Vacuum & Breather Valves
Protect equipment from vacuum collapse and control tank breathing duties.
HHSV’s Leaflet identifies two different low-pressure protection records: HFA72W Vacuum Negative Pressure Safety Valve and HFHX Breather Valve. HFA72W addresses a negative-pressure protection function; HFHX belongs to tank/equipment breathing duty. Neither should inherit the set-pressure, capacity or test assumptions of a conventional positive-pressure safety relief valve.
HFA72W
1/2–10 in
Vacuum negative-pressure safety valve · Gas · Class 150 · ≤200°C
HFHX
2–12 in
Breather valve · Gas · Class 150 · ≤200°C
Inbreathing
Pressure direction inward
Protect against excessive vacuum / negative internal pressure
Outbreathing
Pressure direction outward
Relieve tank breathing flow when internal pressure rises
Pressure Direction Matters
Positive-pressure relief, vacuum protection and tank breathing solve different pressure problems.
A conventional positive-pressure safety relief valve opens to discharge process fluid when internal pressure rises above its set condition. Vacuum protection acts in the opposite pressure direction: it provides an allowed inward gas path when internal pressure falls too far below external/reference pressure. Tank breathing can require both outbreathing and inbreathing capacity as liquid level and vapor temperature change.
Positive-pressure safety relief
Internal pressure ↑ → valve relieves outward. Its set pressure, accumulation and API/ASME safety-relief framework must not be copied automatically to vacuum/breather products.
Negative-pressure protection
Internal pressure ↓ below the allowed vacuum condition → an inward gas path is provided to reduce vacuum and protect the equipment.
Tank breathing
Normal filling, emptying and thermal expansion/contraction can create outbreathing and inbreathing duties that must be sized separately from emergency overpressure relief.
Breathing Flow Map
Size the direction and the duty - not just the tank nozzle.
The blocks below are an editable functional illustration. Actual opening values, pressure-vacuum settings and rated flow must come from the selected HFA72W/HFHX datasheet and flow data; the Leaflet does not publish them.
A
Liquid filling / heating
Vapor space is compressed or warmed; internal pressure tends to rise.
B
Outbreathing →
Required vapor/gas flow leaves the tank through the approved breathing route.
C
← Inbreathing
Liquid withdrawal or cooling can create vacuum; allowed gas enters to limit negative pressure.
D
Liquid withdrawal / cooling
Vapor space expands or contracts thermally; internal pressure tends to fall.
HFA72W · Negative-Pressure Protection
Protect the equipment when internal pressure falls below the allowed vacuum condition.
HFA72W is Leaflet-described as a vacuum negative pressure safety valve for gas service. In a typical negative-pressure protection function, the valve remains in its normal state while pressure is within the allowed operating region. If internal pressure falls to the selected vacuum-opening condition relative to the reference/available gas side, the valve opens an inward gas path. As differential pressure recovers, the closing element returns toward its seated condition. Exact set vacuum, reseating differential and capacity are HFA72W model/configuration data and are not stated in the Leaflet row.
01
Normal pressure
Equipment pressure remains above the selected vacuum-opening condition.
02
Vacuum develops
Withdrawal, cooling, condensation or another scenario reduces internal pressure.
03
Inward path opens
At the model-specific negative-pressure condition, allowed gas enters the protected system.
04
Pressure recovers
The differential reduces and the valve returns toward its normal seated condition.
HFHX · Tank / Equipment Breathing
Breathing demand changes with liquid movement and thermal expansion or contraction.
HFHX is Leaflet-described as a breather valve for gas service. Breather-valve engineering begins with separate outbreathing and inbreathing cases. The selected device must be checked against the tank’s allowable positive and negative pressure limits and the required gas/vapor flows. The Leaflet label alone does not establish HFHX’s positive-pressure set point, vacuum set point, simultaneous two-direction functionality or rated capacity; confirm those from current HFHX controlled data.
Outbreathing duty
Evaluate filling rate, thermal expansion, vapor generation and any normal vent-header pressure. Required outward capacity is direction-specific.
Inbreathing duty
Evaluate liquid withdrawal, cooling/condensation and blanketing-gas behavior. Required inward capacity is direction-specific.
Set conditions
Positive and vacuum opening conditions must protect the tank without conflicting with normal operating/blanketing pressure.
Capacity / pressure loss
Use the selected HFHX flow data and system pressure differential. Do not rate a breather valve from connection size alone.
