Why Low-Temperature Shutdown Creates Freeze and Overpressure Risks

When a refrigeration system is stopped for winter, seasonal downtime, maintenance, or emergency isolation, valves are exposed to conditions that do not exist during stable running:

• Trapped liquid expands as temperature rises.​ Closed piping sections that still contain liquid refrigerant can generate high pressure if warmed. Parker’s refrigeration guidance warns that a fully liquid-filled, valved-off section can rupture gaskets, piping, or valves because liquid expansion under temperature increase creates excessive pressure; hand valves isolating such sections should not be closed until the liquid is removed.
• Low-temperature embrittlement and ice at stems.​ Suction, liquid feed, and evaporator isolation valves may operate well below 0°C. Moisture at packing, sense lines, or gauge connections can freeze and restrict stem movement.
• Ammonia safety constraints.​ Ammonia is toxic; relief discharge, oil draining, and isolated pockets must follow machine-room ventilation, relief routing, and qualified procedures rather than informal site habits.
• Mixed refrigerant compatibility.​ Ammonia, CO₂, and fluorinated refrigerants use different seal, stem, and body limits. A generic “open everything and drain” instruction is unsafe without model-specific data.

Dofun’s refrigeration range—manual PN40/PN63, long-neck, stainless, servo/pilot, motor, gas-powered, solenoid, pressure-regulating, valve stations, and safety auxiliaries such as SRV, DSVD, PGV, CFVD, NRVD—is designed for low-temperature duty, but safe shutdown still depends on procedure, not product alone. Public Dofun product listings for selected models show SVD stop valves DN15–DN300 at 4.0/6.3 MPa and −50°C to +150°C, CFVD cartridge filters DN15–DN80 at 2.5 MPa and −50°C to +150°C, and NRVD neck-flange check valves DN25–DN150 at 2.5 MPa and −50°C to +150°C; these are model examples, not a permission to apply one drainage rule to every valve.

Dofun Valve Groups Relevant to Shutdown and Drainage

A freeze-prevention plan should be organized by valve function, not by room name.

Isolation and Manual Shutoff

Dofun manual families include Manual Operation Valve PN40 Series, PN63 Series, PN40/PN63 Long Neck Series, Stainless Steel Valve Series, Small Stop Valve SVS/SVA, and Double Stop Valve DSVD. Long-neck versions are relevant where stem packing must be kept away from very cold piping; SVD-class listings cover ammonia, fluorinated refrigerants, and CO₂ in selected models with −50°C to +150°C range examples. During shutdown, these valves define isolation boundaries for drainage and purging.

Regulation, Pilot, and Servo Control

Pressure Regulating Valve Series, Servo Main Valve Series, Pilot Valve Series, Inlet Pressure Regulating Valve, Outlet Pressure Regulating Valve, Differential Pressure Regulating Valve, and Motor Valve Series contain sensing lines, diaphragms, springs, and pilot modules. Public listings show IPRV/OPRV/DPAV-class regulators around 2.8 MPa and −50°C to +120°C for selected models, while DFMV motor valves are listed at 4.0/6.3 MPa and −50°C to +150°C for selected models. Pilot sense lines and small-bore impulse tubing are high priority for freeze protection because trapped condensate or refrigerant liquid can block control response.

Automatic On/Off and Safety Auxiliaries

Solenoid Valve Series, Gas Powered Valve Series, Valve Station Series, Safety Valve SRV Type Series, DSVD, PGV, CFVD, and NRVD support automated isolation, overpressure protection, instrumentation, filtration, and reverse-flow control. Solenoid listings such as DEVSC/DEVS are published at 2.5 MPa and −40°C to +105°C for selected models; DGPV pneumatic solenoid listings are at 2.8 MPa and −50°C to +120°C for selected models. Safety items must never be disabled to “drain faster”; SRV set pressure, DSVD changeover position, and PGV isolation status are part of the documented shutdown record.

Pre-Shutdown Planning

Before any drainage or isolation, collect the documents that determine what can be closed, what must remain open, and where liquid can be safely removed.

