Backwards Spec Break 3

[zdas04]

I've got a situation where I'm flowing from an ANSI 300 system (PSV set at 600 psig) into an API 3000 wellhead with a potential build-up pressure of 1200 psig if the well is shut in for a couple of days (it is normally less than 40 psig when flowing). I've come across this situation a couple of times before and the solution we used is described in the attachment. No one trusts check valves for isolation, but this approach allows you to verify that the paired checks hold well enough.

The first time I ever used this we did a HazOp on it and the PSM consultant looked at the design and said "that is a legal spec break". We all took him at his word and went on with our lives. We built it and it worked well. We put it on the schedule to be verified when the PSV was tested, and it passed the test every time.

Now I have a client that is saying "why do you say it is a 'legal spec break', is there a document somewhere that defines legal spec breaks and does it include this?". I looked through my library and can't find any requirements for doing anything in particular at a spec break. Reading through all the threads on eng-tips.com most of the discussion is (of course) going from a high pressure rating to a low pressure rating and that requires some sort of positive block (ESD, manual block valve, etc.). The general Interwebz was equally as unhelpful. Going from high MAWP to low MAWP is not the situation I have here, and I'd rather not put an ESD on this line (the ESD valves that this client uses require a person to pump up a hydraulic pressure to open the valve, and it would be tough to pump up the gas and water inlet ESD's and the gas recycle ESD at the same time and if there is a delay the risk of a trip during start up is too high.

Can anyone point me towards something that defines the range of "legal" spec break technologies or is it down to Engineering Judgement which is what my research this morning hints at?

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[LittleInch]

Zdas04, Although you say " Going from high MAWP to low MAWP is not the situation I have here,", in reality that is not correct, IMO. It might be reverse flow from normal, but the gas and the pipe don't care. Your requirement in legal terms is to not exceed the design /MAWP of your lower pressure system. If you have a higher pressure connected to your lower pressure system then you need to eliminate or mitigate that risk.

I assume you can't reroute your #300 pipe to outside the well shut in envelope but would make it all go away. Your scenario may be acceptable to some, but major operators would have isolation philosophies that prevent this other than very short term use. Why would your isolation have to be an esd? Just make it a normal isolation and then open it just before you need to re start? Risks are minimised and with two nvs and a PSV, presumably sized at what ever flow you could get from the well at 660psig, that would satisfy most people.

Most hazops I've been at would also need the two nrvs to be different types /models to avoid common node failure.

Basically it's a risk based /reliability issue with some specific parameters thrown in such as what is the flow at 660 psig, how much piping volume could be over pressured, how often does it occur, consequence of failure, that sort of thing. In other words a SIL assessment.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[hacksaw]

the fall back in this case is the ASME piping code. 1200 psi would not look good on any drawings you sign off, were failure to occur, etc., etc.

 

[hacksaw]

the asme piping codes govern in this case as it is the lowest rating.

It would not be a good idea to sign-off on a piping design like that even though the customer may have had "good" experience with the design.

 

[zdas04]

LittleInch,
Is there some reason you fell beak to the "he must be an idiot" mode? I thought I had contributed enough here to rate the benefit of the doubt. I guess not.

Normal flow is from ANSI 300 to API 3000. On the other side of the wellhead there are ESD valves since normal flow is from the high MAWP to the low MAWP (API 3000 --> ANSI 300) and the spec break has an ESD. If pressure on the wellhead approaches the hi-hi setpoint the ESDs trip and isolate the wellhead from the lower pressure piping. No problem.

For this one line, normal flow is from low MAWP to high MAWP (I do know that gas doesn't flow from low pressure to high pressure). Typically it will flow from 300 psig to 40 psig. In an upset (no flow), wellhead pressure will build. At the set point, the ESD valves will close. The drawing I attached is the technique I've used to demonstrably prevent the wellhead pressure from getting back to the ANSI 300 piping and vessel.

Where I've used this before was with a pretty major operator (I think they are the second or third largest corporation in the industry), and we had a PSM consultant and the wellsite technical authority approve this approach for a permanent installation. Not trying to pull any wool over on any eyes, this was mainstream approved.

Hacksaw,
Not sure what you mean about 1200 psi not looking good. Reservoir pressure is what it is. That is why we have API 3000 rated wellheads. ASME B31.3 is where I started, but it was silent. Then to JM Campbell's Guidelines for Process Safety, then to a half dozen other PSM references. All were silent.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[LittleInch]

Zdas04, I am well aware of your many contributions here and if it came across as you've interpreted it I apologise profusely. I was just adding my comment based on what you told us in the OP.

As I said it can be done this way, but needs to be specific to the particular location and operating conditions. Double nrvs and a PSV would seem to be sufficient for occasional use. IMHO, an actuated valve would add to the security of the system and could be opened prior to re start where you then rely on the nrvs for a short period.

One thing in addition which I'm sure you are aware of is what type of nrvs these are. If the well head pressure build up is slow then sealing of some types of nrv can be slow to occur if reverse flow is very low.

