## 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. $PS\le PN$

$PS\geq H_{v=0}+∆P+Patm$
where:
 $PS$ maximum allowable pressure $\mathrm{MPa}$ $H_{v=0}$ static pressure before valve in closed position at zero flow $\mathrm{MPa}$ $∆P$ increasing pressure on water hammer $\mathrm{MPa}$ $P_{atm}$ under-pressure behind the valves (atmospheric pressure) $\mathrm{MPa}$

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 $H_{v=0}=1.2\ \mathrm{MPa}$; the length of the pipeline before the valve $L=1000\ \mathrm{m}$; steel pipe $D=2000\ \mathrm{mm}$; thickness of the pipe wall $e=30\ \mathrm{mm}$; max. flow $Q=30\ \mathrm{m^3/s}$; density water $\rho=998.8\ \mathrm{kg/m^3}$; medium compressibility factor $\beta=477.1\cdot10^3$; closing time $45\ \mathrm{s}$; local loss factor for an open valve $\xi=0.106$; the space behind the valve is non-perfect aerated $P_{atm}=0.1\ \mathrm{MPa}$.

Increasing pressure on water hammer:
$∆P=0.39\ \mathrm{MPa}$

was calculated by
Effective closing time factor
Water Hammer

maximum allowable pressure:

$PS\geq H_{v=0}+∆P+Patm= 1.2 + 0.39 + 0.1 = 1.69 \ \mathrm{MPa}$
$PS\le PN\rightarrow PN25$

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