8 replies to this topic

[bag]

Dear All Professionals:

 

I have a doubt on sizing PSV for shell and tube heat exchanger.

 

Here are the process information:

 

Tube side fluid: process liquid

Tube side pressure: operating at 30barg and designed at 40barg

Tube side temperature: 93-95 degC

Tube side fluid boiling point: 102 - 104 degC at ATM

Tube side has a control valve (fail open) at cooling water return line

 

Shell side fluid: cooling water

Shell side pressure: operating at ~3barg and designed at 6barg

 

Based the pressure ratios, i should consider tube rupture case for this heat exchanger.

 

Also, sometimes the cooling water temperature goes up to 80degC when cooling load is small. Considering cooling water has some chlorine content and it could be corrosive over the time.

 

Now I have several questions:

 

1. Is a PSV really required considering the cooling water supply line is always open?

2. When I calculate the relief capacity, shall I consider the cooling water supply line also a pressure relief point? Because the current DP between tube and shell side gives a super large liquid flow through the tube (28mm ID)

 

I am really confused and hope i could get some tips here.

 

Thanks. 

[fallah]

bag,

 

1- You can't take credit for control valve on cooling water return line to be open at the moment of a tube rupture scenario. Nevertheless, even if such control valve wasn't there; without a dynamic study on whole system nobody can suppose the cooling water supply/return lines be able to handle the relief load of tube rupture case such that make the system needless to PSV for tube rupture case. Then it's highly recommended to consider the tube rupture case as a credible scenario for the system you described...

 

2- Yes, you might consider the cooling water supply line as a pressure relief point during a tube rupture case, but to specify how much; needed to a comprehensive dynamic study at the moment of the rupture and next reasonable time duration. Then it's better to avoid considering such pressure relief point in relief capacity of the relevant PSV...

Edited by fallah, 06 December 2014 - 01:00 PM.

[bag]

bag,

 

1- You can't take credit for control valve on cooling water return line to be open at the moment of a tube rupture scenario. Nevertheless, even if such control valve wasn't there; without a dynamic study on whole system nobody can suppose the cooling water supply/return lines be able to handle the relief load of tube rupture case such that make the system needless to PSV for tube rupture case. Then it's highly recommended to consider the tube rupture case as a credible scenario for the system you described...

 

2- Yes, you might consider the cooling water supply line as a pressure relief point during a tube rupture case, but to specify how much; needed to a comprehensive dynamic study at the moment of the rupture and next reasonable time duration. Then it's better to avoid considering such pressure relief point in relief capacity of the relevant PSV...

 

Dear Fallah

 

First, thanks for the suggestion.

 

Second, now i have difficulty on installing a PSV which is sufficient to release the load without considering cooling water supply/return line. I just did a quick sizing for the PSV and it requires 4" to 6", orifice N. There are only two nozzles on the shell of existing heat exchanger both 2", for cooling water supply and return respectively. 

 

Do you have any suggestion, if without making any modification to the heat exchanger.

 

Thanks.

[shan]

Put 2" rapture disks on your cooling water inlet line and outlet line.

[fallah]

bag,

 

Considering two PSVs (or RDs) with inlet nozzle of 2" at cooling water supply/return lines can't handle the relief load of a PSV with inlet nozzle of 4"; then if you intend to do no modification on the heat exchanger, appears the only way to protect the heat exchanger shell against over pressure due to the tube rupture is using a HIPPS configuration at the inlet line of process side taking signal from PTs on the cooling water supply or return line to isolate the exchanger from high pressure source when the PTs set points are going to pass the set point equal to the design pressure of the shell side...

[ahmadikh]

 Bag,

 

I tried to do a quick calculation with some assumptions that I have considered regarding your exchanger and fluid properties. I ended up with an "L" size orifice, so something like 3L4 (Again, all of this is based on some assumptions and might be totally wrong) which comes from about 140000-150000 kg/hr relief load. You may double-check your calculation or advise me if I am wrong. Although you are considering the catastrophic tube rupture, "L" makes more sense to me since it would be a liquid relief case as per its normal boiling point.

 

Best,

Milad

[bag]

 Bag,

 

I tried to do a quick calculation with some assumptions that I have considered regarding your exchanger and fluid properties. I ended up with an "L" size orifice, so something like 3L4 (Again, all of this is based on some assumptions and might be totally wrong) which comes from about 140000-150000 kg/hr relief load. You may double-check your calculation or advise me if I am wrong. Although you are considering the catastrophic tube rupture, "L" makes more sense to me since it would be a liquid relief case as per its normal boiling point.

 

Best,

Milad

 

the tube size in heat exchanger is 28mm, internal diameter.

 

considering both ends are as the orifice, and flow is induced by 40barg from tube side to 6.6 barg at the shell side, I ended up with somewhere 3800 liters/min. Is this similar to yours?

[bag]

bag,

 

Considering two PSVs (or RDs) with inlet nozzle of 2" at cooling water supply/return lines can't handle the relief load of a PSV with inlet nozzle of 4"; then if you intend to do no modification on the heat exchanger, appears the only way to protect the heat exchanger shell against over pressure due to the tube rupture is using a HIPPS configuration at the inlet line of process side taking signal from PTs on the cooling water supply or return line to isolate the exchanger from high pressure source when the PTs set points are going to pass the set point equal to the design pressure of the shell side...

 

This heat exchanger is installed in the reactor circulation loop. I can isolate the pressure source to the reactor, but then it comes back to the same question: how shall I calculate the relief capacity if tube side pressure is reducing after tube is ruptured. I still have to consider the worst case, which is at the beginning of tube rupture, when reactor pressure is not reduced.

 

Also, as the heat exchanger is in the reactor ciruclation loop (reactor is with 30% liquid and 70% gas in head space), I think pressure in reactor could maintain for quite sometime..

[ahmadikh]

 

 Bag,

 

I tried to do a quick calculation with some assumptions that I have considered regarding your exchanger and fluid properties. I ended up with an "L" size orifice, so something like 3L4 (Again, all of this is based on some assumptions and might be totally wrong) which comes from about 140000-150000 kg/hr relief load. You may double-check your calculation or advise me if I am wrong. Although you are considering the catastrophic tube rupture, "L" makes more sense to me since it would be a liquid relief case as per its normal boiling point.

 

Best,

Milad

 

the tube size in heat exchanger is 28mm, internal diameter.

 

considering both ends are as the orifice, and flow is induced by 40barg from tube side to 6.6 barg at the shell side, I ended up with somewhere 3800 liters/min. Is this similar to yours?

 

 I do not have the value for density, so I just assumed a number. As I said, the mass flowrate that I got is about 140000-150000 kg/hr. You can refer to "Crane" for more details regarding the equations and resistance factors.

 

Best,

Milad