A root cause analysis review of relief-valve chatter, based on the instrumentationtools.com article “Repeat Safety Valve Popping caused Urea Plant Shutdown.”
The failure that looked like a valve problem
In the reported case, a ten-year-old urea plant experienced repeated shutdowns when the second-stage inlet safety valve lifted on a three-stage CO2 reciprocating compressor operating from 13 to 220 bar. A freshly calibrated spare lifted within two days. Operators switched between the original and spare valves over the following month, losing roughly ten production days.
The proposed fix was to raise the set point five percent above the licensor’s figure and weld a new nozzle onto the suction knockout drum to relocate the valve. A returning senior engineer instead asked what had changed in a plant that had run acceptably for ten years. That question turned the investigation toward operating conditions rather than a presumed design fault.
The numbers that pointed to the intercooler
The team compared second-stage inlet and outlet pressure and temperature with design data. Second-stage inlet temperature had risen from a design value of 45°C to 70°C; inlet pressure had risen from 35.29 to 37.53 bar. The safety valve set point was 38.82 bar. The operating-to-set pressure gap was 1.29 bar, or about 3.3% of set pressure, leaving little separation under those conditions.
Investigation found heavy fouling in the intercooler downstream of the first stage: oil gum in tube bores and deposits on the cooling-water side. Cleaning the bundle restored design conditions, and the reported repeat popping stopped. This is a case-specific diagnosis, not a reason to assume every relief-valve lift is caused by fouling.
Why redesign was the wrong first move
Changing a set point or moving a relief valve without establishing the cause can compromise protection or introduce piping and approval issues. Here the valve was approaching its set pressure because process conditions had drifted. Cleaning the exchanger addressed that drift before any change to the protected system was made.
The case report also describes reduced concern about third-stage inlet check-valve erosion, restart leaks, and off-spec product after stabilization, with production rising about ten percent. The practical lesson is to investigate process data before replacing another valve or proposing a new nozzle.
A set-pressure margin check for the reliability team
- List each relief valve, its protected equipment, normal operating pressure at the valve inlet, and approved set pressure.
- Review twelve months of available DCS trends and identify the maximum normal operating pressure, distinguishing transients from steady operation.
- Calculate the gap between that pressure and the set pressure. Prioritize tight margins for engineering review; use the equipment design basis, applicable codes, and valve manufacturer guidance rather than treating a single percentage as universal.
- Compare actual suction and discharge temperatures with design values, along with flow and load. Rising temperature may warrant an exchanger-fouling investigation.
- Investigate and correct the underlying cause before changing relief hardware or set pressure, then re-measure the conditions.
- Repeat the check after throughput, catalyst, seasonal cooling, or other process changes.
Instrumentation lessons
Existing pressure and temperature instruments provided the decisive evidence. Trending these measurements in a historian can reveal gradual drift before a relief valve lifts. Consider alerting on narrowing operating margin as well as absolute process limits, with alarm settings approved by the plant’s engineering team. A Honeywell TC-FIAH81 analog input module is an example of catalog hardware relevant to bringing analog process signals into a control system; it was not identified as equipment used in the reported plant.
Capacity control also deserves a check where applicable. For compressors under speed or load control, confirm that unloading responds correctly at low demand. Our catalog includes a Woodward 9905-211 digital speed control and a Woodward 8290-185 powered governor as examples of related control hardware, not as a diagnosis or specified replacement for this compressor. Add intercooler cleaning to the turnaround scope on evidence from temperature and pressure trends rather than habit alone.
Conclusion
Repeated relief-valve lifting calls for a process investigation, not an automatic valve redesign. Compare operating and set pressures, check temperatures against design, examine cooling performance, and document the calculations in the relief-system register. Any change to a valve set point or protected piping should go through the plant’s formal engineering review.
Peng Xiaowei is an industrial automation engineer with over 10 years of experience in PLC, DCS, and control systems.