10 replies to this topic

[Ahmadhamzahperta]

Good day everyone, i have a question concerning flow in flare header and i have attached an image of the header. My question is, how could the flow converge into one stream and proceed to flare stack while having different pressure sources. wouldn't the highest pressure (Flow from PSV 2) force a reverse flow to the other line? or will there be control valves to regulate the pressure so it will equalized?

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[Pilesar]

At the junction of converging flows, there will be only one pressure. Flow will be in the direction of higher pressure to lower pressure. Determine the pressure at the junction and compare it to the source pressures to know the direction of flow.

[breizh]

Hi,

Do you think that the PSV will pop up at the same time? 

Agree with reply above.

Note Probably good to review the details of a PSV.

Pressure Relief Valve

Breizh 

[latexman]

Once the flow gets through the PSV nozzle (sometimes called an orifice), the pressure at the PSV exit is determined by the flow(s) and the  hydraulics of the flare header network and the flare.  The pressure just outside the flare tip is atmospheric pressure, right?  It is common practice to solve the flare header network given the flow(s) from the flare tip backwards toward the PSV exit flange(s).

[Ahmadhamzahperta]

Hi,
Do you think that the PSV will pop up at the same time?
Agree with reply above.
Note Probably good to review the details of a PSV.
Pressure Relief Valve
Breizh

To be honest i don’t know but i feel that for the PSV to be able be credited as an IPL it has to be independent. Therefore i think its more appropriate to consider the scenario in which three of them pop out at the same time

[Ahmadhamzahperta]

Once the flow gets through the PSV nozzle (sometimes called an orifice), the pressure at the PSV exit is determined by the flow(s) and the hydraulics of the flare header network and the flare. The pressure just outside the flare tip is atmospheric pressure, right? It is common practice to solve the flare header network given the flow(s) from the flare tip backwards toward the PSV exit flange(s).

So basically we calculate the pressure drop needed so at the junction all lines have the same pressure?

[Ahmadhamzahperta]

At the junction of converging flows, there will be only one pressure. Flow will be in the direction of higher pressure to lower pressure. Determine the pressure at the junction and compare it to the source pressures to know the direction of flow.

Alright, thanks my friend

[latexman]

Once the flow gets through the PSV nozzle (sometimes called an orifice), the pressure at the PSV exit is determined by the flow(s) and the hydraulics of the flare header network and the flare. The pressure just outside the flare tip is atmospheric pressure, right? It is common practice to solve the flare header network given the flow(s) from the flare tip backwards toward the PSV exit flange(s).

So basically we calculate the pressure drop needed so at the junction all lines have the same pressure?

Yes, at the one point all three PSV exhausts have in common, but that is not what I said.

[Ahmadhamzahperta]

Once the flow gets through the PSV nozzle (sometimes called an orifice), the pressure at the PSV exit is determined by the flow(s) and the hydraulics of the flare header network and the flare. The pressure just outside the flare tip is atmospheric pressure, right? It is common practice to solve the flare header network given the flow(s) from the flare tip backwards toward the PSV exit flange(s).

So basically we calculate the pressure drop needed so at the junction all lines have the same pressure?

Yes, at the one point all three PSV exhausts have in common, but that is not what I said.

My bad i thought when you said solving it backwards you mean calculating the pressure drop from the tip. Nonetheless i kinda understand it now, thank you

[latexman]

 

 

 

Once the flow gets through the PSV nozzle (sometimes called an orifice), the pressure at the PSV exit is determined by the flow(s) and the hydraulics of the flare header network and the flare. The pressure just outside the flare tip is atmospheric pressure, right? It is common practice to solve the flare header network given the flow(s) from the flare tip backwards toward the PSV exit flange(s).

So basically we calculate the pressure drop needed so at the junction all lines have the same pressure?

Yes, at the one point all three PSV exhausts have in common, but that is not what I said.

My bad i thought when you said solving it backwards you mean calculating the pressure drop from the tip. Nonetheless i kinda understand it now, thank you

 

Good.  Ambient pressure just outside the flare tip is known.  The pressure drop calculations backwards toward all 3 PSV outlet flanges is iterative, so they are unknown and will change until successive trials converge.  The pressure drop calculations starting with the 1 known pressure and working backwards towards 3 unknown pressures is inherently more stable than starting with 3 unknown pressures (initially they are guesses) and working forwards toward the 1 known pressure.  It seems intuitive to me, and that's the advice I've always heard and read about solving this type vent header set-up.

[EnggExp]

Why Doesn’t Reverse Flow Happen?

You’re right to observe that:
• PSV-002 is set at 200 psig
• PSV-003 at 150 psig
• PSV-001 at 100 psig

But here’s the key:
At any given time, only one PSV is expected to lift (open) in response to overpressure in its respective vessel. They’re sized based on credible worst-case scenarios, such as:
• Blocked outlet
• Fire exposure
• Equipment failure

So:
• If 15-V-102 is in an overpressure situation and PSV-002 lifts, the other PSVs remain closed (they haven’t reached their setpoint).
• The relief flow from PSV-002 flows into the flare header, which is designed to handle the maximum expected backpressure.
• There is no reverse flow because the other PSVs are non-flowing and check-type by nature (i.e., flow can only exit from the process to the header, not the other way).

⸻

Why Flow Doesn’t Go Backward:
1. PSVs are one-way valves: They only allow flow outward from the pressure vessel. Even if there’s higher pressure in the flare header momentarily, the PSVs are spring-loaded and will reseat once the upstream pressure drops below setpoint + blowdown margin.
2. Flare header pressure is always lower than relieving pressure:
• Relief valves are set to relieve above operating pressure.
• The flare header is at low pressure, often near atmospheric or under slight backpressure (due to frictional losses and flare tip design).
• So during relief, flow is always in the direction from PSV to flare, never backward.

⸻

Pressure Equalization Isn’t Needed:

There are no control valves needed to equalize pressure between branches. Each PSV operates independently, and the flare header simply acts as a collection system.

⸻

Additional Design Safeguards:
• The flare header is carefully sized and sloped to handle multiphase flow and avoid backpressure buildup.
• Dynamic simulations (e.g., Aspen Flare System Analyzer) are used during design to ensure that:
• Backpressure doesn’t exceed PSV allowable backpressure.
• Line velocities are within safe limits.
• No reverse flow or over-pressuring of adjacent equipment occurs.

⸻

What Could Cause a Problem?
• If multiple PSVs lift simultaneously (during fire or major unit upset), flare header backpressure increases.
• If not properly designed, this could cause PSV reseating failure, delayed opening, or flame arrestor issues.
• Hence, relief scenario evaluation and proper dynamic flare load modeling are critical.

⸻

Summary
• No reverse flow occurs because PSVs are one-way and close when not activated.
• No control valves are used to equalize pressure.
• Flare header pressure is lower than the PSV setpoints, so flow direction is always outward to the flare.
• System is engineered assuming only one or a few valves open at a time, based on scenario analysis.