Leakage raey acceptable 1

[pipexp]

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.

 

[BigInch]

That arrangement would be a typical code violation.

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

 

[valvesarefun]

Pipexp,

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

 

[BigInch]

You still need a relief valve.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[valvesarefun]

Biginch,

I do not disagree with your suggestion that a relief valve is required.

Best Regards,

VRF

 

[BigInch]

Right. One would be required with either type of valve, so actually you couldn't disagree... even if you did want to.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[valvesarefun]

Indeed.

 

[zdas04]

Valvesarefun,
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

 

[valvesarefun]

Zdas04,

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

 

[BigInch]

I think your gathering systems are some of those even more thousands of miles of gathering systems that are not regulated by DOT or B31.8, B31.4, or any other code or authority either. The only reason that they may be safe is that their potential to do unsafe things, ie. kill somebody, is totally a function of their remoteness to humans than how they were designed, or not designed, as the case may be.

If they are DOT regulated pipelines, they must have all appropriate protective devices installed.

From B31.8

841.126 Pressure/Flow Control Facilities
(a) Overpressure protection shall be provided by the
use of
(1) a monitor regulator in series with a controlling
regulator (each regulator run)
(2) adequately sized relief valve(s) downstream of
the controlling regulator(s)
(3) overpressure shutoff valve(s) upstream or downstream
of the controlling regulator(s). Installation of
alarm devices which indicate primary (controlling) regulator
failure are useful and should be considered for
monitor regulator systems.

843.441 Pressure relief or other suitable protective
devices of sufficient capacity and sensitivity shall
be installed and maintained to ensure that the maximum
allowable operating pressure of the station piping and
equipment is not exceeded by more than 10%.

843.442 A pressure relief valve or pressurelimiting
device, such as a pressure switch or unloading
device, shall be installed in the discharge line of each
positive displacement transmission compressor between
the gas compressor and the first discharge block valve.
If a pressure relief valve is the primary overprotection
device, then the relieving capacity shall be equal to or
greater than the capacity of the compressor. If the relief
valves on the compressor do not prevent the possibility
of overpressuring the pipeline as specified in para. 845,
a relieving or pressure-limiting device shall be installed
on the pipeline to prevent it from being overpressured
beyond the limits prescribed by this Code.

845.21 Control and Limiting of Gas Pressure in
Holders, Pipelines, and All Facilities That Might at Times
Be Bottle Tight
845.212 Suitable types of protective devices to
prevent overpressuring of such facilities include
(a) spring-loaded relief valves of types meeting the
provisions of BPV Code, Section VIII
(b) pilot-loaded back-pressure regulators used as
relief valves, so designed that failure of the pilot system
or control lines will cause the regulator to open
(c) rupture disks of the type meeting the provisions
of BPV Code, Section VIII, Division 1

845.222 In addition to the pressure-regulating
devices prescribed in para. 845.221, a suitable method
shall be provided to prevent accidental overpressuring
of a high-pressure distribution system.
Suitable types of protective devices to prevent overpressuring
of high-pressure distribution systems
include
(a) relief valves as prescribed in paras. 845.212(a)
and (b)
(b) weight-loaded relief valves
(c) a monitoring regulator installed in series with the
primary pressure regulator
(d) a series regulator installed upstream from the primary
regulator and set to limit the pressure on the inlet
of the primary regulator continuously to the maximum
allowable operating pressure of the distribution system
or less
(e) an automatic shutoff device installed in series with
the primary pressure regulator and set to shut off when
the pressure on the distribution system reaches the maximum
allowable operating pressure or less. This device
must remain closed until manually reset. It should not
be used where it might cause an interruption in service
to a large number of customers.
(f) spring-loaded, diaphragm-type relief valves

Low-Pressure Distribution Systems
845.231 Each low-pressure distribution system
or low-pressure main supplied from a gas source that
is at a higher pressure than the maximum allowable
operating pressure for the low-pressure system shall be
equipped with pressure-regulating devices of adequate
capacity. These devices must be designed to meet the
pressure, load, and other service conditions under which
they will have to operate.
845.232 In addition to the pressure-regulating
devices prescribed in para. 845.231, a suitable device
shall be provided to prevent accidental overpressuring.
Suitable types of protective devices to prevent overpressuring
of low-pressure distribution systems include
(a) a liquid seal relief device that can be set to open
accurately and consistently at the desired pressure
(b) weight-loaded relief valves
(c) an automatic shutoff device as described in para.
845.222(e)
(d) a pilot-loaded, back-pressure regulator as
described in para. 845.212(b)
(e) a monitoring regulator as described in para.
845.222(c)
(f) a series regulator as described in para. 845.222(d)

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[racookpe1978]

The OP is on an oil platform (leakage = pollution, losses and fines) and possible fires or explosions around a habitable space.

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?

 

[BigInch]

No seat requirement. The code requires a thermal relief to prevent overpressure from expansion of the fluid within the hot piping, supposedly the expanded fluid being piped to a safer place, ie. not leaking downstream to overpressure more piping there.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[valvesarefun]

Racookpe1978,

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

 

[pipexp]

Hi Experts,
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

 

[valvesarefun]

Pipexp,

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

 

[blacksmith37]

Is this thread about API 6 well head /tree valves , (all metal/metal, more or less drop tight when new) , or ANSI (I hope the "O" ring holds !) valves? They aren't the same thing.

 

[valvesarefun]

Blacksmith,

My comments were in reference to API 6D/6A ball valves.

-VRF

 

[pipexp]

Hi Experts,

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,

 

[pipexp]

Hi experts,

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