Leaflet-Confirmed Product Records
Keep the published envelope separate from the missing low-pressure set and flow data.
The HHSV Leaflet publishes the records below. Neither row gives a dedicated standard, vacuum/positive set-pressure range, flow-capacity table, body/trim material matrix, exact connection standard/facing or complete dimension schedule.
HFA72W Series
Vacuum Negative Pressure Safety Valve
Medium
Gas
Leaflet record
Size
1/2–10 in
HFA72W only
Pressure Class
Class 150
Connection/pressure-class record
Temperature
≤200°C
HFA72W only
Standard
Not stated
Verify controlled product/project basis
Vacuum Set Range
Not stated
Confirm current datasheet/test data
Material, connection standard/facing, vacuum flow capacity and exact dimensions: confirm current HFA72W BOM, datasheet, flow data and GA.
HFHX Series
Breather Valve
Medium
Gas
Leaflet record
Size
2–12 in
HFHX only
Pressure Class
Class 150
Connection/pressure-class record
Temperature
≤200°C
HFHX only
Standard
Not stated
Verify controlled product/project basis
P/V Set Range
Not stated
Confirm current datasheet/test data
Material, connection standard/facing, inbreathing/outbreathing capacities and complete dimensions: confirm current HFHX BOM, datasheet, flow data and GA.
Sizing & Selection
Start from the tank's pressure limits and breathing flow cases - not from Class 150 or nozzle size.
HFA72W selection needs the negative-pressure protection condition and required inward gas flow. HFHX selection needs the applicable normal inbreathing and/or outbreathing duties plus opening/set conditions. Capacity calculations must state the actual governing standard/method and use approved HHSV product flow data.
Protected equipment
Tank/vessel/equipment type, design basis and available vent/negative-pressure protection nozzles.
Allowable positive pressure
Maximum permitted internal positive pressure for the tank/equipment at the relevant condition.
Allowable vacuum
Maximum permitted negative pressure/vacuum before structural protection is required.
Filling / transfer rate
Maximum liquid inflow and associated vapor displacement used for normal outbreathing duty.
Emptying / pump-out rate
Maximum liquid withdrawal and required inbreathing gas flow.
Thermal breathing
Vapor-space heating/cooling, condensation and ambient/process temperature change affecting flow demand.
Gas / vapor properties
Composition, density/molecular weight, flammability/toxicity, condensation/fouling and corrosion requirements.
Blanketing / vent header
Blanketing-gas set pressure, vent destination, header backpressure and allowed atmospheric discharge arrangement.
Connection / layout
Nozzle size, flange standard/facing, orientation, weather protection, drainage and maintenance access.
Inspection / documents
Required set/function tests, capacity evidence, material records, third-party witness, drawings and MDR scope.
Materials, Connections & Dimensions
Do not turn HHSV's general material capability into an unverified HFA72W/HFHX material list.
The Leaflet rows do not publish exact body/trim/seal materials or flange standards. Material selection must follow gas/vapor composition, corrosion, temperature, environmental exposure and project requirements, then be frozen in the approved BOM/MTC. Connection size alone does not establish flange standard, facing, flow capacity or dimensional envelope.
Body / trim / seal
Confirm exact materials and sealing construction by selected HFA72W/HFHX BOM and MTC.
Connection details
Confirm flange/thread type as applicable, standard, facing/drilling and mating interface by GA.
Flow area
Use approved flow/capacity data for the exact valve configuration; do not infer flow from nominal size.
Dimensional drawing
Use the current GA for overall height, centerline, hood/outlet arrangement, weight and maintenance clearances.
Installation & Tank Interfaces
A low-pressure valve can be selected correctly and still perform poorly if the vent path is wrong.
Follow the approved HFA72W/HFHX GA and installation instruction for actual orientation. The checks below are system-engineering items: minimize unintended pressure loss, keep the gas path unobstructed, account for weather/condensation, and avoid a vent header or flame-arresting device creating pressure drop not included in the sizing basis.
Tank / equipment nozzle
Confirm nozzle location, size, local pressure drop and structural/interface requirements.
Inbreathing source
Define whether admitted gas is atmosphere, blanketing gas or another approved source and account for its pressure loss.
Outbreathing destination
Define atmospheric discharge or vent header and include outlet/header pressure losses in selection.
Weather protection
Review rain, dust, icing, insects, screens and environmental exposure without blocking the required flow path.
Condensate / drainage
Avoid liquid pockets or condensate that can change opening behavior or reduce breathing area.
