The bonnet can be a removable portion of a valve connected to the body by screwing, flange bolting, welding, or a pressure-sealing mechanism. In some cases, the bonnet can be an integral part of the valve body. Removal of the bonnet generally provides access to the valve trim, except in end-entry valves, such as ball and butterfly valves.
For the end-entry valves, access to the trim is through the inlet or outlet ports or through the body joint.
3.2.1.1 Screwed Bonnet
The screwed-in bonnet-type valve, shown in Figure 3-3, is one of the simplest and least expensive constructions; it is commonly limited to valve sizes up to 3 inches. In valves larger than 3 inches, the tools and torque required to tighten the joint become too cumber-some. Threaded joints should be avoided where thread corrosion or galling can make disassembly difficult. There are two variations of the threaded joint: one where the bon-net is screwed directly onto the body, the other where a union is used.
Bonnet
Body
Stem
Threaded Connection
Figure 3-3 Screwed Bonnet
Screwing the bonnet directly onto the body requires that the gasket or ground joint accommodate itself to rotating faces, and frequent unscrewing of the bonnet can dam-age the joint faces. Another disadvantdam-age of these joints is the variability in the circum-ferential alignment between the bonnet and body, because the final assembly position is dependent on the number of turns required during threading. Threaded joints, how-ever, offer the advantage of being easily seal-welded to provide a redundant seal or to eliminate the joint seal altogether.
Joining the bonnet to the body using a union ring offers the advantage of preventing motion between the joint faces as the two are being made up, thereby permitting re-peated unscrewing of the bonnet without damaging the joint faces or seals. Unions also prevent accidental unscrewing of the bonnet by the operator.
3.2.1.2 Flanged (Bolted) Bonnet
Flanged bonnet joints, such as those found in valves shown in Figure 3-4, have an advantage over the screwed joint in that smaller tools and lower torque are required to tighten the joint. Flanged joints can be used on any size valve, under any operating pressure, but they become very bulky and heavy when used on very large valves and under high operating pressures. At temperatures above 650˚F, creep relaxation can, in time, noticeably lower the bolt load and allow the joint to leak. If the application is critical, the flanged joint can be seal welded.
Stem
Packing
Gasket
Bolts
Bonnet Flange
Body Flange
Figure 3-4
Flanged (Bolted) Bonnet
3.2.1.3 Welded Bonnet
Welding the bonnet to the body effectively provides a very economical and long-term seal regardless of size, operating pressure, and temperature. This arrangement can be used to achieve both a sealing function and a load carrying function, as shown in Figure 3-5. When coupled with screwed or flanged joints, the weld joint is designed to seal only against pressure and requires minimal weld material. Except for cast iron, weld-ing can be performed on most materials.
Bonnet
Groove Weld
Body
Figure 3-5 Welded Bonnet
This arrangement is used where the valve is expected to be maintenance-free for long periods, where the valve is a throw-away design due to its relative cost to replace ver-sus repair, or where the required sealing reliability of the valve far outweighs the diffi-culty of gaining access to valve internals, such as in bellows-sealed stem valves.
3.2.1.4 Pressure-Sealed Bonnet
The pressure sealed bonnet design, shown in Figure 3-6, provides the advantage of re-duced weight and size over flanged connections, and allows the internal bonnet pressure to increase the joint sealing contact stress, instead of unloading it as in bolted designs.
This joint is most attractive in larger valves and high pressure applications where the pressure forces are high enough to generate the required contact stress to seal at the metal-to-metal joint. This type of bonnet seal is usually available only on valves of pres-sure Class 600 or higher. It is particularly suited to high temperature applications (660˚F) where bolted bonnet joints can loosen due to bolt creep. One of the disadvantages of this type of bonnet joint is that it provides no positive mechanical location between the bonnet and body and often allows misalignment to occur, which can cause stem binding. Bind-ing can lead to stem gallBind-ing, leakage through the stem packBind-ing, and potential valve inoperability. In large valves, proper assembly of the bonnet usually requires installation of the valve with the stem vertical and pointing upward (see Figure 3-3).
Bonnet Capscrews
Bonnet Retainers
Gasket Retainer
Spacer Ring
Pressure Seal Gasket
Bonnet
Valve Body
Figure 3-6
Pressure-Sealed Bonnet
Another drawback of the pressure seal bonnet joint is that it can start to leak in applica-tions where frequent pressure or temperature fluctuaapplica-tions are experienced; therefore, the bonnet cannot be safely tightened under pressure when a leak occurs because of the possibility of making the leakage more severe when attempting corrective action. In addition, if a leak should occur, it is more difficult to repair and reassemble the valve than with a bolted bonnet due to the required careful alignment and tightening se-quence procedures during assembly.