Valves parallel-seat

Type A The first type consists of a plane surface which moves in a direction perpendicular to the flow coming from an orifice (Fig. 4 a). We can have a circular orifice surrounded by a valve seat and a plane surface which moves parallel to the valve seat until the flow is shut off. 

The flow can be smoothed by using two pistons which work parallel but in opposite directions. Another design is the use of tandem pistons arranged in series, whereby the first one conveys slightly more than the second. This allows to compensate for mobile phase compressibility and helps to suppress cavitation, i.e. the formation of vapour bubbles during the filling stroke. Some pumps use double-ball check valves with a second seat and ball on top of the first one. The electronics of the pumps are also designed to compensate for the compressibility of the eluent (which fortunately is low, e.g. 1% per 100 bar). 

In the borehole producer method (Figure 21.28), parallel galleries (used for air inlet and product gas withdrawal) are constructed in the coal bed. The boreholes are drilled from one gallery to another about 5 ft (1.5 m) apart and are filled with valves at their inlets and with iron seatings at their outlets. Electric ignition of the coal in each borehole is achieved by ranole control. 

A globe valve is a multiturn valve in which closure is achieved by means of a disk or plug that seals or stops the fluid on a seat generally parallel to the line flow (Fig. 5.179). 

Figure 5.1a depicts an old-generation flat valve. In this valve type, the parallel area between the plunger and the valve seat, called land, is larger compared to the land in the newer flat valve system (see Figure 5.1h). The reduction of the land size reduces...

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