13 replies to this topic

[thakur]

hello guys,

can anybody please explaine how to calculate the relieving rate for the psv in case of the reflux failure in distillation column?

if any one has the literature in this regard,please let me know.

[Lowflo]

Thakur,

Loss of reflux triggers a domino effect of other events which ultimately lead to the overpressure. The specifics of those other events can't be generalized for all distillation columns. You'll need to assess this based on the specific design and control details for this particular column. Once you've laid out that sequence of events, following loss of reflux for this particular column, the answer to your question will be clearly revealed to you. BTW, I'm assume this scenarion is based on loss of reflux flow while the column is running normally. Run this scenario through your mind and lay out the sequence of events following (and caused by) loss of reflux.

Here's a typical sequence of events for this scenario:
1.Loss of reflux pump (Note that this could actually be 2 independent failures if there is a spare pump that's programmed to auto-start. If so, consider whether or not this is even a credible scenario.)
2.Without the reflux pumps, the reflux drum will overfill and back up into the condenser, stopping the condenser from operating.
3.The column then pressures up once the condenser stops, but whether or not this happens, and how quickly it happens is a function of the control scheme for the column. Commonly, the steam flow to the reboiler is programmed to maintain a constant temperature at some location in the column. If the reflux pumps and the condenser are out of operation, then the temperature profile in the column will start to rise. If the column uses the control scheme mentioned above, the reboiler controls will respond by cutting back on steam flow. Unless there's a min-flow setting for the steam (usually there's not) then the steam valve might eventually close completely.

a.Be aware that even if the reboiler controls respond by shutting off the steam, you could still reach the relief pressure due to the residual heat in the reboiler and/or slow response of the steam valve controller. A conservative assumption would be that the reboiler continues to operate at some low level of heat input.

4.Review this sequence with your client and revise if as necessary for this particular column design and control scheme.
5.After laying out the specific details for this particular scenario, as described above, the relieving flow is simply the resulting amount of boil-up from the reboiler.

That said, I don't often run the calc's for this particular scenario, because there's another scenario on my list that's nearly identical in its effect, and it's equally as bad or worse than this one. I'm talking about a start-up scenario in which the column is being started while someone has failed to properly line up water flow to the condenser.

Let me also mention two footnotes to this discussion. (1) Before doing anything, check to be sure that the reboiler can actually overpressure the column, even without the condenser running. It not too uncommon to find that the reboiler will pinche out before the process side is at relieving pressure. In other words, the saturation temp of the steam, at supply pressure, might not be high enough to boil the process fluid at the relieving pressure. (2) You can usually make a significant reduction in the heat input for this scenario by doing a simple calculation to reduce the reboiler's heat input, based on the fact that the reboiler dT during relief is lower than the dT used in its design. That, of course, means that the reboiler will be transferring heat at a lower rate. It's a simple calc. Just calculate a "corrected" LMTD using the relieving temp as the cold side outlet temp. Then multiply the reboiler Q by the ratio of the "corrected" Q over the uncorrected Q. This assumes that the U value is constant, and that's a good assumption.

[thakur]

dear,
thanks for your reply.
its really too complex to find out the relieving rate for distillation column psv.
can u suggest me the literature or book for this so i can study on this for cleare understanding?

[ashetty]

dear,
thanks for your reply.
its really too complex to find out the relieving rate for distillation column psv.
can u suggest me the literature or book for this so i can study on this for cleare understanding?

Hello,

Read some previous posts on this topic...might help

http://www.cheresour...x-failure-case/

[thakur]

mr. shetty

i have gone through the link.its pretty worth to enhance the knowledgwe regarding the reflux failure scenario.
however to major extent discussion was revolving around the simulation and balacne.nobody was upfront with the relieving rate calculation.
i need something (i.e. literature,book) which talks about this scenario in very detail.so i can implement in actual practice.

[CMA010]

Thakur,

Loss of reflux triggers a domino effect of other events which ultimately lead to the overpressure. The specifics of those other events can't be generalized for all distillation columns. You'll need to assess this based on the specific design and control details for this particular column. Once you've laid out that sequence of events, following loss of reflux for this particular column, the answer to your question will be clearly revealed to you. BTW, I'm assume this scenarion is based on loss of reflux flow while the column is running normally. Run this scenario through your mind and lay out the sequence of events following (and caused by) loss of reflux.

Here's a typical sequence of events for this scenario:
1.Loss of reflux pump (Note that this could actually be 2 independent failures if there is a spare pump that's programmed to auto-start. If so, consider whether or not this is even a credible scenario.)
2.Without the reflux pumps, the reflux drum will overfill and back up into the condenser, stopping the condenser from operating.
3.The column then pressures up once the condenser stops, but whether or not this happens, and how quickly it happens is a function of the control scheme for the column. Commonly, the steam flow to the reboiler is programmed to maintain a constant temperature at some location in the column. If the reflux pumps and the condenser are out of operation, then the temperature profile in the column will start to rise. If the column uses the control scheme mentioned above, the reboiler controls will respond by cutting back on steam flow. Unless there's a min-flow setting for the steam (usually there's not) then the steam valve might eventually close completely.

a.Be aware that even if the reboiler controls respond by shutting off the steam, you could still reach the relief pressure due to the residual heat in the reboiler and/or slow response of the steam valve controller. A conservative assumption would be that the reboiler continues to operate at some low level of heat input.

