CENTRIFUGAL PUMP HEAD VS FLOW PERFORMANCE CURVES

"Norm, does the discharge pressure from a centrifugal pump

always increase when the discharge valve is throttled back?"

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EFFECT OF REFLUX ON FRACTIONATOR TOP TEMPERATURE - CARL & CLARE

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EFFECT OF REFLUX ON FRACTIONATOR TOP TEMPERATURE

"Norm, is it always true that increasing reflux will decrease tower top temperature?"

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STEAM CONDENSATE DRAINAGE FROM REBOILER BLOWING CONDENSATE SEAL - CARL & CLARE

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STEAM CONDENSATE DRAINAGE FROM REBOILERS BLOWING CONDENSATE SEAL

"Norm, sometimes when I open my reboiler steam condensate outlet valve, 

the reboiler duty decreases. What causes this?"

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ADJUSTING STEAM TURBINE SPEED TO MINIMIZE STEAM CONSUMPTION (VIDEO)

"Norm, how can I set my steam turbine speed to 

reduce energy waste on my centrifugal pump?"

Adjusting Steam Turbine Speed to Minimize Steam Consumption - Carl & Clare

OPTIMIZING FRACTIONATOR PRESURE (VIDEO)

"Norm, how do I optimize my fractionator operating pressure?"

Optimizing Fractionator Pressure - Carl & Clare

STALLING A THERMOSYPHON REBOILER (VIDEO)

"Sometimes the reboiler duty to my splitter dies off, even though 

I increase the steam flow to maximize. What's my problem?"

Stalling a Thermosyphon Reboiler - Carl & Clare

HOT VAPOR BY-PASS PRESSURE CONTROL (VIDEO)

"Should I open or close the tower hot vapor by-pass pressure control valve to lower my tower pressure?"

Hot Vapor By-Pass Pressure Control - Carl & Clare

INCREASING COOLING WATER FLOW thru an ELEVATED CONDENSER or COOLER (VIDEO)

"Norm, should I always keep my water outlet valve, on my water cooler, 

wide open, to maximize cooling water flow?"

Increasing Cooling Water Flow thru an Elevated Condenser or Cooler - Carl & Clare

Seal Drum Problems for Vacuum Systems

            Likely ⅓ of the problems I encounter in refinery vacuum systems are due to difficulties with the seal drum. For example:

1. The seal legs are too short. This causes the bottom portion of the seal leg to be exposed to the vapor, which is then drawn up into the condenser, which no longer drains.
2. The seal legs are too long. Many seal drums have the bottom of the seal leg 4” above the bottom of the vessel. Between turnarounds, sludge covers the bottom of the seal leg and the condensers stop draining.
3. Poor level control in seal drum. The seal baffle should prevent loss of the liquid seal. However, a high level will cause condensate back-up.
4. Corrosion of seal leg inside drum. The corrosion is biological in nature. The holes in the legs above the overflow baffle will then cause the leg to lose its seal and condensate drainage will stop.

Troubleshooting Process Problems

             My mother could never understand my work.

            “Mom,” I explained for the hundredth time, “I’m a field troubleshooter.”

            “So. Norman, how much do they pay you? It’s a good living?”

            “I do okay.”

            “Mrs. Silverman’s son, Bernie. He’s a big lawyer in New York. He charges by the hour. One hour is $200!”

            “Yeah mom. I do okay. I charge $2,400 a day.”

            “And who would pay so much? Only crazy people! If only you had become a doctor – like your cousin Samuel. He has a big practice in Miami.”

            “Mostly I work for Exxon, Chevron, BP, and Mobil. They have lots of problems and lots of money. As long as I can solve their problems, I can charge anything. The people who hire me aren’t spending their own money. They don’t care.”

            And then for the hundredth time, the same inevitable question.

            “Norman. Don’t these companies have their own engineers? They should hire engineers themselves. Mrs. Howotiz’s son, Nathan. He’s an engineer like you. Maybe you could tell Exxon. They could hire him. He’s been looking for a job since summer. He’s such a nice boy.”

            “Look mom. I’ve explained all this to you before. Exxon and Chevron have thousands of engineers already. But they’re office engineers; they’re telephone engineers; they’re computer engineers. They spend their days attending meetings and talking on their cell phones and sending emails. They’re professionals.”

            “So, professional is good. Mrs. Goldberg next door always says, ‘If only my daughter Sarah could marry a professional like your son Norman, I would die happy.’”

            “Mom. I’ve told you before. I’m not a professional. I’m a worker with a trade. My trade is applying Chemical Engineering principles in the field to solve refinery process problems.

            “So Mr. Worker, you want some lunch? Look at you. You’re all skin and bones. I’ll heat a nice bowl of chicken soup for you.”

            “You see mom. Those engineers who work for the big oil companies – they’re all pretty smart. They’re good engineers.”

            “But not as smart as my son. Even Mrs. Silverman in 4-C says your son Norman is really smart, but too skinny. You like noodles in your soup?”

