18 replies to this topic

[chemks2012]

Hi All,

The existing bursting disc [BD] has set pressure of 1barg. The discharge line from bursting disc is quite long with elevation of 2m and many bends. I have estimated the pressure drop for the bursting disc discharge line based on maximum release rate and the pressure drop is about 0.3bar and I believe there are various reasons e.g. elevation of about 2m contributes the maximum pressure drop along with the long pipe and bends.

What should be the maximum pressure drop in the discharge line?

Thanks
KS

[latexman]

Is this a fire case or process upset?

[chemks2012]

Thanks. It is a ‘process upset’ case.

How will it make difference whether it’s a ‘fire case’ or ‘process upset’?

[latexman]

The maximum pressure drop in this relief line is 1.1 x MAWP, per Code.

[chemks2012]

Sorry, I was thinking of back pressure equivalent to 3% of set pressure! Is it not so?

[TS1979]

Chemks,

The back pressure includes two parts: Superimposed and built-up. For conventional PSV, the backpressure should be over 10% set pressure. For balanced PSV, the backpressure should not exceed 50% set pressure based on API 520.

For rupture disc, once the disc is ruptured, the system cannot be closed again. The rupture disc case is very similar to the balanced PSV, therefore, the maximum backup pressure should not exceed 50% set pressure. At same time, you need to do a system pressure drop calculation to ensure the system pressure not exceed 1.1 MAWP at the relief capacity.

You mentioned 3% set pressure is PSV inlet piping instead of discharge piping.

[latexman]

chemks2010,

Does this relief have a rupture disk only, or both a rupture disk and a PSV in a combination?

[chemks2012]

Hi Latexman,

There are two identical vessels;

One with bursting disc only
Other with busting disc in combination of RV

Thanks.

[latexman]

Is your question answered to your satisfaction?

[chemks2012]

Hello Latexman, TS1979 & others

Sorry, client has just confirmed that both vessels have stand alone bursting disc open to atmosphere i.e. without any relief valve in combination.

And from your previous post of TS1979, I understand that the maximum allowable back pressure [i.e. built up + superimposed pressure] is 50% of set pressue. However, I have realised that the busting disc is ‘Reverse Acting’ busting disc and the supplier’s [busting disc supplier] literature says that ‘Due to the reverse acting compression mode of the disc it is also totally immune to backpressures and vacuum’ – What does this mean?

Thanks
KS

[fallah]

‘Due to the reverse acting compression mode of the disc it is also totally immune to backpressures and vacuum’ – What does this mean?

Hi,

It could be interpreted in mechanical standpoint; means because this type of disc is compression loaded can withstand even full vacuum without any need to vacuum support and also can withstand pressures even higher than the burst pressure at the outlet side.

Fallah

[latexman]

both vessels have stand alone bursting disc open to atmosphere i.e. without any relief valve in combination.

Each tank has a separate and independent RD relief line to atmosphere? If so, there is no concern of BP when RD is intact. After the RD blows, it's just a pipe flow problem and the max. BP in top of vessel is 1.1 x MAWP.

[chemks2012]

Thanks Fallah & Latexman,

Yes, Each tank has a separate and independent RD relief line to atmosphere

This means that I have to check following

1) Fallah - Back pressure i.e. built up pressure + superimposed pressure at the discharge of bursting disc should NOT be more than burst pressure. Please correct me if I am wrong. If my understanding is right, the total back pressure in this case would be 0.3barg which is < burst pressure of 1barg.

2) Latexman- what do you mean by ‘BP in top of vessel is 1.1 x MAWP’? Do you mean ‘ back pressure exerted [i.e. pressure drop due to release flow & atmospheric pressure ] on top of vessel should be maximum 1.1 X MAWP [max allowable working pressure]’? We do not have info on MAWP but the design pressure is 1barg and therefore if I consider MAWP as design pressure, I can say 0.3 barg < (1.1 x 1barg). Please confirm. Do I need to check the same for discharge pipework?

3)Client has also informed a short while ago that they have 500mm long flexible just at the discharge of bursting disc and he informed that this flexible has been installed as the reactor is on the load cell but I have never seen such arrangement before and it sounds very strange to me. I have asked the info on maximum allowable pressure through this flexible anyways. What do you think on this?

Thanking you in anticipation
KS

[latexman]

2) Yes, I meant the back pressure in the vessel that is pushing flow to atmosphere. In general, consider MAWP as design pressure. Check pipework? Yes, but probably design pressure of pipework >> 1 barg.

3) A flex hose in a horizontal run of pipe is common practice when vessel is on load cell.

[chemks2012]

Thanks very much Latexman

[fallah]

1) Fallah - Back pressure i.e. built up pressure + superimposed pressure at the discharge of bursting disc should NOT be more than burst pressure. Please correct me if I am wrong. If my understanding is right, the total back pressure in this case would be 0.3barg which is < burst pressure of 1barg.

Hi,

The burst pressure of the rupture disk is the value of static pressure difference across the disk just before when the disk bursts. Then it don't depend on the bulid-up back pressure. If the discharge line of the rupture disk terminated to atmosphere, the burst pressure of the RD would be 1 barg.

Fallah

[latexman]

In my experience, the term "built-up back pressure" is not used with a RD. That is a term used with a PSV, because the functioning of the PSV itself depends on the back pressure at it's outlet nozzle. A RD does not, it functions more like an ordinary pipe fitting once it has burst open. Chemks2012, just as in earlier posts in this thread, you seem to confuse the terminology of PSVs and RDs.

Edited by latexman, 15 August 2012 - 07:33 AM.

[chemks2012]

Hello All,

As stated earlier, the reactor is having an independent bursting disc installed [without any relief valve] on it. And as per the discussion earlier the back pressure should be less than 50% of set pressure. I have now re-estimated the pressure drop across the bursting disc discharge line and it is more than 50% of the set pressure i.e. set pressure is 1barg and pressure drop is 0.6bar. However, I think we can get away with this restriction [50% of set pressure] as the bursting disc is reverse acting [i.e. the bursting disc manual says : [i]Due to the reverse acting compression mode of the disc it is also totally immune to backpressures and vacuum]. [/i]Any comment/help?

Please note that it is a low pressure reactor and has bursting disc set at 1barg. As this is the case, how such discharge line [with pressure drop of 0.6bar] will be affected if we install a relief valve in combination i.e. relief valve at the downstream of the existing bursting disc? I think it will cause more problem as the set pressure [1barg] is too low [?]. Any comment?

Thanks in advance.
KS

[fallah]

KS,

The statement from manual relates to RD stability against back pressure (and also vacuum) while appears you are referring to pressure drop in RD discharge line due to fluid relieving after disc rupture. Am i wrong?

Fallah