PSV Backpressure After a Flare Header Tie-In
A 60 m3 propane bullet with an 18 barg design pressure is protected by a single conventional spring-loaded PSV set at 17.2 barg, sized for an external fire with credit for fireproofing. A new flare header tie-in will raise the built-up backpressure at the valve outlet from 0.35 barg to 2.9 barg. The superimposed backpressure from the header is a constant 0.4 barg. The relief load for the fire case is 12,400 kg/h of propane vapor at 17.2 barg. The PSV inlet line loss is 2.5% of set pressure. The discharge piping downstream of the PSV is rated for the new pressure. The plant wants to avoid replacing the valve, and the tie-in is scheduled for next month.
1. Check the existing valve against the new backpressure. Show the numbers and identify which acceptance limit is broken.
Read the worked answer
Backpressure check
- PSV set pressure: 17.2 barg
- New built-up backpressure: 2.9 barg
- Built-up backpressure as a fraction of set: 2.9/17.2 = 16.9%
For a conventional spring-loaded PSV, the accepted limit on built-up backpressure is 10% of set pressure for set pressures up to about 70 barg. At 16.9%, the limit is exceeded by a wide margin.
Superimposed backpressure
The constant 0.4 barg in the header is superimposed backpressure. A conventional valve is unbalanced, so this pressure acts on the disc in the closing direction. The valve will not open at 17.2 barg; it will open at roughly 17.2 + 0.4 = 17.6 barg.
Check that against the vessel:
- Design pressure 18 barg
- Opening at 17.6 barg is below design, so the vessel is not overpressured by the shift.
- But the margin to design is only 0.4 bar, and the 10% accumulation allowance (19.8 barg) is not the governing issue here.
The limit that is broken is the 10% built-up backpressure rule. The 0.4 barg set-pressure shift is a second, separate effect that also has to be documented.
2. Explain what a conventional valve will do under 2.9 barg built-up backpressure, and rank the options for fixing it. What other checks must be completed before the tie-in is approved?
Read the worked answer
What the conventional valve will do
When the valve lifts, the flow through the tailpipe creates the 2.9 barg built-up backpressure. That pressure acts on the disc and adds to the spring force. The valve sees a higher closing force, so it starts to close; the flow falls, backpressure falls, the closing force falls, and the valve opens again. The result is chatter: the disc hammers the seat at high frequency.
Consequences:
- The seat and disc are damaged, often within minutes.
- The effective relieving capacity is reduced because the valve does not stay at full lift.
- The valve may not reseat properly after the event.
- In a fire case, a chattering valve is not a reliable safeguard.
Options, ranked
- Retrofit a balanced-bellows PSV of the same size and set pressure. The bellows isolates the disc from the backpressure, so the set pressure is not shifted by the 0.4 barg superimposed value and the valve tolerates much higher built-up backpressure. A bellows valve typically allows built-up backpressure up to 30% of set, with a capacity derating factor from the manufacturer's curve. Check the derating: at 16.9% backpressure the capacity may need to be re-rated, and the fire-case load of 12,400 kg/h must still be met. Bellows also require a vented bonnet.
- Pilot-operated PSV. A pop-action pilot can tolerate backpressure up to about 70% of set and is unaffected by superimposed backpressure. Propane is clean and dry, so the pilot is not at risk of plugging. This is technically the better solution but costs more and needs a pilot supply line.
- Reduce the header backpressure by enlarging the flare header or routing this PSV to a separate line. This is a capital project and probably not available in the tie-in window, but it is the only option that fixes the root cause for every user on the header.
- Re-set the valve or install a rupture disk upstream. Re-setting is not acceptable because the vessel design pressure is 18 barg; a rupture disk upstream adds a device but does not solve the backpressure.
Other checks before approval
- Inlet line loss: 2.5% of set, within the 3% rule. Confirm at the actual relief flow, not at the fire-case estimate.
- Discharge piping and flange rating at 2.9 barg plus the relief temperature.
- Reaction forces on the tailpipe from the relief flow.
- Whether the fire case is vapor or two-phase relief. If the bullet is liquid-full and the relief is two-phase, the backpressure effect on capacity is much stronger than for vapor, and the valve may need to be re-sized rather than just re-trimmed.
- The rest of the header: the 2.9 barg must be the backpressure with all simultaneous reliefs flowing, not just this one.