            “It’s not a matter of being smart. Or having an engineering degree, or having lots of experience. Those things don’t help solve problems all that much. It’s something else.”

            But mom was no longer listening, “Where’s your father? He must have fallen asleep in the park again.”

            “You see mom. It’s a matter of determination. When I have a refinery process problem to solve, it’s a matter of life and death. Just last week, I risked my life climbing a rickety scaffold to get a skin temperature on a jet fuel draw-off line. If I need a sample, and it’s against the plant’s safety practices to get such a sample, I’ll wait until nobody is watching, and get the sample myself. If I’m working on a problem, and I’m tired, hungry, and cold, I still keep going. I’ll never give up. Better death than defeat. The Shell and Conoco engineers don’t look at their jobs my way. They’re bound by safety practices and company rules as to what engineers and operators are allowed to do. But rules never apply to me. I can do anything.”

            “But why is that?” mom asked.

            “Because I’m determined.”

            “Norman. Go and look for your father. He’s probably playing cards with the Mexicans in the park again. Did you know that your father speaks Spanish?”

            “Yeah mom. I know. I’ll go and look for him. Save the soup. Don’t give it to the kids.”

Distillation-Effect of Feed Preheat

           Increasing feed preheat to a distillation tower typically makes fractionation worse, assuming that the reflux rate is fixed. That means, that if the concentration of the heavy component in the overhead product is held constant, then the concentration of the light product in the tower’s bottom product will increase.

            But why?

            The reason is the reboiler must decrease. If the reflux rate is held constant, and the concentration of the heavy component in the overhead is held constant, then the condenser duty (i.e., heat removal) is also constant. Thus, if I increase the heat input with the feed, it follows that the reboiler duty must go down to maintain the tower heat balance.

            A decrease in the reboiler duty will also decrease the vapor flow through the trays between the feed tray and the bottom tray. As this vapor flow goes down, the stripping section trays don’t work as hard. Or, we can say that the stripping factor is reduced. Or, perhaps, the rate of tray deck leakage increases. Either way, the amount of the lighter component slipping into the bottom of the distillation tower will increase.

            What then are the benefits of increased feed preheat? There must be such benefits, otherwise most of our towers would not have feed preheaters:

 

• Saves Energy – The feed preheater normally uses waste heat or low pressure steam, with little value. This saves more valuable reboiler steam.
• Supplements Reboiler Capacity – If the tower is not condenser limited, but limited by the capacity of the reboiler, then more feed preheat will permit higher reflux rates, and hence better fractionation efficiency.
• Stops Flooding in Bottom Section Trays – If the tower is limited by flooding or entrainment in the bottom stripping trays, then more feed preheat, which reduces the reboiler duty, will improve fractionation. This happens because the vapor velocities will go down, as the feed preheat goes up.

 

So, depending on the hydraulic capacity of the stripping trays, feed preheat can make fractionation better or worse.

Effect of Steam Superheat

            You would think superheating steam would always increase the usefulness of the steam. This is not always true. A few examples:
 

·         Steam superheat is bad for amine reboilers. The superheated steam increases the shell side tube temperature and accelerates H₂S corrosion.

·         Steam superheat is bad for jets. The superheated steam increases the volumetric flow through the steam nozzle, and thus reduces the height flow of steam.

·         Steam superheat is neutral for reboilers in general. The LMTD is not increased by using superheated steam. It’s the saturation temperature which determines the LMTD.

·         Steam superheat is slightly helpful when stripping a product. But the effect is so minor that I typically ignore superheat in my steam stripper calculations.

 

The real value of superheating steam is the guarantee that it is dry. Dry steam is critical for steam strippers, jets, and turbines.

Centrifugal Compressors

Mr. Norman P. Lieberman, Process Improvement Engineering, Metairie, Louisiana, USA. This is Ashok from India. Explain please why compressors surge.

--S.R. Ashok, Process Engineer

 

Ashok – Read my book, Process Equipment Malfunctions (McGraw Hill Publisher) for my best explanation. Process Factors promoting surge are:

n  Low molecular weight

n  Rotor fouling

n  Higher temperature gas

n  Low suction pressure

n  High discharge pressure

n  Low flow

n  Closing spill-back

n  Suction throttling

--Regards, Norman Lieberman

Centrifugal Compressors

Norm – A quick question. A relatively minor surging incident has severely damaged our compressor. We have experienced much worse and longer periods of surging without noticeable damage. Why this extreme damage now?

--Harry Hambin

P.S. Regards to Liz

 

Dear Henry – Hope you had a good holiday. It could be that the main bearing lube pump is driven off of the compressor shaft itself. Then, if the auxiliary lube oil pump failed to come-on automatically, with the resulting low lube oil pressure (assuming the compressor itself tripped-off due to surging and was then restarted by the operators), this could have damaged the radically support bearings. If the thrust bearing is not particularly damaged, this would support my theory. Hope this helps you.

