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Steam Flow Through Reboilers
15 Steam Flow Through Reboilers “Mr. Lieberman, sir. May I ask you a question? I’ve only worked in the plant for a few months. I’m totally lost.” I was teaching a Refinery Troubleshooting Seminar at the Etton Refinery in the United Kingdom. Tad, who looked like he was 16, but was likely 23, was not my brightest student. “What’s the question, Tad? We only have a couple minutes before the class starts.” “Well, I got this packed stripper that’s reboiled with 3 bar steam. We used to be able to strip 120 tons per hour, but lately all I can strip is 90 or 100 tons an hour. I’m not getting enough heat out of the steam reboiler.” “What’s the reboiler look like?” “Well, sir, I have a sketch of it right here (see Fig. 15.1).” “Tad. Could you not call me ‘sir’? That makes it sound like I’m an old person. Call me Norm!” “Okay, sir. I guess what really bothers me, is that none of the senior engineers is willing or able or has time to help me. This is a big company, but I feel like I’m all on my own with my problems.” “Look. It’s time to start the class. Let’s go out into the plant at 5:00 P.M. and look the problem over in the field. What have I taught all of you, Tad, about solving refinery process problems?” “Well, sir…er, Norm. It’s like picking up girls at a bar. You have to get closer to the problem.”
Lost Reboiler Duty Most refinery reboilers, at least in the United States, are horizontal shell and tube exchangers. The process fluid is circulated through the shell side either by thermosyphon circulation, or less commonly, with a pump. The steam is on the tube side. For low-pressure steam, as in this case, the steam flow is controlled by varying the rate of condensate flow from the condensate pot (Fig. 15.1). As the condensate outlet control valve closes, the steam condensate (water) backs-up and fills some of the tubes. Tubes full of water do not condense any steam. In a steam reboiler, 90%+ of the heat is derived from the latent heat of condensation of the steam and less than 10% is from sensible heat of the hot water. Since the Etton Refinery was a subsidiary of a US company, the reboiler was horizontal. A European refinery would have used a vertical reboiler, with the steam of the shell side, and the circulating process fluid on the tube side. Understanding Process Equipment for Operators and Engineers. https://doi.org/10.1016/B978-0-12-816161-6.00015-1 © 2019 Elsevier Inc. All rights reserved.
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UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
Condensate Back-Up The most common problem with steam reboilers using low pressure steam is condensate back-up. Something is restricting the flow of steam condensate drainage. One problem could be the condensate drain valve is too small. That’s uncommon. More likely, is the pervasive problem of “Vapor Lock.” When hot steam condensate drains out of the channel head (see Fig. 15.1) of the reboiler, it is close to its boiling point. As the hot water flows through the condensate drain line, it can lose pressure due to: 1. Frictional losses in the drain line. 2. Acceleration losses, as the condensate flows through a smaller fitting or pipe. 3. Increases in line elevation.
Stripper 3 bar steam
Vapor line
″A″ Vent
Reboiler ″B″
Balance line
Reset level
Channel head
Bottoms product
Condensate drum
4″ Steam cond.
Cold water
FIG. 15.1 Thermosyphon reboiler.
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The hot condensate will have a tendency to partly vaporize. The evolved vapor may increase the volume flow by a factor of a hundred. This huge volume of evolved steam chokes off the flow and backs-up the condensate in the channel head. Result is “VAPOR LOCK” and the loss of reboiler duty.
Blowing Condensate Seal Every operator on the planet knows how to handle the problem of condensate back-up. They divert the condensate to the deck which creates two problems: 1. The valuable condensate, which is supposed to be returned to the deaerator, or boiler plant, is lost to the sewer. 2. If the drain valve is opened a little too much, steam escapes with the condensate. This is called “Blowing the Condensate Seal,” which is bad. Bad, because the steam is wasted. But far more important, the heat transfer efficiency (“U” value) is severely degraded. Whereas high velocities aid sensible heat transfer, high vapor velocities may retard latent heat transfer. Blowing the condensate seal can instantly reduce reboiler duty by 20%–50%. This is based on my experience and those of field operators.
