Bursting disc/RV/RO 2

[Thatllberight]

Hello folks,

I wonder if anyone knows the answer to this little observation.
I have been looking at a gas terminal and on the liquid side equipment there are a number of RVs downstream of pumps (NGL service). The RV sets have a pilot operated RV and a standby, but also a conventional valve with the same set pressure (eg. 113 barg) but a bursting disc in the inlet with much higher set pressure (eg 151 barg at 20 C)and an RO in parallel with the conventional RV. I am pressuming the bursting disc is set for a fire case which is the only thing that seems to make sense, even though the main case is a pump blocked discharge. This plant is early 80's and the philiosophy seems to be lost on everyone.

Also heard that the new PED regs dont allow overpressure of more than 10% for bursting discs - can anyone confirm this?

Be grateful for any thoughts,

Dave

 

[EGT01]

Here's a few thoughts but there's still a lot of info missing...

What is NGL service? Is this liquefied natural gas? If so, then the operating temperature of your relief systems is likely rather low, an important factor to note especially in regards to the stated burst conditions of your rupture disc.

What do the relief systems protect? If protecting just piping, what is the pressure limit of the piping and the design code to which the piping is constructed? If ASME/ANSI B31.3 applies, then the piping is likely not within the scope of what is considered high pressure piping (2500# class flange rating). Are there other special requirements that would apply for your service?

It appears that the pilot operated RV's are your primary relief system and the rupture disc/conventional RV arrangement is a secondary relief system. I presume this means that the primary system alone is not adequate for all cases and requires supplemental relief capacity.

It makes sense if you need multiple relief devices to have them at staggered settings. I assume the pilot operated RV's are adequately sized to handle the more likely blocked pump discharge. For a less likely event that requires the supplemental relief capacity (such as fire), and to avoid having to replace the rupture disc unnecessarily, it makes sense to set it as high as possible but within the limits allowed by the applicable code.

Now it looks suspicious that the rupture disc is set about 33% higher than the pilot operated RV's. That happens to be an allowable overpressure condition as given in B31.3 but I wonder what effect the operating temperature would have on the rupture disc burst pressure.

You don't say how the RO is setup but I suspect that it is a means of venting the space between rupture disc and relief valve to avoid pressure buildup. Without a means of avoiding pressure buildup, the relief system may not open until the upstream pressure equals the sum of rupture disc set plus relief valve set.

Once the rupture disc has blown, the conventional valve will already experience an overpressure condition and the question then becomes whether the conventional valve provides adequate supplemental capacity without the need of the system pressure increasing above the rupture disc set point.

I can't say I'm familiar with the PED regulations but I can't imagine why there would be any restriction to the amount of overpressure that is used to size a rupture disc as long as the maximum relieving pressure does not exceed that allowed (allowable accumulation) by the code to which the equipment was constructed.

 

[MortenA]

PED is evry specific - no more than 10% overpressure. BUT there is an expection for the fire case. Here its still possibel to use 21% as pr. API. But note that existing plant do not have to comply with PED and you can maintain the plant without having to upgrade when replacing equipment.

Check:

http://ped.eurodyn.com

search for "fire" and check item 4 (or go directly to guidelines 5/2

The ped.eurodyn.com site contains the full text, the guidelines and other PED stuff.

Best regards

Morten

 

[Thatllberight]

EGT01, hey, thanks for the detailed reply.

Yes you are right NGL is natural gas liquids or condensate. Pipeline gas enters plant , slug catcher and separator and off comes the liquids that are then pumped via filters and coalecsers to mol sieves and recombined with the treated gas stream. The particular RVs are downstream of the pumps and on the vessels. Operating temperature is lowish (sea temp ~ 1C).
One set of RVs is for pump shut in protecting piping (900#) and vessels (117 barg). The two pilots are actually modulating type – which the plant didn’t even know! They will take care of the full pump run out so are ok. The reason I am am looking at them is because the suction pressure in the separator can now be increased.
It is this supplemental case you mention requiring a bursting disc/Conv. RV and parallel RO that is baffling – again the plant have no idea why it is there and there is no further information available. I was thinking a fire case but also had in mind the 33% for piping but that I have never seen done before. Another thing that crossed my confused mind was hydrates but that isn’t sensible either. And for the fire case why would the conv. RV be set a 113 barg as you say.

Interesting about the R0 for venting the space, I have not seen that before either but seems very logical now. By the way I asked BS&B about the RO and they didn’t understand it!?!

I agree on PED, it was BS&B again who said they could no longer supply discs with 21% accumulation. Seems strange. I’ll have to read up and then get on to them again.

Thanks for your help and time.
Best,
Dave

 

[Thatllberight]

MortenA, thanks for info.

BS&B said they can't supply 21% discs now. Looks like I'll have to go back to them again.

Best,
Dave

 

[Thatllberight]

EGT01, sorry, the conv. RV could be set at 113 barg for the fire case.
Now have discovered a note on the BD data sheet to protected equipment design pressure of 113 barg and a MAWP of 141.2 barg, now I am confused.

 

[1974vet]

Just some thought jotted down:

The pilot valves or modulating type, may be required by whatever applicable code is for your specific application to deliver the full pump capacity. Don’t know what they are, but an example is an NFPA requirement that these type valves be required on the discharge of some firewater pumps. Do a data check; these relief devices are not ASME Code Certified. Note, a conventional relief device is not normally sized to relieve full pump capacity, because this results in extremely large relief devices. Think of all the pumps out there. Minimum flow bypasses, etc., are better.

Pressure vessels and some piping will require a code certified relief device. I am not familiar with the 33% overpressure indicated above (please reference). Typically 10% overpressure (design or rated pressure of protected equipment) is allowed for all scenarios other than fire, which can be 21%. API 520 presents these rules and for staggered settings for multiple devices. One may be accepting some additional liability by installing or certifying a relief system that protects equipment at a higher pressure (and capacity) than the vendor certified values. Generally this can be avoided.

It also can happen that maintenance looked at a burst disk tag with a burst pressure, and ordered more "just like that one". The problem then is that no account was taken for manufacturing and burst tolerances, leading immediately to incorrect burst settings for the replacement disks.

When a rupture disk device is installed at the inlet of a pressure relief valve, the specified burst pressure and set pressure should be the same nominal value.

David