6 replies to this topic

[CalvinG]

Hello all,
A new member here. I have a few questions regarding our gas dehy system. We have had problems for quite some time. We have no NGL or water dump on our contactor (705 psi vessel). A piece of 3/8" tubing has been installed to skim off the NGL's. This skim line is going to the glycol skimmer. (55 psi vessel) We move about 556 mmcsfd through our system. The burner bundle was replaced in Jan of this year. When pulled it was clean, however it had burned through, and leaked. So our problem now is that our reboiler stack temp is hovering around 990 F with a setpoint of 395 F and a flow rate of 31 gpm. We strip out about 7,000 bbls of NGL per day. If the TEG gets loaded up with NGL's it causes havoc. The skimmer does not seem to skim the NGL's like it should. We have a dampner on the exhaust, although it does not seem to do anything. Our Reboiler fires at 100% and can not reach the setpoint of 395 F, yet the stack temp reaches 1000 F at times.
Any ideas???

Thanks in advance.

[Art Montemayor]

Calvin:

You are welcome as a new member, but please don’t go against our Forum guidelines. I deleted your duplicate posting in the other Forum. Multiple postings of the same thread are very confusing and non-productive. They only achieve chaos.

I presume you are running a TEG dehydration unit that doesn’t have a vapor-liquid separator ahead of your TEG contactor. If that is true, it is a serious mistake to start off. With all the Natural Gas Liquids (NGL) that you are reporting, it is strange that you haven’t reported that your product gas is not up to the dew point specifications. You must be using your TEG circulation system as the means to separate the NGLs. This is not the normal way to handle a rich gas saturated with NGLs.

You state you burned out your reboiler tube bundle. I also presume you have a direct-fired reboiler. If your fired tubes were clean on the TEG side, then you must have had a defective bundle or you over-fired it and literally “burned” it out. Who designed and fabricated your TEG unit? What was the original heat transfer flux used on your reboiler and what was the designed flux used on the replacement bundle?

Your flue gas stack is running much too high and you are either over-firing your bundle or you don’t have enough heat transfer area to transfer the heat to the TEG in the reboiler. Something is either wrong in the design or in the operation and someone should take immediate steps to find out the real cause of why you are not transferring the heat in the reboiler. I don’t know who designed and fabricated your TEG unit, but I would expect a design heat transfer flux of at least 8,000 Btu/hr – preferably 10,000 Btu/hr. You should have the calculations – especially of the latest bundle replacement. What has been the history of your TEG unit? What were the original temperatures on the unit when it first started operations successfully (if it ever did)?

Where is your temperature sensor located within the reboiler? Is it functioning in accordance with modulating your burner rate? Or are you controlling your burner manually? What type of burner and fuel are you using?

I can accept the 395 ^oF reboiler temperature (I’d like to see 400 ^oF) since I don’t know your dew point specs on the product gas – nor do I know the stripping gas rate you are using (if you are). However, I don’t believe you are circulating 31 gallons per minute of TEG. 31 gallons per hour may be more like it.

I’m sorry to ask so many questions, but we need basic data in order to dig into this serious problem.

[CalvinG]

My Bad!!! I thought I had removed one of the posts. I will keep it clean in the future.

So, I need to clear up a few things first. It is indeed 31 gpm TEG flow. We treating over 1/2 (one half) a billion cubic feet of gas per day. The raw gas has a dewpoint of about 70 F and we are trying to get it to 10 F. There are actually two vessels before the contactor, a scrubber and a seperator. The reboiler is direct fired. Not sure of the manufacturer. The TEG system is not being used to skim NGL's, the NGL's are taken off downstream of the contactor. There is quite a bit of NGL carry over from the contactor to the skimmer by way of entrainment in TEG.

My current concern is reboiler temps. We are trying to maintain 395 F firing temp and keep the exhaust stack temp down as well. What is a standard for back pressure in the stack? Is the damper in the stack used to put back pressure on the unit to keep the heat from blowing through the reboiler? The bundle is new, however when the old one was removed it was discovered that there was not a "liner" in it to spead the heat out.

We also have a discussion going on amongst the crews abou the proper way to "skim" NGL's from the contactor, as there is not oil bucket on the contactor. One crew likes to skim to the Glycol skimmer, the other crew like to skim to a vessel that can not introduce the ngl's back to the TEG system. This system has been a pain since it's start date, personally I don't think it's very well designed, however, it's what I have to work with......Any more idea's?

