Page 204 - Valve Selection Handbook

Pressure Relief Valves 191

• Three-valve Method for capacity certification of one-valve model or
type in one size at one set pressure.
• Four-valve or Slope Method for capacity certification of a type of
valve of one size at various pressures.
• Coefficient Method for certification of the coefficient of discharge of a
type or model of valve of various sizes over a range of pressures.

All values derived from the testing must fall within plus/minus 5% of
the average value. This value is to be multiplied by 0.90 and the product
be taken as the certified value. In the case of the coefficient of discharge,
the average actual value is the K d-value and the certified value the K-
value. The derating factor of 0.90 compensates for minimal inlet pressure
losses, for minus deviations from the average actual test value of up to
5%, wear and tear on the valve, and other unseen, unaccountable factors.
The certified valve capacities and coefficients of discharge are published
by the National Board in the red book titled Pressure Relief Device Certi-
]
fications.
The blowdown of pressure relief valves with nonadjustable blowdown
is to be noted and recorded as part of the performance test.
During all certification tests the valve must not flutter or chatter and
shall properly reseat after the overpressure has receded.
Determining the permissible level of built-up and superimposed back
pressure for satisfactory valve operation and its effect on mass flow or
coefficient of discharge within the permissible back pressure range is not
part of the performance test.

Force/Lift Diagrams as an Aid for Predicting the Operational
Behavior of Spring-Loaded Pressure Relief Valves

Force/lift diagrams, such as those shown in Figure 5-29 to Figure 5-33,
are important aids for predicting the operational behavior of pressure
relief valves. They are also used for focusing solely on the effect of inlet
pipe pressure loss and back pressure on the lifting force of the valve or on
the mode of blowdown adjustment using a particular blowdown adjust-
ment device.

1
The coefficients of discharge contained in the red book of the National Board are
correlated to the actual valve flow area. This area may be larger than the standard
"effective" orifice area specified in the API Standard 526. Where the "effective" orifice
area is employed in the sizing equations as in API RP 520, the correlated "effective"
coefficient of discharge must be used in sizing the valve as derived from the
realtionship (AK) Certified = (AK) Effective