Maintenance access
Keep setting/sealing elements, covers, nameplate and inspection interfaces accessible for test and service.
Standards & Compliance Boundary
The Leaflet does not publish a dedicated HFA72W or HFHX standard. Treat standards as project/product-specific inputs.
API 2000 / ISO 28300 may be relevant engineering bases for atmospheric and low-pressure storage-tank venting when the project requires them, but they must not be presented as HFA72W/HFHX certification merely because they are common tank-venting standards. Likewise, API 520/526/527 and ASME UV apply to different pressure-relief contexts and should not be copied automatically onto these Leaflet records.
API 2000 / ISO 28300
Use as sizing/venting basis only where the project and exact HHSV product data support it; state the actual applied edition/basis in the calculation.
Tank design code
The tank/equipment’s allowable positive pressure and vacuum come from its actual design basis and are primary selection inputs.
API 520 / 526 / 527
Do not apply automatically. These are not Leaflet-listed HFA72W/HFHX standards and may address a different relief/test context.
ASME UV
Do not infer Section VIII UV certification for HFA72W/HFHX from company-level authorization; model and scope must be explicitly verified.
Project specification
Owner/EPC requirements for venting, blanketing, emissions, materials, weather protection and inspection may control the supplied configuration.
Certificate matrix
Only claim certification when the exact legal entity, factory, model/range, standard and marking are inside the active certificate scope.
Designed to
State the actual design/sizing basis applied to the selected HFA72W/HFHX configuration.
Tested in accordance with
State the actual pressure/vacuum/function/leakage or flow test method recorded for the order.
Certified to
Use only when the exact product configuration and legal scope are covered by the cited authorization/certificate.
Inspection & Functional Testing
Test near-atmospheric pressure behavior with the acceptance basis that actually governs the valve.
The Leaflet does not publish set/vacuum ranges or universal leakage/flow acceptance values for these records. Test limits therefore come from the selected HFA72W/HFHX product data, project specification and applied venting standard. Third-party witnessing can be included when specified.
Negative-pressure opening
For HFA72W, verify the vacuum/negative-pressure opening condition and tolerance to the approved test basis.
Vacuum reseating / leakage
Verify return toward the closed condition and leakage criterion at the specified differential.
Outbreathing function
For HFHX where the selected design includes outward pressure relief, verify the positive-pressure opening/function requirement.
Inbreathing function
For HFHX where the selected design includes vacuum admission, verify the negative-pressure opening/function requirement.
Flow capacity evidence
Where contractually required, provide direction-specific capacity/flow data at the stated pressure differential – not a generic safety-valve capacity.
Dimensional / material traceability
Confirm interfaces, critical dimensions, nameplate/flow direction and agreed MTC/PMI/NDT/serial records before release.
Visual Release Check
Make flow direction, vents and product identity obvious before shipment.
Final inspection photos are most useful when they show the HFA72W/HFHX nameplate, connection interface, hood/outlet or inlet path, any flow-direction marking, external protection and supplied accessories. Photos should support the approved GA/BOM, not replace them.
Dimensions & Technical Documentation
The approvable deliverable is the valve plus its breathing-duty evidence.
Because the Leaflet does not provide complete low-pressure set, capacity, material or dimension tables, contractual values should come from controlled HFA72W/HFHX datasheets, calculations, drawings and test records.
Valve datasheet
Selected HFA72W or HFHX configuration, service basis, pressure/vacuum settings, materials and connection details.
Breathing / vacuum calculation
Separate inbreathing and outbreathing cases, pressure limits, flow assumptions and selected capacity basis.
GA / dimension drawing
Overall dimensions, connection, orientation, outlet/hood arrangement, weight and service clearances.
Flow / capacity data
Direction-specific gas/vapor capacity at the stated pressure differential for the selected configuration.
Material records
BOM/MTC plus agreed PMI/NDT/heat/serial traceability for pressure/wetted components.
Functional test reports
Vacuum opening/reseating, pressure opening where applicable, leakage and other agreed acceptance records.
QAP / ITP
Inspection stages, hold/witness points, calibration and third-party requirements where contractual.
MDR / final dossier
Controlled document index, approved revisions, certificates/conformity documents and shipment-release records.
Typical Application Routes
Application fit begins with the tank pressure envelope, not an industry label.
Every installation still requires the actual positive/vacuum limits, gas/vapor properties, breathing flow cases, material compatibility, vent destination, project standard and approved HFA72W/HFHX data.
