Valves balanced-bellows

Superimposed Back Pressure - Is the pressure at the outlet of the pressure relief valve while the valve is in a closed position. This type of back pressure comes from other sources in the discharge system it may be constant or variable and it may govern whether a conventional or balanced bellows valve should be used in specific applications. 

When the pilot exhausts to the atmosphere, a pilot-operated PR valve is fully balanced. Like the balanced bellows valve, therefore, its opening pressure is unaffected by back pressure, and high built-up back pressure does not result in chattering. 

Balanced bellows valves need no reduction in spring pressure to compensate for superimposed back pressure, and they can tolerate variable superimposed back pressure without an effect on opening pressure. 

It is important to note that back pressure affects balanced PR valve capacities in the same way as for conventional valves, and appropriate factors are included in the sizing procedures. They are subject to the same recommended limits of maximum total back pressure (superimposed plus built-up) as conventional valves. In the case of balanced bellows valves, mechanical considerations must also be evaluated, since they may limit the maximum permissible back pressure. 

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

As an alternative, a balanced bellows valve could be used since it is not affected by a variable superimposed back pressure. 

Conventional Valve, Balanced-Bellows Valve, or Pilf>t-Operated Valve... 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

For the balanced-bellows design the bellows on the backside of the valve seat ensures that the pressure on that side of the seat is always atmospheric. Thus the balanced-bellows valve will always open at the desired set pressure. However, the flow through the balanced-bellows relief... 

The bonnet is closed for a conventional valve and is vented for a balanced bellows valve, and it is usually bolted onto the body. 

In this sectioned view of a balanced bellows valve (Figure 5.18), we can see that the bellows also protects spindle and guiding surfaces from corrosive fluids. It isolates the spring chamber or bonnet from the process fluid as well. Therefore, bellows provide good corrosive protection. 

The basic design of a balanced bellows spring valve is the same as a conventional valve, but a bellows is added to compensate for variable backpressures and the bonnet is vented to atmosphere A leak at the bonnet vent indicates a failed bellows. Because balanced bellows valves must have their bonnets vented to the atmosphere, a safe location for piping the vent must be determined. The balanced bellows valve, with or without a supplementary balancing piston, only discharges process medium from the bonnet vent in the case of failure of the bellows (Figure 5.26). 

Theoretically, the balanced bellows valve can handle any backpressure but is usually limited up to 50% of set pressure. Balanced SRVs may be used anywhere the backpressure is either constant or variable... 

The difference between a balanced bellows valve and a balanced piston valve is that additional assurance for safe operation is built into the latter by adding a balanced piston on top of the guide (Figure 5.29). 

Despite the clear notice that the valves shown in Figure 6.35 are balanced bellow valves, the vent hole in the bonnet is plugged. This valve will not work properly and is a great installation hazard. ... 

Below -70°C and backpressures needing a balanced valve Pilot-operated, soft-seated valves are the only valid solution here. Balanced bellows valves are not acceptable at these temperatures due to the high risk of bellows failure ( cold working) and the risk of condensates freezing inside the bellows. This phenomenon is discussed in detail in Chapter 11. 

Price of a new standard balanced bellows valve 4P6 3.790,-Price of a new pilot-operated 4P6 valve 3.950,-... 

Constant pressure variations in inlet or/and outlet piping 1. Rearrange piping. 2. Use balanced bellow valves if pressure variations occur in outlet piping. 3. Use POSRV. 4. Use multiple valves with different sizes in stagger set. 

No vacuums are created in the outlet piping (if any) Use balanced bellows valves or POSRV ... 

Backpressure A conventional valve might see too much backpressure, in that case use a balanced bellows valve or POSRV... 

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