4.Review this sequence with your client and revise if as necessary for this particular column design and control scheme.
5.After laying out the specific details for this particular scenario, as described above, the relieving flow is simply the resulting amount of boil-up from the reboiler.

That said, I don't often run the calc's for this particular scenario, because there's another scenario on my list that's nearly identical in its effect, and it's equally as bad or worse than this one. I'm talking about a start-up scenario in which the column is being started while someone has failed to properly line up water flow to the condenser.

Let me also mention two footnotes to this discussion. (1) Before doing anything, check to be sure that the reboiler can actually overpressure the column, even without the condenser running. It not too uncommon to find that the reboiler will pinche out before the process side is at relieving pressure. In other words, the saturation temp of the steam, at supply pressure, might not be high enough to boil the process fluid at the relieving pressure. (2) You can usually make a significant reduction in the heat input for this scenario by doing a simple calculation to reduce the reboiler's heat input, based on the fact that the reboiler dT during relief is lower than the dT used in its design. That, of course, means that the reboiler will be transferring heat at a lower rate. It's a simple calc. Just calculate a "corrected" LMTD using the relieving temp as the cold side outlet temp. Then multiply the reboiler Q by the ratio of the "corrected" Q over the uncorrected Q. This assumes that the U value is constant, and that's a good assumption.

Three quick remarks:
3) Favourable instrument response is not taken into account per API STD 521, assume control valves stay in their normal position.
Be very carefull with taking credit for a temperature pinch (see below) and in my opinion a clean U should be used.

For more information about (hand) calculation of the relief load of a tower:
"Distillation column relief loads part 1 & 2", P.L. Nezami, Hydrocarbon Processing Magazine, April 2008
"Tower pressure relief calculation", S. Rahimi Mofrad, Hydrocarbon Processing Magazine, September 2008
"Avoid common mistakes in sizing distillation safety valves", M. Bradford, D.G. Durrett, Chemical Engineering Magazine, July 1984
"How to size relief valves", A. Mukerji, Chemical Engineering Magazine, June 1980

[Lowflo]

dear,
thanks for your reply.
its really too complex to find out the relieving rate for distillation column psv.
can u suggest me the literature or book for this so i can study on this for cleare understanding?

Thakur,

Whatever you are designing as a process engineer, always be sure you understand it first. If this seems too "complex" then you probably haven't had much experience with distillation. Consult with someone who has that knowledge rather than looking for a simple recipe. Always....understand it before you design it.

When you get some time, play around with a distillation simulation. Change the temperature of the feed, change the feed location, raise and lower the column pressure, raise and lower the reflux. While doing those things, look at the effect it has on composition and the impact on heat duty (condenser and reboiler). Most importantly, study each change until you understand why it had the effect that it did.

You probably need to get your relief design done now, so work with someone who can help with this problem. An ultra conservative approach to sizing the relief load for loss of reflux is to simply use the design Q of the reboiler, with the condenser out of service. However, in most cases that's far too conservative and a waste of money. That's why I suggest a more reasonable approach, but that takes a basic understanding of distillation dynamics.

[sheiko]

Look at this: http://www.cheresour...rent-scenarios/

I had attached some of the above mentionned articles.

Edited by sheiko, 22 August 2009 - 09:05 AM.

[SSWBoy]

Tower relief is one of the more complicated PSV sizing cases you will come across, don't worry if you are a little confused.

There's some good advice above, but simply at the end of the day if you want to get a number you can rely on and not have a long list of assumptions a dynamic simulation is the only way.

One of the downsides of everyone at a company have access to simulation packages is that they tend to simulate different relieving conditions for columns to death, one must be careful to remember that orifice sizes go up by a factor of ~2 from one size to another. Even if your answer is +- 20% it is unlikely to affect orifice size too much.

Of course a simple way of sizing for PSV case is just considering 2nd stage vapours. Ultimately if you're confused your best asking a older and wiser engineer, your company probably has a typical approach to tackling column relief, best to read this too

Edited by SSWBoy, 22 August 2009 - 01:09 PM.

[Lowflo]

From SSWBoy: ".....they tend to simulate different relieving conditions for columns to death, one must be careful to remember that orifice sizes go up by a factor of ~2 from one size to another. Even if your answer is +- 20% it is unlikely to affect orifice size too much."

Good point SSWBoy! One of the most common mistakes made by process engineers is not knowing when to quit....knowing how good is good enough I suppose it's an occupational hazard for us, but it's one that can greatly limit an engineer's productivity and effectiveness.

[thakur]

dear,
thanks for your reply.
its really too complex to find out the relieving rate for distillation column psv.
can u suggest me the literature or book for this so i can study on this for cleare understanding?

