25 replies to this topic

[chemks2012]

Dear all,

I have a dilemma and need your help

I have an existing bursting disc [independent] on the reactor in the existing plant which I want to check if it has got enough relieving capacity for the two phase release scenario I am looking into.

 

Reactor design pressure: 6barg

Bursting disc: 10” bursting disc set at 2.75barg and discharge line 10”

Bursting disc discharge pipe: 1.5m vertical (1off) rise straight from reactor nozzle, 3off long radius elbows and total discharge length of 6m discharges to a dump tank. [Note: I am sure, you don’t need sketchJ]

Two phase release scenario: complete failure of a reactor steam coil below liquid level causing two phase flow release

Two phase release rate: 480kg/sec [of which liquid release rate is 478 kg/sec and vapour is 2kg/sec]

 

I believe, there are two options and it seems only one option could be right?

 

Option#1: With above two phase release rate [480kg/sec], the two phase flow pressure drop [using Lokhart Martinelli model for two phase flow pressure drop],  in the discharge pipe line is about 90% of set pressure i.e. 2.5barg.  This proves that the given 10” bursting disc OK.

 

 

Option#2: If I use Omega Method or HEM Method to estimate the required bursting disc diameter, it seems that the 10” bursting disc is not enough and I need about 23” bursting disc!!!

 

 

Which option is correct? Am I missing anything?

 

Waiting for expert comments

Thanks.

Edited by chemks2012, 04 June 2014 - 04:10 AM.

[serra]

I suspect something wrong in your calc's,

a simple test would be to calculate the speed of sound at the different conditions in discharging piping,

speed of sound with two phase flow is discussed in this thread

 

http://www.cheresour...with-hem-model/

 

by the way you may use the same software to verify your HEM model

[chemks2012]

Hi Serra

 

Thanks very much for your reply but… are you saying I have to go for Option#2 and NOT Option#1?

 

 

PS: For HEM model,  I have taken all care to calculate mass flux which is 1754 kg/m2 s.  I have then divided the actual release rate [480kg/sec] by the mass flux to get required area which is 0.27m2 which equates to 23inch bursting disc.

[chemdoc]

I think serra is correct,

use speed of sound criteria to validate your calc's,

btw, I can't say if the correct solution is #1 or #2 as I have no data for calc's.

[chemks2012]

Thanks chemdoc,

 

I am after a reason why can I not say Option 1 is sufficient to prove the bursting disc has enough capacity?

Any idea please?

 

Input from other memebers would be highly appreciated. Thanks.

[serra]

given the very different values as output from your calc's,
you should validate results to understand where your method fails,
the suggested criteria (speed of sound or peak flux limit) is a
important step as you can't exceed that limit on each segment,
for a specified mass flux given an area (piping diameter, RO diameter)
you can't exceed speed of sound,
differently local pressure will change to reach the
maximum (peak) flux allowed by speed of sound in that segment.
Typical mach numbers are in the range 0.3-0.5, only once you are sure to be within those limits

(i.e. you selected a suitable piping diameter for estimated local conditions)
you can compare the different methods for evaluating pressure drop in piping.
Possibly you have not considered those limits and that could explain the results.
Finally, do not forget that where required (for example with fluids with relative high density as liquids etc.) you should check also against stress limits on piping.

[chemks2012]

Thanks for your input Serra.

 

Now the basic question - Can I use the Omega method given in the API 520 Part 1 Appndix D?

I am asking this as it is specifically given for relief valve ans I am assessing for bursting disc?

 

API 520 (PART 1, SECTION D.2.1) SIZING FORTWO-PHASE FLASHING OR NONFLASHING FLOW  THROUGH  A PRESSURE RELIEF VALVE

[serra]

in principle you could use a simplified HEM method (as Fauske's Omega)
for rupture disks, see

https://www.fauske.c...o-PhaseFlow.pdf

however personally I prefer a rigorous approach as in HEM, HNE, NHNE models
included in PRODE PROPERTIES,
see also the discussion about the limits of Omega model in API.

 

Finally, you may be required to give some proof
about the correct design and that may suggest to prefer
a conservative approach.

[chemks2012]

Thanks Serra,

 

Does this mean, just checking of pressure drop at the bursting disc discharge line [stated above as Option 1] is not sufficient?

I have calculated required bursting diameter by 3 methods with and without  safety factor and results are as below.

 

Safety factor of 2, actual rate is about 240kg/sec, with safety factor of 2, it is 480kg/sec, vent diameter calculated are

 

Omega: 23”

HEM: 23”

Fauske: 18”

 

With  actual rate is about 240kg/sec and without any safety factor, vent diameter calculated are

 

Omega: 16”

HEM: 16”

Fauske: 13”

 

Again, the release rate of 240kg/sec has been calculated assuming total failure of a coil which I believe way too conservative!!