--Norm

Centrifugal Compressors

To Mr. Norman Lieberman – In your book, Process Engineering for a Small Planet, you write about damage to a compressor because of failure of the bearing lubrication pump. The same failure occurred in our plant. How could this have been prevented?

--Oscar Harrod

 

Oscar – Are you checking that the back-up pump will start automatically? Bleed-off the pressure to the auto-start switch for the pump and see if it will start by itself and that it develops the required lube oil pressure as stated by the manufacturers.

--Norm

Centrifugal Compressors

Dear Norman – Our vacuum tower is now working fine. Thanks for your help. But I have another question. I have a combustion air blower which surges during hot weather. The surges do not damage the compressor (blower). But when we have surge, there is a loud banging sound. That banging is coming from the check valve on the compressor discharge. Should we consider removing this check valve?

--Chuck Hendrics

 

Chuck – No. The check valve dampens down the force of the surge and helps protect the stationary elements in the compressor case from damage.

--Norm Lieberman

Centrifugal Compressors

Dear Mr. Norman Lieberman – Thank you and Mrs. Lieberman for writing Working Guide to Process Equipment. A question I have is when our compressor begins to surge, the surging seems to get louder. Then when I lower the discharge pressure to stop the surge, it does not help, even though it’s the same discharge pressure we had before the surging stared. Please explain to me.

--Kia Vala

 

Mr. Vala – Once surging starts, the gas inside the compressor case goes forward, then back, then forward,  and then back, etc. This causes the gas to become hotter and less dense. The reduced density reduces delta P developed by the compressor, which promotes gas flow reversal.

--Regards, Norm Lieberman

Centrifugal Compressors

I attended your seminar in Sasol, Secunda in 2008. Our problem is that when we open our compressor spill-back, which we thought would stop surge, it sometimes makes the compressor surge. What is our problem?

--H.K. Masa

 

Your problem is that the spill-back flow is hot. This reduces vapor density. Which reduces compressor delta P. Which reduces compressor discharge pressure. Which allows gas flow to stop and reverse. I had this exact problem in 1974 on my alky unit in Texas City. I desuperheated compressor suction by injecting a spray of liquid isobutane into the suction. This would quickly bring the compressor out of surge. Hope this helps.

--Norm

Reboilers

Question:

            Should we de-superheat our steam to our amine regeneration reboiler?

Answer:

            Yes. Use of saturated steam will retard the thermal degradation of the amine, and reduce the rate of reboiler tube failure. It’s a standard industry practice, especially critical if you are using MEA.

Reboilers

Question:

            Our steam supply is contaminated with non-condensibles (maybe CO₂?). We opened the vent on the top of the channel head to purge out the non-condensibles which we think are reducing steam condensation rates and our reboiler duty. This did not help. What do you suggest?

Answer:

            The non-condensibles will not accumulate in the top of the channel. They accumulate below the bottom pass partition baffle in the channel head. That is, the portion of the channel head that must be vented. Opening the vent on top of the channel head will vent out steam, not the non-condensible CO₂.

Reboilers

Question:

            I retubed a butane splitter (iso vs. normal) reboiler recently. Twenty percent of the tubes were plugged. Yet I lost reboiler capacity with the retubed bundle. Why did we lose capacity, rather than increase capacity by twenty percent?

Answer:

            The problem is smooth tubes that lack nucleate boiling sites. This happens all the time. The old tubes were rough and pitted. Increasing the condensing pressure of the reboiler steam will only make the problem worse. One possibility is to sand blast the tubes to roughen their surface. Alternately, you can use “Linde Hi-flux,” sintered metal coating. Whichever you find most cost effective.

Reboilers

Question:

            Does superheating steam aid or retard heat transfer?

Answer:

            I had a reboiler on a debutanizer reboiler in Aruba, supplied with 400 psig, 650°F superheated steam. There was a de-superheating station (i.e., a condensate injection point). As I reduced the steam supply temperature (at a constant pressure) with the condensate injection, my reboiler duty did not change. I believe the conventional answer is that superheat retards heat transfer. But, this was not the case in my only field experiment.

Reboilers

Question:

            Sometimes, when I OPEN the channel head condensate drain valve, my reboiler duty increases. But, sometimes when I partly CLOSE off the same valve, my reboiler duty decreases. What’s happening?

Answer:

            There are two problems with draining condensate from the channel head:

1.      Blowing the condensate seal.

2.      Condensate back-up.

If you open the drain valve too much, steam and water will blow-out together. The high velocity steam, blowing though the tubes, will reduce the condensing heat transfer coefficient. Hence, the observed loss in heat transfer and reboiler duty.

If you close the drain valve too much, water will back-up and submerge the lower few rows of tubes, which will reduce the surface area exposed to the condensing steam, which also diminishes the rate of heat transfer.

Without some method of observing the steam condensate level in the channel head, the optimum position of your condensate drain valve can only be found by trial and error.

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