1. P&ID and valve schedule​ with Dofun model, DN, PN, medium, temperature class, function, and tag number.
2. Refrigerant-specific data—ammonia R717, CO₂ R744, or fluorinated blends; confirm seal and body compatibility per model.
3. SRV/DSVD information—set pressure, spring range, orifice, seat material, and relief routing.
4. Low-point and high-point list—receivers, separators, intercoolers, surge drums, pump headers, evaporator manifolds, suction and liquid headers.
5. Drain, purge, and recovery connections—approved recovery unit, oil pots, bleed valves, and vent paths to safe location.
6. Local code basis—pressure-equipment rules, ammonia machine-room standards, ventilation interlocks, and permit-to-work requirements.

Using Dofun’s DOFUN-CTP modular platform, maintenance teams should also prepare standardized kits—stem/seal kits for manual valves, pilot/spring kits for regulators, actuator kits for solenoid/motor valves, cartridges for CFVD, and spring/seat kits for SRV—so shutdown service does not stop because of missing parts.

Step-by-Step Low-Temperature Shutdown Procedure

The sequence below is generic. Actual isolation, pumpdown, recovery, draining, and purging must follow the plant’s approved procedure and Dofun model instructions.

Controlled Depressurization and Refrigerant Recovery

• Stop compressors and associated controls according to the plant sequence; do not close isolation valves first.
• Where the design uses pumpdown or recovery, transfer refrigerant from low-pressure vessels to receiver or recovery unit per OEM instruction. Never trap liquid in a closed section before isolation.
• For ammonia machinery rooms, keep ventilation, gas detection, and alarm systems active until the isolated circuit is confirmed safe.
• Record pressures and temperatures at receivers, separators, evaporators, and suction/liquid headers before opening any drain.

Isolate by Functional Zone, Not by Individual Valve Alone

Use Dofun manual and double-stop valves to create clear boundaries:

• Receivers and liquid headers—isolate with SVD/PN40/PN63 or DSVD where dual safety changeover is required.
• Evaporators and cold rooms—isolate liquid supply and suction according to defrost/shutdown mode; avoid closing both ends of a liquid-filled coil.
• Instrument branches—isolate through PGV rather than removing gauges under pressure.
• Safety continuity—if one SRV must be removed, use DSVD changeover so the second safety path remains active; otherwise reschedule work to avoid losing overpressure protection.

Drain Liquid from Approved Low Points

Drainage means removing refrigerant liquid, oil, and condensate from designed drain connections—not opening random valves.

• Open approved drain/bleed valves only after the zone is isolated, recovered, and verified at safe pressure.
• For ammonia oil pots or drop legs, follow double-valve or self-closing valve procedures; IIAR-based guidance uses a shutoff in series with a self-closing or spring-return valve and recommends draining only by qualified personnel after isolation and safe boil-off/purge as designed.
• Route all discharged refrigerant to recovery or approved ventilation/relief path. Relief valve discharge must go to a safe outdoor location; do not connect relief vents to building drains or occupied spaces.
• For CO₂ or fluorinated systems, use recovery cylinders and oil separation according to refrigerant type; do not mix ammonia and CO₂ drainage procedures.

Purge Sense Lines, Pilot Modules, and Small Valves

Pilot-operated and regulating valves are especially sensitive to blockage:

• Remove and blow/clean pilot cartridges using methods approved by Dofun application engineering; replace contaminated cartridges from CTP kits.
• Check IPRV/OPRV/DPAV impulse lines for liquid slug, ice, or oil sludge.
• Verify solenoid valves DEVSC/DEVS or DGPV pneumatic sets are free of trapped liquid if the duty cycle and ambient condition require it; otherwise keep them in the manufacturer-recommended position.
• Inspect CFVD cartridge elements and replace if contaminated; filtered protection reduces pilot drift after restart.

Protect Stems, Packing, and Long-Neck Bodies

• For long-neck manual valves, confirm packing temperature rating and insulation coverage; extended bonnet designs keep packing farther from the coldest pipe surface, but packing compound must still be compatible with refrigerant and ambient humidity.
• Exercise infrequently used SVS/SVA, PGV, and DSVD handles periodically during planned shutdown so stems do not seize; full stroke frequency should follow site maintenance rules, not a universal weekly rule.
• Do not hammer, lever, or force frozen handwheels. Investigate binding, repack or warm the bonnet using approved methods, and verify stem straightness before restart.