 

Do you think, code allows orifice leak let’s say about 12mm? If so, the bursting disc seems OK

 

Thanks

Edited by chemks2012, 05 June 2014 - 09:37 AM.

[serra]

similar results from different methods should mean that results can be accepted,

presuming there are no similar errors, say, for example a wrong density, in all calc's :-)

 

discharge line may require a iterative procedure due to speed of sound limit,

there are examples of such procedures in API, 

basically you assume a diameter, calculate pressures and mach number,

update values etc.

[chemks2012]

Thanks Serra.

 

Just last few but important queries

 

1) It seems that if I use HEM method, I need not to calculate rupture disc discharge line two phase pressure drop separately. Is this correct?

2) If I use Omega method, we need to calculate the rupture disc discharge line two phase pressure drop as it is asking for down stream back pressure [Pa]. Is this correct?

3) Can I not consider a scenario of partial rupture of a coil [e.g. 12mm] rather than completer rapture?

[serra]

make sure to select a suitable method, for rupture disks you may consider,

 

COEFFICIENT OF DISCHARGE METHOD
suitable for simple systems, different authors give different limits of application,
personally I consider this approach with less than 4-5 pipe diameters of inlet piping and 4-5 pipe diameters of discharge piping but these may change with RD reduced diameters
the RD is sized as a PSV with a specified discharge coefficient (Kd) (in preliminary design I assume 0.6-0.7 as value), you can utilize the same methods (HEM, HNE, NHNE etc.) used for PSV

RESISTANCE TO FLOW METHOD
(when COEFFICIENT OF DISCHARGE METHOD is not applicable)

once burst, the RD becomes a single component in the discharge piping and it is the complete piping system that has to be designed to allow the required relief without exceeding the accumulated pressure limits. A conservative estimate of pressure drop for rupture disk can be obtained by assuming it has a pressure drop equivalent to the pressure drop in a pipe 75 nominal disc diameters in length. This approximation is used to compensate for the disc material protruding into the flow stream,
manufacturers provide resistance values (KR) that represent the expected resistance to
flow (independent of the fluid flowing) but I do not know if these are available for two phase flow.
API discusses several methods for two phase flow in piping, I prefer the adiabatic approach which is iterative and requires solution of a sequence of flash operations (HPF or PIPE methods in PRODE Properties)

[chemks2012]

If I simply calculate maximum allowable discharge flow through a bursting disc based on following criteria [which again not sure if applicable], calculation suggests that the bursting disc is OK. But if this is the case, then why even we need Omega and HEM method for sizing?

 

OR

 

Below criteria is just an initial check and then Omega/HEM is the detailed check which considers pressure drop across whole discharge line etc.,..?

 

Criteria: While for stress, ρV2 < 200000 kg/ms2 where ρ is average density for two phase conditions in kg/m3 and V is the velocity in m/s

 

Please see attached calculation based on above criteria...

 

Thanks

Attached Files

•  Calculation.rtf   99.74KB   24 downloads

Edited by chemks2012, 06 June 2014 - 03:43 AM.

[breizh]

Consider this resource to support your work.

 

Breizh

[chemks2012]

Thanks very much Breizh..

 

You always point to a good resource. Really appreciated.

 

Yes, I have used this method and it seems the existing bursting disc is bit small for the release scenario [total failure of a coil] based on Fauske's method [again this is bit less conservative than HEM/Omega]

 

Regards

 

PS: I really think, I need to limit the rupture diameter of a coil...assuming gross leak and not full rupture...as I dont want to put 23inch or 16inch rupture disc, while coil is desinged for 15barg pressure and all control measures are in place so that coil doesn't get pressurised...etc..

Edited by chemks2012, 06 June 2014 - 04:07 AM.

[serra]

ρV2 < 200000 kg/ms2 allows to verify maximum allowable stress on piping

 

HEM/HNE etc. can model the piping (including RD) as isentropic nozzle,

that is possible for a relatively short pipe,

for example if you have a short (say  6-8 diameters) 2" line including the RD

from a 10" line (HP) to another 10" line (LP) you can model that line as isentropic nozzle

with the above discussed COEFFICIENT OF DISCHARGE METHOD

 

Differently you need to consider the RESISTANCE TO FLOW METHOD

[fallah]

Hi,

 

IMO, to check the ability of existing system of RD configuration if handles such two phase relief load, appears you should use "Resistance to Flow" method; because it is the responsibility of system designer while using "Coefficient of Discharge " method is limited by 8*5 rule and can be applied for RD sizing itselef not the system included the RD...

[chemks2012]

Thanks Fallah,

 

HEM being more conservative, I would be using that method for sizing the bursting disc.

