5 replies to this topic

[FMS]

Can anyone please let me know the capacity requirement for a Vacuum Breaker on a Deaerator vessel (No guidelines of condensation rate in API2000 rather it only specifies about pumpout rate and thermal inbreathing with a comment that any condensation impact will be in adidition to these two contingencies). Secondly API2000 is primarily for non ASME VIII vessels only, while usually deaerators are designed as per ASME VIII standards. API 520 also not comprehensively covers it.

[Bill B]

I am a bit confused here, as I am not sure if you have a API 520 situation. What is the MAWP of the protected vessel. If > 15 psig, then this is probably a Section VIII vessel (which you seem to indicate). If so, API2000 would not apply and you would default back to normal analysis - can vessel be overpressured from pump, fire, etc.

If the MAWP < 15, then you probably have an API2000 case. If so, the first order of business would be to determine if it is built to API 620 or 650, as this gives you info regarding the actual pressue design range.

Assuming a low pressure vessel (API2000), then apply the standard you have found. You did not mention a fire case - if indeed you could have one, then apply the info in API 2000 to that also.

[Lowflo]

Here's a good explanation, and procedure for sizing deaerator vacuum relief (<www.flowserve.com/files/Files/Literature/ProductLiterature/FlowControl/Gestra/810670.pdf>)

Deaerators are pressure vessels, so you ought not be looking in API 2000.

Theoretically, the vacuum can get as low as the vapor pressure of the water feed stream, at its coldest temperature. Typical practice is to use a check valve as the relief device. That always seemed odd to me, but it's common industry practice, and apparently it has proven to be successful. Alternatively, you can use a common vacuum relief vent device if you can find one rated for the operating conditions (T & P) of the deaerator. Unless the operating pressure is very low, it'll probably be hard to find a VRV with the necessary T & P rating - I think that explains the common practice of using check valves.

[FMS]

@Bill: Yes my Deaerator vessel is ASME VIII vessel with MAWP of 70 psig and design vacuum of 7.5 psia. However, I was consulting API-2000 as I couldn't find any other standard giving guidelines for capacity calculation under different contingencies under VACUUM conditions.
As a recommended practices Deaerator should have been designed for full vacuum which unfortunately is not. Now I want to prepare a data sheet for vacuum relief device for which I need to know required capacity under all contingencies. I could not get the recommended guidelines that if vessel if bottled up with only steam inside and steam starts to condense owing to ambient or cold water in, what rate should I assume.
As long as fire is concerned, it will be required for internal pressure case not for vacuum case.
Hope this clarifies by issue in detail.

[FMS]

@ lowflo: Thanks for the reference. It seems quite helpful. Use of a check valve for pressure / vacuum relief is an interesting proposition. I will study the link in detail and then comment. However, apart from Tyco recommended standard, is there any prudent engineering standard available for the purpose (e.g. API, Exxon Eng. Design Practice, Shell Standards, GE standards, GPSA etc.)? What sizing bases do these standards recommend and what kind of relief device do these standards suggest. As referring to those standards give you more comfort rather than a vendor standards. Anyway will study the link in detail, apparently it makes sense. One more thing that might be of importance, is that will the check valve hold under normal operating conditions of pressurized vessel or leak some of precious steam into atmosphere.

[fallah]

As a recommended practices Deaerator should have been designed for full vacuum which unfortunately is not. Now I want to prepare a data sheet for vacuum relief device for which I need to know required capacity under all contingencies. I could not get the recommended guidelines that if vessel if bottled up with only steam inside and steam starts to condense owing to ambient or cold water in, what rate should I assume.

FMS,

You rightly stated the practice would be applied on Deaerator design against external pressure and it should normally be designed for full vacuum conditions, but i was faced with a case in which the Deaerator was designed for partially vacuum conditions and consequently equipped with vacuum breaker.

The worst case for vacuum design of a vessel could likely be sudden condensation of contained steam due to rain fall,..., and naturally air handling capacity of the required vacuum breaker depends on the total volume would subject to vacuum. Therefore you can reflect/add this volume (and also possibility of sudden condensation) on the relevant data sheet and ask the vendor to select proper model/size to meet your specified conditions.

Fallah