pressurized deaerator safety valve setting 1

[amazing azza]

Hello friends, I have a pressurized deaerator. Operating pressure 0.2 barg, max allowable operating pressure 2 barg. All instruments and gauges are rated for max 1 barg. Feedwater pumps are also rated up to 120 C (~1 barg).

What should the safety valve be set to? I am planning to set it to pop at 0.5 barg. Is that reasonable?

 

[MJCronin]

Amazing .....

If the deaerator is built and certified in accordance with the ASME or other pressure vessel standards, the setpoint should be set according to those standards.

However, it is vitally important that the CAPACITY of that safety valve be set in accordance with the worst case possible for the particular plant in which the DA is installed. Typically (but not always) this would include maximum possible steam flow into the DA

Different plant configurations could mean different safety valve configurations....

Multiple safety devices are permitted and the safety device can be located on steam piping

http://www.heatexchange.org/pdf/techsheets/techsheet119.pdf

"What doyathink" engineering is not acceptable ....

Tell us more about your specific configuration.

MJCronin
Sr. Process Engineer

 

[Ehzin]

First I'd find out what the actual vessel is rated for. There shouldn't be an issue with having your set pressure some margin under your vessel MAWP it just can't exceed it but there's a lot more that needs to be checked such as what MJCronin mentioned. You mention the pumps are rated for ~1 bar. What do they deadhead at? Your instruments are rated for a max of 1 bar. Is that their max design pressure or the max range at which they function properly. Setting your PSV 0.5 barg may make these questions somewhat null but I would still say it's important information.

I would first get a better understanding of what scenario can actually cause you to reach your vessel MAWP. Would recommend digging into additional details on your system to have a better understanding of it and work from there.

Thanks,
Ehzin

 

[amazing azza]

If the deaerator is built and certified in accordance with the ASME or other pressure vessel standards, the setpoint should be set according to those standards.

ASME specifies no more than 10% over MAWP. They do not specify a lower limit.

You mention the pumps are rated for ~1 bar. What do they deadhead at?

The deaerator is 7m above the pumps for maximizing inlet pressure. What is more of an issue is temperature. According to the nameplate 120 C is max, but even at lower temperature I can hear signs of cavitation.


I guess what I am trying to ask is this. The MAWP is much greater than the actual working pressure. It is most certainly an error condition if the actual working pressure even starts to approach the MAWP. The instruments won't survive it, the pumps will cavitate. So shouldn't the safety valve setting be set to prevent this from happening? Why are people only concerned with mawp?

 

[MJCronin]

Amazing ...

You are conflating two issues. Safety valves on pressure vessels were never intended to protect pumps or facilitate operations. They are meant to protect vessels from catastrophic failure.

Adequate NPSH for feedwater pumps is ensured by proper pump design, pump selection and plant configuration ... not by playing around with DA Safety valve setpoint.

What is your NPSH margin and were these FW pumps sized properly ? Are these pumps being re-used from another facility ?

The ASME pressure vessel code also says that the code Safety Valves shall be set at the vessel MAWP. Nothing says you are forbidden from adding additional relief devices that actuate at a lower pressure.... Do what you want.

https://www.esmagazine.com/articles...for-deaerator-installations--how-much-relief-

Amazing,...I believe that you questions should have been posted in the "Safety Relief Valve Engineering" forum

Good Luck ....

MJCronin
Sr. Process Engineer

 

[amazing azza]

https://www.esmagazine.com/articles/95550-safety-v...-

Yup, read that one already.

Nothing says you are forbidden from adding additional relief devices that actuate at a lower pressure....

Thus, the dilemma. Why have 2 when 1 will suffice? In a boiler there are two to increase the discharge capacity progressively. But here, is there such a need? I was hoping for people experienced with deaerator installation/operation to share their experience, but perhaps you're right. Maybe they all hang out in the safety valve forum.

 

[MJCronin]

Well Amazing, you seem to know better than everyone else. Many engineers prefer the option of dual device configuration on their Deaerators.

There is a clear discussion of this topic by people who have been selling DAs for decades

https://deaerator.com/deaerator-products/deaerator-accessories/

Call up the Kansas City DA people and tell them they don't know what they are doing.

One more question, please tell us just where in the third world you plant is located ...


MJCronin
Sr. Process Engineer

 

[amazing azza]

From your link:

One or more relief valves are provided to either protect the pressure vessel against over-pressurization or act as sentinel relief

Seems it is not quite as cut and dry as you make sound.

It is the responsibility of the purchaser to evaluate all possible conditions and determine the worst case scenario for sizing the relief valve.

Hence, I came here looking for feedback. Instead all I got was condescending remarks and personal insults from a "senior process engineer". The Kansas City DA people indeed know what they are talking about, while you clearly don't.

Good day.

 

[Ehzin]

So I think this will be my final comments on this subject for reasons and they may be somewhat jumbled/repetitive but...

1)

Why are people only concerned about MAWP? Because that's the primary purpose of a safety device as mentioned. You shouldn't be relying on a PSV to protect your pumps from cavitation which I don't see as being an issue you would encounter during high pressure excursions. Protecting the pumps from its own issues should be done by low level alarms and min flow systems. Using your only PSV to keep your pumps working properly is a general misapplication, in my opinion. There's nothing stopping you from lowering your PSV set pressure but it should be a device meant to address safety issues during rare excursions, not to keep your pumps from cavitating once a month. Maybe I'm missing something but I'm still confused by the idea of high pressure in your drum leading to cavitating pumps, that just seems counterintuitive to me.​

2)

Do what you want at this point. Nothing you're intending to do is explicitly prohibited. Set pressure can be some margin under MAWP. Everything mentioned up to this point is simply preferred practice and/or opinion of people related somewhat to this field. It's not recommended to use a PSV as a catch all for any related problem you could encounter whether it's an actual safety issue or just operations related. The only item that's required that was mentioned is that it at least has to be able to handle the primary reason it's there for which is to prevent overpressure and relieve the required capacity for its controlling event.​

Thanks,
Ehzin

 

[amazing azza]

Ehzin, thank you for a well-reasoned and articulate response!