Spring type balance

Flow through spring-type reliefs is approximated as flow through an orifice. An equation representing this flow is derived from the mechanical energy balance (Equation 4-1). The result is similiar to Equation 4-6, except that the pressure is represented by a pressure difference across the spring relief ... 

Link-Suspended Basket Centrifuges In centrifuges with diameters larger than 762 mm (30 in), the basket, curb, curb cover, and drive form a rigid assembly flexibly suspended from three fixed posts (also known as a three-column centrifuge). The three suspension members may be either chain hnks or stiff rods in ball-and-socket joints and are spring-loaded. The suspended assembly has restrained freedom to oscillate to compensate for a normal out-of-balance condition. The drive is vertical with more efficient power transmission compared to the base-bearing type. 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

Other types of pressure-relief valves do not depend upon the back pressure for their performances. However, to ensure that the safety valves work at their maximum capacity, back pressure is limited to 50 percent of the relief valve set pressure. In the balanced bellows type valve, the spring does not act directly on the disk. Instead, it serves on a bellows first, which in turn acts on the disk. In case of the piston type, it works on the same principle as the bellows type, except that the bellows is replaced by a piston (see Figure 17B). The cross-sectional area of both the piston and the bellows is the same as the inlet nozzle of the valve and the effect of the back pressure on the top and the bottom of the disk creates equal balancing forces. That is, P,A is always equal to F, as shown in Figure 17B. 

I here are three types of relief valves conventional, balanced-bellow s, and spring loaded. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Before initiating any calculations, it is necessary to establish the general category of the pressure relief valve being considered. This section covers conventional and balanced spring-loaded types. 

There are two general categories of relief devices (spring-operated and rupture discs) and two major types of spring-operated valves (conventional and balanced-bellows), as illustrated in Figure 8-7. 

The backpressure correction Kb is used only for balanced-bellows-type spring reliefs and is given in Figure 9-4. This correction compensates for the absence of backpressure on the back... 

Kb is the backpressure correction and depends on the type of relief used. Values are given in Figure 9-5 for conventional spring reliefs and in Figure 9-6 for balanced-bellows reliefs. 

As mentioned above, mainly two types of balances are used beam balances and spring or torsion balances. Spring or torsion balances show a special relation between... 

Type Value (% of Conventional Balanced Spring Pilot Operated ... 

Therefore, as seen earlier, to avoid this highly hazardous situation, a balanced bellows spring-loaded valve must be used. This type includes a bellows assembly as described in Sections 5.2 and they subtract the disc of the valve from the influence of the backpressure so that the valve opens at the correct predetermined pressure (Figure 11.4). 

Perhaps the biggest problem with spring balances is that the linear motion of the spring is typically converted to some type of circular motion for reading the weight. Such gear reductions introduce friction, which decreases readability, pre-... 

The weight of gas adsorbed is sometimes measured, and for this purpose various kinds of microbalance have been used. McBain s sorption balance consists of a delicate helical quartz spring,5 suspending the adsorbing solid the upper end of the spring is fixed, and the lower end observed with a cathetometer. Bradley6 has used a balance with a beam, supported in the centre by a fine horizontal quartz fibre and references to other types of balance are given by McBain.7... 

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