PSV vs Liquid Seal

[david6245]

Hello All,

I have a liquid seal that is used as overpressure protection on a vent system off of a reactor set at 16.5 psig. I am wondering, in general, what promts the use of a liquid seal vs a PSV or rupture disk for this application? We have a pressure control valve on the vent piping so it does not appear that it is there to maintain the header pressure.

Are there situations where one would be advantageous over another?

Thank you very much.

David

 

[Chance17]

1.) Is the seal leg on a vessel or on vent pipe from a vessel ??

2.) If on a vessel, is the 16.5-psig the actual MAWP or is it relieving pressure -- looks like 15-psig +10% ???

 

[david6245]

The seal leg is on the vent pipe from the vessel.

16.5 psig is the relieving pressure. We are unsure what the actual MAWP is on the vessel as it is 40 years old, fabricated in Germany, and little documentation exists.

Please let me know if I can provide any more information.

 

[Chance17]

In days gone by, a liquid seal leg was used when the vent is wet and contains "small" amounts of corrosive compounds (HCL, Cl2, SO2, NH3)in the vent gas.
The wet vent would corrode normal materials used for vent pressure relief.
Admittedly, the liquid seal did not "scrub" a high percentage of gases.

More recently, vendors offer FRP devices for corrosive gases.

I once saw a single liquid seal connected to the vent of eight tanks. The service was wet aldehydes(smelly).
The focus was odor control and "being cheap" by not having individual PV devices on eight tanks.

The "activation" accuracy of a liquid seal could be cited as an advantage when compared to either a 15-psig PSV or 15-psig rupture disk.

I recommend hiring a professional to rate your vessel to determine MAWP.

 

[pleckner]

If the pressure in this vessel (appears not to be for water storage) can exceed 15 psig (as it appears to be able to do) then it falls within ASME Section VIII, Div. 1 scope. You either have to show the vessel is designed for containment against any credible overpressure scenarios or you will have to put on a relief device. I don't believe the authorities allow you to 'grandfather' an exception on this .

 

[david6245]

pleckner,

Are you suggesting that the seal leg does not qualify as a relief device?

I was wondering when one would choose between a seal leg and a PSV.

Regards,

 

[pleckner]

It will only qualify as a relief device if you can show the vent size will relieve against all credible scenarios and there is nothing in the seal pot that can ever create an obstruction against flow. I've heard of one case where some obstruction (don't have a clear definition of what it was) in a common seal pot blocked the vent pipe and it caused several low pressure tanks to buckle (this was a pressure/vacuum protection system). It was not a pretty sight.

 

[SeanB]

I have seen liquid seals used in one instance. It was going to be replaced to prevent venting to atmosphere in a relieving situation. However, it was found that it would take multiple, very large relief valves to have the same relieving capacity and would have been a very expensive piece of work. Thus the liquid seal was kept and a HIPPS system was installed to monitor the pressure and to shit down the unit of a high pressure situation.