Thakur,

Whatever you are designing as a process engineer, always be sure you understand it first. If this seems too "complex" then you probably haven't had much experience with distillation. Consult with someone who has that knowledge rather than looking for a simple recipe. Always....understand it before you design it.

When you get some time, play around with a distillation simulation. Change the temperature of the feed, change the feed location, raise and lower the column pressure, raise and lower the reflux. While doing those things, look at the effect it has on composition and the impact on heat duty (condenser and reboiler). Most importantly, study each change until you understand why it had the effect that it did.

You probably need to get your relief design done now, so work with someone who can help with this problem. An ultra conservative approach to sizing the relief load for loss of reflux is to simply use the design Q of the reboiler, with the condenser out of service. However, in most cases that's far too conservative and a waste of money. That's why I suggest a more reasonable approach, but that takes a basic understanding of distillation dynamics.

u r absolutly right lowfo
i need to keep Patience
have to understad the basics of psv for such scenario

[thakur]

Look at this: http://www.cheresour...rent-scenarios/

I had attached some of the above mentionned articles.

thanxs a lot dear
i was wanting this only
i think this is the end my quest

[thakur]

Thakur,

Loss of reflux triggers a domino effect of other events which ultimately lead to the overpressure. The specifics of those other events can't be generalized for all distillation columns. You'll need to assess this based on the specific design and control details for this particular column. Once you've laid out that sequence of events, following loss of reflux for this particular column, the answer to your question will be clearly revealed to you. BTW, I'm assume this scenarion is based on loss of reflux flow while the column is running normally. Run this scenario through your mind and lay out the sequence of events following (and caused by) loss of reflux.

Here's a typical sequence of events for this scenario:
1.Loss of reflux pump (Note that this could actually be 2 independent failures if there is a spare pump that's programmed to auto-start. If so, consider whether or not this is even a credible scenario.)
2.Without the reflux pumps, the reflux drum will overfill and back up into the condenser, stopping the condenser from operating.
3.The column then pressures up once the condenser stops, but whether or not this happens, and how quickly it happens is a function of the control scheme for the column. Commonly, the steam flow to the reboiler is programmed to maintain a constant temperature at some location in the column. If the reflux pumps and the condenser are out of operation, then the temperature profile in the column will start to rise. If the column uses the control scheme mentioned above, the reboiler controls will respond by cutting back on steam flow. Unless there's a min-flow setting for the steam (usually there's not) then the steam valve might eventually close completely.

a.Be aware that even if the reboiler controls respond by shutting off the steam, you could still reach the relief pressure due to the residual heat in the reboiler and/or slow response of the steam valve controller. A conservative assumption would be that the reboiler continues to operate at some low level of heat input.

4.Review this sequence with your client and revise if as necessary for this particular column design and control scheme.
5.After laying out the specific details for this particular scenario, as described above, the relieving flow is simply the resulting amount of boil-up from the reboiler.

That said, I don't often run the calc's for this particular scenario, because there's another scenario on my list that's nearly identical in its effect, and it's equally as bad or worse than this one. I'm talking about a start-up scenario in which the column is being started while someone has failed to properly line up water flow to the condenser.

Let me also mention two footnotes to this discussion. (1) Before doing anything, check to be sure that the reboiler can actually overpressure the column, even without the condenser running. It not too uncommon to find that the reboiler will pinche out before the process side is at relieving pressure. In other words, the saturation temp of the steam, at supply pressure, might not be high enough to boil the process fluid at the relieving pressure. (2) You can usually make a significant reduction in the heat input for this scenario by doing a simple calculation to reduce the reboiler's heat input, based on the fact that the reboiler dT during relief is lower than the dT used in its design. That, of course, means that the reboiler will be transferring heat at a lower rate. It's a simple calc. Just calculate a "corrected" LMTD using the relieving temp as the cold side outlet temp. Then multiply the reboiler Q by the ratio of the "corrected" Q over the uncorrected Q. This assumes that the U value is constant, and that's a good assumption.

Three quick remarks:
3) Favourable instrument response is not taken into account per API STD 521, assume control valves stay in their normal position.
Be very carefull with taking credit for a temperature pinch (see below) and in my opinion a clean U should be used.

For more information about (hand) calculation of the relief load of a tower:
"Distillation column relief loads part 1 & 2", P.L. Nezami, Hydrocarbon Processing Magazine, April 2008
"Tower pressure relief calculation", S. Rahimi Mofrad, Hydrocarbon Processing Magazine, September 2008
"Avoid common mistakes in sizing distillation safety valves", M. Bradford, D.G. Durrett, Chemical Engineering Magazine, July 1984
"How to size relief valves", A. Mukerji, Chemical Engineering Magazine, June 1980

thanx dear

[atanukc]

Hello everyone!
I happen to working on a relief valve design for a Lube vacuum distillation column, based on a top CR failure scenario. Wanted to confirm if the method suggested by Nezami, P.L, et al (Hydrocarbon processing ,Apr- May 2008 )would be suitable in this case too?
Also, what would be the basis of deciding the relieving temperature in such a scenario?