Atmospheric / low-pressure storage tanks
Normal breathing and negative-pressure protection where the selected product pressure/vacuum range and capacity match the tank limits.
Oil & chemical storage
Gas/vapor breathing where material compatibility, emissions/vent destination and fire/environmental requirements are reviewed.
Tank farms & terminals
Filling/emptying rates and shared vent/blanketing systems require direction-specific breathing calculations.
Process vessels with vacuum risk
HFA72W may be evaluated where cooling, draining or condensation can create unacceptable negative pressure.
Blanketed tanks
Coordinate breathing-valve settings/capacity with blanketing regulator behavior and the tank’s permitted pressure/vacuum envelope.
OEM tank / package builders
Repeat equipment can standardize approved valve configuration, breathing calculation, GA and document package after validation.
Project Evidence
Show a real breathing calculation and delivery record - not a generic tank photo pretending to be a case study.
When a HFA72W or HFHX reference is approved for publication, show the tank/equipment type, allowable pressure/vacuum, in/out breathing duties, selected configuration, test scope and delivered documents while protecting confidential customer information.
Tank / equipment
Type, service and country/region, with customer identity anonymized when required.
Pressure envelope
Normal pressure, allowed positive/vacuum limits, blanketing and vent-header conditions.
Breathing duty
Filling, emptying, thermal cases and selected direction-specific flow capacity.
Delivery evidence
Approved datasheet/GA, calculation, material records, functional tests and MDR outcome.
FAQ
Vacuum and tank-breathing questions
The answers keep low-pressure breathing duties separate from conventional positive-pressure safety relief assumptions.
What is the difference between HFA72W and HFHX?
HFA72W is identified in the HHSV Leaflet as a Vacuum Negative Pressure Safety Valve and is presented here primarily for negative-pressure protection. HFHX is identified as a Breather Valve and is selected from the tank/equipment breathing duty. Their pressure direction, set conditions and flow requirements must be handled separately.
What ranges are actually published in the Leaflet?
HFA72W: gas, 1/2–10 in, Class 150, ≤200°C. HFHX: gas, 2–12 in, Class 150, ≤200°C. The Leaflet rows do not publish a dedicated standard, vacuum/positive set range, capacity table, material schedule or full dimensions.
What is inbreathing?
Inbreathing is inward gas flow required when tank pressure tends to fall, for example during liquid withdrawal or vapor-space cooling/condensation. Required flow must be calculated from the real tank scenario and selected valve data.
What is outbreathing?
Outbreathing is outward gas/vapor flow required when tank pressure tends to rise, for example during liquid filling or vapor-space heating. The required positive-pressure opening condition and capacity must stay within the tank design envelope.
Can I apply API 520 or API 527 automatically?
No. Neither standard is shown in the Leaflet records for HFA72W or HFHX. Use the actual project/product design and test basis. API 2000 / ISO 28300 may be relevant to tank venting when specified, but this does not itself mean the HFA72W/HFHX product is certified to those standards.
Can Class 150 tell me the vacuum setting or breathing capacity?
No. Class 150 is not a vacuum set-pressure or flow-capacity value. Vacuum/positive opening settings and capacity must come from the selected product configuration, test data and breathing calculation.
What should I send for a quotation?
Send tank/equipment type, normal and allowable positive/vacuum pressure, filling and emptying rates, thermal breathing information, gas/vapor properties, blanketing and vent-header data, temperature, connection/nozzle, materials, governing standard, inspection/document requirements and available tank datasheet/P&ID.
Related Safety Valve Families
Choose the pressure-protection function before choosing the product family.
Spring-Loaded Safety Relief Valves
Positive-pressure direct spring-loaded relief family; not a substitute for vacuum/tank-breathing protection.
Pilot-Operated Safety Relief Valves
Pilot-controlled positive-pressure relief architecture with its own pressure/capacity data.
Impulse & Main Safety Valve Systems
Impulse/main high-capacity steam protection system; a different service and pressure-relief architecture.
All Safety Valves
Return to the HHSV family map and select by protection function and real operating data.
Request a Quote
Send the tank pressure envelope and breathing cases - HHSV will turn them into a valve selection.
Provide allowable positive pressure/vacuum, filling and emptying rates, thermal breathing, gas/vapor data, blanketing/vent header, temperature, connections, materials, standard, quantity, inspection, documents and available tank datasheet/P&ID.