12 replies to this topic

[MJ94]

A shell and and tube Exchanger with TEMA Designation AHU(channel and removable cover,U tube ,double split flow) needs to be hydro tested :

 

Tube side fluid : BFW(operating pressure =85 kg/cm2 , temp= 160'C)
Shell side fluid : h2,co,co2,water vapour( operating pressure = 21 kg/cm2, temp = 280'C)

 

Design Pressures : TUbe :106kg/cm2

                                Shell : 26.5 kg/cm2

 

how will this exchanger be checked for tube leaks(Hydrotested)before boxing up ??

 

How will the shell be hydrotested??

 

since it does not follow the 2/3 rule , should there be a PSV on the shell side ??

 

 

 

[fallah]

MJ94,

 

Please find below my answers to your question as red color:

 

 

A shell and and tube Exchanger with TEMA Designation AHU(channel and removable cover,U tube ,double split flow) needs to be hydro tested :

 

Tube side fluid : BFW(operating pressure =85 kg/cm2 , temp= 160'C)
Shell side fluid : h2,co,co2,water vapour( operating pressure = 21 kg/cm2, temp = 280'C)

 

Design Pressures : TUbe :106kg/cm2

                                Shell : 26.5 kg/cm2

 

how will this exchanger be checked for tube leaks(Hydrotested)before boxing up ??

 

As far as i know, before boxing up and hydrostatic test, leak test shall normally be done on tube to tubesheet connections, let say, using 15 psig air and soap bubble...

 

How will the shell be hydrotested??

 

If design code is ASME Sec VIII Div 1, it should be done per UG-99 of the mentioned code...

 

since it does not follow the 2/3 rule , should there be a PSV on the shell side ??

 

The PSV, with the set pressure of 25.6 kg/cm²g, is needed on the shell side at least for tube rupture scenario...

 

 

 

[MJ94]

Fallah , I have seen the hydrotest of this exchanger , this was after repetetive leaks in the tubes , the whole bundle got replaced , and after putting in the new bundle , before boxing up the channel cover the shell was pressurized to 1.5 times of tube design pressure to check whether the tubes leak or not . Is this right since the shell's design pressure is only 26.5kg/cm2. ?
If you suggest that this is wrong then how can a leaking tube be specified , (for plugging)  ??

Also tell me if this exchanger had a the high pressure stream in the shell and low pressure stream in the tubes , the tubes would then be surrounded by High Pressure BFW  , so in this case do we have to use tubes with design pressure mathing BFW pressure or can we use tubes with design pressure 26.5 kg/cm2 g ????

[MJ94]

All the experts here please have a look into this problem!!!

[MJ94]

Do I need to get some an extra info here ..??
Please reply !!!

[Mayank Joshi]

I think this 2/3 rule or 10/13 rule is really not given clearly anywhere , a lot of engineers including me have confusion in this region ...
 

[PingPong]

since it does not follow the 2/3 rule , should there be a PSV on the shell side ??

If the shell side of the exchanger has block valves, there has to be a PSV within these blockvalves to relief any load from a possible tubeburst.

 

If there are no blockvalves around the shellside of the exchanger, but the shellside is open with the rest of the process system, there needs to be a PSV somewhere on that system able to relief the load from a possible tubeburst, unless the process system pressure is not going to reach its design pressure in case of tube burst. In this case a tubeburst will introduce water and steam into the process system, so if there is enough volume to accumulate the water (for say 15 minutes) and the steam is condensed further downstream in an aircooler (or similar) then the pressure in the process system is not going to reach PSV setpressure in case of a tubeburst.

 

 

 

I think this 2/3 rule or 10/13 rule is really not given clearly anywhere , a lot of engineers including me have confusion in this region ...

Indeed.

A lot of people seem to think that designing for the 2/3 rule will prevent a tubeburst, but that is NOT so.

