Pressure relief valves are essential safety devices that protect equipment in pressurized systems from rupture or damage due to accidental overpressure. They achieve this by opening at a preset pressure to release excess pressure and then closing when the normal working pressure is restored.
There are two main types of pressure relief valves – full nozzle and semi nozzle:
What is a Full Nozzle Pressure Relief Valve?
In a full nozzle valve, only the nozzle and disc are exposed to the fluid media when the valve is closed. The full nozzle inlet is machined from a single piece of material and forms the entire inlet tract and seat area.
As the name suggests, the full nozzle material sees the full pressure and flow during operation.
The key advantages of full nozzle valves are:
- Made from materials highly resistant to the fluid media
- Suitable for handling aggressive, toxic and corrosive process fluids
- Can be used for high pressure applications
- Highly polished flow surfaces prevent deposits and blockages
Full nozzle valves also tend to have better alignment due to the unitary nozzle/seat construction.
What is a Semi Nozzle Pressure Relief Valve?
In semi nozzle valves, the nozzle is made as a separate component with a flange or threads that fit into the valve body. The nozzle, disc and parts of the valve body are thereby exposed to the process fluid during operation.
Semi nozzle PRVs have certain advantages over full nozzle valves:
- The removable/replaceable nozzle simplifies maintenance
- Body can be made from cheaper, standard materials
- Used in less demanding applications involving non-toxic fluids at lower pressures
The semi nozzle PRV may suffer from minor misalignment issues and is more prone to corrosion over time.
How Are Full Nozzle and Semi Nozzle PRVs Different in terms of Design and Construction?
While both types of valves have similar overall construction in terms of body, cap, disc and nozzle; there are some notable differences:
Nozzle Construction
In full nozzle PRVs, the inlet nozzle forms an integral one-piece component with the seat.
The semi nozzle valve has a removable flanged nozzle that fits into the valve body. Misalignment between the nozzle and seat can therefore occur over time.
Threads and Connection
Full nozzle valves may have top, middle or bottom inlet threads to connect the nozzle to the body. Each method has pros and cons in terms of ease of assembly, alignment, drainability, etc.
Semi nozzle PRVs often use a crimped or pressed fit for securing the nozzle to the valve body. The nozzle can also be screwed into place in some designs.
Guiding Surfaces
The polished flow surfaces prevent accumulations and blockage in full nozzle valves. Semi nozzle PRVs are more prone to corrosion and deposits over time which may jam the valve.
Blowdown Rings
Full nozzle PRVs can be easily fitted with internal blowdown rings to fine tune the lift characteristics. It is more challenging to incorporate such blowdown adjustment devices in semi nozzle valves.
Standards Compliance
Full nozzle valves more easily comply with stringent performance standards specified by ASME Section I. Semi nozzle PRVs are more commonly used with ASME Section VIII and EN standards.
When Should You Choose a Full Nozzle PRV Over a Semi Nozzle Valve?
You should select a full nozzle pressure relief valve when:
Your Process Involves Aggressive or Toxic Fluids
Full nozzle valves allow only the disc and nozzle to contact the corrosive fluid media. The body can be made from cheaper materials not resistant to the fluid. This provides a cost saving while protecting the integrity of the valve internals.
You Have a High Pressure Application
At higher pressures, the unitary construction provides better alignment of critical sealing surfaces like the nozzle and disc. Misalignment in semi nozzle PRVs can lead to leakage issues.
Your Process Media Tends to Solidify, Crystallize or Polymerize
The highly polished flow paths prevent any build-up or accumulations that could restrict flow or jam the valve. Semi-nozzle valves are more prone to fouling and clogging over extended use.
When is a Semi Nozzle PRV the Better Choice?
You should go for a semi nozzle pressure relief valve in the following situations:
You Want Easy Maintenance and Lower Costs
The replaceable nozzle is simpler and cheaper to maintain compared to the integrated nozzle in full-nozzle PRVs. Additionally, the body in semi nozzle valves can be fabricated from lower cost materials.
Your Design Codes Mandate a Particular PRV Configuration
Sometimes, the governing pressure vessel code may dictate the use of a semi nozzle PRV. For example, EN and DIN codes often allow semi nozzle valves unlike ASME Section I.
You Have a Relatively Low Pressure Application
While full nozzle PRVs tend to be used for pressures above 400 bar, semi nozzle valves reliably operate below 100 bar. So if your system design pressure is low, a semi nozzle valve represents better value.
You Operate Non-Toxic, Less Aggressive Fluids
Regular fluids like water, steam or air can be handled by semi nozzle PRVs. The consumable nozzle is cheaper to replace compared to an entire full nozzle valve.
Full Nozzle vs Semi Nozzle: Making the Optimal Choice
When sizing and selecting a pressure relief valve, engineers have to choose between full nozzle and semi nozzle configurations carefully after considering these key selection criteria:
- Fluid properties – Toxicity, corrosion potential, solids content, etc.
- Design and max allowable pressures
- Prevailing industry standards and code compliance needs
- Level of maintenance access and cost considerations
- Desired valve service intervals before replacement
While full nozzle PRVs are favored for high pressure, toxic applications; semi nozzle valves provide a maintenance friendly and lower cost option for less demanding applications.
By matching the pressure relief device to your specific process conditions and constraints; you can maximize valve service life while optimizing safety, reliability and costs.
Full Nozzle vs Semi Nozzle: Key Takeaways
Full nozzle and semi nozzle describe the pressure relief valve inlet/nozzle construction which exposes internals to the process media during flow.
In full nozzle PRVs:
- Entire inlet nozzle is machined from a single piece of resistant metal
- Provides smooth surface and better alignment
- Used for toxic, aggressive and high pressure applications
Semi nozzle pressure relief valves have:
- A removable inlet nozzle fitted to the valve body
- Exposure risks with standard body materials
- Used for easier maintenance and cost savings
Engineers have to weigh parameters like process conditions, codes and standards, maintenance needs and lifecycle costs when choosing between full nozzle and semi nozzle PRVs.