Noncondensables in Steam A very small amount of noncondensable vapor contaminating the steam can reduce reboiler duty by half. The noncondensables are due to: • •
•
CO2—Residual carbonates left in the boiler feed water from the demineralization plant. The carbonates then break down into CO2 in the boiler. N2—In Texas City, I once found nitrogen in a low pressure steam supply to my alky DIB tower reboiler. Someone had inadvertently cross-connected nitrogen into the steam at some other unit. Light Hydrocarbons—A small tube leak may cause noncondensable ethane, propane, or butane to enter the steam tubes.
The most common problem is CO2, formed from carbonate dissociation. The noncondensables will accumulate in the tubes and retard the rate of steam condensation if they are not vented off frequently. At one plant in Australia, the accumulation of CO2 was reducing the reboiler duty in a debutanizer by 20%–30%, over a period of several days, before it was vented off from the channel head. The problem is described in the literature [1] as “Vapor Binding.” I have always been amazed how a little bit of noncondensable venting has such a dramatic effect on steam condensation rates—provided the venting is from beneath the channel head pass partition, and not on the channel head top vent [2].
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UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
Lack of Thermosyphon Circulation The reboiler shown in Fig. 15.1 is called a “Circulating Thermosyphon Reboiler.” The liquid circulates through the shell side of the reboiler due to density difference between the cold inlet side and the hot outlet side. The cold inlet side is denser because it is all liquid. The hot outlet side is less dense because it is a vapor-liquid mixture. The density difference, multiplied by the height of the liquid in the vessel, as compared to the reboiler elevation: ðLiquid ht:Þ ðReboiler ht:Þ ðSGÞ ¼ DP 2:31
(15.1)
where: • • •
Ht. is in feet SG ¼ specific gravity of the liquid (i.e., 1.00 for water) DP ¼ psi
The DP is the thermosyphon circulation driving force. Note that I’ve neglected the SG of the outlet fluid as its volume is largely low density vapor. If the thermosyphon circulation rates get too low, reboiler heat transfer duty will suffer. Operators also call this vapor lock. If the vessel level rises above the reboiler return nozzle, this will also impede the rate of thermosyphon circulation.
The Big Fix Tad and I did not begin our field investigation until 8:00 p.m. It required 3 hours for my safety orientation. “Tad,” I explained, “let’s first open the non-condensable vent. The CO2 will never accumulate above the channel head pass partition baffle, but only beneath the lower channel head partition.” “Yes, Norm, I can see that. The upper part of the channel head will be continuously purged with steam. CO2 could only accumulate after the steam is condensed. I’ll crack open valve ‘B’ (see Fig. 15.1), and continuously purge the channel head.” The flow of reboiler steam, as indicated on the local flow indicator, climbed by 5%–10%. “Looks like we did some good, Tad,” I observed. “Yeah! That venting helped. But not by 50%. What’s the next step?” “We need to determine whether we’re suffering from condensate back-up or have blown the condensate seal.” “How can we do that?” “Real easy,” I explained. “You monitor the reboiler vapor line skin outlet temperature with your infrared temperature gun. I’ll close the steam condensate outlet gate valve part way. Either the reboiler shell outlet temperature will go up or down.”
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“If it goes down, that means we’re losing reboiler duty due to condensate back-up in the tubes. If the reboiler outlet temperature goes up, that means we have been blowing the condensate seal.” “Hey Norm. The reboiler outlet temperature is dropping. We must be hurting from condensate back-up, and not blowing the water seal.” I saw that Tad was checking the water level in the condensate drum, which was well below the bottom of the channel head (see Fig. 15.1). “But Norm, now I’m really confused! How can you say we’re losing reboiler duty due to condensate back-up, when the level in the damn pot is a foot lower than the bottom of the channel head?” “I’ll explain; it’s complicated. It’s due to a common design error: 1. Note that the balance line between the condensate drum and the channel head is connected above the channel head pass partition baffle, at the top vent (valve ‘A’). 2. Let’s assume the pressure drop of the steam through the tubes is 0.1 BAR or one meter of water. 3. Therefore, the pressure in the bottom half of the channel head will be 0.1 BAR less than in the condensate drum. 4. Therefore, the water level in the channel head is going to be about 3 ft, or one meter, higher than the water level in the condensate drum. 5. Which means that almost half the tubes will be full of water.”