It merely means that in case of a tubeburst all low pressure side equipment that is designed using 2/3 rule or 10/13 rule will not exceed their test pressure (1.5 or 1.3 times design pressure) and therefor need no PSV to survive a tubeburst. However all other low pressure side equipment that is not designed for the 2/3 rule still has to be protected in some way against the consequence of tubeburst.

 

In this case designing the shellside using the 2/3 rule would only protect that shellside, but not the rest of the low pressure process system that is open with that shellside.

 

 

 

the shell was pressurized to 1.5 times of tube design pressure to check whether the tubes leak or not . Is this right since the shell's design pressure is only 26.5.

If the shellside is really designed for only 26.5 kg/cm² then it should not be able to withstand 1.5 times tubeside design pressure, which would be 159 kg/cm². If it can withstand 159 kg/cm² then it obviously was designed for a much higher pressure than that 26.5 kg/cm².

 

 

 

Also tell me if this exchanger had a the high pressure stream in the shell and low pressure stream in the tubes , the tubes would then be surrounded by High Pressure BFW  , so in this case do we have to use tubes with design pressure mathing BFW pressure or can we use tubes with design pressure 26.5 kg/cm2 g ????

The chosen thickness of the tubes has to be such that they can withstand the design pressure of the shellside with no pressure on the tubeside, as well as the design pressure of the tubeside with no pressure on the shellside.

 

There is no such thing as a "tube with a design pressure of .... kg/cm² ". Required thickness depends on selected tube material, corrosion allowance, design temperature and design pressure. The design pressure of the shellside of an exchanger is usually limited by the thickness of the shell itself, tubesheet thickness, and the flanges and bolts with which it is connected to the tubesheet(s) and channel(s).

 

Determining for what pressure (and temperature) an exchanger will be designed, and whether or not the 2/3 rule will be used in a particular case, is decided by experienced process design engineers like me.

Once the design pressure (and temperature) are set, determining the test pressure is merely a matter of following the applicable codes, as already explained by fallah. There is no degree of freedom with that, no "yeah but ....", no "I would prefer to ....".

Edited by PingPong, 15 September 2013 - 10:35 AM.

[Mayank Joshi]

Ping Pong thanks a lot for replying , yes you are absolutely right that when no Blockvalves are given then PSV is not given on the lower pressure side on the equipment , I saw this in one of the plants P&ID that iss why this doubt came to me , can you elaborate a more on this point or give the reference to  :
'unless the process system pressure is not going to reach its design pressure in case of tube burst. In this case a tubeburst will introduce water and steam into the process system, so if there is enough volume to accumulate the water (for say 15 minutes) and the steam is condensed further downstream in an aircooler (or similar) then the pressure in the process system is not going to reach PSV setpressure in case of a tubeburst.'

 

 

 

If the shellside is really designed for only 26.5 kg/cm² then it should not be able to withstand 1.5 times tubeside design pressure, which would be 159 kg/cm². If it can withstand 159 kg/cm² then it obviously was designed for a much higher pressure than that 26.5 kg/cm².

 

The shell side desgin pressure on the face plate of the exchanger is given as 26.5kg/cm2 but even then to check for tube leaks , Shell was pressurised to 1.5 times Tube pressure , keeping the channel cover of exchanger open . If this method is not right how are tube leaks detected in an exhanger with such a configuration.????

 

If shell side has higher pressure for egs 40kg/cm2 and tubes have operating pressure 15 kg/cm2 , then should the design pressure of tube side be more equal to 1.5 times of 40kg/cm2 (approx).??

Thanks a lot for replying by the way .

[PingPong]

Tube side fluid : BFW(operating pressure =85 kg/cm2 , temp= 160'C)
Shell side fluid : h2,co,co2,water vapour( operating pressure = 21 kg/cm2, temp = 280'C)

 

Design Pressures : TUbe :106kg/cm2

                                Shell : 26.5 kg/cm2

 

What is the service of this exchanger? I suspect it is a steam boiler in a Steam Reformer (Hydrogen Plant) that cools the heater effluent.

