Pilot-Operated Safety Relief Valves
Pilot control for precise pressure relief - with HFXD limits kept model-specific.
Built for industrial engineers, OEMs, EPC teams and procurement. HHSV’s Leaflet identifies HFXD as a pilot-operated safety relief valve for gas and liquid service. Selection starts from the relieving case, sensing arrangement, backpressure and required compliance – not from line size alone.
Representative pilot-operated pressure safety valve image from the current TSV product page. Replace with the approved HFXD project/product image when available.
How Pilot Operation Works
The pilot senses pressure; the main valve supplies the relieving flow area.
A pilot-operated relief valve uses system pressure and a pilot/control circuit to hold the main valve closed under normal conditions. The sensing pressure is monitored by the pilot. At the set condition, the pilot changes the control pressure acting on the main valve so the main disc or piston opens. When system pressure falls to the model-specific reseating condition, the pilot restores control pressure and the main valve closes. The exact pilot action, blowdown and pressure limits belong to the selected design.
Visual reference: identify the main relief body, external pilot and connecting control/sensing tubing. Exact HFXD internal routing remains drawing-specific.
01
Sense pressure
The pilot receives pressure from the protected system through the defined sensing route.
02
Hold closed
Control pressure keeps the main valve seated during normal operation.
03
Reach set
At the set condition, the pilot changes the control-pressure balance.
04
Main valve opens
The main element lifts and provides the relieving flow path sized for the case.
05
Reseat
As pressure falls, the pilot restores the closing condition and the main valve reseats.
System Architecture
Pilot, sensing path and main valve must be selected as one pressure-relief system.
Treating only the main-valve body as the product misses the control path that determines opening and reseating behavior.
Pilot assembly
Senses the pressure condition and switches the control pressure that operates the main valve.
Main valve
Provides the primary relieving flow path; rated capacity and effective area require the selected model/orifice data.
Sense line
Carries the pressure signal. Source location, line loss, isolation and fouling risk are engineering inputs.
Control / dome volume
Pressure acting on the main closing element is managed by the pilot circuit; details are design-specific.
Seat & sealing surfaces
Main and pilot sealing performance must be verified against the project test requirement.
Inlet / outlet connections
The Leaflet gives the HFXD size range and API 526 basis, but the exact flange combination and dimensions require the model drawing.
Leaflet-Confirmed Product Record
HFXD is the published product record - not a blanket Pilot-Operated rating.
The values below are transcribed from the HHSV Leaflet. Capacity, allowable backpressure, materials and dimensional combinations are not published in this Leaflet row and must be confirmed by the selected datasheet, sizing calculation, GA drawing and certificate matrix.
Series
HFXD
Pilot operated safety relief valve
Medium
Gas · Liquid
Leaflet record
Standard shown
ASME VIII · API 526
Design/product-standard statement; certification verified separately
Size
1–10 in
HFXD published range
Pressure
Class 150–2500
HFXD published range
Temperature
-196 to 200°C
HFXD published range
Opening, Reseating & Backpressure
Backpressure is an input to selection, not an assumed Pilot-Operated advantage.
Pilot-operated designs are often considered where system pressure, tight operating margins or outlet conditions make control behavior important. However, the HHSV Leaflet does not publish one HFXD allowable-backpressure percentage. Superimposed and built-up backpressure, pilot exhaust arrangement and sensing configuration must be checked against the exact HFXD design.
Set pressure
The pilot set condition is established for the selected service and applicable standard; operating margin is reviewed.
Opening
The pilot changes the control-pressure balance so the main valve can open and relieve the required load.
Reseating / blowdown
Closing behavior and blowdown are design-specific and must be taken from the approved model data/test basis.
Backpressure
Quantify superimposed + built-up pressure and its variability; confirm allowable limits from the selected HFXD data.
Engineering Choice
Pilot-Operated vs Spring-Loaded: choose from the pressure-relief system, not a hierarchy of products.
Neither architecture is automatically better. The correct choice depends on service stability, pressure margin, backpressure, fluid cleanliness, maintenance philosophy, code requirements and the exact certified product scope.
Pilot-Operated
Pressure-controlled main valve
Consider when the service and project benefit from pilot-controlled opening/reseating behavior. Confirm sensing, pilot exhaust, cleanliness, backpressure and maintenance requirements.
HHSV Leaflet anchor: HFXD. Published HFXD limits remain HFXD-specific.
Spring-Loaded
Direct spring force on the disc
Consider conventional or balanced-bellows spring-loaded designs when their mechanical behavior, backpressure capability, service and code scope fit the relieving case.
