The pressure reading is not the diagnosis
Pressure is routinely measured in natural-gas and LPG midstream stations for control, metering, protection, and maintenance. Around an isolation ball valve, however, a pressure indication alone cannot establish whether the valve seals correctly. A closed ball may separate two line-pressure regions while its internal cavity follows a third pressure history. The challenge is to distinguish leakage across a seat from expected cavity behaviour, changing operating conditions and, in liquid-LPG service, thermal or phase effects. Internal seat leakage is different from external loss of containment: an isolation barrier can deteriorate without any product escaping to atmosphere.
Three pressure regions, several possible explanations
For a double-seated ball valve, a useful starting model distinguishes upstream pressure P1, body-cavity pressure Pc and downstream pressure P2. Their relationship is determined by the actual seat-loading arrangement, pressure-relief provision and position of any vent or bleed connections. DBB or DIB terminology alone is not a substitute for checking the certified drawings and pressure paths.
Published experimental studies by Shi and colleagues have demonstrated that cavity-pressure responses can support internal-leakage detection and diagnosis under controlled conditions.1, 2 Qin and colleagues subsequently examined unilateral and bilateral seat-leakage identification.3 These findings support using cavity pressure as diagnostic evidence; they do not supply a universal pressure-to-leak-rate conversion for every installed valve. Initial pressure, cavity volume, test sequence, seat design and fluid conditions affect the signal.
Why the operating state matters
Consider a valve whose cavity is deliberately depressurised through an approved bleed connection. If Pc subsequently rises while P1 remains relatively stable, pressure communication across an upstream seat is one possible explanation. Before assigning that cause, an engineer must verify the bleed sequence, closure of the bleed connection, downstream pressure and any intended cavity-relief route. If P1, Pc, and P2 change together, a compressor, regulator or broader station transient may be a more plausible explanation than progressive seat damage.
The distinction matters operationally. A position indicator confirming that the ball has turned to its closed stop does not establish seat tightness. Nor does a pressure trend replace a specified isolation-verification procedure before personnel access. A measurement becomes useful only after the monitored boundary and preceding operating state are known.
PG introduces a different pressure mechanism
Natural gas is compressible, whereas LPG may be present as liquid, vapour or a two-phase inventory. A liquid-filled section trapped between closed isolation points can develop substantial pressure on warming even without a newly leaking seat. Such thermal pressurisation is an established process-safety hazard.4 In a vapour-filled or two-phase volume, temperature and pressure respond differently, so one generic “LPG pressure-rise signature” is not technically defensible.
For LPG, the interpretation should begin with composition, phase state, temperature, initial filling condition, available expansion space and the relief arrangement. A pressure increase that looks suspicious when viewed alone may be consistent with warming of an isolated liquid volume. Conversely, attributing an unexplained change to thermal expansion without checking the valve pressure paths could conceal a real isolation problem.
A practical interpretation sequence
One useful distinction is between a pressure signal collected for routine station monitoring and a controlled test intended to evaluate seat isolation. In the first case, the operator may not control the upstream pressure, downstream demand or temperature. In the second, the test sequence and boundary conditions can be documented in advance. Published experiments show the value of the controlled approach, but site data are only comparable when their acquisition conditions are sufficiently understood. Trend quality also matters: synchronised measurements, instrument ranges, calibration status and the time resolution of recorded events can determine whether a brief station disturbance is visible or missed.
A disciplined review can proceed in six steps:
Identify fluid and phase.
Define the measured pressure boundary.
Establish the specific seat and cavity architecture.
Reconstruct the vent, bleed and station operating sequence.
Compare simultaneous pressure and temperature trends against expected behaviour.
Investigate deviations that remain unexplained.
The sequence can guide monitoring priorities and maintenance preparation without making a pressure measurement an automatic leakage verdict.
What a manufacturer and operator should document
Before accepting any pressure-based interpretation, the equipment file should identify seat type, expected pressure-energising and cavity-relief behaviour, vent and drain arrangements, permitted test directions and the conditions of the original factory acceptance. The operator should record pressure taps and their isolation states, instrument locations, the process conditions at the time of the event and the sequence of commands. These details are more useful than a generic label such as “double block and bleed” without an accompanying pressure-path drawing.
For a compact or monoblock configuration, the same questions remain. Shorter face-to-face dimensions do not prove better monitoring or easier depressurisation; cavity access and testability are design-specific. Photographs of manufactured valves show physical product variety, but cannot by themselves demonstrate leakage-detection capability. The engineering record and intended operating procedure must make that connection.
Pressure-response interpretation at a glance
Pc rises after controlled bleed
Possible explanation: seat communication may be present.
Before concluding: check seat arrangement, P1/P2 and bleed state
P1, Pc, and P2 move together
Possible explanation: station or pipeline transient.
Before concluding: correlate with compressor/regulator events.
Closed indication, pressure equalises
Possible explanation: movement confirmed; seal uncertain.
Before concluding: perform specified isolation verification.
Trapped liquid-LPG pressure rises
Possible explanation: thermal expansion or phase effect.
Before concluding: Review temperature, inventory and relief path.
Conclusion
The diagnostic value of pressure lies in its context. In gas service, controlled cavity-pressure responses may help identify internal communication across a seat. In liquid-LPG service, temperature and phase behaviour may create pressure changes without leakage. For both, engineers should interpret P1, Pc and P2 in the light of actual valve construction and station operation. A credible isolation decision still depends on the site’s approved testing, depressurisation and safety procedures.
Selected references
Shi, M. et al., Flow Measurement and Instrumentation 83 (2022), 102112. DOI: 10.1016/j.flowmeasinst.2021.102112.
Shi, M. et al., Flow Measurement and Instrumentation 89 (2023), 102303. DOI: 10.1016/j.flowmeasinst.2022.102303.
Qin, L. et al., Engineering Failure Analysis 153 (2023), 107584. DOI: 10.1016/j.engfailanal.2023.107584.
Ramirez, J.C. et al., “Preventing overpressure hazards from trapped liquids,” Process Safety Progress 29(4) (2010), 313–317. DOI: 10.1002/prs.10413.