[Art Montemayor]

Calvin:

I am also concerned with the extremely hot temperatures you are running in your reboiler tube bundle as evidenced by your very high stack temperatures. You are cooking the TEG as well as the new tube bundle. You are headed for tube bundle failure at this rate if you constructed the new, replacement tube out of carbon steel. Your stack temperature should be reading 500-600 ^oF. With 900 ^oF you are either over-firing the reboiler or you simply don’t have enough heat transfer area (bad heat flux) – or possibly your fired tube is fouled and can’t transfer heat fast enough. The fault has to be found before you arrive at a tube failure again. A damper on your flue stack may be of no use. It depends on what type of burner you are using. Normally, just natural draft is used on direct-fired TEG reboilers. What do you have?

I can’t help or furnish "ideas" if I don’t have or get any basic data from the field. Can you help us help you by answering the following:

What are your product gas dew point specifications? Are they being met?

If you can’t tell us who designed and fabricated your TEG unit, can you tell us how old it is? What was the original heat transfer flux used on your reboiler and what was the designed flux used on the replacement bundle? If you don’t know, can you furnish us the fuel gas flow rate burned in the reboiler (in Scfm or any other unit) and an idea of its composition? Also, tell us the dimensions of the replacement fired tube that is presently working. Is it constructed out of carbon steel – as is the custom?

What has been the history of your TEG unit? What were the original temperatures on the unit when it first started operations successfully (if it ever did)? Have you always been condensing NGLs within the TEG system? Or is this a recent happening?

Where is your temperature sensor located within the reboiler? Is it functioning in accordance with modulating your burner rate? Or are you controlling your burner manually? What type of burner and fuel are you using?

Are you using dry stripping gas and if so, what rate are you using? Is your TEG process like what I show in the attached Excel workbook? If so, fill in the pressures and temperatures at the appropriate places on the flow diagram.

What is the temperature of the Lean TEG entering the top of the contactor and what is the temperature of the wet, feed gas entering the bottom of the contactor?  TEG_Dehydration.xls   52.5KB   107 downloads

[mishra.anand72@gmail.com]

Can you route high temperature stack gas somewhere in the process to utilise wasted energy associated with this?

[CalvinG]

I have edited your drawing as per my system. Our tube bundle is carbon steel. I believe we are over firing the reboiler due to unknown heat flux. We have a blower at the burner, the burner is throttled by means of the TEG temp setpoint. The TI-XXX is low on the vessel like a thermoweld.

The Dewpoint specs are 70 F inlet and 10 F outlet. The discharge dewpoint is not being met. The TEG system including the reboiler is about 7 years old. I have only been here 3 years and the system has run like this since I started here.

Our gas supply has been getting wetter, therefore the load on the system has increased. The stripping gas rate is somewhere around 1100 cu.ft/hr and the fuel gas rate to the burner is designed at 0.15 mmscfd. Now whether that is what actually gets used is anyone's guess. We have not had a analisys on the burner and exhaust.

The vessel is Natco and still not sure who designed the burner. The burner blower is rated at 800 cfm at 1.7" WC I believe the tube that was replaced was about 11' long. I also suspect that our exchangers are getting plugged off from parrafain entrained in the TEG. The reason is that the HEX are not up to design temps. The parrafains are coming from the oil mist being carried over from the wet gas at the oil seperators.

In case the pic does not come back to you with the proper temps and pressures, I will list them here as per your drawing

Wet gas inlet temp 75 F
Wet gas inlet psi 745
Lean TEG 109 F and 31 gpm
Dry gas to feed gas coolers and NGL system 500 mmcfd 75 F, 710 psi and dewpoint 28 to 38
TEG/Hydrocarbon skimmer 50 psi 100 micron sock filters, charcoal filter bypassed, horizontal vessel and the hydrocarbon bucket dumps to waste water stream
Burner is natural gas fired with a blower. the firing is determined by TI-xx on the vessel in a thermaweld
We do not have a TEG expansion vessel.

Please ask more questions. I think we are headed in the right direction Tons of thanks

Oh yeah, we can not divert waste heat from exhaust to be used anywhere else.  TEG_drawing.xls   54.5KB   76 downloads

[Art Montemayor]

Calvin:

Thanks for the information on the Excel Flow Diagram. This is the most effective and efficient way of exchanging vital and critical engineering information.

Attached, find my revision #2 of the same PFD. Please note the calculation for what I estimate would be the natural gas firing rate in your burner. I come up with 50-55 Scfm of gas. If you are burning more that that, that proves that you are overfiring the tube - and permanent damage is being done with eminent failure just around the corner.

With the information you've furnished I form the opinion that either the fire tube is fouled or it is just too small. You haven't furnished answers to the size or the past operating history, so I can't form a firm judgment on that. However, from what you report, that's what I would diagnose and look into right away.

With approximately 1,000 ^oF in the stack gas, I don't give your stack and your fire tube much of a future life span. You are burning both of them up - with the fire tube being a very critical item.

I hope this helps you out and gives you some leverage on what to do next in order to remedy your potentially dangerous situation.

Good Luck.
 TEG_Dehydration_Rev2.xls   57KB   140 downloads