Normally the hydrogen rich gas (heater effluent) is then on the tube side and the BFW/Steam is on the shellside. I have never seen it the other way around.

 

I think you mixed up shell side and tube side conditions.

[Mayank Joshi]

I have seen both configurations , High Pressure in tube side is for Halder Topsoe reformers and high pressure on shell side is in KTI Technip...and yes this is for Reformers .

 

Do you know which API should I read to find details about this ???

and if possible can you please have a look into my yesterdays reply , ie my post which is above this post in the same topic

[PingPong]

'unless the process system pressure is not going to reach its design pressure in case of tube burst. In this case a tubeburst will introduce water and steam into the process system, so if there is enough volume to accumulate the water (for say 15 minutes) and the steam is condensed further downstream in an aircooler (or similar) then the pressure in the process system is not going to reach PSV setpressure in case of a tubeburst.'

It simply means that if the load from the tubeburst is not able to overpressure the low pressure system, the there is no relief load for the PSV's on the low pressure system. If the water can be accommodated by hold-up in the vessels, and the steam is simply condensed in the heat exchangers and aircoolers without the system then the PSV set pressure will not be reached. What safety philosophy is used in a unit depends on that of the licensor, the client, and the engineering contractor that built the unit and probably did a HAZOP.

 

The shell side desgin pressure on the face plate of the exchanger is given as 26.5kg/cm2 but even then to check for tube leaks , Shell was pressurised to 1.5 times Tube pressure , keeping the channel cover of exchanger open . If this method is not right how are tube leaks detected in an exhanger with such a configuration.????

Once again: it is impossible to pressurise a shell to 159 kg/cm² if it was only designed for 26.5 kg/cm².

 

That is apart from the fact that there is absolutely no reason or requirement to use such extreme overpressure.

If the shell survived 159 kg/cm² it must have been designed for a much higher pressure than 26.5 kg/cm². Look up the official documents belonging to the exchanger.

Note however that surviving does not mean that the metal structure could not have been affected by this excessive overpressure due to slight deformation, creep, cracks.

 

If shell side has higher pressure for egs 40kg/cm2 and tubes have operating pressure 15 kg/cm2 , then should the design pressure of tube side be more equal to 1.5 times of 40kg/cm2 (approx).??

Of course not. But what exchanger are we talking about here? A real one? Or a fictitious one?

 

Do you know which API should I read to find details about this ???

No.

 

High Pressure in tube side is for Halder Topsoe reformers

If you say so. I never saw a HT reformer design.

But frankly I doubt it, as this kind of waste heat boilers are not simply taken off the shelf. The heater effluent is extremely hot, almost 900 ^oC. If that would flow through the shell side, then the shell would have to withstand that temperature. With BFW/Steam on the shellside the shell as well as tube temperatures are close to that of the BFW.

Could you upload a process datasheet of this exchanger? Or part of the P&ID with this exchanger on it?

[thorium90]

I worked with a HT reformer and it had a waste heat boiler with BFW on the shell side making 50bar steam by natural circulation and cools ~800C syngas at ~20bar to ~250C in the tubes. Im also curious to see this unique design that Mayank is talking about.

Edited by thorium90, 18 September 2013 - 11:18 AM.

[Mayank Joshi]

i am really sorry :

Tube side fluid : BFW(operating pressure =85 kg/cm2 , temp= 160'C)
Shell side fluid : h2,co,co2,water vapour( operating pressure = 21 kg/cm2, temp = 280'C)

 

Design Pressures : TUbe :106kg/cm2

                                Shell : 26.5 kg/cm2

this is not a steam boiler , it is a bfw preheater in a hydrogen plant ..High Pressure in tube side is for Halder Topsoe reformers and high pressure on shell side is in KTI Technip

really srry I read  steam boiler as bfw preheater.....

Edited by Mayank Joshi, 18 September 2013 - 06:49 PM.