In short: It is the variant of the STD disc (solid domed metal, tension-loaded) prepared for mounting in reusable screw-type disc holders or in disposable disc holders. It works the same as the flanged STD; what changes is the mounting system and, in small flat-seat sizes, the recommended operating margin.
How it works
The principle is that of the conventional STD: an unscored metal dome receives pressure on its concave face and, on reaching the differential burst pressure, opens completely in milliseconds. There are no moving parts or soft seals.
The difference lies in where it is housed. In a screw-type disc holder the disc is clamped between two threaded parts, a compact solution common in small lines, laboratory equipment, pilot plants or process skids. The threaded disc holder is reused and only the disc is replaced. In the disposable option, the disc and its support form an assembly that is replaced as a whole, which speeds up emergency changeouts and avoids reassembly errors.
Like any tension-loaded disc, it can be inverted if pressure arrives on the convex side. The manufacturer is categorical: if there is any possibility of full vacuum, the vacuum support is essential.
Operating ratio
The operating ratio is the quotient of normal operating pressure and the marked burst pressure. For this model, the manufacturer recommends not exceeding 70 % of the nominal burst pressure on ½" and 11/16" flat-seat STD discs, as with the 30° seat version. Operating above that value accelerates metal fatigue and promotes premature bursts, especially when pressure fluctuates frequently.
When it makes sense (and when it does not)
- Recommended when the connection is threaded and there is no room or justification for a flanged disc holder.
- Suitable if quick changeover is valued, whether of the disc (screw-type disc holder) or of the complete assembly (disposable).
- Suitable in static, pulsating or cyclic service within the indicated 70 % margin.
- Not recommended if you need to operate very close to the burst pressure; a reverse-acting disc allows more margin.
- For insert-type disc holders (for example, with LL seat), the recommended version is the base STD.
Data to specify it
- Disc holder type: reusable screw-type or disposable, and its size.
- Seat type (flat or 30°).
- Burst pressure, temperature and normal operating pressure.
- Possibility of vacuum, so the support can be included.
- Material compatible with the fluid.
To go deeper: Installation and disc holders and Rupture disc maintenance and replacement.
Descriptive sheet for educational purposes. For availability and a quote for this model, consult P&V Control Devices.