Freeze Prevention Measures for Idle Valves

Insulation and Heat Tracing

• Insulate valve bodies, bonnets, and connection necks according to the design temperature and ambient condition. For long-neck ammonia valves, insulation should cover the specified neck length without blocking packing inspection.
• Where specified, use trace heating with temperature control and alarms. Self-regulating traces, sensor location, and explosive-atmosphere certification must match ammonia machine-room classification.
• Keep PGV, small SVS/SVA, and pilot sense lines inside insulated and traced runs where the P&ID defines winter idle temperature.

Elimination of “Dead Pocket” Sections

Closed sections full of liquid are the main freeze/overpressure hazard. For each Dofun valve group:

• Identify two-valve pockets—e.g., isolation valve plus check valve, regulator plus downstream stop, solenoid plus manual bypass.
• Provide approved drain/bleed or continuous circulation where the process design allows a small flow.
• For NRVD-protected lines, remember a check valve is not an isolation device; add a proper stop valve when maintenance drainage is required.
• Review valve station DFCV arrangements so idle branches are either fully drained or kept in controlled circulation.

Stem and Seal Material Management

Dofun product data for selected models list 304 stainless stems and broad ammonia/fluorine/CO₂ compatibility, but seat and packing compounds vary by model. For winter shutdown:

• Confirm packing remains flexible at the lowest expected ambient temperature around the bonnet.
• Confirm diaphragm and seal elastomers for regulators/solenoids are compatible with retained refrigerant traces and any methanol/water remediation policy.
• For stainless hygienic or pharmaceutical branches, document seal compound, surface condition, and clean-drain procedure separately from industrial ammonia lines.

Safety Valve, Double-Stop, and Instrument Isolation During Shutdown

SRV and DSVD

• Do not blind, wire, or disable SRV to speed drainage. Overpressure protection must remain valid until the circuit is fully depressurized and locked out.
• Where online SRV service is required, use DSVD changeover: rotate the changeover to isolate one safety valve while the other remains connected to the protected equipment. DSVD model data include DN20–DN32 secondary sizes and 4.0 MPa nominal pressure examples with −46°C to +150°C range for selected configurations; exact set and changeover procedure must match the installed model.
• After reinstallation, record set pressure, spring code, seat material, and test date.

PGV and Small Instrument Valves

• Use PGV to isolate pressure gauges, transmitters, and switches before removal or calibration. Do not expose instrument impulses directly to ambient if they contain liquid refrigerant.
• Drain instrument sense lines through approved bleed points; if the sense line serves a pilot regulator, refit the pilot module only after the line is verified clean and dry per Dofun instruction.

CFVD and NRVD

• CFVD protects regulators, solenoids, and pilot systems from debris. During shutdown, remove the cartridge, inspect for oil sludge/ice particles, and replace with the CTP-standard element grade.
• NRVD prevents reverse flow but does not isolate. For pump or compressor discharge shutdown, keep NRVD in service and add manual isolation only where the drainage procedure requires it.

CTP-Based Spare Kits for Winter Maintenance

DOFUN-CTP reduces shutdown inventory complexity by standardizing modules and enabling quick function changes without pipeline disassembly. For freeze-protection maintenance, organize kits by duty:

• Manual isolation kit—SVD/long-neck/PN40/PN63 stem, bonnet gasket, packing, handle hardware by pressure class and connection.
• Regulation kit—IPRV/OPRV/DPAV/TVD pilot cartridges, springs, diaphragms, sense-line fittings.
• Automatic kit—DEVSC/DEVS or DGPV solenoid coils, pistons, seals; DFMV motor actuator couplings, position feedback, seal set.
• Safety kit—SRV spring by set band, disc/seat set, test label; DSVD changeover seal set; PGV gland kit.
• Filtration and protection kit—CFVD cartridges by DN and micron class; NRVD disc/spring kit.

Stock by criticality and lead time rather than by one whole valve per tag. Safety items should retain model-specific set-pressure records; CTP standardization applies to compatible modules, not to mixing unverified models.