I haven't seen a code requirement for any particular design or requirement at a spec break other than the general requirement not to exceed the design pressure. How you do it is up to you. To revitalise this is one reason why SIL Assessments have been used for this on many occasions. Your testing method is a good one and if it is used well then there should be no issue. The question I always ask myself is how do I know the operator will do what he is supposed to do once I've left and could I make the system safer by design.

If it came across wrong then I'm sorry, but you haven't said why this one line is connected to the welled and not the #300 d/s pipework. I'm just curious to understand why it is connected this way as removal of the risk is the best way.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

Thinking about this a bit more, it seems that our difference of view, if indeed there is one, is based on direction of flow.

To me there is no real difference, only the type of valve you can use. Your double NRV and PSV system is analogous to a high to low pressure flow using two control valves which seal on 0% open with a D/S PSV. So the question then is would you think a double control valve (fail closed) is acceptable for a 1200 psig to 300 psig pressure drop. If yes then you're fine with your current arrangement, if not then you some additional protection.

I'll leave to others to comment further.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

LittleInch,
I was a bit stressed yesterday (too much to do and too little time), I probably over-reacted.

If you look on my web page under "Samples" in the right-hand column towards the bottom there is an animation of my patented GasBuster. Downhole pump discharge comes into the vessel (high MAWP into low MAWP with an ESD). When gas accumulates in the GasBuster it is let out to a sink (which can be a traditional gas/liquid separator if there is one, or back into the wellbore). In either of these cases an upset that sent water through the gas outlet would just go to a place where we a set up to handle produced water without putting it into the gas-gathering system.

Gas outlet from the GasBuster can also go downstream of the annulus ESD and into the gas gathering system (many of the wells that get GasBusters do not have any traditional separators). This last option has a near certainty that sooner or later the gas-outlet valve will stick open or the top level float will stick down and the downhole pump discharge will go into the gas system. I'm using pretty reliable equipment, but mechanical things all eventually fail.

The "will they really test it" question is exactly the same for this doubled check valve as it would be for an ESD or PSV. There is always a point where you put your design in the hands of the field and get on with your life. Where I've done this doubled-check valve approach the maintenance records have been 100% complete and successful.

I never consider a throttle valve to be part of a spec break. The potential for asymetric wear and significant leakage is just too great. On the other hand the potential for asymetric wear on a non-return valve is much lower. I'm OK with a verifiable check valve manifold as a spec break because even if both leak, the PSV will protect the vessel from overpressure (and I see the probability of leakage of both NRV to be the same order of magnitude as an ESD leaking).

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[SNORGY]

Not sure of the code requirements or where it's written down, and perhaps before I post I should review that carefully.

What I CAN say is that we do this sort of thing all the time: low pressure (low ANSI rating) piping flowing into high pressure (higher ANSI rating) piping via two check valves in series. While the two check valves (designed to the higher of the two ratings) cannot constitute overpressure safeguards on the low pressure piping, they can be used in combination with a tie-in ESD (on pressure) or PSV to safeguard the low pressure side. We're looking at this right now for a gas lift scheme on a well that ties into a production manifold at higher pressure. The rationale is that with two check valves in series you can take credit for net leakage backwards, something like the maximum net will be 10% of what one single check would pass if it failed. I believe it's in API 521...

I'll look into it a bit later today.

 

[BigInch]

API 521

4.3.4.1 Causes of overpressure due to check-valve leakage or failure
A single check valve is not always an effective means for preventing overpressure by reverse flow from a high-pressure source. For example, if a fluid is pumped into a system that contains vapour at significantly higher pressure than the design rating of the equipment upstream of the pump, loss of pumped flow with leakage or latent failure of a check valve in the discharge line results in a reversal of the liquid’s flow. When high-pressure fluid enters the low-pressure system, overpressure can result.

In most cases, focus should be on prevention of reverse flow. It is important to note that, in addition to
overpressure of the upstream system, reverse flow through machinery can destroy mechanical equipment,
causing loss of containment. If this hazard is of concern, additional means of backflow prevention should be provided.

I hate Windowz 8!!!!

 

[BigInch]

4.3.4.2 Pressure consideration for single check-valve latent failure
Overpressure protection shall be provided for single check-valve latent failure (e.g. stuck open or broken flapper) where the maximum normal operating pressure of the high-pressure system is greater than the design pressure or MAWP of vessels, equipment and piping in the upstream low-pressure system and there is enough stored energy in the high-pressure system to cause an overpressure in the low-pressure system (e.g. large vapour cap in the high-pressure system).

When sizing a pressure-relief device to prevent exceeding the allowable accumulation of the protected equipment for the latent check-valve failure, the reverse flow rate through a single check valve may be determined using the normal flow characteristics (i.e., forward-flow Cv) of the check valve. If the check valve Cv is unavailable, one may conservatively assume that the check valve is not there by taking no credit for its flow resistance.

