Pressure Relief & Vacuum breaker in storage tank

[ER_Azza]

Hi All

I am checking the requirement if there is a need for Pressure/vacuum relief valve on a slurry tank.
API650 design check, internal pressure was calculated at 8.1kPa. I believe API650 allow for up to 6.9kPa.

However, there is tank overflow to a seal pot downstream (built to AS1210),
which probably mitigate the overpressure and vacuum effect in the tank.

I wonder if a pressure/vacuum relief valve is still required?
Previous experience with tanks - they have their own relief valve. But not this one.

Tank has a frangible roof to shell connection. Would it make a difference if it is not, in this case?

 

[LittleInch]

You need to find out what your tank was designed to. There should be a name plate.

Anything

designed or listed as "atmospheric" pressure is limited to 1kpA (4in wc) Really.
API 650 tanks CAN be designed up to 2.5psi / 18kPa, but need to be specially designed for this to Appendix F

Vacuum is 0.25kPa (1in wc) unless otherwise designed.

API 650 tanks ( how big is this??) really really don't like negative pressure and will and do collapse with very small vacuum pressures.

What is this mysterious "Seal pot"? This looks to me like your vent and vacuum relief system. What is AS1210? This is coming up as a Pressure vessel code in Australia. nothing to do with seal pots.

A frangible roof just means that the roof to shell connection is very weak and will give way before the tank gets overstressed. But then you've damaged the tank if the pressure times square are of the roof exceeds the weight of the roof.

Like I said - find the design data on your tank or look at the nameplate before you can determine if the controls you have for pressure and vacuum ( this seal pot thing ) are good enough or not. From the very limited data supplied it doesn't look like a good idea to me at the moment. Is it operating?

Where did the rather exact 8.1kPa come from??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mk3223]

IMO, the pressure/vacuum breaker is a simple and cheap device to provide the required protection for the tank during operation. It is difficult to explain why the tank is damaged if without a pressure/vacuum breaker.

 

[ER_Azza]

Hi All

Tank is 5m diameter.
Have not gone to the site to have a look at the nameplate.
From GA drawing, Internal Pressure = 8" WG
and Design = Full of slurry (SG 1.25 x 8" WG.
(Material is 316).

The "Seal Pot" they call it, is a pressure vessel to AS1210. (The location is in Australia).

That's why I find it a bit odd. They deemed the seal pot to take any over-pressure and vacuum effect on the tank.
Not sure if API650 would allow that.
From previous experience, normally there always a PVRV install on the tanks.

The "8.1kPa" was design calculation (found) from the project team that install it many year back (from API650)

I'll be onsite next week to get more info.

This is a review of a client process safety and one of the things the team pick up was the absence of the PVRV.

 

[LittleInch]

Well 8 in wc is 2 kPa...

Maybe the roof has already detached from the shell and all the air inflow and outflow is happening though a gap between the roof and the shell?

As you've drawn it it is very odd, but much depends on what this seal pot actually looks like / size, flowrate, pressure etc and if it has any liquid in it on how it "seals"....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

The tank needs to be vented, but that can be accomplished in different ways.
For example, it can have an open vent and no pressure/vacuum valve at all.
It can be vented via piping away from the tank, but whether that works all depends on the details of what is out there.
If I remember correctly, the way API 650 is worded, the pressure should not exceed the design pressure when venting, which require a set point at some lower pressure. IE, don't confuse the design pressure and the set point pressure(s).

To complicate things, there have been quite a few revisions through the years to the pressure/wind overturn criteria, so what the maximum pressure can be may depend on which version of API-650 you use (generally, lower allowed pressure with newer versions).

 

[Snickster]

The tank needs to be vented to handle vapor displcement rates based pumping of liquid in and out. For a slurry tank I would just use a goose neck open pipe vent. Refer to API 2000 for sizing and other requirements.

 

[PersonalProfile]

The following may be of interest, though does not directly address the enquiry:

Ex:

https://www.amazon.com.au/Chemical-Process-Safety-Fourth-Histories/dp/0128014253

Regards.

 

[georgeverghese]

Are you sure this sketch correctly shows the seal pot feed line from the tank as an exit line from the top of tank rising up before it descends to seal pot? Or is it from the side of the tank ?
If it is from the side of the tank, it would provide level overflow protection also.
Seal pots can in theory be used as pressure relief but are not used much these days due to reliability concerns - seal pot has to be kept filled at all times, risk of blockage from internal corrosion, biological growth. Low vacuum relief protection is almost impossible due to slug flow up the long ascent when liquid seal is overcome by vacuum conditions in the tank.