Safety Valves · Purchase-decision comparison
Balanced bellows vs pilot-operated safety valves
Both executions protect a vessel or line at a set pressure — the procurement decision is which one handles your back pressure, your size and your testing basis. This table compares the two safety-valve subtypes we publish specification pages for, using only fields that mean the same thing and are collected on the same basis for both. Where a figure is not established by the collected evidence, the cell says so and asks you to confirm it at RFQ.
Last updated: 2026-11
Comparison table
A row appears only where the field means the same thing and is collected on the same basis for both columns. A cell without collected evidence says "confirm at RFQ" — it is never filled with an assumption, and it never implies either execution cannot be supplied.
Quick decision
These conclusions apply to the two executions compared on this page — not to the safety valves family as a whole. Other executions of the same family differ, and any value marked "confirm at RFQ" is settled by the quotation, not by this table.
- Back pressure varies or is hard to pin down → state the calculated back pressure on the RFQ; neither execution tabulates a verified back-pressure tolerance in the current evidence.
- A short-blowdown, full-lift operating profile matters → evaluate the pilot-operated execution, whose stated profile is full-lift at set pressure with short blowdown.
- The final selection is always confirmed against your relief capacity, back pressure, medium and code basis — state them on the RFQ.
| Field | Balanced Bellows Safety Valve | Pilot-Operated Safety Valve |
|---|---|---|
| Working principle | ||
| Working principle Same meaning across columns: how overpressure is relieved and what balances back pressure. | Not established by the collected evidence for this execution — confirm the working principle and back-pressure behaviour at RFQ Unverified — confirm at RFQ | Main valve + external pilot; full-lift at set pressure with short blowdown |
| Ratings & size | ||
| Size range Same basis (nominal diameter). The pilot-operated range is tabulated as inlet × outlet, with the outlet flange the larger of the pair. | Size range is not established by the collected evidence for this execution — confirm at RFQ Unverified — confirm at RFQ | DN25–DN200 (1 in × 2 in – 8 in × 10 in, inlet × outlet) |
| Pressure class | Pressure class is not established by the collected evidence for this execution — state your class requirement and confirm at RFQ Unverified — confirm at RFQ | ANSI Class 150–600 / PN16–PN100 |
| Set pressure range Same basis (set pressure span); the sizing element differs between the executions and is confirmed with the selection at RFQ. | Set pressure range is not established by the collected evidence for this execution — state your set pressure and confirm at RFQ Unverified — confirm at RFQ | Set pressure range is not established by the collected evidence for this execution — state your set pressure and confirm at RFQ Unverified — confirm at RFQ |
| Performance | ||
| Superimposed back pressure Same meaning: how much superimposed back pressure the execution tolerates. Neither execution tabulates a verified tolerance band in the current evidence — state your calculated back pressure on the RFQ. | Back-pressure tolerance is not established by the collected evidence for this execution — state your calculated back pressure and confirm at RFQ Unverified — confirm at RFQ | The tabulated back-pressure tolerance is not established by the current evidence — state your calculated back pressure and confirm at RFQ Unverified — confirm at RFQ |
| Blowdown Same meaning: how far the inlet pressure falls after lift before reseating. Not tabulated for the bellows execution in the collected evidence. | Unverified — confirm blowdown setting at RFQ Unverified — confirm at RFQ | Short blowdown (stated as part of the pilot-operated design) |
| Materials & connections | ||
| Body materials Same meaning (available body constructions); the balanced bellows list is not established in the current evidence — confirm the offered grade at RFQ. | Body materials are not established by the collected evidence for this execution — confirm at RFQ Unverified — confirm at RFQ | WCB, CF8, CF8M |
| End connections Same meaning; the balanced bellows execution's connection options are confirmed at RFQ. | End connections are not established by the collected evidence for this execution — confirm at RFQ Unverified — confirm at RFQ | Flange (RF) |
| Service & verification | ||
| Maintenance scope Not tabulated for either execution — recorded as unverified, not as a supplier limitation. | Unverified — confirm maintenance scope and spare parts at RFQ Unverified — confirm at RFQ | Unverified — confirm maintenance scope and pilot calibration interval at RFQ Unverified — confirm at RFQ |
- Specified
- Differs — read the note
- Unverified — confirm at RFQ
- Conditional — confirm at RFQ
How to choose
Choose the balanced bellows valve when back pressure varies
The balanced bellows execution is the candidate for installations where outlet pressure is not constant, but the collected evidence currently establishes no tabulated figures for it — size, class, set-pressure span, materials and connections all render as confirm-at-RFQ below. Describe the installation (medium, required capacity, back-pressure case) on the RFQ and the execution and its figures are confirmed in the quotation.
Choose the pilot-operated valve when back pressure is high or blowdown matters
The pilot-operated execution states a full-lift, short-blowdown operating profile; its tabulated back-pressure tolerance is currently pending confirmation, so state your calculated back pressure on the RFQ rather than assuming a band. Note the mechanical consequence visible in the size table: the connection is tabulated as inlet × outlet, and the outlet flange is the larger of the pair, so piping and supports should be dimensioned for it. Ends are flange (RF).
Verification boundary
Values in this table are stated only where the collected evidence supports them; cells marked unverified name the property to confirm instead of a figure. The binding figures for your installation — set pressure, blowdown, capacity and materials — are confirmed at quotation. Each column links to the full specification page for its execution.
Frequently asked questions
When does back pressure decide between the two types?
When your outlet sees built-up or variable pressure — a shared flare header, a long outlet line, or a discharge into a process at pressure. Neither execution tabulates a verified back-pressure tolerance in the collected evidence, so the comparison on this row is by name and principle, not by figure: state the calculated back pressure, the set pressure and the medium with the RFQ and the suitable execution — and its tolerance — are confirmed in the quotation against figures.
Both columns show the same subtype names — are the valves interchangeable?
No. The two columns also stand on very different evidence footing in the current table: the pilot-operated figures are as tabulated on its specification page, while several balanced-bellows cells — size, class, set-pressure span, materials, connections — are marked unverified pending confirmation. Overlapping names never imply interchangeability in any case: the pilot-operated valve is a main valve plus an external pilot that opens full-lift with short blowdown. Installation orientation and testing basis are confirmed per case — this table is a selection orientation, not a substitution approval.
Why do some cells say "Unverified — confirm at RFQ" instead of a figure?
This page only states figures the collected evidence supports. Where a property is not established for an execution — the balanced bellows ratings and materials, the back-pressure tolerance of either execution, blowdown for the bellows valve, maintenance scope for both — the cell is marked unverified rather than filled with an assumed number. Attach your set pressure, back pressure, medium and temperature to the RFQ and the figures are confirmed in the quotation.
Individual execution specification pages
Each execution compared above has its own page — full specification table, selection guidance and RFQ details.
Ordering & documentation
Documents
Ask for the documentation set with the inquiry — typical items (material certificates, test reports to the listed inspection standard, dimensional drawings) are confirmed and supplied with the quotation.
Quotation
Send the medium, temperature, pressure, size and connection for a quotation-ready inquiry. Lead time, MOQ and price are confirmed at quotation, together with the documents to be supplied with the order.
Sample & supply
Supply terms, order quantities and lead time are agreed per order — state the destination country and required schedule with the inquiry.
Storage & shipping
Store valves with seats and seals protected from UV, ozone and deformation, and keep end-face protectors on until installation; storage limits and shipping arrangements for the selected materials are confirmed at quotation.
Knowledge: selection guides
Request a quote
Send size, pressure class, set pressure and service conditions for a quotation-ready inquiry — the execution is confirmed against figures, with documents and terms.
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