If the single check valve is inspected and maintained to ens ure its reliability and capability to limit reverse flow, the user may determine that the check-valve latent failure is unlikely. In this case, overpressure protection should be provided where the maximum normal operating pressure of the high-pressure system is greater than the upstream equipment’s corrected hydrotest pressure (see 3.21 and 4.3.2). The user is cautioned that some systems can have unaccept able risk due to latent failure of the check valve and resulting consequences due to loss of containment. In these cases, limiting the overpressure to the normally allowable overpressure can be more appropriate. Note that the entire system, including all of the auxiliary devices (e.g. gasketed joints, instrumentation), should be considered for the overpressure during the la tent failure of the check valve.

I hate Windowz 8!!!!

 

[BigInch]

and....

4.3.4.4 Pressure consideration for series back-flow prevention
Experience has shown that when inspected and maintained to ensure reliability and capability to limit reverse flow, two back-flow-prevention devices in series are sufficient to eliminate significant reverse flow. [highlight #EF2929]As the differential pressure increases, the use of additional safeguards should be considered to reduce the risk of check-valve latent failures resulting in mechanical equipment damage causing loss of containment. The user might want to consider diverse back-flow prevention devices.[/highlight]

If reliability of the series back-flow prevention cannot be assured, then it can be necessary to estimate the reverse flow. The quantity of back-flow leakage through check valves in series depends on the types of check valves, the fouling nature of the fluid and other system considerations. Therefore, it is the responsibility of the user to determine an appropriate technique for estimating the reverse flow through check valves in series. The maximum expected differential pressure across the check valve at the time of demand should be used as the basis for relief rate calculations.

Where no specific experience or company guidelines exist, one may estimate the reverse flow through series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal flow diameter. A lower value may be used if a condition-monitoring system for the check valves (e.g. pressure indicators with appropriate volumes between the check valves) is installed to monitor the condition to ensure that the leakage rate is below the capacity of the low-pressure side relief device.

I hate Windowz 8!!!!

 

[SNORGY]

BigInch...

Thanks...that's what I was thinking of. I have been out removing snow with my tractor so I can actually get to work tomorrow, otherwise I might have found it.

 

[BigInch]

You have to go to work if it snows?
I think I'd buy a snowmobile.
My grandparents lived in Upper Michigan. Calumet|Laurium. On the line between the two towns. They had a door off the second floor; no stairway ... for use in the winter.


I hate Windowz 8!!!!

 

[hacksaw]

the issue with 1200 psi involves concern with un-anticipated pressure build up from the well head.

have seen related situations, perhaps I am overly cautions.

Your safety concerns are ultimately resolved by your customer HAZOP review as the others have suggested.

My concern is that only "single mode failures" are considered in safety reviews. This is done under the provisio of adequate inspections.

Windows 8 forget it. Along with windows 7, it is designed for right handed individuals with eyesight that is compatible with low contrast pastel color options, and every feature behind at least two doors...It is great if you use prepackaged software exclusively under the direction of others, but does not allow quick access to the sort of tools involved in engineering that tends to cover several disciplines and projects.

It is good for trained hamsters more interested in facebook and blog sites...not serious work

Will be using a copy of xp64 at some point that hopefully will eliminate the nonsense.

 

[zdas04]

BigInch,
That is exactly what I was looking for. Thank you.

Snorgy,
Thanks for the anecdote. I was starting to think that this was bizarre when I know it is reasonable.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[BigInch]

I think that if one valve fails, the 2nd valve is twice as likely to fail as the first.

I hate Windowz 8!!!!

 

[zdas04]

How do swing checks fail?
[ul]
[li]Flapper falls off (if it is the upstream flapper, then the loose part will certainly stick in the downstream valve, probably disabling it)[/li]
[li]Flapper breaks off chunks (shouldn't hurt the downstream valve)[/li]
[li]Rubber on flapper comes off (shouldn't hurt downstream valve)[/li]
[li]Debris builds up on the seating surface (no reason to expect that it wouldn't affect both valves the same so they'd both "fail", but so would a ball valve)[/li]
[/ul]

Other than the first one, the failure modes I've seen in swing checks would mostly be independent of each other.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[BigInch]

After the downstream valve breaks from slamming closed because of a reverse pressure surge, the now elevated pressure wave smashes through the second one even easier than the first.

I hate Windowz 8!!!!

 

[zdas04]

Think about this one. A well is flowing normally. The pressure is 40 psig all the way from the wellhead to the shut control valve, but there is no flow so both check valves are at rest (shut). The well ESD's for some reason and wellhead pressure starts increasing, but the check valves are closed. 48 hours later the wellhead pressure is 1200 psig, but the check valves and control valve are all shut.

If the one-in=a-billion happens and the control valve is open and the checks are open when the well ESD's. Wellhead pressure starts to build, and as soon as wellhead pressure is higher than vessel pressure the checks will snap shut.

There is no scenario where the check valves are open and the wellhead pressure instantly changes to slam the checks with 1200 psig dP.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat