Spring-loaded pressure relief valves

Where it is used

Spring-loaded valves are the default choice for most process relief duties. The type tests on file cover steam, gas and liquid, and cryogenic service down to -196 °C. The materials list runs from carbon steel to nickel alloys.

Choose LFO, the conventional type, when back pressure is low and steady and the medium does not attack the spring and guide. Choose LFB, the bellows type, when back pressure varies or the medium is corrosive. The bellows isolates the spring and the upper trim from the medium and keeps the set pressure steady as back pressure changes.

Technical data

Series

LFO conventional
LFB balanced bellows

Inlet size

NPS 1 to 8 (DN25 to DN200)

Orifice

API 526 letters D to T, 1"D2" to 8"T10"

Pressure rating

Class 150 to Class 2500 ASME flanges
PN16 to PN400 PN flanges

Temperature range

-268 to +720 °C (-450 to +1,328 °F)

Set pressure

Confirmed in the sizing for each tag number

Set pressure tolerance

±3% of set pressure

Overpressure

10% of set pressure, maximum

Blowdown

10% of set pressure, maximum
Adjusted with the nozzle ring only

Seat tightness

API 527

Seat and disc

Stellite hard facing on the nozzle seat and the disc

Nozzle

Full nozzle, screwed into the body

Body and trim materials

Carbon steel, austenitic and martensitic stainless steel, duplex and super duplex stainless steel, 6Mo, Alloy 400, Alloy B, Alloy C, Alloy 625, Alloy 825

These are the limits of the series. The limits of one valve depend on its size, rating, and materials. Service near either end of the temperature range needs special materials. Our own cryogenic tests go down to -196 °C. For service below that, we source the materials through our supplier network and arrange the tests with a third-party laboratory. The sizing confirms the limits and the materials for each tag number. The Class range and the PN range are the ranges we offer with ASME flanges and with PN flanges. One is not a conversion of the other.

Structure

The full nozzle screws into the body and forms the inlet passage and the seat in one part. The seat face of the nozzle and the disc are hard faced with Stellite.

Blowdown is adjusted with the nozzle ring only. Blowdown is the drop below the set pressure at which the valve reseats.

On the LFB, a bellows sits between the disc holder and the bonnet. It seals the spring, spindle, and guide from the medium. It also balances the back pressure that acts on the disc.

A lifting lever, shown in the cutaway, lets the operator lift the disc off the seat for a function check.

Options

Choose these at the inquiry stage. The sizing confirms which ones apply to your tag:

• Lifting lever, as shown in the cutaway
• Balanced bellows, series LFB
• Body and trim materials from the table, including nickel alloys for corrosive media
• Inlet and outlet ratings from Class 150 to Class 2500

Standards and certifications

Seat tightness is tested to API 527. Orifice sizes use the API 526 letters D to T. These certificates apply to the series:

What we need to quote this valve

Send these with your inquiry. A completed data sheet covers all of them, and the PSV data sheet page explains each line:

1. Medium and phase (gas, vapor, liquid, or two-phase)
2. Required relieving capacity
3. Set pressure
4. Back pressure at the outlet
5. Operating and relieving temperature
6. Inlet and outlet size and rating
7. Material requirements
8. Design code (ASME, API, EAC, or the project specification)
9. Quantity, per tag number
10. Project country

No data sheet yet? Send what you have. We ask for the rest by email.

Related series