Sizing & Selection
Required relieving area comes from the credible case and the applicable sizing method.
HHSV can prepare API 520 or ISO 4126 sizing where applicable to the selected product and project. The calculation should state input data, assumptions, correction factors and the selected model/orifice. Do not infer HFXD capacity from line size or Class.
Relieving case & capacity
Required mass/volumetric flow plus the credible scenario used to establish it.
Medium & properties
Gas or liquid plus density, molecular weight, k/Cp/Cv, viscosity or other properties required by the sizing method.
Pressure basis
Operating pressure, set pressure, allowable overpressure/accumulation and pressure reference.
Temperature
Normal and relieving temperature; verify material/rating at the actual condition.
Backpressure
Superimposed + built-up, constant or variable, plus outlet-system assumptions.
Pilot / sensing arrangement
Sense location, line routing, exhaust arrangement, isolation requirements and contamination risk.
Materials, Connections & Dimensions
Do not convert company capabilities into unverified HFXD options.
The Leaflet HFXD row does not state a material matrix, exact inlet/outlet flange combinations, or a dimension table. Those details should be pulled from the current HFXD datasheet, BOM and GA drawing for the selected pressure/temperature condition.
Material selection
Specify medium composition, corrosion allowance, low-temperature/high-temperature requirements and any NACE/PMI requirement. HFXD material compatibility is then confirmed by BOM.
Connection selection
API 526 is shown for HFXD in the Leaflet, but the exact inlet/outlet size, flange class, facing and connection combination require model-level confirmation.
GA / dimensional drawing
Use the approved drawing for face-to-face/centerline dimensions, overall height, pilot/sense-line orientation, weights and installation clearances.
Standards & Certification
Designed to, tested in accordance with and certified to are different statements.
HHSV holds ASME UV and National Board authorization at company/product-scope level. For an HFXD quotation, the legal entity, factory, model, pressure range, marking and current authorization scope must be matched before the certification claim is made.
ASME BPVC Section VIII
Shown with the HFXD Leaflet record as its product/code basis. Any ASME marking claim must match the actual authorization scope.
API 526
Shown with HFXD in the Leaflet; apply the exact dimensional/orifice requirements only to the selected product configuration.
API 520
Sizing/selection methodology where applicable; relieving case and correction assumptions must be explicit.
API 527
Seat-leakage test basis where specified; testing to API 527 does not itself mean product certification.
ISO 4126
Company/project design and sizing basis where applicable; do not assume every HFXD configuration is certified to it.
Project / owner requirements
Material, inspection, documentation and deviation requirements are contractual inputs in addition to product standards.
Designed to
The code or standard is used as the design, sizing or dimensional basis for the selected HFXD configuration.
Tested in accordance with
The supplied valve is tested using the stated method and acceptance criteria for the agreed scope.
Certified to
Use only when the actual legal entity, factory, model, range and marking are within the current authorization/certificate scope.
Inspection & Performance Testing
A pilot-operated valve must be verified as a complete control-and-relief assembly.
The actual test sequence, witness points and acceptance limits are defined by the selected product, code and contract. Third-party witnessing can be added where specified.
Pressure boundary
Shell/body integrity test at the specified procedure and acceptance criteria.
Pilot set function
Verify pilot set behavior against the approved test procedure and configuration.
Main-valve opening
Confirm main-valve response and the required test points for the selected design.
Reseating / blowdown
Record closing behavior where required by the applicable standard/project test plan.
Seat leakage
API 527 or the specified leakage basis where applicable to the supplied configuration.
Control-path integrity
Inspect pilot tubing, sensing path, fittings and required leak/functional checks before release.
Visual Inspection Point
Use photography to make the delivered configuration auditable.
For Pilot-Operated valves, final inspection photos are most useful when they clearly show the pilot assembly, control tubing, inlet/outlet orientation, nameplate and tagging. The image should support the approved GA/BOM rather than serve as decoration.
Dimensions & Technical Documentation
The deliverable is the selected valve plus the evidence needed to approve it.
Because the Leaflet does not contain a complete HFXD dimension/capacity table, contractual dimensions and performance values should come from the controlled datasheet, calculation and approved GA drawing.
Valve datasheet
HFXD configuration, service basis, set pressure, materials and selected options.
Sizing calculation
Relieving-case inputs, assumptions, correction factors and selected area/capacity basis.
GA drawing
Inlet/outlet dimensions, overall envelope, pilot/sense-line arrangement and installation interfaces.
