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SUMMARY OF VALVE SIZING FOR GASES & VAPOURS

The following notation is used in the formulae in the section:

= Flow area (i.e. valve seat bore) in mm²

= Function of k (Isentropic exponent of expansion) taken from BS 6759, and varying from a maximum of 2.8 for monatomic gases down to 2.4 for polyatomic vapours.

d_o

= Safety valve seat bore in mm

= Tabulated factor

K_dr

= Derated coefficient of discharge (declared on data sheets). For type test according to German TÜV rules the working coefficient of discharge is designated α_w and is also declared.

= Molecular mass of gas in kg/k mol.

= Set pressure in bar gauge

= Actual flowing (inlet) pressure (in bar absolute). P =(1.1p+1).The actual flow pressure being given at 10% pressure accumulation (i.e. 1.1p).

= Mass flow of gas in kg/hr.

= Volumetric flow of gas in Std. litres/sec.

= Temperature in °C of flowing gas at valve inlet

= Absolute flow temperature in °K given by (273 + t°C)

= Compressibility factor, which may be taken as approximately 1 for significantly superheated vapours

= Specific volume at actual flowing temperature and pressure m³/kg

Please refer to these tables for the values of C, f, F, M

The rated flow of gases is often given in standard litres per second, which is the volume occupied by the mass per second at 1 atmosphere pressure (1.013 bar absolute) and 15°C. SCFM (or CFM) is an alternative measure also used, it denotes Standard Cubic Feet per Minute, again at 15°C and 1 atmosphere.

If for a given range of safety valves the value of the coefficient of discharge (K_dr) is not uniform, then by putting the lowest value into formulae (b) & (d), the minimum safe seat diameter (d_o) will be obtained.

For a detailed explanation of how the formulae are derived please see this section.

Worked examples can be found here.

(a) To find the MASS FLOW of gas, q, discharged by a safety valve use the formula;

q=A\cdot K_{dr}\cdot P\cdot C\cdot \sqrt{\frac{M}{Z\cdot T}}=0.2883\cdot A\cdot K_{dr}\cdot C\cdot \sqrt{\frac{P}{v}}\;(kg/hr.)

(b) To find the minimum safety valve seat bore, d_o for a given MASS FLOW, q (taking Z=1) use the formula;

d_{\circ}\geq \sqrt{\frac{0.08756}{F\cdot K_{dr}\cdot (1.1p+1)}\cdot \sqrt{273+t}\cdot q}\;(mm)

(c) To find the VOLUMETRIC FLOW of gas, Q, discharged by a safety valve use the formula;

Q=6.566\cdot \frac{K_{dr}\cdot A\cdot C\cdot (1.1p+1)}{\sqrt{M\cdot Z\cdot (273+t)}}\;(std.litres/s)

(d) To find the minimum safety valve seat bore, d_o for a given VOLUMETRIC FLOW, Q (taking Z=1) use the formula;

d_{\circ}\geq \sqrt{\frac{0.38665\sqrt{273+t}}{f\cdot K_{dr}\cdot (1.1p+1)}\cdot Q}\;(mm)

If volumetric flow is given in European std. litres/s, Q_e, then in equation (d), Q must be replaced by;

Q=\frac{Q_e}{0.948}

If volumetric flow is given in European N/m₃ hr units, V, then in equation (d), Q must be replaced by;

Q=\frac{V}{3.6\cdot 0.948}=\frac{V}{3.413}

In both these cases the coefficient 1.1 of p in denominator of equation (d) should be replaced by unity if required to conform to German standard calculations.