Also, I believe, resistance to flow criteria has already been covered by HEM as it requires all head loss constants K for the whole discharge system including bursting disc.

 

Sorry, when you said, " to check the ability of existing system of RD configuration if handles such two phase relief load, appears you should use "Resistance to Flow" method;" which method or equation you are talking about. I meant is there any equation which could be used for two phase flow sizing similar to those available for liquid or gas? 

 

Regards,

Edited by chemks2012, 07 June 2014 - 04:40 AM.

[fallah]

 

Sorry, when you said, " to check the ability of existing system of RD configuration if handles such two phase relief load, appears you should use "Resistance to Flow" method;" which method or equation you are talking about. I meant is there any equation which could be used for two phase flow sizing similar to those available for liquid or gas? 

 

 

Hi,

 

Yes, you are right; the methods such as one included in API 520 Part I as "Resistance to Flow" are applicable to single phase flow, but i did focus on system integrity considered in such methods rather than the state of fluid flow. As far as i know, DIERS Project Manual, and CCPS Guidelines for Pressure Relief and Effluent Handling Systems are included the methods can be applied to two phase flow sizing...

Edited by fallah, 08 June 2014 - 02:03 AM.

[breizh]

Chemks2012.

 

Consider reading this booklet (yellow book) attached .

Hope this helps

 

Breizh

[fallah]

 I meant is there any equation which could be used for two phase flow sizing similar to those available for liquid or gas? 

 

 

Hi,

 

Below link provide a sample resource on your qurey:

 

https://www.google.c...7SkpxzruUlQQGRA

[fallah]

 

Seems to be the same document I posted # 14!

 

 

Hello breizh,

 

Good day...

 

Yes, and also seems to be the same document posted by serra at#8!!!

 

But it's surprizing the OP still follow to find an equation for sizing of the RD for two phase flow...!

[chemks2012]

Thanks very much Fallah and Breizh

Yes, I think I would show sizing by 2 or 3 different methods as the flow nature would be unpredicted.

Few more things to ask lease

The vessel design pressure is 6barg and as per vessel design code we can allow 10% accumulation i.e. vessel can withstand total 6.6barg.

Now the 10" BD set pressure is 2.75barg and supplier has given the BD tolerance data as +0% and -10% and suggested that bursting disc maximum operating temperature is 2.75 X 0.9 (tolerance) X 0.95 (safety factor) = 2.35barg.

However, for the release rate and sizing calculation, I have ignored all above data thinking irrelevant to my calculation and considered relieving pressure of 2.75barg + 10% overpressure = 3.025barg = 4.025bara. However, as you can see 10% has not been mentioned by supplier. Can I assume 10% overpressure here?

In the HEM method where is says "upstream stagnation pressure", I have assumed it as the relieving pressure (4.025bara). Also I have considered all the fluid properties at this pressure and its corresponding temperature as we do normally. Am I correct?

Once, HEM model confirm disc is OK, do you think I need to perform other calculations to show bursting disc discharge line pressure drop? This is because HEM model includes pressure drop criteria.

Thanks and regards
KS

PS: Agree I have that document but again it does not show equation for my particular case. It says single component as two phase I.e. Steam + water.

Edited by chemks2012, 08 June 2014 - 03:43 AM.

[breizh]

^{Naser, thanks for pointing me to #8 and Serra's answer , I did not realize , all the credit to him ! LOL}

 

^Breizh

[fallah]

Hi,

 

My comments are in Bold as follows:

 

The vessel design pressure is 6barg and as per vessel design code we can allow 10% accumulation i.e. vessel can withstand total 6.6barg.

Now the 10" BD set pressure is 2.75barg and supplier has given the BD tolerance data as +0% and -10% and suggested that bursting disc maximum operating temperature is 2.75 X 0.9 (tolerance) X 0.95 (safety factor) = 2.35barg.

However, for the release rate and sizing calculation, I have ignored all above data thinking irrelevant to my calculation and considered relieving pressure of 2.75barg + 10% overpressure = 3.025barg = 4.025bara. However, as you can see 10% has not been mentioned by supplier. Can I assume 10% overpressure here?

Yes, i think you should consider 10% overpressure...

In the HEM method where is says "upstream stagnation pressure", I have assumed it as the relieving pressure (4.025bara). Also I have considered all the fluid properties at this pressure and its corresponding temperature as we do normally. Am I correct?
I think you are correct...
Once, HEM model confirm disc is OK, do you think I need to perform other calculations to show bursting disc discharge line pressure drop? This is because HEM model includes pressure drop criteria.
I think you need to perform pressure drop calculation for BD discharge line...
Thanks and regards
KS

PS: Agree I have that document but again it does not show equation for my particular case. It says single component as two phase I.e. Steam + water.

 

Edited by fallah, 09 June 2014 - 01:41 AM.