Huddling chamber

This valve provides an internal design (usually bellows) above/on the seating disk in the huddling chamber that minimizes the effect of backpressure on the performance of the valve (opening pressure, closing pressure and relieving capacity) [35]. See figures 7-4, 7-6, and 7-6A. 

An approved safety valve is usually of the huddling chamber design. In this valve the static pressure acting on the disk area causes initial opening. As the valve pops, the air space within the huddling chamber between seat and the blowdown ring fills with pressurized air and builds up more pressure on the roof of the disk holder. This temporary pressure increases the upward thrust against... 

Safety valves are spring-loaded valves of various designs, typically with a steam huddling chamber feature that increases the total force of the steam, thereby causing the valve to pop up (pop-type SV) rather than rise slowly in response to increased pressure. Boilers operating at higher pressures (say, 600-3,000 psig) tend to have super-jet SVs. 

Huddling chamber An annular pressure/boosting chamber in an SRV located above the seat area for the purpose of generating a rapid opening (Figure 3.12). 

To achieve, however, the adjustable reseat pressure required by the codes, a nozzle ring is added in the design, as shown in Figure 5.3. The nozzle ring, the nozzle and disc holder form the so-called huddling chamber, the strength of which determines not only the opening action but also the reseat pressure. 

As per the General Gas Law, if pressure decreases (which occurs when the valve opens), the volume will increase proportionally with a constant temperature. Now, this is exacdy what happens in the huddling chamber. When the valve opens, pressure is reduced and the enormous increase in volume is trapped in the huddling chamber. This volume acts on the secondary skirt (increased surface area), which forces the valve to snap open rapidly. 

The liquid trim modifies the shape of the huddling chamber so that the full capacity at 10% overpressure can be achieved with a non-compressible fluid. Some manufacturers provide an all media trim which fulfils the requirements for gas and liquid with one single trim others have an interchangeable trim for each fluid. 

Disc holder Has a dual function First, it holds the disc, and second, its skirt or hood shape determines the opening characteristics of the valve as it forms the huddling chamber. It also determines the flow path and hence can have an influence on the flow coefficient. The design of disc holders is very different from manufacturer to manufacturer. 

Therefore, an SRV has a backpressure correction factor which is applied in the calculation and which is different for each manufacturer depending on the design of the valve and, in particular, the nozzle, huddling chamber arrangement and the shape of the body bowl. 

The operating principle is similar to that of API valves with the exception that the blowdown ring and resulting adjustable huddling chamber are usually semi-nozzle design (Figure 5.47). 

Design for lift-assistance is associated with pressure recovery downstream of the valve seal. Generally, two approaches are adopted 1) a valve arrangement with huddling chamber as shown in Fig. 2(A) and 2) a valve arrangement with a deflector plate as shown in Fig. 2(B). Both approaches require extra disk area outside the sealing area on which the recovered pressure may... 

Fig. 2 (A) Illustrative valve arrangement with huddling chamber. (B) Illustrative valve arrangement with deflector plate/skirt.

A shock wave will exist downstream of the throat and is typically located where flow enters the huddling chamber. A sketched example is shown in Fig. 10. Evidence of shock wave location has been found from pressures measured on the disk surface. Between the throat (usually close to the seal) and the shock wave is a supersonic region of low pressure that detracts from the general upward thrust. If the shock wave can be induced soon after the mainstream flow passes the seal then pressure recovery downstream of the shock wave can act on a greater area (and low pressure immediately upstream of the shock on a smaller... 

The design of the valve is such that when the pressme approaches the set pressure the fluid moves past to another chamber called the huddling chamber. Because of the larger area available, the pressme force exerted over the disk exceeds the spring force, and the valve pops open. 

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