Leakage raey acceptable
Leakage raey acceptable
(OP)
Hi Experts,
Can any body suggest whether in a offshore platform all the valves including Shut Down Valve can be made metal seated. In such case what should be the criteria in case of spec. Break. Because with metal seated valve, zero leakage is not possible and hence, if there is a spec. break between #600 and #300 with a metal seated valve, the down stream piping shall get pressureized over a epriod of time and as a result the low perssure piping shall be subjetced to high pressure.
Can any body suggest whether in a offshore platform all the valves including Shut Down Valve can be made metal seated. In such case what should be the criteria in case of spec. Break. Because with metal seated valve, zero leakage is not possible and hence, if there is a spec. break between #600 and #300 with a metal seated valve, the down stream piping shall get pressureized over a epriod of time and as a result the low perssure piping shall be subjetced to high pressure.





RE: Leakage raey acceptable
If there is any possibility of lo pressure piping becoming pressured above its design pressure, leakage across seals from a high pressure region, pump backing up on its curve, a failed downstream pressure control valve or regulator, blockage downstream, thermal expansion of the fluid within, etc. you must have a relief valve installed to protect the low press piping.
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Leakage raey acceptable
I disagree with your statement that zero leakage is not possible in a metal seated valve, or atleast as it might apply to ball valves. There are several companies who are capable of manufacturing a metal seated ball valve that will provide bubble tight shutoff. When you send out for bid, specify that the valves must be metal seated have "zero leakage".
You can always request a 3rd party test/certification if you are worried the manufacturer's product wont live up to their claim.
Best Regards,
VRF
RE: Leakage raey acceptable
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Leakage raey acceptable
I do not disagree with your suggestion that a relief valve is required.
Best Regards,
VRF
RE: Leakage raey acceptable
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Leakage raey acceptable
RE: Leakage raey acceptable
Are you contending that a metal-seated valve will remain bubble tight in real flows? I've seen many metal-seated valves that start out bubble tight, but then with the accumulation of scale, trash, and slime develop progressively worse through-leakage over time. Resilliant seats are also subject to fouling, but they can often be flushed--I've found hard seated valves to be resistant to sealing after they have once leaked.
As to PSV downstream of a spec break, that is more engineering judgement than code. I know of thousands of miles of gathering system with HDPE and a sub-100 psig MAWP that are downstream of an ANSI 300 vessel (with a 600 psig PSV) and upstream of an ESD valve with no PSV anywhere between the spec break and the ESD. Happens all the time. It may not be great engineering, but its not illegal, imoral, or fatening.
David
RE: Leakage raey acceptable
From a manufacturing (my) point of view, it is impossible to say that any valve will remain bubble tight in service. All that can be guaranteed is that it doesnt leak from the start.
As you mentioned, problems with soft seated valves are usually easier to repair in the field, like with flush or sealant. Metal seated valves on the other hand, require a trip to the repair shop for the sealing surfaces to be cleared of debris, and then lapped and polished. They may even need their weld overlay repaired, or new trim parts, to seal properly, but a good repair shop should be able to restore a bubble-tight metal to metal seal.
That said, I think comparing soft -vs- metal is like comparing apples and oranges. Both are equally prone to fouling and damage in different ways. Typically metal seated valves will only be selected when the use of a less expensive soft seated valve is precluded for some reason, so comparing their modes of failure and ease of repair is mostly academic IMHO.
Best Regards,
VRF
RE: Leakage raey acceptable
If they are DOT regulated pipelines, they must have all appropriate protective devices installed.
From B31.8
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Leakage raey acceptable
After a fire (during a fire) is there a code-required (or noticeably significant) difference between a metal-seated valve and a soft-seated valve?
Does the damage to other gasket material (if not all flexitallic) during a fire out-weight any advantage of a metal-seated valve over a soft-seated valve?
RE: Leakage raey acceptable
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Leakage raey acceptable
Its tough to speak in generalities, because valves vary so much depending on the manufacturer. I would say, most of the time, a valve that has a metal seat will not have any soft goods that would melt in the event of a fire. Thus a metal seated valve would be considered in most cases inherently fire-safe. I say that because many/most metal seated valves are designed to go into services that are 450F or greater, where a thermoplastic seat is just no longer an option.
Soft seated valves, on the other hand, are usually certified fire-safe by complying with API Spec 6FA or API607, or some other similar inernational variant of those specifications. This is accomplished by designing the valve so that if the soft goods were to melt in a fire, a metal to metal backup seal will engage and stop the valve from leaking appreciably.
While a "fire-safe" rating is possible for both soft & metal seated valves, I think that a metal seated valve will probbably seal better after a fire than a soft seated one will. However, I dont think that anyone will select a metal seated valve based on this fact alone.
Best Regards,
-VRF
RE: Leakage raey acceptable
While going through API RP 14C, I have noticed that there a valve leakge upto 400cc/min is categorized as criteria for deciding whether a valve is required to go for maintanance. Thereby can we assume this much leakage in operation is acceptable.
Regards
RE: Leakage raey acceptable
I am not really familliar with RP14C, but I would say that an acceptable leakage rate should be determined not by reading an API Spec, but by carefully examining all aspects of the system to determine the implications of leakage. There are many applications where NO leakage is acceptable, despite what API RP 14C may reccomend.
Again, I am not familliar with that particular spec, but I would think that the "acceptable" leakage rate would also greatly depend on the pipe size. 400cc/min is a very bad leak for a 2" line, while it is relatively minor for a 36" line.
Best Regards,
VRF
RE: Leakage raey acceptable
RE: Leakage raey acceptable
My comments were in reference to API 6D/6A ball valves.
-VRF
RE: Leakage raey acceptable
Thanks for guideline suggetsed by various experts. But I would like to suggest API RP 14C, is a Recommended Practice for Analysis, Design, Installation and testing of Basic surface safety systems for offshore Platforms. Therefore, this may be applicable for all the types of Valve design i.e. API 6D/API 6A etc.
Regards,
RE: Leakage raey acceptable
I would like to add further to above that in the case, I suggested, all safety systems i.e. Primary and secondary protection of complete piping as required by API RP 14C are in place.
Regards,
Pranjal