Pressure Relief Valve Symbol

Introduction

Looking for a clear reference on the Pressure Relief Valve (PRV) symbol used in P&ID and engineering drawings? Shrinathji Infracon explains common drafting standards, how to read PRV callouts, and typical variations (spring-loaded, pilot-operated, and pressure-vacuum types) so designers and maintenance teams can interpret diagrams quickly and accurately.

Description

In most P&ID conventions, a PRV symbol shows a valve body connected from the process line to a relief outlet. A diagonal spring mark over the valve indicates a spring-loaded setpoint. A small sensing line drawn to a pilot box indicates a pilot-operated relief valve. For storage silos and tanks, a pressure-vacuum relief valve (PVRV) symbol includes bidirectional arrows or dual seats to show relief on both over-pressure and vacuum conditions.

Symbol Elements & Interpretation

Symbol Element	Meaning on P&ID
Valve body with outlet to flare/vent	PRV diverts fluid when set pressure is exceeded; outlet typically to safe vent, flare, or atmosphere.
Diagonal spring mark over valve	Spring-loaded PRV; opening set by calibrated spring (set pressure noted in callout, e.g., 0.2 bar(g)).
Pilot box + sensing line	Pilot-operated PRV; pilot controls main valve lift for higher capacity and precision.
Dual-seat/bi-directional arrows	Pressure-vacuum relief (PVRV) for tanks/silos; relieves over-pressure and vacuum conditions.
Tag bubble e.g., PSV-101	Instrumentation tag: device type + loop number (PSV/PRV/PVRV); spec sheet details set pressure, size, service.

How PRV Symbols Differ by Type

Common Variants

Safety Valve (SV) / PSV: Typically used on gas/steam; symbol resembles PRV with emphasis on rapid lift.
Relief Valve (RV): Often for liquids; similar symbol but called out as RV with gradual opening characteristic.
Pilot-Operated PRV: Pilot box + sensing line shown; suitable for high backpressure or large capacity.
PVRV (Tank Breather): Dual action symbol indicating pressure and vacuum pallets for tanks/silos.
Rupture Disc (RD): Adjacent triangle/diaphragm symbol; may be shown in series with PRV for fire case protection.

Where These Symbols Are Used

P&ID Drawings: Process plants, utilities, and storage systems to define over-pressure protection.
Silo/Tank GA Drawings: PVRV and flame arrestor representation on roof nozzles for bulk powder storage.
Cause & Effect (C&E): Interlocks and alarm references linked to PRV/PVRV locations and service.

Why Correct Symbols Matter

Accurate PRV/PVRV symbols avoid ambiguity in design, procurement, and maintenance. They ensure the installed valve type, size, set pressure, and outlet routing match the safety basis, helping auditors, contractors, and plant teams align on intent without costly rework.

Manufacturer & Support Information

Shrinathji Infracon supports selection and integration of pressure-vacuum relief valves for silos and tanks along with accessories like dust collectors, level switches, aeration pads, and safe access systems—ensuring your drawings and installed hardware match.

FAQs

1. PRV vs PSV vs RV symbols—are they interchangeable?

They look similar, but tags differ by service: PSV (gas/steam, pop action), RV (liquid, modulating), PRV (generic). Follow your project legend.

2. How is a PVRV symbol shown for tanks?

As a bi-directional relief device (pressure & vacuum). Many legends show dual seats or arrows; tag as PVRV with set points for ±pressure.

3. What does the pilot box mean on a PRV symbol?

It indicates a pilot-operated PRV where a small pilot valve senses pressure and controls the main stage for precise opening.

4. Where do set pressures appear on drawings?

Typically in the equipment list or valve spec sheet referenced by the tag (e.g., PSV-101: Set 0.2 bar(g), 2" x 3", service N2).

5. How is the discharge destination shown?

The outlet line leads to flare, vent stack, or safe area; look for line spec and destination notes on the P&ID.

Conclusion

Understanding the pressure relief valve symbol helps engineers read P&IDs correctly, choose the right device (PRV/PSV/PVRV), and verify safe routing to vent/flare. Use your project’s symbol legend to stay consistent across drawings and site execution.

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Pressure Relief Valve Symbol