Vacuum Systems

Mr. Norman P. Lieberman – Why does an increase in the tower bottom’s level in my vacuum tower cause a loss in vacuum? Does this indicate an excessively high tower bottom’s level?

--George

            George – No. The level is not the problem. It’s that your boot temperature is too hot. The problem is excessive cracked gas formation in the boot. Assuming your level change is within the level span of the boot, increase the level from 30% to 70%. If the vacuum is adversely affected, try lowering the vacuum tower bottom’s temperature, using the circulating boot quench, by 10°F. You are also promoting coke formation in the boot which may coke-up the suction of your vacuum tower bottom’s pumps.

--Norm

Vacuum Systems

Mr. Norman Lieberman – What does it indicate if we have a gradual increase in CO₂ in the vacuum tower waste gas to the vacuum heater? A year ago the CO₂ concentration was one percent and now it is ten percent. Our vacuum is still quite acceptable. Also, the nitrogen (but not oxygen) concentration of the waste gas stream is also increasing, which I assume indicates an air leak.

--Carl

            Carl – You most likely have an air leak in the vacuum heater transfer line feeding the vacuum tower. The leak is getting progressively worse. Since the leaking line is under a slight vacuum, relatively small amounts of air are drawn into the vacuum tower. The oxygen then reacts with the hydrocarbons to form CO₂ (and perhaps some CO). Regardless, if you have a sudden loss of vacuum, the area of the leak will pressure-up. Hot vacuum tower feed will erupt from such a leak and auto-ignite. This is a very dangerous situation and I’ve been involved in such incidents three times in my career.

            I don’t really know how to advise you. Shutting down to repair the leak in the transfer line may very well result in a fire. One of my clients burned down their vacuum tower due to such a failure. Sorry for the bad news, but I believe my analysis is quite accurate.

--Norm

Vacuum Systems

Norm – I was reading your chapter on vacuum system operation. You talked about how raising the level in the seal drum can improve vacuum. I have observed this on our vacuum tower myself.  Certainly, this is not normal. We have to flood our seal drum to sustain a reasonable vacuum.

--Harvey C.

            Harvey – Unfortunately, you have developed a leak in the barometric drain line (i.e., your seal leg inside the seal drum). This is caused by biological corrosion. Raising the level to submerge this leak stops the seal drum gas from getting sucked-up into the leg and interfering with this drainage. I know it seems backwards, but flooding the drum will lower the condensate back-up level in the condenser. This lowers the vapor outlet temperature from the condenser and thus unloads the downstream ejectors.

            I suggest that you add some biocide to your seal drum to suppress the biological corrosion. Then, during the next turnaround, replace the carbon steel seal legs with 316 (L) S.S., or whatever allon is suitable for your service.

            I’ve discussed a similar incident in my book, Troubleshooting Process Operations, 4^th Edition, PennWell Publications.

--Norm

Vacuum Systems

Mr. Lieberman – I have a vacuum tower question. Reducing the ejector steam supply below its design (vendor specified) pressure improves vacuum. How could this happen? I suppose it’s best to operate at the lower seven bar pressure, rather than the design ten bar of steam pressure.

--Sriganesh

            Sriganesh – Likely one third of the jets I look at have the property that reducing the motive steam supply pressure below the design value improves vacuum or at least does not harm the vacuum.

            The two most common reasons for this are that the ejector steam nozzle is badly worn, or that the downstream condenser is overloaded. Either way, it’s an indication of poor system performance.

--Norm

Vacuum Systems

To Norman Lieberman: Norman. We met in India last year. I have a serious problem. Our vacuum on our asphalt vacuum tower is always bad. But sometimes it suddenly becomes much worse. It seems to correlate with increased naphtha product from the seal drum of the vacuum tower overhead system. As a result, we cannot produce proper grade of asphalt for sales. Any advice would be greatly appreciated.

--Kumar

            Kumar – If the naphtha production rate is very high, the naphtha liquid level in the first stage condenser (or precondenser) may start to back-up. This reduces the surface area exposed to the condensing vapor, which consequently will increase the vapor load to the downstream ejector. However, if the naphtha liquid level rises to the bottom edge of the air or vapor baffle inside the condenser shell, then the vapor is trapped inside the condenser. Pressure will build inside the condenser shell until the pressure is great enough to push the liquid level down below the bottom edge of the air baffle, so that the vapor can escape out of the condenser.

            On one occasion, I found this problem to be caused by a high liquid level in the seal drum, which was a consequence of an erroneous seal drum level indication. Also, the seal drum naphtha pump had lost capacity due to a worn pump impeller wear ring. Hope this helps.

