Valve Bonnet – A Complete Beginner’s Guide

Valve bonnet


Have you ever dealt with applications like wastewater treatment, water supply, gas supply, and fire protection in large industries?

If the answer is yes, you may also have seen the valves.

These are primarily used in power generation and fuel handling systems.

Valves are the components that help you control a system’s pressure and flow.

Moreover, valves are essential components in the piping system.

Any system that conveys gases, liquids, vapors, and slurries, requires a valve bonnet.

Like there are different types of valves in the market, they are composed of other parts.

The valve bonnet is an essential part of a valve.

In this article, we will explain the valve bonnet.

But let’s take a look at some main functions of a valve. These are used for:

  • Stopping and starting flow
  • Controlling the flow direction
  • Increasing or decreasing a flow
  • Relieve a pipe system of a certain pressure
  • Regulation of a flow or process pressure

You must know that the valve body has several parts, but the valve bonnet is the main element.

The whole functioning of a valve depends on the valve bonnet.

So let’s look at a valve bonnet and what it does.

Video – Valve Parts Explained (Industrial Engineering)

What Is A Valve Bonnet?

The valve bonnet is a part of the valve body that covers the opening on top of the valve.

You must know that the valve bonnet is also called the pressure-retaining part of the valve.

Moreover, it is interesting to note that the bonnet and its parts are exposed to the operating fluid.

Therefore, they must have the ability to withstand operating pressure.

It is also said that your bonnet must be able to withstand the corrosive effects of the fluid.

Another essential thing to know is that the bonnet is made from the same material grade as the body.

In this case, it is either made from forged steel or cast one. This is done to avoid the placement of dissimilar metals in contact.

You must realize that the valve bonnet is the most critical part of a valve. The required level is much similar to the internal components of the bonnet. Some examples are the disc and stem in the valve body.

The reason is that the bonnet holds all these parts together.

All the parts are joined together through different mechanisms—for example, welding, bolting, or threading.

It is important to note that when maintenance is needed, the mechanics carry out the process by dismantling the bonnet.

As a result, they get access to the internal mechanisms and replace components like the stem and the seat.

You must know that there are multiple models and designs of valve bonnets.

Moreover, they are manufactured in forged steel or cast steel.

Important Fact: The internal components of a valve are collectively known as trim. They can be replaced for maintenance purposes.

Let’s explain different methods of joining the bonnet to the valve.

What Are Different Methods Of Joining Bonnet To The Valve?

First of all, it is essential to realize that the methods vary depending on the severity of the service.

This is because the valve bonnet is the part that carries pressure.

Moreover, it seals the fluid from the outside. Let’s look at different methods to join the bonnet to the valve:

  • Threaded connections
  • Manges
  • Unions
  • Welding
  • Bonnets as an integral part of valve bodies
  • Pressure sealed mechanisms

Based on the above methods, the types of valve bonnets also vary.

Let’s explain the different types of valve bonnets.

What Are the Different Types Of A Valve Bonnet?

Threaded Valve Bonnet

If you are looking for a low-cost design, you must learn about threaded valve bonnets.

It is important to note that this type has a straightforward structure.

However, you will find fragility in this setup. The rotating surface is closely attached to the gasket of the valve bonnet.

As a result, it causes friction during the loosening and tightening process.

And there are chances of leakage since the joint surface might be damaged.

At the same time, it has one more downside: the torque will increase when the size of the valve increases.

You must know that torque is required for tightening the valve.

Therefore, the threaded valve bonnet is limited to a tiny size of DN80 and even less than it.

Separate threaded collars are used in some versions to improve this design.

It helps to fasten the bonnet and the body of the valve.

Now even if the bonnet tightens, there will be no relative motion between the body and the bonnet at the joint surface.

This process can reduce the risks of leakages, but there will still be a restriction on the size of the valve body.

Therefore, threaded bonnets are primarily used in non-critical services.

Some examples are heating shut-off and plumbing.

Moreover, threaded bonnets are applied in low-pressure valves managing noncorrosive and non handling fluids with valves sizes less than 2 inches.

Threaded bonnets are very rare for control valves.

Having an economical construction, threaded bonnets also contain a low weight.

One major downside is that it is difficult to remove after a long service.

Welded Valve Bonnet

An interesting fact about welded valve bonnets is that they are more reliable than threaded connections.

This is because there are fewer chances or risks of leakage in welded joint connections.

Moreover, the template and pressure don’t limit the welded connections.

At the same time, it doesn’t restrict the size of the valve body.

However, there is one downside to it. And that is difficulty in its maintenance.

The valve body with welded valve bonnet is difficult to access at internal parts.

Therefore, welded valve bonnets are suitable for valves that don’t require maintenance after short intervals.

These bonnets are also suitable for valves that are difficult to assemble and disposable valves.

Important Fact: Some experts use threading and welding in valves to prevent leakage and provide strong protection.

Bolted Valve Bonnet

To span all sizes and operating pressure, you must look at the bolted valve bonnet.

However, it increases the weight and size of the valve with the increased operating pressure.

Apart from that, it faces another challenge load-bearing capacity has been reduced drastically. But it happens when the operating temperature exceeds 600 ℉ (350 ℃).

In such a case, welding of flanged connections is performed. It helps to bolster the bolts.

You must know that a bolted valve bonnet makes use of different gaskets.

But it depends on temperature, operating pressure, and process media.

Pressure Seal Valve Bonnet

If you are looking for an ideal design for your high-pressure applications, then you must look for this type.

A pressure seal bonnet is a unique bonnet design.

