Reduce the liquid pressure across the transfer control valve, then control the second vessel at 18 bar by balancing gas removal, liquid removal, and heat transfer. These are separate duties. The transfer valve creates the hydraulic pressure drop; the gas outlet valve controls vapor-space inventory. Neither valve, by itself, guarantees that the liquid remains at -23 °C.
Control-method comparison
Before anything else, confirm whether 150 bar and 18 bar are gauge or absolute pressures. Use one pressure basis for the hydraulic calculation and absolute pressure for ammonia property calculations.
| Method | Controlled quantity | What it accomplishes | Primary limitation |
|---|---|---|---|
Transfer-line LIC valve |
First-vessel level | Throttles liquid from the upstream pressure toward the downstream system pressure | May produce flashing, cavitation, noise, or temperature change |
| Second-vessel gas outlet valve | Second-vessel pressure | Removes compressible gas or vapor as it accumulates | Cannot independently select pressure and temperature for pure ammonia at vapor-liquid equilibrium |
| Liquid outlet from the second vessel | Second-vessel inventory or level | Balances most of the incoming mass | Incorrect flow balance causes rising or falling level |
| Cooling or heating duty | Second-vessel temperature | Holds -23 °C after throttling and phase change | Required duty depends on inlet enthalpy, flashing, and heat leakage |
Use the LIC valve for the liquid pressure drop and retain the gas outlet valve for pressure control. Add or confirm a separate temperature-control duty when -23 °C must remain fixed. Do not assign the gas valve the hydraulic duty of reducing the incoming liquid from 150 bar.
Recommended control architecture
The nominal pressure difference is 132 bar if both stated pressures use the same reference and line losses are neglected:
ΔP = 150 bar - 18 bar = 132 bar
This difference identifies the operating case for valve and piping evaluation; it is not proof that the entire 132 bar appears permanently across the valve. Actual valve differential equals the measured upstream pressure minus the measured downstream pressure at the valve connections, including line-pressure losses and static head.
Place the transfer valve under the first vessel's level controller so that upstream inventory determines liquid transfer. Let the second vessel's pressure controller manipulate its gas outlet. Control the second vessel's liquid outlet from its own level or inventory requirement. Use the vessel cooling or heating system to hold -23 °C.
If the second vessel contains both liquid and vapor ammonia at equilibrium, temperature fixes ammonia saturation pressure. An imposed pressure different from that saturation pressure requires subcooled liquid, superheated vapor, noncondensable gas, another component, or a nonequilibrium condition. Measure vapor composition and obtain ammonia saturation pressure at -23 °C from the approved property package before deciding which condition applies.
Commissioning procedure
- Normalize the pressure basis. Record whether each transmitter reads gauge or absolute pressure. Convert both readings to absolute pressure for phase calculations. Do not move on until the upstream value, downstream value, transmitter ranges, and valve connection pressures use documented units and references.
- Confirm the downstream phase condition. Look up ammonia saturation pressure at -23 °C in the project property method. Compare it with the second vessel's absolute pressure. Pressure above saturation pressure permits liquid ammonia to remain as compressed or subcooled liquid; pressure at saturation supports equilibrium liquid and vapor; pressure below saturation drives boiling until energy, composition, or inventory limits intervene.
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Close the mass balance. At steady inventory, total outlet mass flow must equal the stated inlet flow of 42,117 kg/h. If the 20 kg/h gas stream and the other liquid stream are the only outlets, and all flows use the same mass basis, the required liquid outlet is
42,117 - 20 = 42,097 kg/h. Verify the liquid flow measurement before changing the pressure loop. - Validate the transfer valve. Evaluate the valve at measured flow, inlet pressure, outlet pressure, temperature, and ammonia properties. Check body and actuator pressure ratings, shutoff differential, required flow coefficient, rangeability, outlet velocity, noise, flashing, and cavitation using the valve manufacturer's sizing method. A high downstream bulk pressure does not rule out cavitation because pressure at the valve's vena contracta can fall below local vapor pressure and recover downstream.
