Page 203 - Valve Selection Handbook

190 Valve Selection Handbook

cylinders with a narrow gap between them, which are filled with a spe-
cial viscous grease. As the valve opens or closes, the cylinders glide rela-
tive to each other and introduce speed-dependent forces on the stem to
damp out motion. The forces can be regulated by the width of the gaps
between the cylinders and by the cylinder surface area.
Viscous damping adjusts itself to demand and is capable of preventing
chatter regardless of the size of flow and pressure disturbance. The effect
of viscous friction on the set pressure can be taken into account when
setting the valve.
The combination of pressure relief valves and oscillation dampers as
shown in Figure 5-27 and Figure 5-28 have been tested and certified by
TiiV, the technical supervisory organization in Germany.

Certification of Valve Performance

The ASME Boiler and Pressure Vessel Code requires pressure relief
valve manufacturers to demonstrate performance and relieving capacity
of pressure relief valves by a test to be carried out by an ASME-approved
facility. The body that conducts the testing for certification of perfor-
mance and capacity is the National Board of Boiler and Pressure Vessel
Inspectors. 1 The methods and procedures to determine relieving capaci-
ties and additional characteristics that may be required by other codes are
contained in the ASME Performance Test Code on Pressure Relief
Devices PTC 25-1994.
The minimum design pressure of equipment covered by the ASME
Code is for practical purposes 15 psi (1.03 bar). The ASME rules there-
fore do not generally apply to pressure relief valves for lower pressures. 2
The certification tests should be conducted, with one exception, at an
overpressure of either 3% or 10%, but minimum 3 psi (0.207 bar), as
required by the applicable section of the code. Depending on the range of
valves for which certification is sought, the following test methods are
employed:

1
The National Board of Boiler and Pressure Vessel Inspectors, 1055 Crupper Avenue,
Columbus, Ohio 43229-1183.
2
For methods of capacity testing for conditions of subsonic nozzle flow refer to API
Standard 2000.