Application-Specific Notes

Ammonia Cold Storage and Food Plants

Prioritize receiver/liquid-header drainage, evaporator isolation without trapping liquid, SRV/DSVD continuity, and PGV instrument safety. Ammonia relief must be routed safely; oil draining uses double-valve or oil-pot procedure under qualified ammonia safety rules. Stainless or sanitized branches should follow separate clean-drain records.

CO₂ and Transcritical Systems

High-side CO₂ shutdown may involve higher static pressure than subcritical ammonia/fluorine loops. Confirm Dofun model pressure class, SRV orifice, and relief routing before any drainage. Do not use ammonia SRV spring data for CO₂ high side without model confirmation.

Fluorinated Refrigerant Systems

Blended refrigerants can fractionate during recovery. Follow recovery-machine instructions, model-specific seal compatibility, and leak-tight isolation using SVD/SVS/PGV. Replace contaminated CFVD cartridges before restart.

Pharmaceutical and Hygienic Refrigeration

Dofun’s Public Welfare & Health segment covers food/pharma hygienic duty; client references mention sanitary-grade valves and polishing, but exact surface-finish and cleaning requirements vary by project. Shutdown drainage should preserve cleanliness—use designated drains, avoid cross-contamination, and keep material certificates for stainless trim and seals.

Mining, Heavy Industry, and Remote Sites

Abrasive and high-run-hour sites benefit from CTP standardized kits and CFVD protection. During winter shutdown, focus on check-valve free closure, regulator sense-line cleanliness, and pre-staged motor/solenoid modules so restart does not wait for full valve replacement.

Restart and Post-Shutdown Verification

Before returning Dofun valves to service:

1. Verify all drained zones were re-evacuated, charged, or purged per refrigerant type.
2. Check SRV set pressure and DSVD changeover position; confirm only one intended safety path is isolated during service.
3. Test regulators and pilots under low/medium/high load; confirm no sense-line lag from residual moisture.
4. Operate manual, solenoid, motor, and valve-station functions through full stroke; record torque and feedback.
5. Inspect CFVD differential pressure; replace cartridge if restriction appears after startup.
6. Review packing and long-neck insulation for frost after stabilization; repack or reconfigure heat tracing if stem binding recurs.

FAQ

Can I just close all Dofun stop valves and leave the system for winter?​

No. Closing both ends of a liquid-filled section can create thermal-expansion overpressure as temperature rises. Remove liquid through approved recovery/drain points before isolation.

Should SRV safety valves be disconnected during drainage?​

No. Keep overpressure protection active until the circuit is fully isolated, depressurized, and locked out. Use DSVD changeover if one SRV must be serviced online.

How often should unused low-temperature valves be exercised?​

Follow the site maintenance plan. Infrequently operated SVS/SVA, PGV, DSVD, and long-neck manual valves should be cycled during planned shutdowns to prevent stem seizure, but frequency depends on ambient, refrigerant, and service history.

Do all Dofun valves use the same seals for ammonia, CO₂, and fluorinated refrigerants?​

No. Selected models share broad refrigerant ranges, but seat, packing, diaphragm, and gasket compounds must be confirmed by Dofun model number and refrigerant/temperature data.

Can CTP modules reduce winter spare inventory?​

Yes, for compatible families. Standardized stems, seals, pilot springs, solenoid/actuator kits, CFVD cartridges, and SRV kits reduce whole-valve stock, but interchangeability must be confirmed by Dofun application engineering.

Conclusion

Dofun low-temperature shutdown protection is a procedure-led program: recover and depressurize by zone, avoid trapping liquid, drain only through approved points, protect pilot/sense lines and stems, keep SRV/DSVD/PGV safety functions documented, and use DOFUN-CTP standardized kits for fast winter maintenance. With manual PN40/PN63, long-neck, stainless, servo/pilot, motor, solenoid, regulating, and safety auxiliary valves, the objective is not to “open everything” but to create isolated, drained, traceable zones that will not freeze, seize, or overpressure during idle periods. For any plant-specific shutdown sheet, send Dofun the P&ID valve schedule—model, DN, PN, refrigerant, temperature class, and safety setpoints—so the company can return a CTP spare list and model-specific drainage checklist.