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Establish temperature duty. Calculate the downstream state with an isenthalpic valve model,
h1 = h2, using the actual upstream state. If the calculated outlet temperature or vapor fraction differs from the target, set the vessel heat-transfer duty to return the contents to -23 °C. Throttling is approximately constant enthalpy, not constant temperature. - Commission the loops separately. Stabilize liquid transfer and vessel level first. Place the gas outlet pressure loop in service only after a usable vapor space exists and the gas path is open. Introduce pressure-controller action gradually and verify that increasing pressure causes the controller to open the gas outlet rather than close it.
Pressure and temperature mechanism
A liquid does not need to boil merely because its pressure decreases. If its final pressure remains above its vapor pressure at the resulting temperature, it stays liquid. The transfer valve converts static pressure into velocity and irreversible losses; the downstream vessel and piping establish the final static pressure.
Flash gas forms when the post-throttle enthalpy cannot be accommodated as liquid at downstream pressure. Cavitation differs from sustained flashing: vapor bubbles can form at the valve's local minimum pressure and then collapse when pressure recovers. This distinction determines trim selection, damage risk, and where temperature and pressure measurements must be taken.
For a vessel containing equilibrium liquid and vapor of one component, pressure and temperature are linked by the saturation relationship. Venting vapor briefly lowers pressure, but liquid evaporation can restore saturation pressure while cooling the inventory. Holding both 18 bar and -23 °C therefore depends on phase condition, vapor composition, and heat transfer—not solely on the 20 kg/h gas outlet.
Diagnostic checks
| Observation | Likely mechanism | Deciding check |
|---|---|---|
| Pressure remains high with gas valve open | Gas path restriction, inadequate valve capacity, excess vapor generation, or noncondensable accumulation | Compare pressure at the vessel and across the gas valve; inspect gas flow and composition |
| Second-vessel level rises | Liquid outlet is below the required mass balance | Compare measured liquid outlet with the derived 42,097 kg/h case |
| Transfer valve is noisy or vibrates | Cavitation, flashing, excessive velocity, or unstable valve position | Run the valve sizing calculation with measured connection pressures and ammonia vapor pressure |
| Temperature departs from -23 °C after throttling | Isenthalpic temperature change, flashing, or inadequate heat-transfer duty | Calculate outlet enthalpy and compare required duty with measured cooling or heating duty |
| Pressure and temperature appear incompatible | Gauge/absolute error, transmitter error, noncondensable gas, mixed composition, or nonequilibrium measurement | Calibrate instruments, use absolute pressure, sample the vapor, and compare with the property model |
Do not infer vessel condition from valve position alone. Trend upstream pressure, valve-inlet pressure, valve-outlet pressure, vessel pressure, both vessel levels, liquid inlet flow, liquid outlet flow, gas outlet flow, temperature, and controller outputs on the same time base.
Verification criteria and recurring pitfalls
Accept the arrangement only after it reaches steady operation with the second vessel at 18 bar and -23 °C, stable levels, and a closed mass balance. Confirm that the measured pressure drop occurs across the intended transfer restriction and piping rather than across an unintended blockage.
Recurring errors include mixing gauge and absolute pressure, treating throttling as isothermal, overlooking the vena-contracta pressure, using the 20 kg/h gas flow as the total outlet balance, and attempting to control pure-component equilibrium pressure independently of temperature. Another common failure is tuning both level and pressure loops before confirming that the valves have the correct action and enough operating range.
Perform a controlled disturbance test: make a small permitted change in liquid transfer, observe the first-vessel level response, then verify that the second-vessel pressure controller rejects the disturbance through the gas outlet without sustained temperature or level drift.
FAQ
How do I reduce ammonia liquid from 150 bar to 18 bar?
Throttle the transfer through the correctly sized LIC valve. Using the same pressure reference and neglecting line losses gives a nominal differential of 132 bar; use measured valve-connection pressures for final sizing.
How do I keep ammonia at -23 °C while reducing pressure?
Model the valve as isenthalpic with h1 = h2, determine the downstream phase and temperature from ammonia properties, and apply the required vessel cooling or heating duty. The pressure valve alone does not hold temperature.
How do I verify the second vessel is operating correctly?
Confirm 18 bar and -23 °C on calibrated instruments, stable vessel levels, and steady mass balance. For the stated streams, the liquid outlet should approach 42,097 kg/h when the gas outlet is 20 kg/h and no other flow or accumulation exists; finish by repeating the controlled disturbance test and confirming recovery without sustained drift.