PSV Discharge Orientation 1

[Henry S]

Hi,

I have a few questions regarding the orientation of PSV discharge. Medium is dry air. No insulation, No heat trace, and PSV discharge open to the atmosphere.
Reference API 520 Part 2 Section, the PSV venting to the atmosphere is generally vertically up with a weather cap, low point drain, and a support at the elbow. In all the below cases the support is present. No issue on the piping stress side. However, there are other concerns:
1= Due to extremely cold temperatures (-45C) and snow blowing vent up is not suitable. It can fill up with snow. The low point drain is also expected to freeze up and not function as intended due to temperature variations.
2= The second recommended orientation is to vent vertically down. Again stress-wise no issue. But when the relief valve operates it will blow over the vessel. In winter, the vessel is expected to have snow on top of it and during the relief valve operation, the snow can dislodge and fall onto the nearby person. A possible safety concern.
3= Third recommendation is to have the discharge vent horizontally straight with a 45-degree cut at the end, and the cut facing down. This will prevent any rain/snow accumulation and due to free-draining, no risk of blockage. Additionally having a birdcage to prevent any bird entry. The issues with this 3rd recommendation are:
==A) The exact dimensions of the bird cage are unknown. If not done correctly, it can come off and with the speed of discharge air, it can be a safety risk. Or restriction to the flow at the time of operation.
==B) There is building/piping infrastructure on all four sides of PSV with the closest one at 1m. I cannot find if there is any recommendation in the code regarding the free space distance in the direction of PSV discharge.

Any recommendation on what should be done in such a scenario?

 

[georgeverghese]

Go for (3) - use bug screen or insect screen on PSV discharge - many types shown on Google. Choose one.

 

[The Obturator]

What is the size of the Pressure-relief Valve and what is the set pressure?

Why not simply add a 90deg. bend to the recommended vertical discharge set up with a 45deg. downside cut and add a mesh screen? Consider the vertical length high enough not to impact on any settled snow below.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

Other options - Vent line up to say 2m above the vessel then put on either 90 or 180 degree elbows to point horizontal or down high enough to dissipate air.

Make the last portion of the vent line 2 or 3 times the diameter of the vent to reduce velocity.

Cap the end of the vent line with a strong cap and rill holes in the vertical section to at least 1.5 times the square area of vent line. And/ or use a larger pipe for this section.

Never seen a guide for horizontal distance as dependant on fluid, velocity and what the vertical surface is.

Anything two phase coming put will be worse than single phase gas.

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

 

[Henry S]

Thanks @georgeverghese @The Obturator @LittleInch for the feedback.

The vent has been installed as option 3 (straight horizontal w 45 degree cut downwards). The next question I am trying to answer is if there is any recommendation that say do not have any infrastructure within X meter of the discharge. One peer engineer recommended 3m based on his experience but there is no code or documented past recommendation. Reason of getting clarification on this is because there is a 2in line crossing the path of the discharge vent. It is located 1m away from the end of the discharge vent. Is it okay or not?

Any thought?

 

[The Obturator]

You've not advised the size of valve and set pressure...


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Henry S]

@The Obturator
My apologies, the PSV in question is 1.5" 150# RF INLET, 3" 150# RF OUTLET, with set pressure of 1280 KPag, Orifice H.

 

[LittleInch]

Codes do not getthat specific.

It's up to you.

But I wouldn't do it.

Think always how would you justify this in court it it all went bang?

Some random guy on the Internet said it was OK??[sub][/sub]

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

 

[georgeverghese]

Not a problem, I would say, its a non flammable gas release, and this pipe is 1m away. PSV setpoint is quite high at 1280kpag, so even if there some minimal backpressure created at PSV exit, it wont affect relief rate.
Think you wont find screwed end insect / bug screen to fit a 45 degree cut pipe end. It will have to be a straight 90deg cut with threaded end. Check with vendor.

 

[pierreick]

Hi,
Take a look at the document attached.
Note: Pay attention, not to release in a pathway where operators could be in trouble, i.e. the best way is to have release vertically at a certain distance from obstacles, pathway. Discharge should be protected from birds (nest) with a drain at the bottom to prevent ice formation during cold weather.

Pierre

 

[Henry S]

@LittlInch I definitely won't be following blindly. You are right that won't hold up in case things go sideways. I am in fact gathering as much evidence as possible for the said case.

@georgeverghese Thanks. I agree but having it 45 degree cut helps. Will check with the field staff and vendor of similar past cases.

@pierreick thanks for the document. It has good info.

 

[The Obturator]

FYI: The pressure drop through the Pressure-relief Valve discharging to atmosphere, will be something like 90%. Depending on the size and length of the outlet piping, further pressure drop can be expected at the vent end. Note also that the outlet piping needs to be supported properly, as indicated in the first attachment, and not be anchored to the valve - this is often overlooked.

I'm not sure about the 2" pipe crossing the outlet vent (why?) as nothing else is advised on this.

Also, have you considered the local noise level should the valve open? Any caution/signage in the area, in view of it being mentioned that there are buildings in close vicinity?

Suggest that your local Health & Safety authority be consulted for peace of mind.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

Don't know what Orifice H gives you in terms of flow but it will be somewhere.

one thing to work out is what the exit velocity is which will go somewhere to giving you an idea about jet velocity. It's likely to be in the hundreds of m/sec if you stick to 3". As the O says, also consider reaction forces on any bends - they can be severe.

See the example photo in this thread

https://www.eng-tips.com/viewthread.cfm?qid=470184

If there is any chance of particles or liquid in that gas stream from condensation or otherwise then I would not have that gas stream impinging on anything

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