The joint surface faces compression due to the configuration of sealings rings in the bonnet and valve body.

Note that this condition happens when the internal pressure increases.

It is interesting to note that the sealing capability is increased due to the higher pressure of a pressure seal bonnet design.

The material for making pressure seal bonnet rings includes graphite with stainless steel wire re-enforcement.

You must know that graphite is highly flexible.

At the same time, stainless steel has good strength and anti-corrosive features.

This combination gives you a better seal in pressure-seal bonnets.

It is also interesting to note that the pressure seal bonnets remove the need for large and heavy structures.

You don’t need to have large valves to increase pressure levels.

The best part is easy assembling and dissembling since the valve body orifice helps insert and remove sealing rings.

The most important thing to note is that this design is mainly applied for valves that require frequent maintenance.

You must have to replace the pressure seal rings after the repair process.

Generally, pressure seal bonnets and bolted bonnets give the best results in high-pressure applications.

But you must look for pressure seal bonnets if there are very high operating temperatures.

There exists a bridge lock threaded engagement with the pressure-activated metal seal sign in this design.

The absence of a large bonnet flange is the most significant advantage of this design.

It has a stem seal or valve packaging in the upper portion.

The seal of this bonnet becomes tighter when there is an increase in fluid pressure.

How To Deal With Leakages & Repair Of Valve Bonnet?

Even if you are maintaining your bonnets, they may still face leakages.

It is essential to repair them immediately to prevent any significant loss.

So let’s look at the leakages and repair of valve bonnets:

You must know that the valve bonnet is the pressure-retaining part of the valve body.

Therefore, it is more prone to leakages. As a result, you need to perform its maintenance regularly.

The leakage may occur from the connection of the bonnet to the valve body or from the bonnet itself.

There are so many things that result in leakages. Let’s take a look at them:

  • First, it is essential to know that bonnets are manufactured by forging or casting.
    • If it is made from poor casting, it will result in leakage. Some examples are loose structures, blisters, and slag inclusions.
    • Impact loads may also cause leakages. These impact loads are mainly due to the hammer strikes.
    • Therefore, it is recommended to avoid impact strikes because cast iron is more prone to cracks.

  • The bonnet temperature becomes too low if you have ever dealt with cryogenic valve applications.
    • It can cause the packing to leak because of freezing.
    • As a result, the manufacturers specify extended bonnets with enough room for insulation.
    • An interesting part is that cracking occurs below minus 20 degrees F.

  • Leakages mainly occur at the bonnet body joint.
    • This happens primarily due to slag inclusions and stress cracks.
    • Such situations come into action due to poor welding.

For pressure seals, threaded, or bolted valves, leakage may occur from the uneven contact surface.

And that, too, happens due to corrosion: improper gasket installation, factory defects, and inadequate contact pressure.

Now that you know the causes of leakages, you must also look for ways to prevent them. Let’s explore the ways to avoid leakages.

Steps To Prevent Leakage In Valve Bonnets

  • First, it is essential to ensure the quality of the casting.
    • This is possible by performing some tests according to the industry’s regulations.
    • But you must ensure the tests before the whole installation process.
    • At that point, it is essential to take help from expert manufacturers and testers.

  • Secondly, the technicians should never place heavy loads over the bonnets.
    • Moreover, they should also avoid striking the bonnet.
    • If your valves have large diameters with heavy actuators, you must install them with brackets.
    • In such a case, a mounting kit is also recommended since it can bear the load easily.

  • You must know the fact that insulation is required in valves during cold temperatures.
    • Or it may also require supplemental heat.
    • Moreover, the technicians should ensure no liquid is in valves when they are out of service.

  • You must follow the right procedures when you fasten the bonnet to the valve body.
    • Moreover, you must specify the gaskets compatible with the process media.
    • At the same time, proper welding tests should be carried out.

This is how you can prevent leakages in your valve bonnets.

Final Thoughts

Now that you have read about bonnet valves, you must also learn about bonnetless valves.

However, you must realize that it is not a type of bonnet.

But bonnetless valves are those valves that don’t have any bonnet.

So you find that the valve body is the only pressure-retaining part.

The exciting part of knowing here is that there exists no point of leakage in the bonnet-body joint or gasket.

Moreover, you get quick maintenance of your bonnet.

You need to remove the body parts quickly.

The removal process takes only one step and allows for in-line repair.

A box opening helps us to perform the removal process.

This opening is large enough to pass all parts of the valve.

The best part to know here is that bonnetless valves help you to achieve a compact design with light weight.

Moreover, it is still considered suitable for applications with high pressure.

If you are looking for the proper valve manufacturer, we at INOXCN can provide a wide variety of stainless steel valves to choose from, which are from the finest quality material and offer various functions.

We are always ready to cater to you with the right ideas as to which material and type of valve will suit your business or industrial projects.

So get in touch today and have the accurate valves chosen for your systems.


What Is the Bonnet of a Valve?

The bonnet of a valve is the central part of the valve that works as a pressure retainer.

What Is the Bonnet in the Gate Valve?

It helps to give protection to the internal parts of the gate valve.

Moreover, it performs its function through a leak-proof seal.

You can also remove it for maintenance or repair purposes.

Gate valves can have different types of bonnets depending on the application.

What Are the Parts of the Valve?

Regardless of type, the valve body consists of the following parts:

  • Bonnet
  • Trim (internal parts)
  • Actuator
  • Packing

How Does a Pressure Seal Bonnet Work?

These are used for services having to pressure more than 100 bars.

You can improve the body-bonnet joints’ seal with increased internal pressure within the valve body.

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