Full Nozzle & Semi Nozzle Safety Relief Valves
The nozzle is a primary pressure- containing component in a safety valve that forms a part or all of the inlet flow passage.
- Full Nozzle: the nozzle forms all of the inlet flow passage and is typically threaded into the valve body.
- Semi Nozzle: the nozzle forms only a part of the inlet flow passage and is typically not removeable from the valve body.
Most nozzles and discs are made from stainless steel to avoid corrosion and to ensure trouble free valve performance. Selection of other corrosion resistant materials may be advisable depending on the application.
The valve inlet (or approach channel) design can be either a full-nozzle or a semi-nozzle type. A full-nozzle design has the entire ‘wetted’ inlet tract formed from one piece. The approach channel is the only part of the safety valve that is exposed to the process fluid during normal operation, other than the disc, unless the valve is discharging.
Full-nozzles are usually incorporated in safety valves designed for process and high pressure applications, especially when the fluid is corrosive.
Conversely, the semi-nozzle design consists of a seating ring fitted into the body, the top of which forms the seat of the valve. The advantage of this arrangement is that the seat can easily be replaced, without replacing the whole inlet.
The disc is held against the nozzle seat (under normal operating conditions) by the spring, which is housed in an open or closed spring housing arrangement (or bonnet) mounted on top of the body. The discs used in rapid opening (pop type) safety valves are surrounded by a shroud, disc holder or huddling chamber which helps to produce the rapid opening characteristic.
The basic elements of the design are right angle pattern valve body, inlet can be either a full nozzle or a semi-nozzle type. With a full nozzle design has the “wetted” inlet tract formed from one piece (as per figure 6) with the seat integrated into the top of the nozzle. The internal bore of the nozzle and the disc is the only part of the valve that is exposed to the process fluid with the valve in the closed position.
A full-nozzle design has the entire 'wetted' inlet tract formed from one piece. The approach channel is the only part of the safety valve that is exposed to the process fluid during normal operation, other than the disc, unless the valve is discharging.
A semi-nozzle design consists of a seating ring fitted into the body.The disc is held onto the seat by the stem, with the downward force coming from the compression on the spring mounted in the bonnet. The amount of compression on the spring is adjusted by the spring adjuster under the cap.
Criteria for Selecting a Full Nozzle or Semi Nozzle Design
Criteria | Full Nozzle | Semi Nozzle |
---|---|---|
Regional Preference | USA and regions where API is the dominant standard | USA for ASME I applications |
Design Standard | Design per API 526 / API 520 required | No design standard required |
Application(corrosion) | Together with a disc in stainless steel, all permanently wetted parts have excellent corrosion resistance. In corrosive process applications selection of a carbon steel body material is possible. | The valve body will be permanently in contact with the medium. A carbon steel body material can be selected for non-corrosive applications, e.g. utility, air, steam, water. A stainless steel body material should be selected for corrosive process applications. |
Pressure | All pressure ranges | Max set pressure approx. 100 bar / 1450 psig |
Capacity | In most cases according to API orifice designations | In most cases full bore designs with maximum capacity relative to the valve size |
Repair | A full nozzle is typically removable and can either be replaced or repaired outside of the valve body | A semi nozzle can typically not be removed. Repair is possible inside of the valve body with lapping tools. To repair major damages of the seat, the complete body must be taken on a lathe. |
Cost | Full nozzle designs require more machining and material and are less cost effective than semi nozzle designs | A semi nozzle design requires less machining and less material than a full nozzle design thus being more cost effective |