Let’s be honest—valve bonnet types don’t often get the spotlight. Yet, ask anyone in maintenance, operations, or engineering, and they’ll tell you: the wrong bonnet can lead to leakages, access headaches, or worse—total valve failure. The bonnet isn’t just a cap or cover.
It’s a critical part of the body-bonnet joint that houses the stem and connects everything from packing to actuators. In short, the bonnet is the unsung hero of leak prevention and valve maintenance success.
In this guide, we’re going beyond the textbook. We’ll explore welded bonnet vs bolted designs, screwed bonnet use, and how pressure-seal bonnet types perform under different conditions. More importantly, we’ll explore how bonnets interact with other parts, and see how bonnet types affect serviceability—two things that really matter in the real world.
So whether you’re an engineer selecting new units or a technician replacing old ones, this bonnet selection guide will give you clarity where it counts.
Bolted Bonnet Design: The Reliable Workhorse of Valve Bonnet Types
When most people picture a valve bonnet, they’re probably thinking of a bolted bonnet. It’s one of the most common valve bonnet types used across industries because of its straightforward assembly, solid strength, and easy accessibility.
Why it works
- Maintenance access is simple—just unbolt and inspect.
- It’s great for moderate design pressure
- You can inspect and reseal packing or replace gaskets easily.
But here’s the flip side: bolting torque needs to be just right to maintain bonnet sealing. Too loose and you risk leaks; too tight and you can distort the flange. And like all bolted joints, they’re vulnerable to thermal expansion and contraction, especially in systems with big temperature swings. Still, for most medium-pressure, frequently serviced applications, the bolted bonnet is your go-to.
Welded Bonnet Vs Bolted: A Tough Choice for High-Stress Environments
Now let’s talk about welded bonnet vs bolted configurations. When the environment is high-pressure, high-temperature, or high-risk, welded bonnets shine. Why? Because they offer a permanent, leak-free seal.
Key strengths of welded bonnets
- No gasket or bolting to fail—strong, seamless construction
- Ideal for internal pressure extremes and hazardous materials
- Superior leak prevention in corrosive or radioactive environments
You do lose access to maintenance, though. After welding, it cannot be opened again without the use of cutting tools and downtime. Therefore, a welded bonnet can be the best option if you are working with a crucial safety valve in a deep-sea operation or nuclear plant.
However, what if a municipal water plant’s gaskets are changed once a month? Choose the fastened option instead.
This is where we really see how bonnet types affect serviceability. It’s not just about strength—it’s about access, downtime, and practicality.
Screwed Bonnet Use in Compact, Low-Pressure Valve Applications
Not every valve needs to stand up to 1,500 psi or temperatures over 800°F. For smaller, low-pressure systems, screwed bonnet use still makes perfect sense. These bonnets are cost-effective, space-saving, and fast to assemble—especially in systems where you’re not expecting frequent access.
Use cases for screwed bonnets
- Compact valves in instrumentation or utility lines
- Systems with minimal vibration or pressure fluctuation
- Temporary setups or low-duty cycle operations
That said, be mindful: bonnet sealing in screwed bonnets depends heavily on thread precision. Any wear or misalignment can cause micro-leaks. Also, if the valve is exposed to thermal expansion, threads can lock up, making disassembly a chore.
As always, consider how this bonnet application in valves aligns with the bigger picture—what pressures you’re dealing with, how often you need to service, and what failure modes you can afford.
Understanding Pressure-Seal Bonnet Types For High-Pressure, High-Temperature Service
Of all valve bonnet types, none is quite as specialized—and powerful—as the pressure-seal bonnet. These bonnets actually use internal pressure to seal themselves tighter. The higher the pressure inside the valve, the better the seal. Genius, right?
What makes pressure-seal bonnets unique
- Exceptional at sealing under extreme pressure
- Ideal for steam systems, power plants, and boiler feed water lines
- Reduced need for bolting torque or massive gaskets
But as with all things genius, they’re not always user-friendly. You need experienced hands to install and inspect them properly, and they aren’t ideal for applications needing quick or frequent valve maintenance. Still, when leak prevention is a non-negotiable—and it often is in critical systems—these bonnets deliver peace of mind.
The Bonnet May Be Small, but It Holds The Whole System Together
You wouldn’t build a house with the wrong foundation—so don’t run your system with the wrong bonnet. Whether you’re deciding between a bolted bonnet for routine maintenance or a pressure-seal bonnet for safety-critical use, your choice will directly affect leak prevention, service time, and long-term reliability.
By taking time to understand welded bonnet vs bolted pros and cons, proper screwed bonnet use, and where bonnet sealing matters most, you’re protecting your team, your system, and your bottom line. And remember: bonnets don’t exist in isolation. They must fit perfectly into a system—aligned with internal pressure, thermal expansion, and every other stress your valve sees.
Still unsure which bonnet your operation truly needs? Don’t worry—we’ve got your back.
Get expert advice on choosing the right bonnet for your unique application. Let’s keep your valves tight, your system flowing, and your team safe.