--Norm

Vacuum Systems

Mr. Lieberman – Which should be hotter – the vapor or liquid outlet of my vacuum tower pre-condenser? Currently the liquid outlet is approximately 20°C and the vapor outlet is 40°C. That difference is increasing and our tower top vacuum is becoming progressively worse.

--L.T. Lee

            To Mr. L.T. Lee – By design, the vapor outlet of a normal surface condenser (where the vapor outlet is located on the side of the condenser shell) is cooler by about 20°F-30°F. This is accomplished by an internal baffle located inside the tube bundle. It’s called the air or vapor baffle. It forces the vapor to flow down and then up across the tubes.

            There are two possible reasons for your problem. First this air baffle may be leaking. This is always caused by defective air baffle seal strips (please see my book, Process Equipment Malfunctions, McGraw Hill, 2011.

            Secondly, there may be condensate back-up due to inadequate drainage from the seal leg. Your seal leg may be plugging or your seal drum may be filling with corrosion products or sludge. Blowing out the seal legs with steam will help in this case. Perhaps the seal leg is sucking in air, due to a seal leg leak. Such a leak will prevent proper drainage through the seal leg.

            Regardless of the cause, an increase of the vapor outlet temperature is certain to cause a loss of vacuum and could possibly result in the jets making a surging (i.e., erratic) sound, which is a certain indication of a loss in the sonic boost of the jets.

--Norm

Vacuum Systems

     Dear Dr. Lieberman – I have purchased many of your books, in which you have kindly invited questions. My question pertains to our new vacuum tower operation. Every morning about 10:00 AM, our vacuum ejectors start to make a regular sound. The operators say the ejectors start to breathe. At the same time our flash zone pressure increases from 10 to 12 mm of mercury to about 20. Any suggestions you may offer would be sincerely appreciated.

--Amed

     Amed – Your jets are surging. Most likely, because of higher discharge pressure. The cause of the higher discharge pressure is probably an increase in the plant’s cooling water temperature as the sun comes up. When I say your jets are surging, I mean that they are losing their sonic boost as described in my book, Process Equipment Malfunctions.

     To start with, try back-flushing the condenser water side on the discharge of the surging jet. Unfortunately, there are dozens of different things that can cause jets to surge. But anything you can do to improve water flow through the downstream condenser is sure to help to some degree.

--Mr. (not doctor) Norman Lieberman

Vacuum Systems

Norm – I was reading Troubleshooting Process Operations. In your chapter on vacuum towers, you talked about gas composition of the sea drum off-gas. I sampled this stream. It is 40% hydrogen sulfide. Is this possible? My boss said that I have done something wrong.

--Raymond

            Raymond – First, let me warn you to be careful. One breath of that off-gas will knock you out. Yes, I’ve measured up to 40% myself in Aruba, when the vacuum tower bottoms was about 4% sulfur. There is not much to be done to reduce the H₂S concentration. More typically, I see off-gas in the 10%-20% H₂S range. I hope this helps with your supervisor.

--Norm

Vacuum Systems

Mr. Norman Lieberman: I attended your Troubleshooting Seminar in 2001 in Ft. McMurray. I have a question relating to my vacuum tower (I’m now working in a refinery on the Gulf Coast). What does it mean if ice forms on the outside of the ejector body?

--Thank you for your help. I really enjoyed your seminar. Frank.

           

Frank – This is a sure indication of water in the motive steam supply. Not just a little water, but probably over 10%. If you can eliminate this moisture, I’m sure that you will see an immediate improvement in your vacuum tower performance.

--Norm

Vacuum Systems

Dear Mr. Norman – Our vacuum tower top pressure is 40-45 mm of mercury. We have two large vacuum jets on top of the tower. The front part of both jet bodies is around 80°F.  The steam supply is 150 psig and 360°F. The temperatures are obtained using my infrared gun. Is that 80°F normal, or good, or bad?

-- Henry R.

           

Henry – The 150 psig, 360°F motive steam may be saturated or it may be wet. When steam expands through the ejector nozzle, its temperature is converted to velocity. The more efficient the conversion, the higher the velocity of the steam. The greater the velocity of the steam, the more efficient the compression of the vacuum tower off-gas. Hence, in that sense the cooler the mixing chamber (i.e., the front end of the jet), the better.

            However, if the mixing chamber is cool, that could be due to moisture in the supply steam, flashing as it enters the mixing chamber. This conversion of latent heat to sensible heat just robs energy from the motive steam and thus slows down the motive steam as it enters the diffuser. And this is bad.

            If the part of the diffuser immediately downstream of the mixing chamber doesn’t get too hot to touch within one or two feet of the mixing chamber, that’s an indication of wet steam.

--Norm