Need a safety relief valve on dearator? 3

[sheiko]

Hello,

I have a deaerator opened to atmosphere via 6-inch nozzle and operating at 0 barg.

The design pressure is 5 barg.

I have the following questions:

1/ Is the deaerator considered as a Pressure Vessel?

2/ Shall i check all the overpressure scenarii to determine if i need a safety relief valve?

Thank you



"We don't believe things because they are true, things are true because we believe them."

 

[brandonb73]

What you are referring to sounds like an atmospheric DA. Unlike pressurized DA's that are designed to operate under a certain pressure (usually 5psi), atmospheric DA's are vented to atmosphere. Therefore, it is usually not required that they be designed as a pressure vessel.

Also, if you have a 6" nozzle open to atmosphere, it is unlikely that there would ever be an overpressure situaton.

 

[boilerone]

What kind of scenario can pressurize the deaerator? Overflow not working? If the direct opening to atmosphere cannot make sure to keep the pressure low so that you have to design the vessel for 5 barg which is 72.5 psig greater than 15 then it is 'qualified' to be a pressure/code vessel and you also need a safety relief valve.

 

[davefitz]

A 5 bar g dearator might be considered to be a pressure vessel, depending on the location , jurisdiction, and design code. This determination will afffect more than just relief valves- it also affects welding prcedures, QC, PWHT, stress relieving, documentation, etc.,hydrotest , and cost.

The need for a relief device, either releif valve or bursting disc, will be determined by the comparison of the max steam flow that can exit thru the 6" nozzle versus the max "caualty" steam flow into the dearator from the pegging steam nozzle or other sources of steam, under the assumption that the source steam control valve has failed open. If you had made any cycle changes to reduce heat loss which involved routing a drain or vent to the dearator, then you will need to conirm the new source will not overpressure the DA.

 

[MJCronin]

Answers to your questions:

1) Deaerators are a unique type of pressure vessel because they are typically designed to a pressure >15psig, (usually 30-50 psig) but operate at near atmospheric. The reason for this is that during operation there are internal pressure and flow transients that require a robust construction.

But the ASME and other codes typically require only very little wall thickness at pressure near atmospheric.

Several older DAs failed in the early 1990s in paper mills and the Heat Exchange Institute (HEI) published a revised guideline on recommended strength requirements for new DAs.

2) A "sentinel" relief valve is typically installed on a DA to let the operator know something exciting is going on. It is not a "full capacity" RV.....although a full capacity device can be ordered as an option.

http://www.kansascitydeaerator.com/deaerator-accessories.htm

To the best of my recollection......

-MJC

 

[sheiko]

Thanks all!

What if i find that the 6-inch nozzle opened to atmosphere is enough to releive the maximum contingency? is a safety valve still required?

P.S.: The maximum safety valve API nozzle is smaller than 6 inches.

"We don't believe things because they are true, things are true because we believe them."

 

[vpl]

Sheiko

Is there anything that would isolate the 6" nozzle? If there is, you might still need a relief valve.

Patricia Lougheed

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[sheiko]

I can't see any isolation valve on it...so still need a PSV?

"We don't believe things because they are true, things are true because we believe them."

 

[MJCronin]

shieko...

IMHO:

- If you have an ASME (or equivalent)code plate with an MAWP> 15 psig.....then YES

- Otherwise, ...NO

-MJC

(Like I said, DAs are a little wierd..)

 

[skearse]

My thoughts:

1) "Is it a PV?" may largely be a jurisdictional issue. we have a number of vessels that we operate below 15 psi. For our particular jurisdiction, the operating condition determines the answer to "Is it a PV?" So, event though the vessels are designed and manufactured to ASME Code, and are stamped Div VIII, Section 1, the jurisdiction does not consider them pressure vessels.

2) Having said that, though, I typically take a conservative approach to ensure that, in the case that something did go wrong, there is some form of protection from an overpressure event. Depending on the service and vessel, that could be a PSV, RD, etc. with the appropriate set pressure. It may be possible to use the atmospheric vent capacity (see UG-127(d)) and/or apply the provisions of UG-140 (Overpressure Protection by System Design) for your application.

 

[sheiko]

Thanks!

In Europe, we consider (Pressure Equipement Directive) pressure vessel as vessel with a PS (maximum allowable pressure or design pressure) above 0.5 barg.

However i cannot find any precise reference where it is mentionned that putting a relief valve on it is mandatory.

Please let me know if you know/find something about it...

P:S: BTW, what are the differences between the words: Directive, Juridiction, Law, Code? Because they sounds the same to me, i.e. legal requirements that we must follow...

"We don't believe things because they are true, things are true because we believe them."

 

[skearse]

Sorry sheiko; forgot that you aren't in the US when I wrote that.

From my perspective (and everybody feel free to correct me) "directive" is used more in the EU, and I'm not sure exactly what it pertains to-whether it be law, recommended practice, etc.

In the US, technical bodies such as the ASME write the "Code" (ie, the Boiler and Pressure Vessel Code). The state (or in some cases the federal) governments then author the "laws" which are truly the legal requirements that must be followed. Oftentimes what happens is that instead of re-writing a brand new law from scratch, they will instead adopt the Code (either in part or in their entirety), essentially giving it the weight of law. They will then also determine who the "jurisdictional authority" is-basically, who has the responsibility of enforcing the law. In some (most?) but not all cases, that is the State, State Fire Marshall, or some other body. Confused yet-it gets better-the Boiler Code only applies to new construction. For vessels in service (i.e., once installed at the owner's location) another Code applies: the National Board Inspection Code. The National Board then has their own responsibility to ensure that Pressure Vessels are operated and inspected according to their requirements, and they have their own "army" of authorized inspectors (AIs) who have the ultimate final sign off on whther or not the pressure vessel is being operated and maintained properly. In some cases, the State will delegate the "jurisdictional authority" to an AI-that's the case where I am. So, ultimately, if the AI is OK with what's going on with a particular vessel (or boiler) then he signs off and everything's OK. If not, we have to do something or he will shut us down. It's his ability and responsibility for having the final say on something that makes some of these things come down to being called "jurisdictional issues."

Sorry for the long post-it's really confusing, and it varies from state to state (and in some places locality to locality, I believe).

So, where I am located, when I install a new Pressure Vessel I have to ensure that it is designed and constructed according to the ASME Code, and then I have to ensure that it gets installed in accordance with the requirements of the NB, and also, what are commonly referred to as recommended practices or other standards, such as those published by API or NFPA...and before any startup activities, the Authorized Inspector comes to witness the installation and signs off on it being a proper installation.

Wow-that made my head hurt.