Correcting Design Error “They should have connected the balance line below the channel head pass partition baffle,” Tad observed. “At valve ‘B.’ Then the pressure in the condensate drum, and the lower half of the channel head, would have been the same. I guess even big companies can make mistakes.” “Okay Tad. Close off the valve connecting the top of the channel head to the drum. Then, I’ll open the atmospheric vent on the drum just a little to reduce the condensate drum pressure by a bit.” “Look Norm, look! The reboiler steam flow just jumped up. And the reboiler vapor outlet temperature is also rising fast! This is great! You got any other ideas we could try to get more heat out of the reboiler?”
Vapor Lock “I’m not sure, but we may still be suffering from some condensate back-up due to vaporlock in the 400 condensate drain line. As the condensate loses pressure in the line, it will tend to partly vaporize back to steam. Even a few percent vaporization can restrict condensate flow and cause the condensate to back-up in the reboiler’s channel head.” “So that’s vapor-lock. But what can we do about it? Install a 600 line? Buy a pump?”
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UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
“No, Tad. We want to fix the problem tonight. We’ll inject some cold water into the 400 condensate line. This will suppress the tendency of the hot condensate to flash. Let’s just remember to call ‘Utilities’ and have them divert the condensate to the boiler feed water treatment plant.”
Increasing Thermosyphon Circulation “Norm. That cold water injection into the 400 condensate drum outlet has also increased reboiler heat. But, it’s getting kind of late. Should we call it a night?” “It’s only 11:30! Let’s try one more thing.” “Okay, but then let’s go to the gate.” “Tad, increasing the rate of flow through the reboiler shell side will often, but not always, improve heat transfer. If a lot of the reboiler duty is in the form of sensible heat transfer, higher circulation rates will help. But, if it’s all latent heat transfer, higher circulation rates may not help at all.” “But how can we tell for sure if the higher circulation rate is doing us any good?” “If the tower bottoms temperature increases, we’re moving in the right direction,” I answered. The chief operator came over to us, “You guys want to order lunch? They deliver at about 1:00 A.M.” “No,” Tad said, “Mr. Henry, we’ll be leaving before then.” “Henry, raise up the bottoms level in the tower from 50 to 80 percent. Tad, increasing the tower level will increase the liquid head driving force that causes the flow through the reboiler. That is, it will increase the Thermosyphon Circulation driving force (see Equation #1 above).” As I waited for plant security to return my passport at the gate, Tad said, “Mr. Lieberman, I forgot to tell you that my grandfather, Harry Jenkins, took your Troubleshooting Seminar back in 1983. Sir, I suppose that you don’t remember my granddad? Anyway, he said hello.” “No, Tad. I don’t remember your grandfather. But I do remember that I’ve asked you not to call me Sir.”
References [1] C.L. Henderson, J.M. Marchello, Film condensation in the presence of a non-condensable gas, J. Heat Transf. 91 (1969) 447–450. [2] R.E. Putman, Steam Surface Condensers, ASME Press, 2001.
Lost Reboiler Duty Most refinery reboilers, at least in the United States, are horizontal shell and tube exchangers. The process fluid is circulated through the shell side either by thermosyphon circulation, or less commonly, with a pump. The steam is on the tube side. For low-pressure steam, as in this case, the steam flow is controlled by varying the rate of condensate flow from the condensate pot (Fig. 15.1). As the condensate outlet control valve closes, the steam condensate (water) backs-up and fills some of the tubes. Tubes full of water do not condense any steam. In a steam reboiler, 90%+ of the heat is derived from the latent heat of condensation of the steam and less than 10% is from sensible heat of the hot water. Since the Etton Refinery was a subsidiary of a US company, the reboiler was horizontal. A European refinery would have used a vertical reboiler, with the steam of the shell side, and the circulating process fluid on the tube side. Understanding Process Equipment for Operators and Engineers. https://doi.org/10.1016/B978-0-12-816161-6.00015-1 © 2019 Elsevier Inc. All rights reserved.
113
114
UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
Condensate Back-Up The most common problem with steam reboilers using low pressure steam is condensate back-up. Something is restricting the flow of steam condensate drainage. One problem could be the condensate drain valve is too small. That’s uncommon. More likely, is the pervasive problem of “Vapor Lock.” When hot steam condensate drains out of the channel head (see Fig. 15.1) of the reboiler, it is close to its boiling point. As the hot water flows through the condensate drain line, it can lose pressure due to: 1. Frictional losses in the drain line. 2. Acceleration losses, as the condensate flows through a smaller fitting or pipe. 3. Increases in line elevation.
Stripper 3 bar steam
Vapor line
″A″ Vent
Reboiler ″B″
Balance line
Reset level
Channel head
Bottoms product
Condensate drum
4″ Steam cond.
Cold water
FIG. 15.1 Thermosyphon reboiler.
Chapter 15 • Steam Flow Through Reboilers
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The hot condensate will have a tendency to partly vaporize. The evolved vapor may increase the volume flow by a factor of a hundred. This huge volume of evolved steam chokes off the flow and backs-up the condensate in the channel head. Result is “VAPOR LOCK” and the loss of reboiler duty.
Blowing Condensate Seal Every operator on the planet knows how to handle the problem of condensate back-up. They divert the condensate to the deck which creates two problems: 1. The valuable condensate, which is supposed to be returned to the deaerator, or boiler plant, is lost to the sewer. 2. If the drain valve is opened a little too much, steam escapes with the condensate. This is called “Blowing the Condensate Seal,” which is bad. Bad, because the steam is wasted. But far more important, the heat transfer efficiency (“U” value) is severely degraded. Whereas high velocities aid sensible heat transfer, high vapor velocities may retard latent heat transfer. Blowing the condensate seal can instantly reduce reboiler duty by 20%–50%. This is based on my experience and those of field operators.
Noncondensables in Steam A very small amount of noncondensable vapor contaminating the steam can reduce reboiler duty by half. The noncondensables are due to: • •
•
CO2—Residual carbonates left in the boiler feed water from the demineralization plant. The carbonates then break down into CO2 in the boiler. N2—In Texas City, I once found nitrogen in a low pressure steam supply to my alky DIB tower reboiler. Someone had inadvertently cross-connected nitrogen into the steam at some other unit. Light Hydrocarbons—A small tube leak may cause noncondensable ethane, propane, or butane to enter the steam tubes.
The most common problem is CO2, formed from carbonate dissociation. The noncondensables will accumulate in the tubes and retard the rate of steam condensation if they are not vented off frequently. At one plant in Australia, the accumulation of CO2 was reducing the reboiler duty in a debutanizer by 20%–30%, over a period of several days, before it was vented off from the channel head. The problem is described in the literature [1] as “Vapor Binding.” I have always been amazed how a little bit of noncondensable venting has such a dramatic effect on steam condensation rates—provided the venting is from beneath the channel head pass partition, and not on the channel head top vent [2].
116
UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
Lack of Thermosyphon Circulation The reboiler shown in Fig. 15.1 is called a “Circulating Thermosyphon Reboiler.” The liquid circulates through the shell side of the reboiler due to density difference between the cold inlet side and the hot outlet side. The cold inlet side is denser because it is all liquid. The hot outlet side is less dense because it is a vapor-liquid mixture. The density difference, multiplied by the height of the liquid in the vessel, as compared to the reboiler elevation: ðLiquid ht:Þ ðReboiler ht:Þ ðSGÞ ¼ DP 2:31
(15.1)
where: • • •
Ht. is in feet SG ¼ specific gravity of the liquid (i.e., 1.00 for water) DP ¼ psi
The DP is the thermosyphon circulation driving force. Note that I’ve neglected the SG of the outlet fluid as its volume is largely low density vapor. If the thermosyphon circulation rates get too low, reboiler heat transfer duty will suffer. Operators also call this vapor lock. If the vessel level rises above the reboiler return nozzle, this will also impede the rate of thermosyphon circulation.
The Big Fix Tad and I did not begin our field investigation until 8:00 p.m. It required 3 hours for my safety orientation. “Tad,” I explained, “let’s first open the non-condensable vent. The CO2 will never accumulate above the channel head pass partition baffle, but only beneath the lower channel head partition.” “Yes, Norm, I can see that. The upper part of the channel head will be continuously purged with steam. CO2 could only accumulate after the steam is condensed. I’ll crack open valve ‘B’ (see Fig. 15.1), and continuously purge the channel head.” The flow of reboiler steam, as indicated on the local flow indicator, climbed by 5%–10%. “Looks like we did some good, Tad,” I observed. “Yeah! That venting helped. But not by 50%. What’s the next step?” “We need to determine whether we’re suffering from condensate back-up or have blown the condensate seal.” “How can we do that?” “Real easy,” I explained. “You monitor the reboiler vapor line skin outlet temperature with your infrared temperature gun. I’ll close the steam condensate outlet gate valve part way. Either the reboiler shell outlet temperature will go up or down.”
Chapter 15 • Steam Flow Through Reboilers
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“If it goes down, that means we’re losing reboiler duty due to condensate back-up in the tubes. If the reboiler outlet temperature goes up, that means we have been blowing the condensate seal.” “Hey Norm. The reboiler outlet temperature is dropping. We must be hurting from condensate back-up, and not blowing the water seal.” I saw that Tad was checking the water level in the condensate drum, which was well below the bottom of the channel head (see Fig. 15.1). “But Norm, now I’m really confused! How can you say we’re losing reboiler duty due to condensate back-up, when the level in the damn pot is a foot lower than the bottom of the channel head?” “I’ll explain; it’s complicated. It’s due to a common design error: 1. Note that the balance line between the condensate drum and the channel head is connected above the channel head pass partition baffle, at the top vent (valve ‘A’). 2. Let’s assume the pressure drop of the steam through the tubes is 0.1 BAR or one meter of water. 3. Therefore, the pressure in the bottom half of the channel head will be 0.1 BAR less than in the condensate drum. 4. Therefore, the water level in the channel head is going to be about 3 ft, or one meter, higher than the water level in the condensate drum. 5. Which means that almost half the tubes will be full of water.”
Correcting Design Error “They should have connected the balance line below the channel head pass partition baffle,” Tad observed. “At valve ‘B.’ Then the pressure in the condensate drum, and the lower half of the channel head, would have been the same. I guess even big companies can make mistakes.” “Okay Tad. Close off the valve connecting the top of the channel head to the drum. Then, I’ll open the atmospheric vent on the drum just a little to reduce the condensate drum pressure by a bit.” “Look Norm, look! The reboiler steam flow just jumped up. And the reboiler vapor outlet temperature is also rising fast! This is great! You got any other ideas we could try to get more heat out of the reboiler?”
Vapor Lock “I’m not sure, but we may still be suffering from some condensate back-up due to vaporlock in the 400 condensate drain line. As the condensate loses pressure in the line, it will tend to partly vaporize back to steam. Even a few percent vaporization can restrict condensate flow and cause the condensate to back-up in the reboiler’s channel head.” “So that’s vapor-lock. But what can we do about it? Install a 600 line? Buy a pump?”
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UNDERSTANDING PROCESS EQUIPMENT FOR OPERATORS AND ENGINEERS
“No, Tad. We want to fix the problem tonight. We’ll inject some cold water into the 400 condensate line. This will suppress the tendency of the hot condensate to flash. Let’s just remember to call ‘Utilities’ and have them divert the condensate to the boiler feed water treatment plant.”
Increasing Thermosyphon Circulation “Norm. That cold water injection into the 400 condensate drum outlet has also increased reboiler heat. But, it’s getting kind of late. Should we call it a night?” “It’s only 11:30! Let’s try one more thing.” “Okay, but then let’s go to the gate.” “Tad, increasing the rate of flow through the reboiler shell side will often, but not always, improve heat transfer. If a lot of the reboiler duty is in the form of sensible heat transfer, higher circulation rates will help. But, if it’s all latent heat transfer, higher circulation rates may not help at all.” “But how can we tell for sure if the higher circulation rate is doing us any good?” “If the tower bottoms temperature increases, we’re moving in the right direction,” I answered. The chief operator came over to us, “You guys want to order lunch? They deliver at about 1:00 A.M.” “No,” Tad said, “Mr. Henry, we’ll be leaving before then.” “Henry, raise up the bottoms level in the tower from 50 to 80 percent. Tad, increasing the tower level will increase the liquid head driving force that causes the flow through the reboiler. That is, it will increase the Thermosyphon Circulation driving force (see Equation #1 above).” As I waited for plant security to return my passport at the gate, Tad said, “Mr. Lieberman, I forgot to tell you that my grandfather, Harry Jenkins, took your Troubleshooting Seminar back in 1983. Sir, I suppose that you don’t remember my granddad? Anyway, he said hello.” “No, Tad. I don’t remember your grandfather. But I do remember that I’ve asked you not to call me Sir.”
References [1] C.L. Henderson, J.M. Marchello, Film condensation in the presence of a non-condensable gas, J. Heat Transf. 91 (1969) 447–450. [2] R.E. Putman, Steam Surface Condensers, ASME Press, 2001.