Page 179 - Valve Selection Handbook

166 Valve Selection Handbook

Figure 5-1. Valve Elements of Pressure Relief Valve, Ritchie Type.

An Englishman, Charles Ritchie, achieved the first significant improve-
ment in valve lift in 1848 with a valve that exploited the expansive prop-
erty of the gas for raising the disc. This was achieved by providing the
disc with a peripheral flow deflector, which, together with a lip around the
seal, formed an annular chamber with a secondary orifice around the seat,
as shown in Figure 5-1. When a valve thus designed begins to open, the
discharging gas expands in the annular chamber, but cannot readily
escape. Therefore, the static pressure in this chamber rises sharply and,
acting now also on an enlarged area on the underside of the disc, causes
the valve to open suddenly. But as the escaping gas deflects on the disc
only through around 90°, only a portion of its kinetic energy is converted
into lifting force so that the valve can open only partially within the nor-
mally permissible overpressure. When the overpressure recedes, the static
pressure in the annular chamber builds up again, causing the disc initially
to huddle above the seat until the operating pressure has dropped to below
the set pressure. The resulting difference between the set pressure and
reseating pressure is referred to as the blowdown. Liquids, being incom-
pressible, cannot develop a sudden pressure rise in the annular chamber,
and the valve initially will open only a little with rising overpressure.