Material records
MTC and specified PMI/NDT/heat-lot traceability for the agreed scope.
Test reports
Pressure-boundary, set/function, reseating/blowdown and leakage records as applicable.
Certificate matrix
Maps the quoted HFXD configuration to the claimed authorization/conformity scope.
QAP / ITP
Inspection stages, hold/witness points and third-party requirements where contractually required.
MDR / final dossier
Controlled document index, approved revisions, certificates and shipment-release records.
Typical Application Routes
Application relevance is a starting point, not proof of suitability.
Every project still requires the actual relieving case, material compatibility, pressure-temperature rating, backpressure review, certificate scope and installation conditions.
Pressure vessels & process equipment
Gas or liquid relief where the HFXD service range and applicable code scope fit the relieving case.
Refining & petrochemical
Candidate applications where operating margin, backpressure and engineering documentation require careful review.
Gas processing & compression
Pilot-operated architecture may be evaluated for gas systems after sensing, cleanliness and discharge conditions are defined.
LNG / low-temperature systems
The Leaflet publishes HFXD down to -196°C, but material/BOM and actual service compatibility must be verified for the selected configuration.
OEM / packaged equipment
Repeat supply can standardize approved sizing basis, GA, BOM and document package for recurring equipment builds.
Liquid pressure relief
HFXD is Leaflet-listed for liquid service; sizing method, viscosity, backpressure and stability inputs remain application-specific.
Project Evidence
Publish verified delivery evidence; do not invent a project case to fill the page.
When a suitable HFXD reference is approved for publication, the case should identify the application, service data, supplied configuration, quantity, test/witness scope and delivered documentation without exposing confidential customer information.
Application & country
Industry, installation type and country/region, with customer identity anonymized when required.
Service conditions
Medium, set pressure, temperature, backpressure and relieving basis that drove selection.
Supply & testing
HFXD configuration, quantity, material, inspection and third-party witness scope.
Delivery evidence
Approved drawings, calculations, MTC/MDR, test records and schedule outcome.
FAQ
Pilot-operated selection questions
The answers keep HFXD product claims separate from general pilot-operated engineering principles.
How does a pilot-operated safety relief valve differ from a direct spring-loaded valve?
A pilot-operated design uses a pilot/control circuit to manage the pressure acting on the main valve, while a direct spring-loaded design applies spring force directly through the moving assembly. Selection depends on the actual relieving case, pressure margin, backpressure, fluid condition, maintenance philosophy and compliance scope.
What is the published HFXD range?
The HHSV Leaflet lists HFXD for gas and liquid, 1-10 in, Class 150-2500 and -196 to 200°C, with ASME VIII / API 526 shown. These limits belong to the HFXD record and must not be generalized to every pilot-operated valve.
How much backpressure can HFXD tolerate?
The Leaflet row does not publish one allowable-backpressure percentage. Provide superimposed and built-up backpressure plus its variability and the pilot exhaust/sensing arrangement; the selected HFXD data must then be checked.
Can I size HFXD from line size and pressure Class?
No. Required capacity comes from the credible relieving scenario and the applicable API 520 / ISO 4126 sizing method. Line size and flange Class are not substitutes for a sizing calculation.
Does API 527 mean HFXD is API certified?
No. API 527 is a seat-leakage test standard. "Tested in accordance with API 527" is different from a certification claim.
Does ASME VIII in the Leaflet automatically mean every HFXD is ASME UV marked?
No. The legal entity, factory, exact model/configuration, pressure range and marking must be within the current authorization scope before "Certified to" or UV-marking language is used.
What should I send for a quotation?
Send the RFQ/datasheet or existing valve nameplate plus medium, required capacity, operating and set pressure, relieving temperature, backpressure, connections, materials, code/certification, quantity, inspection, documentation and delivery requirements.
Related Safety Valve Families
Compare architectures before locking the RFQ.
Spring-Loaded Safety Relief Valves
Direct spring-loaded conventional and balanced-bellows designs for model-specific services.
Back-Flow Safety Valves
Separate functional family for back-flow protection; not the same as a balanced-bellows spring-loaded valve.
Impulse & Main Safety Valve Systems
System architecture using impulse/control and main-valve elements for applicable services.
All Safety Valves
Return to the product-family map and compare selection routes.
Request a Quote
Send the relieving case. HHSV will turn it into an approvable HFXD selection.
Provide medium, required capacity, operating/set pressure, temperature, backpressure, connections, materials, standards/certification, quantity, inspection, documentation, delivery target and available RFQ/datasheet files.