Valve Selector - D thru T Orifice (Metric Units)

The Series R/RB pressure relief valve
consists of a nozzle threaded into a cast body housing which is flanged to a pressurized system. A disc is held against the nozzle by a spring, which is contained in a cast bonnet. The spring is adjusted by a compression screw to permit the calibration of opening or set pressure

An adjustable nozzle ring, threaded onto the nozzle, controls the geometry of the fluid exit control chamber (huddling chamber). The huddling chamber geometry is very important in controlling valve opening and closing pressures, and stability of operation. The nozzle ring is locked into position by a ring pin assembly. A cap attached to the top of the bonnet seals the internal calibration adjustments. Refer to the illustration
above for the location of these
important components.

Under normal system operation the valve
remains in the closed position because
the spring force (Fs) is greater than the
system pressure acting on the internal
nozzle seating area (PA). If system pressure
increases to a point when these
forces are equal, set pressure is reached.
The disc lifts and fluid flows through the
valve. When pressure in the system returns to a safe level, the valve closes.

Just prior to reaching set point, the pressure relief valve leaks system fluid into the huddling chamber. The fluid now acts on a larger area of the disc inside the huddling chamber (PAh), causing the valve to experience an instantaneous increase in the opening force. Refer to the Figure on next column to see relationship between Nozzle Area (A) and the Hudding Chamber Area (Ah). System pressure acting on the larger area will suddenly open the pressure relief valve at a rapid rate.

Although the opening is rapid and dramatic, the valve does not open fully at set point. The system pressure must increase above the set point to open the valve to its full lift and capacity position. Maximum lift All pressure relief valves are allowed an overpressure allowance to reach full rated flow. The allowable overpressure can vary from 10% to 21% on unfired vessels and systems, depending on the sizing basis and whether a fire condition is encountered.

Once the valve has controlled the pressure excursion, system pressure will start to reduce. Since the huddling chamber area is now controlling the exit fluid flow, system pressure must reduce below the set point before the spring force is able to close the valve. The difference between the set pressure and the closing pressure is called blowdown, and is usually expressed as a percentage of set pressure. The typical blowdown of the series R/RB can vary from 7% to 10%, the industry standard.

In the seriesR/RB, the nozzle ring adjustment changes the shape and volume of the huddling chamber, and its position will affect both the opening and closing characteristics of the valve. When the nozzle ring is adjusted to its top position, the huddling chamber is restricted to its maximum. The series R/RB will usually pop very distinctly with a minimum simmer (leakage before opening), but the blowdown will increase. When the nozzle ring is lowered to its lowest position, minimal restriction to the huddling chamber occurs. At this position, simmer increases and the blowdown decreases. The final ring position is somewhere between these two extremes to provide acceptable performance.

Liquid Service Operation
On liquid service, a different dynamic situation exists. Liquids do not expand when flowing across orifices, and a small amount of fluid flow across the nozzle will produce a large local pressure drop at the nozzle orifice. This local pressure drop causes the spring to reclose the valve if the fluid flow is minimal. Liquids leaking into the huddling chamber can quickly drain out by gravity and prevent fluid pressure from building up on the secondary area of the huddling chamber. Liquid relief valves are thus susceptible to a phenomenon called chatter, especially at low fluid flow rates. Chatter is the rapid opening and closing of the pressure relief valve and is often destructive in nature.

The unique design of the Series R/RB disc holder and nozzle ring retains the fluid within the huddling chamber, without conversion to a special liquid trim. ASME Code Section VIII performance criteria of full rated liquid flow at 10% overpressure is met with the standard trim, unlike all other major manufactur- ers.

Since no visible or audible pop is heard at set point, the series R/RB liquid set pressure is defined as the pressure when the first heavy flow occurs (a stream of water falls off the outlet flange and remains unbroken for about one inch).