Page 104 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.54 CHAPTER TWO

Firesafe valves require testing to meet minimum recommended performance standards
when operating in a firesafe environment. These recommendations are:
1. Minimum internal leakage. A valve must offer acceptable seating prior to and after
exposure to high temperatures without depending on supplementary pressure from
spring-loaded or other devices and without depending on a critical seal.
2. Minimal external leakage. The valve body design should minimize external leakage by
using fire-resistant stem seals and avoiding large gasketed body joints.
3. Continued operability. A valve must be operable despite fire damage. The body and
actuator must resist warpage and damage from high temperatures.

VALVE RATINGS

There are a number of designations used to indicate the pressure ratings of valves. Valves
are pressure rated by their ability to withstand pressure within a range of temperatures.
Standard pressure ratings have been established to match ANSI ratings of flanges and
fittings and are designated by class, conforming to ANSI B 16.34 ratings. Two types of
designation are WSP and WOG. WSP, or working steam pressure, rates the ability to handle
steam at the specified working pressure. WOG, or water, oil, and gas, rates the ability to
handle cold water, oil, and gases at the assigned working pressure. When the two ratings
are given, WSP is called the primary rating. When only one rating is given, the valve is
not generally used for the service not mentioned. The rating 150 lb refers to the working
pressure in psig for which the valve is rated. If a valve is primarily used for water service, a
common designation is WWP, or water working pressure. This designation rates the ability
to handle cold water. The valve class designates the working pressure of a valve. A class
300 rating indicates a valve with a working pressure of 300 psig.
Cold temperatures mean ambient temperatures from 32 to 90°F. For high temperatures,
the valve pressure shall be derated. For high pressures, the temperature rating shall be
derated. The temperature limitation on most metallic valves is generally based on the capa-
bilities of the seat and interior trim materials.

PRESSURE LOSS THROUGH VALVES

In general, valves used for utility piping are rarely selected based on pressure drop through
the valve but rather for their suitability in service. Calculations are not needed since estab-
lished equivalent lengths of pipe for each type of valve are sufficiently accurate for deter-
mination of the approximate pressure drop through the valve. Refer to Fig. 9.1 to determine
the equivalent length of pipe for common valves.
There may be occasions where precise determination of the pressure drop through any
valve would be desired, such as in cases where pressure drop must be kept to a minimum
or the exact determination of the pressure drop is necessary. This is done by using the
standard measure of valve flow, the coefficient C . This coefficient is the flow in gallons
v
per minute that will pass through a valve in the wide-open position with a pressure drop
of 1 psi (6.9 kPa). This coefficient is determined by the valve manufacturer using actual
flow tests. With the C known, the pressure differential can be found using the following
v
formula:
ΔP = SF 2 (2.1)
C 2
v

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