EN
Shanghai Xiazhao Series TOXD Cryogenic Pilot-Operated Safety Relief Valve (-196°C to +80°C, Class 150 to 2500)
1. Sealing Force Mechanics: Dome Pressure vs. Spring Relaxation
In a conventional spring-loaded relief valve, downward seating force is provided exclusively by the compressed mechanical spring: F_net = F_spring - (P_in × A_seat). As operating pressure approaches set pressure, contact stress drops below the plastic deformation threshold. At 88%~92% of set pressure, micro-leakage (simmering) begins, causing seat erosion and boil-off gas losses.
In the TOXD Series pilot valve, inlet pressure is transferred into the upper dome chamber above the main piston. Because the dome piston area A_piston is 1.28 ~ 1.35 times larger than the nozzle seat area A_seat: F_net,pilot = P_in × (A_piston - A_seat). As system pressure increases, downward seating force increases proportionately, guaranteeing API 527 zero-bubble tightness up to 98% of set pressure.
Cross-Sectional Working Principle: Dome Pressure Balancing below Set Pressure vs. Pilot Actuation
2. Engineering Comparison Matrix
Engineering Parameter |
Direct Spring-Loaded Cryogenic PRV |
Cryogenic Pilot-Operated PRV (Model TOXD) |
Min Operating Temp |
-196°C (Requires heavy extended bonnet) |
-196°C (Thermal decoupling architecture) |
Max Operating Pressure Ratio |
Max 85% ~ 90% of set pressure |
Up to 95% ~ 98% of set pressure |
Seat Leakage Rate (API 527) |
Simmer leakage begins above 88% set pressure |
0 bubbles/min up to 98% set pressure |
Reseat Blowdown (ΔP) |
10% to 15% (Spring hysteresis loss) |
≤ 7% (Fast, crisp reseat, minimal BOG loss) |
Allowable Backpressure |
≤ 10% (Conventional); ≤ 30% (Bellows) |
Up to 70% of set pressure (Balanced Piston) |
Backpressure Sensitivity |
Set point shifts as header pressure drifts |
Set point remains 100% constant |
Structural Envelope |
Heavy (Large forged springs & bonnets) |
Compact: 40% ~ 50% lighter than spring PRV |
External Thermal Decoupling Loop and Balanced Pilot Block on Series TOXD
3. Boil-Off Gas (BOG) Economic Analysis
On a 160,000 m³ LNG storage tank with relief set pressure at 1.0 MPa (10.0 bar) and a required relieving rate of 25,000 kg/h:
• Conventional Spring Valve (12% Blowdown): Reseats at 0.88 MPa; 480 s duration; vents 3,333 kg LNG = $2,166 USD loss per event.
• Xiazhao Pilot Valve (6% Blowdown): Reseats at 0.94 MPa; 220 s duration; vents 1,527 kg LNG = $992 USD loss per event.
• Direct Savings: $1,174 USD saved per relief cycle; over $9,300 USD annualized savings, eliminating fugitive boil-off emissions.
