Page 188 - Pipeline Rules of Thumb Handbook

Control Valves 175

reactive forces are equal but opposite, resulting in zero force Testing
on the valve outlet, but if redirected can still impose loads
that must be reacted in some manner. Using DOT Title 49, Part 192.739 as a guideline, each valve
An approximate method for calculating the reactive force should be inspected at least once a year to determine that it:
is to multiply the following pressure (psig) by the orifice area
2
(in. ); the reactive force obtained will be on the high side. • Is in good mechanical condition
• Will operate at the correct set pressure
• Has adequate capacity and is operationally reliable
2
Example A-G Type 22312R68 “R” Orifice (16.00in. ) • Is properly installed and protected from dirt, liquids, or
Set pressure— 1,000psi, 10% overpressure other conditions that might prevent proper operation.
Flowing media— Natural gas, 60°F
M = 17.4 (G = 0.60) The effectiveness of a safety relief valve installation
C = 344 (k = 1.27) depends greatly on proper sizing and selection of the valve
Reactive force = 10,143lb, using equation (1) type, suitable installation conditions and proper and timely
Reactive force = 17,600lb, using approximate method. testing of the valve.

Rupture disc sizing

A review of the ASME Section VIII requirements Sizing methodologies

Changes to Section VIII, Division 1 of the ASME Code The code describes three methods to provide adequate
have impacted the methods used to size pressure relief relieving capacity for a pressure vessel. These methodologies
systems that include rupture discs. The objective of this are applied based on the characteristics of the relieving
article is to clarify the sizing methodologies. system, not individual components.
When a rupture disc is installed upstream of a pressure
relief valve, the Code requires the capacity of the valve be
de-rated by a factor of 0.9, or by a combination capacity factor
Key terminology determined by test for that disc/valve combination. This
method assumes the disc has a capacity equal to or greater
• K D (Coefficient of Discharge)—This is a unit-less factor than the valve, and does not interfere with the normal func-
used to de-rate the theoretical flowing capacity of a tioning of the valve. From the rupture disc standpoint, this
device or system. For rupture discs, this value is defined means the rupture disc should be the same nominal size as
by the Code as 0.62. the relief valve inlet (or larger), and that the disc be of a full
• K R (Resistance to Flow Factor)—This is a unit-less factor opening design. This method is unchanged from the previous
used to characterize the resistance to flow of a rupture versions of Section VIII.
disc device. The larger the K R value, the more restrictive The coefficient of discharge method (K D ) is not a rupture
the device. The K R value has no direct correlation to the disc sizing method, but a method for sizing a relatively simple
K D , coefficient of discharge. relief system. The calculations for this method remain un-
• Combination Capacity Factor—This is a unit-less factor changed from previous versions of the Code, but guidelines
used to de-rate the capacity of a pressure relief valve, for use have been added. This method calculates the theo-
when a rupture disc is installed upstream. The combina- retical capacity of system, and de-rates it by the coefficient of
tion capacity factor may be the default value of 0.9, or a discharge, K D = 0.62. This method is used only when each of
higher value, if the disc/valve combination has been the following conditions is true:
tested.
• MNFA (Minimum Net Flow Area)—This is a derived 1) the rupture disc discharges to atmosphere
area based on the area of the device, and/or the piping 2) the rupture disc is installed within 8 pipe diameters of
area, and is validated during ASME certification testing. the vessel
The MNFA is used only in the coefficient of discharge 3) there are no more than 5 pipe diameters of discharge
method. piping on the outlet of the device.