector system resources, until the leak problem is solved.

#24 Qalander (Chem)

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Posted 07 August 2008 - 11:38 PM

QUOTE (C.Cesar @ Aug 6 2008, 06:32 PM) <{POST_SNAPBACK}>
Sophie,

I spent the last 3 days in a mining company, with little access to the internet, so I could not post any additional suggestion.

1. My suggestion to turn on both first stages, or even the four stages, is just a temporary measure, until you can solve the leak problem. Turning on the additional ejectors will increase your vacuum level, since they can handle an additional load - and your turbine could work with a more favorable delta H.

2. To check if your manifold can handle the extra steam load (extra ejector motive steam load), since you do not have any technical data about the ejector system, take a look at the data attached to this message. This is from a German jet ejector company (Wiegand), and give the amount of steam that can flow in different tubes, at different pressures (with v=30 m/s). Assuming the same pressure in the incoming tube (from your boiler) and in the tubes going to the ejectors, check if the tube arriving at your manifold can handle the load corresponding at the sum of all the tubes going to the ejectors.

3. Suposing you have manometers between the valves and the ejectors (all the ejectors), and keeping the default ejectors working, try to open the valve of the other first stage, slowly, and check if the motive pressure of the default first stage is being kept constant. If this is the case, open it until the pressure of the extra ejector is the same as the default first ejector stage.

4. If the vacuum increases, OK. If not, try to work the same way with the other second stage. If, after these procedures, the results are still the same (same bad vacuum), you do not have an alternative: solve the leak problem.

5. Again: I´m only trying to give you some alternative to keep your turbine working properly. It is much more important (and cost effective) to keep your turbine working in good conditions than to spend extra motive steam to turn on all the ejectors on your vacuum system.

6. Have you talked to Graham (your ejector system manufacturer)?

Finally, sorry for the prolixity; I´m not exactly fluent in English, and I tend to detail the procedures a lot.

My e-mail is: ccesar@estadao.com.br


Dear C.Cesar Hello/Good Morning,
I have been busy with domestic urgencies.
I do endorse your concept for initial trouble shooting/problem solving phase,However subsequently the optimum and economical operating conditions/set of equipments in service can be and should be kept.
I myself not master of English language please be rest assured.
Best Regards to all.
Qalander




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