Relief and flare networks, worked back from the flare tip
Rate a relief system in the simulator you already built the process in. Type the pressure at the end, and every relief valve gets the back pressure the network builds up, on the case’s own fluid package, with no second program and no data to carry across.
Set up and solved in about three minutes
Two relief valves, their tail pipes, a tee and a header to the flare tip, on a natural gas. One tail pipe is too narrow, and the network says so.
- A relief valve goes to Relieving, a pipe to Inlet pressure, a mixer to From the outlet (tee)
- Type the pressure at the end of the header: nothing else is set up, the app finds the network from those units
- Solve All works the pressures back and hands each relief valve its back pressure
- A tail pipe too narrow is choked at its exit, its relief valve is over its allowable back pressure, and a larger diameter clears both
Captioned, no sound needed.
Setting up a network, start to finish
Work back from the flare tip, the way the network does. Each step is one choice on a unit you already know, and every picture is the app itself, on the case in the video.
Draw it the usual way
Each relief valve’s inlet stream holds the relieving fluid and its load: give it the relieving pressure, the temperature, the flow and the composition. Then a relief valve, a pipe for each tail pipe, a mixer where two lines join, and a pipe for the header. Nothing about them is special yet.
Start at the end
Open the last pipe. In the Pressure group set Calculate to Inlet pressure and type the Outlet pressure: the flare tip’s, often the atmosphere’s. given by reads Typed: it ends the network.
Make the mixer a tee
Open the mixer and set its outlet pressure rule to From the outlet (tee). The Tee losses group appears: Crane’s K by default, the run and the branch chosen inlet by inlet, or a K you type, or Off.
Set each tail pipe
Set each tail pipe’s Calculate to Inlet pressure. Its outlet pressure is now read-only, and given by names the unit downstream. In Limits, its Service is Tail pipe, for a Mach limit of 0.7; a header’s is 0.5.
Set each relief valve relieving
On each relief valve’s Design tab set Steady State to Relieving. Relieving flow passes the inlet stream’s load, or the capacity at full open. A valve left on Closed passes nothing and takes the back pressure the others build up.
Solve All
The app finds the network from those units and solves it as one, working the pressures back from the end. Every unit in it shows SOLVING, and each view has Cancel and Elapsed. A line in Messages names the network, its units and the passes it took.
Read what each pipe found
A pipe too narrow for its flow shows WARN. Its Results tab says Choked at its exit, with the pressure the gas leaves at and a Mach number of 1.00. The Speed of sound and limits group gives the rest: Mach number and ρv² at each end, the highest Mach number and the coldest point.
Read each relief valve’s back pressure
The Rating tab shows the back pressure the network builds up, read-only, with given by naming the network’s end. Type an Allowable back pressure and it warns when the network passes it. Here the choked tail pipe has put this valve well over.
Change a diameter, and it solves again
Type a larger Inner diameter on the tail pipe. The network solves again by itself in about a second, the choke clears, and the relief valve’s back pressure falls below its allowable one.
Add fittings and swages when you have them
A pipe’s Fittings table takes HYSYS’s 67 fittings or a K of your own, spread along the pipe. Swage at inlet is Auto, from the pipe upstream: two pipes of different sizes joined directly take the loss by themselves.
No separate tool: four ordinary units, set a certain way
A pressure network is not a block of its own. The app finds it from the units on your flowsheet and how they are joined, and solves them as one.
Pipes in Inlet pressure
A pipe works its inlet pressure out from its outlet’s, so the pressure is carried back along each line. The last pipe or tee takes a typed outlet pressure, the flare tip’s, often the atmosphere’s. A pipe too narrow is choked at its exit: its gas reaches the speed of sound, and the relief valve behind it takes the back pressure that choke gives.
Tees that take their losses
A mixer on From the outlet (tee) joins lines. Each inlet takes the outlet’s pressure plus its own loss, Crane’s K through the run and the branch by default, or a K you type, and its change of velocity, in its pressure and its energy balance.
Relief valves that relieve
On Relieving a relief valve passes its inlet stream’s flow or its capacity at full open, and takes the back pressure the network builds up. It gives the orifice area needed and the smallest API 526 letter that has it, and warns when its back pressure passes the allowable one.
A valve for a water seal
A valve on Pressure drop passes the pressure back with its drop, which stands in for a water seal. A relief valve left on Closed passes nothing, and its back pressure is what the others build up at its tail pipe.
A pipe that knows the speed of sound
Every increment of a pipe has its speed of sound, from the fluid package, its Mach number and its ρv², with the highest Mach number and the coldest temperature along the line.
- Limits give warnings only: Mach 0.5 for a header and 0.7 for a tail pipe, a ρv² limit if you type one, and a minimum metal temperature by material
- A fast gas line is solved in sub-steps that close momentum and energy at both ends
- A gas that reaches the speed of sound says where; it no longer gives an answer past Mach 1
- A Fittings table with HYSYS’s 67 fittings or a K of your own, and a swage at the inlet on Crane’s A-26
Capacity by direct integration
The capacity at full open is direct integration along the isentrope by default: the package’s own isentropic flash at each pressure, as API 520 Part I’s Annex C sets out. It needs no exponent, so it holds for a dense gas and for a gas that condenses in the nozzle.
API 520’s equation with the isentropic expansion coefficient n, and with the ideal gas’s k, sit beside it, each labelled, and the group shows the exponent in use beside both.
What it was checked against
| Check | Against | Result |
|---|---|---|
| Two whole networks, every node | The answers stored in DWSIM’s two flare samples, rebuilt like for like | Each node’s pressure within 0.083 %, friction and fittings within 0.48 %, Mach numbers within 0.19 % |
| Back pressures on the app’s own settings | The same samples, with relief valves, adiabatic pipes and Crane’s tee losses | The four back pressures within 1 % |
| The speed of sound | NIST’s reference equations | On Peng-Robinson within 1.5 % for every gas checked |
| A choked exit | Fanno flow in closed form, on an air line | The sonic pressure to 3 parts in 100 million |
These are comparisons with another program and with reference data, and they appear in the validation report inside the app. None is against a plant’s measured response.
What it does not do
This rates one relief scenario at a time, on a network that branches and does not loop. It is a rating tool, not a flare system design tool. Not in this version:
- Several scenarios in one run, with a governing one picked out
- Header sizing: you give the diameters, it tells you what they do
- Knockout drums, seal drums and the flare tip as units of their own
- Radiation and noise
- API 521 relief loads: the load is the flow you give the relief valve’s inlet stream