[NEFSS Technical Series #03] How Does a PSV Operate? A Complete Comparison: Full Bore vs. Lift

nick so
Jan 15
2 min read

Hello, this is Nick from NEFSS.

In our last session, we explored the primary types of Pressure Safety Valves (PSVs) and the critical concept of Back Pressure.

Today, we will examine the step-by-step operating mechanisms of PSVs and how the flow of fluid varies by type. Furthermore, I will explain the differences between Full Bore and Lift types—concepts that are often confusing—using simple analogies.

1. Step-by-Step Operating Principle of a PSV

To help you understand, I have illustrated the areas in contact with the fluid during each stage. (Please note that the fluid-contact areas differ between the CONVENTIONAL TYPE and the BELLOWS TYPE during operation.)

■ CONVENTIONAL TYPE

**Operating Principle of Conventional Safety Relief Valve (by NEFSS)**

■ BELLOWS TYPE

**Operating Principle of Bellows Safety Relief Valve (by NEFSS)**

[Summary] The key difference in fluid flow: In a CONVENTIONAL TYPE, the fluid flows into the bonnet, contacting the spring, stem, and other internal components. In contrast, in a BELLOWS TYPE, the fluid is isolated from the bonnet, protecting the internal parts.

2. Full Bore vs. Lift Types

According to industrial codes such as KS B6216, these are defined by the ratio of lift to seat diameter. To make it easier to understand, let’s use a "Water Faucet" analogy.

[Analogy: Faucet and Flow]

Photo C: Even if the main passage (A) is wide, if the nozzle (B) is narrow, the flow is limited by B. (A > B → Flow = B)
Photo D: Even if the nozzle (B) is wide, if the main passage (A) is narrow, the flow is limited by A. (A < B → Flow = A)

The takeaway is that the flow rate is determined by the most restricted part of the path.

① Low Lift Type

In a Low Lift valve, the space created by the lifted disc is narrower than the internal seat area. Therefore, the Lift (L) determines the total discharge capacity.

Calculation for Effective Area : A = π × D × L (L: Lift / Distance the disc is raised)

② Full Bore Type

In a Full Bore valve, the space created by the lifted disc is much larger than the internal seat area. Therefore, the Nozzle Throat (Orifice) Area determines the flow.

Calculation for Effective Area : A = π · d² / 4 (d: Throat diameter)

💡 Closing Thoughts

Generally, most FLANGE TYPES are Full Bore and most THREAD TYPES are Lift. However, this is not a universal rule. Some manufacturers offer Full Bore designs even in Threaded types. We recommend consulting with the manufacturer (MAKER) for precise selection.

🛠️Technical Consultation & Request for Quotation

If you are having difficulty selecting the right safety valve or need a fast and reliable quote, please feel free to contact NEFSS at any time!

Technical Consultation & Request for Quotation

[Coming Next] Now that we understand how they work, it's time to identify them on-site. In our next session, [Part 4: External Components and Trim for Field Practitioners], we will cover the importance of Face-to-Face dimensions, the four types of Bonnet/Lever configurations, and the purpose of the Test Gag.

At NEFSS, we go beyond simple material supply to become a partner that cares about your site's safety.

Thank you. Best regards, Nick, NEFSS