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For both metal and O-Ring seated valves in liquid service, Farris’ standard
acceptance criteria is no visible leakage at 90% of set pressure for valves
with a set pressure of 50 psig and higher. For valves with a set pressure
below 50 psig, the test is performed at a 5 psig differential. API Standard
527, Seat Tightness of Pressure Relief Valves, does allow for a more liberal
leakage acceptance criteria of 10 cubic centimeters per hour per inch of
inlet size. Ultimately, the acceptance criteria selected and the amount of
leakage that can be tolerated is the responsibility of the end user based on
his experience with the specific application.

Blowdown Ring Setting

Tables 10.2 and 10.3 give blow down ring settings in the number of
notches down from disc holder contact. The higher the ring, the sharper
the pop and longer the blowdown. The lower the ring, the poorer the pop,
the shorter the blowdown.

The 2600L series is now ASME Code Section VIII Certified for use in air, steam
and vapor service. Regardless of the service, 2600L Series valves should be
adjusted using the liquid service blowdown ring settings in Table 10.3.

Field Settings

The above settings are final average field settings for good performance
under actual operating conditions on the installation. Use recommended
number of notches, or maximum adjustment possible if less than table.
Finer adjustments may be desirable or necessary because of individual
piping and service differences. On applications where the valve will
operate at substantially reduced flow, fewer notches may help eliminate
chatter and ensure seat tightness.

Compensation for Temperature

Any increase in temperature causes a reduction in set pressure of a
safety-relief valve. The primary factors are the linear expansion of the body
and top works, which reduces the spring loading, and the direct effect of
temperature on the spring itself.

It is customary to compensate for this effect by increasing the setting
when a valve is set at ambient conditions on a test stand and the valve is
intended for a higher operating temperature in service. Although the
adjustment is approximate and may be outweighed by other differences
caused by variations in media and blowdown ring adjustments, compen-
sation for temperature as an independent variable improves the accuracy
of the setting. It is not recommended to reduce a spring setting for valves
intended for sub-zero service. (Table 10.4)

COLD DIFFERENTIAL TEST PRESSURE (CDTP) is the inlet pressure at which
the valve is adjusted to open on a test stand at ambient temperature
conditions, discharging to atmosphere. Computation includes compensa-
tion for constant back pressure (if required) and inlet operation tempera-
ture. Cold Differential Test Pressure is the sum of the Spring Selection and
the Increase in Setting to compensate for temperature. The abbreviation is
Cold Set. (Table 10.4)

Compensation for Backpressure

Back Pressure is the pressure existing at the valve outlet and in the valve
outlet and in the downstream portion of the valve body. It may be
superimposed, built-up, constant, or variable. The spring setting should be
compensated for constant and superimposed back pressure, except for
BalanSeal valves. For these type valves, variable back pressure is not com-
pensated for, but the maximum pressure limit of the bellows is stamped on
BalanSeal tag (Figure 1.1).

Liquid Service Applications

Orifice

Notches

D/ E

G /H/J

K/L

M/N/P

Q/R/T/U

Table 10.3

CDTP Temperature Compensation

Normal Operating Temperature

% Increase in Set Pressure at