Page 187 - Valve Selection Handbook

174 Valve Selection Handbook

safety valves. This modification converts the valve to a partially balanced
pressure relief valve. As such, the valve may permit a built-up back pres-
sure of 20% subject to confirmation by the valve manufacturer. The same
conversion occurs also when a conventional safety relief valve with
closed bonnet is provided with a regular or unpacked lifting lever used
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for hand lifting the valve disc from its seat.
Balanced safety relief valves. Balanced safety relief valves such as
those shown in Figure 5-10 through Figure 5-12 minimize the back pres-
sure limitations of conventional safety relief valves by means of balanced
bellows mounted between the valve disc and vented bonnet. By this
means, the unbalanced disc area is exposed to constant atmospheric pres-
sure. On valve opening, back pressure acts only on that portion of the
disc in the closing direction that overhangs the valve seat. Although,
back pressure acting on the convolutions of the bellows tends to elongate
the bellows, manifesting itself in an increased spring rate of the bellows.
As they are now stiffer, they resist valve lift accordingly.
Balanced safety relief valves are quoted to achieve their rated capacity
at back pressures between about 20% to 40%, depending on the design
and size of valve. Then the valve begins to close as the back pressures
rises. Allowing for reduced capacity, balanced safety relief valves are
offered for back pressures of up to about 50% (consult manufacturer).
The bellows seal may be supplemented by an auxiliary mechanical
seal as in the valve shown in Figure 5-11 to safeguard against bellows
failure. Depending on the nature of the fluid handled by the valve, the
vent may have to be directed toward a safe location.
Not all balanced safety relief valves offered in the market are truly bal-
anced, though the balance achieved is quite acceptable. Possible exceptions
are pressure relief valves to API Std. 526 of orifice sizes D and E, where
the required diameter of the stem guide in relation to the nozzle diameter is
too big to incorporate balanced bellows. Such valves are therefore affected
by variable back pressure. Some manufacturers restrict the lift of balanced
valves with F orifice to obtain an orifice D or E flow area. 45

Slowdown adjustment of safety relief valves. Slowdown adjustment
of safety relief valves is commonly carried out by an adjustable nozzle
ring as found in the valve shown in Figure 5-7. Locating the adjustment

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Refer to ASME Code Section VIII, Div. 1, UG-136 (3).