# Maximum allowable pressure PS of a valves

Maximum allowable pressure PS of a valves:

Maximum allowable pressure PS must be based on the most stringent conditions, which act on the valve. According to the maximum allowable pressure, the PN valve must be selected.

$PS\le PN$
$PS\ge {H}_{v=0}+\Delta P+{P}_{atm}$

PS - maximum allowable pressure - [MPa]

Hv=0 - static pressure before valve in closed position at zero flow - [MPa]

ΔP - increasing pressure on water hammer - [MPa]

Patm - under-pressure behind the valves (atmospheric pressure) - [MPa]

Under-pressure behind the valves:

For non-perfect aerated space behind the valve

${P}_{atm}=0,1MPa$

For perfect aerated space behind the valve

${P}_{atm}=0MPa$

Example:

We have to determine maximum allowable pressure PS of a valves (Butterfly valve DN2000 the valve is controlled by a gearbox with a drive) with the following parameters:

Static pressure before valve in closed position at zero flow Hv=0 = 1,2MPa; the length of the pipeline before the valve L = 1000m; steel pipe D = 2000mm; thickness of the pipe wall e = 30mm; max. flow Q = 30 m3/s; density water ρ = 998,8Kg/m3; medium compressibility factor β = 477,1*10-12; closing time 45s; local loss factor for an open valve ξ = 0,106; the space behind the valve is non-perfect aerated Patm = 0,1MPa

Increasing pressure on water hammer:

$\mathrm{\Delta }P=0,39MPa$

was calculated by

Effective closing time factor

Water hammer

was calculated by

$PS\ge {H}_{v=0}+\Delta P+{P}_{atm}$ $=1,2+0,39+0,1$ $=1,69MPa$

$PS\le PN\to PN25$

Literature:

ČSN EN 13445-3: Netopené tlakové nádoby – část 3: Konstrukce a výpočet.

MET-Calc: Effective closing time factor

MET-Calc: Water hammer   