Emergency Vent

[C26M13]

Hello,

I need help to checked the size of the vent, flow capacity and fluid properties at reliving conditions for Emergency Vent installed on Waste Water Tank.

I am trying to use HYSYS simulation to do this, but it doesn't really work well with the below conditions. How is the best way to confirmed this?

Relieving Conditions:
Fluid: Air
Relief Temperature (T1): 38 degC
Set Pressure (P1): 2.6 kPag
Over Pressure: 10%
Discharge Pressure (P2): 0 kPag

Tank Information:
Tank Design Pressure: 3.0 kPag
Tank Diameter: 3 m
Tank Height 4 m

Please kindly give some of your guidance on this matter?


Thanks in advance

 

[RVAmeche]

I assume this is an API 2000 type vent due to the pressure. If so, you can do the sizing simply enough with the API 2000 equations. It's a function of the latent heat of the fluid, wetted surface area, etc of the tank.

If it's just a water tank I typically wouldn't worry about simulations since it's not volatile, etc.

 

[JStephen]

Note that the overpressure used in venting may be specified by the tank code (as in API-650) and is not just automatically assumed as a percent over.

 

[RVAmeche]

Another thing to note - if it is an API 2000 vent, they typically require significant overpressure to achieve their rated flow. For example, a vent set at 10" WC may not have reach your required flow until 20" WC. Be careful about ensuring the required flow is met and your tank pressure won't exceed the MAWP

 

[C26M13]

I have done the calculation for the vent as per API 2000. And the results show the existing vent with the size of 16" is adequate to handle the relief rate during relieving scenario.

Just a follow up question regarding this discussion.
I have scenario where the inlet fluid to the tank I coming from an upstream pump. If there is a trip at the pump, how can I validate the size of the vent on the tank.

Inlet Fluid: Water with contaminant (hydrocarbon)
Pipe Size (Pipe 1 & 2): 4"
Emergency Vent (EV) size: 16"
Pump flow (rated) : 28 m3/hr
Pump suction and discharge pressure (rated): 2 bara and 7 bara
PBV: Breather Vent

Please the attached sketch for illustration.

I got a reference saying that, since the inlet line to the tank (Pipe 1) has smaller size (3") than the Emergency Vent size (16") thus, if trip at upstream pump "Overpressure will unlikely to happen and NO issue with the vent sizing capacity".

Can anyone explain how is the correlation between this rule of thumb (Pipe Size vs Vent size?


Thank you

 

[Ehzin]

Regarding a trip at the pump, wouldn't this indicate that the pump is no longer pumping in and therefore remove having to worry about this scenario.

In regards to the pump in rate versus relief size, I'm assuming this is based on some general logic about flow restriction. If I'm pumping into a tank at 1000 lb/hr thru a 3" line and my emergency vent has a 20" opening it is very unlikely that I can actually build pressure in my tank to exceed design pressure. API-2000 should have some calculations on how to quantify this in conjunction with the relief valve documentation. Volumetric pump in rate versus emergency vent relief rate should solve this one. If my pump in rate is 1000 lb/hr and my relief rate of my relief device at some margin less than my design pressure is the same then I'd flow balance and never exceed tank design.

If you have any questions on the subject please let us know.

Thanks,
Ehzin

 

[C26M13]

Thanks Ehzin for you comments.

What I meant with the trip is failure on the pump which leads to maximum discharge pressure of the pump flowing to the tank.