In short: The POS-A-SET® is a dual-action rupture disc: at a single installation point it relieves very low positive pressures, from 1 inWC, and protects the equipment against moderate vacuum. Its flat, easy-to-clean seat makes it well suited to hygienic processes.
How it works
Many low-pressure tanks and vessels need two protections: one against overpressure, for example when filling or heating, and another against vacuum, when emptying or cooling. The usual approach is to install two devices. The POS-A-SET combines both functions in a single piece.
In the positive direction, the disc has perforations and a pre-cut section that allow it to open at very low pressures; the manufacturer states values as low as 1 inWC (about 0.036 psi), that is, the pressure of a one-inch water column. In the negative direction, relief is provided by a band-and-blade assembly, which acts at a vacuum somewhat greater than the positive opening pressure. Both the pre-cuts and the blade aim for a controlled opening with no fragments.
The manufacturer presents it as the inverted configuration of its CAL-VAC model. It supports static, cyclic or pulsating service and is offered with fluoropolymer or metal seals depending on the required chemical compatibility.
When it makes sense (and when it does not)
- Suitable for very low-pressure tanks and equipment that must be protected in both directions and have little room for connections.
- Suitable for food or pharmaceutical plants, thanks to its flat seat that limits product buildup and eases CIP/SIP cleaning. According to the manufacturer, it helps meet FDA sanitary requirements; that assessment is the responsibility of the user and their quality system.
- Recommended to reduce cost and installation effort compared with mounting two separate discs.
- Not suitable if what you need is for the vacuum side to be the more sensitive one; in that case it is worth asking about the inverted configuration (CAL-VAC).
- Not suitable for medium- or high-pressure vessels: it is a device designed for very low pressures.
Data to specify it
- Positive burst pressure and coincident temperature.
- Vacuum pressure at which the negative side must open.
- Normal operating pressures and whether the service is static, cyclic or pulsating.
- Nominal diameter (from 1" / 25 mm) and connection type.
- Seal material: fluoropolymer or metallic.
- Whether a non-fragmenting version is required.
| Characteristic | Manufacturer data |
|---|---|
| Function | Dual action: overpressure and vacuum |
| Minimum positive pressure | From 1 inWC (0.036 psi) |
| Vacuum side | Knife-blade band; opens at a vacuum slightly higher than the positive pressure |
| Sizes | From 1" (25 mm) |
| Seals | Fluoropolymer or metal |
| Identification | Flow direction tag |
Learn more: Sanitary rupture discs and Types of rupture discs.
Descriptive sheet for educational purposes. For availability and a quote for this model, consult P&V Control Devices.