Bonnets

The plug, cage, seat ring, and associated seals are laiovvn as the trim, A kev feature of globe valves is that thev allow maintenance of the trim via a removable bonnet without removing the valve bodv from the line. Bonnets are tvpicallv bolted on but rnav be threaded in smaller sizes. 

Gate valves are used to minimize pressure drop in the open position and to stop the flow of fluid rather than to regulate it. The problem, when the valve is closed, of pressure buildup in the bonnet from cold liquids expanding or chemical action between fluid and bonnet should be solved oy a relief valve or by notching the upstream seat ring. 

Extracted from Face-to-Face and End-to-End Dimensions of Ferrous Valves, ANSI B16.10—1973, with permission of the publisher, the American Society of Mechanical Engineers, New York. To convert inches to millimeters, multiply by 25.4. tNot shown in ANSI B16.10 but commercially available. jTliese dimensions apply to pressure-seal or flangeless bonnet valves only. 

The bonnet (Fig. 11-35B) bolts to the shell. It is necessary to remove the bonnet in order to examine the tube ends. The fixed-tube-sheet exchanger of Fig. 11-36Z has bonnets at both ends of the shell. 

Other types of headers include the bonnet-type header, which is constructed similarly to the bonnet construction of sheh-and-tube... 

Stationary Head—Bonnet 21. Floating Head Cover—... 

The markets for polyetherimides arise to an extent from stricter regulations concerning flammability and smoke evolution coupled with such features as high strength, toughness and heat resistance. Application areas include car under-the-bonnet uses, microwave equipment, printed circuit boards and aerospace (including carbon-fibre-reinforced laminated materials). The polymer is also of interest in flim, fibre and wire insulation form. 

Batteries not located under bonnet to be fitted in vented box... 

Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes of the back pressure on the valve. 

The pressure relief device used most often in refinery and chemical plant equipment is the spring-loaded, top-guided, high-lift, nozzle-type safety relief valve, which is illustrated in Figure 2. The spring is usually external and enclosed by a bonnet for weather protection, and the bonnet chamber is vented through an internal passage to the valve outlet. 

In addition to the above back pressure limitations based on valve capacity, balanced bellows PR valves are also subject to back pressure limitations based on the mechanical strength of the bellows or bellows bonnet, or the valve outlet flange rating. The back pressure specified for the valve is governed by the lowest back pressure permitted by these various criteria. 

Manufacmrer s shipping plugs must be removed from the bonnet vent holes before a new valve is commissioned. 

Each PR valve must be installed so that the bonnet vent does not allow released vapors to impinge on lines or equipment, or towards personnel walkways. Where necessary, a short nipple and elbow should be added to direct flow away from such areas. In these cases, the vent piping should discharge horizontally to avoid entry of rainwater and debris, and should terminate in a position which is accessible for leak testing. 

A balanced bellows PR valve is one in which the closing force exerted by back pressure on the top of the valve disc and the back pressure exerted on the bottom of the disc are balanced by equal surface exposure, thus cancelling each other. The bellows shields the top of the disc from back pressure and the bellows area is vented to the atmosphere via the bonnet vent. 

Balanced bellows PR valves need not be restricted to the same built-up back pressure limit (10% of set pressure) as are conventional valves, since they are not subject to chattering from this cause. However, maximum back pressure is limited by capacity and in some cases by the mechanical design strength limitations of parts such as the outlet flange, bellows, or valve bonnet. 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

Stationary Head—Bonnet 2J. Floating Head Cover—External... 

Stationary Head Flange—Channel or Bonnet 22. Floating Tubesheet Skirt... 

Balanced relief valves are spring-loaded valves that contain a bellows arrangement to keep back-pressure from affecting the set point. Figure 13-4 shows a cross section of a balanced relief valve, and Figure 13-5 is a schematic that shows how the valve operates. The bonnet is vented to atmosphere and a bellows is installed so that the back-pressure acts both downward and upward on the same area of the disc. Thus, the forces created by the back-pressure always cancel and do not affect the set point. 

---