1. SCOPE
2. REFERENCE DOCUMENTS
3. DEFINITIONS 4. GENERAL
4.1 Analysis Methods 4.2 Water Hammer 4.3 Pressure Safety Valve Loading 4.4 Rupture disk 4.5 Slug Flow 4.6 Vibration Control
1. Scope
This article establishes design requirements for piping systems subject to the dynamic effects of
impulse loads and supplements the requirements for ASME B31.1, B31.3, B31.4, and B31.8.
2. Reference Documents
Reference is made in this standard to the following documents. The latest issues, amendments, and
supplements to these documents shall apply unless otherwise indicated.
P01-E01 Design Conditions and Basis for Pressure Piping
P01-E02 Design of Piping Systems for Stress and Pressure Criteria
P01-E03 Flexibility, Support, and Anchoring of Piping Systems
P01 E07 Piping Loads on Equipment Nozzles
American Society of Mechanical Engineers (ASME)
B 16.5 Steel Pipe Flanges, Valves, and Fittings
B 31.1 Power Piping, Appendix II, Non Mandatory Rules for the Design of Safety Valve Installations
B 31.3 Process Piping
B 31.4 Pipeline Transportation Systems For Liquid Hydrocarbons and Other Liquids
B 31.8 Gas Transmission and Distribution Piping Systems
Sect VIII Div. 2 Boiler and Pressure Vessel Code
American Petroleum Institute (API)
RP 520 Part 1. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part 1 – Sizing and Selection
3. Definitions
Force Spectrum.
A frequency vs. force spectrum constructed from predefined force sets that are based
on load versus time data. Information from predefined force sets usually consist of load magnitude,
direction, and location.
Response Spectrum.
A plot giving the maximum responses in terms of displacement, stress, or
acceleration, of all possible linear one degree systems due to a given input. Usually plotted with the
abscissa of the spectrum as the natural frequency or period of the system and the ordinate as the
maximum response in terms of displacement, stress, or acceleration.
Time History Analysis.
An analysis of the dynamic response of a piping system at each increment of time
when a portion of the system is subjected to a specific motion time history.
4. Dynamic Effects on Piping Systems Due to Impulse Loads General
Impulse loading is a time dependent loading condition usually characterized by a suddenly applied single
point load that may take the form of some sort of shock, such as water hammer, pressure safety valve
relief, rupture disk opening, and slug flow. Various static methods exist to determine a piping system’s
response to these types of impulse loading. These methods are usually more than adequate for most
applications. However, in some instances, it may become necessary to obtain a more accurate system
response by applying dynamic analysis procedures such as time history analysis and response spectra
analysis.
4.1 Analysis Methods
4.1.1 The requirement and methodology to use dynamic analysis to evaluate a piping system’s response
to impulse loading shall be at the discretion of the Owner’s engineer.
4.1.2 Time history analysis that incorporates the mode superposition technique to summarize the history
response of the system shall utilize a sufficient number of modes, a short time step, and a long enough
duration time to ensure accuracy of results. A minimum of eight modes, a time step equal to 1/12 of the
highest frequency used, and a duration equal to the time step times the number of steps is recommended
for solution convergence.
4.1.3 Dynamic stresses resulting from impulse loading such as water hammer, pressure safety valve relief,
rupture disk bursting, or slug flow shall be treated as occasional stresses and shall be added to sustained
stresses for the purposes of meeting ASME B31 Code compliance. The correct allowable stress shall be in
accordance with the appropriate ASME B31 code. See also Engineering Standard P01-E02,
Par. 7.1 Design of Piping Systems for Stress and Pressure Criteria.
4.2 Water Hammer
4.2.1 Pump piping configurations shall be designed to minimize dynamic loads associated with hydraulic
transient effects due to pump power startup, outage, and rapid valve closure.
4.2.2 The pressure wave considered in water hammer analysis shall be considered as a single pass event,
not repetitive or harmonic.
4.2.3 Swing check valves in discharge pump piping shall be located and supported so as to minimize the
damaging hammer effects resulting from pump outages. See STD. P01 E07; 4.2.4.
4.3 Pressure Safety Valve Loading
4.3.1 Pressure safety valve piping and supports shall be designed to withstand the thrust loads and
vibration due to discharge.
4.3.2 Thrust loads associated with safety valve releases in pressure relief systems shall be calculated as
follows:
F = (W(KoT/(Ko+1)M)**1/2)/366 (with no impact factor)
or
F = ((CK/183)(Ko/Ko+1)**1/2)AP (with impact factor of 2)
where:
W = CKAP(M/T)**1/2 and:
F = Reaction force, lbs.
A = Orifice area of valve, square inches
C = 520 ((Ko (2/Ko+1)**(Ko+1)/(Ko-1))**1/2
Ko = Ratio of specific heats, Cp/Cv
M = Molecular weight of gas or vapor
P = Inlet pressure at time of opening, psia
T = Absolute temperature, degrees Rankin
K = Coefficient of discharge
W = Flow rate, lbs/hr
A simplified formula that may be used for the PSV reaction force is:
F = 0.6 (Ko+1) AP
4.4 Rupture disk
4.4.1 Dynamic recoil forces resulting from bursting rupture disks shall be considered in the design of vessel
shells and the piping to which they are attached. In the absence of a formal dynamic analysis, a suitable
dynamic load factor or impact factor shall be incorporated into the static analysis. The rupture disc reaction
force may be calculated as follows:
F = 0.378 (Ko+1) AP
where:
F = Reaction force, lbs.
Ko = Ratio of specific heats, Cp/Cv
A = Orifice area of the disc, square inches
P = Inlet pressure at time of opening, psia (Set pressure+ 14.7)
4.4.2 Exhaust stacks attached to rupture disks on vessels discharging to atmosphere shall be routed as
straight as possible and guided where appropriate to reduce vibration due to bursting effects. Bevelling of
stack tips shall be discouraged to minimize dynamic overturning moments on flange nozzles.
4.4.3 A formal dynamic analysis consisting of a time history and force spectrum analysis shall be
performed on piping systems with rupture discs operating at pressures greater than 2500# rating.
4.5 Slug Flow
4.5.1 Lines in which the possibility of liquid accumulation may occur, such as blowdown lines and flare
headers, shall be investigated for the possibility of slug flow.
4.5.2 Slug flow forces at elbows shall be calculated with the following formula:
F = (1.414RhoA(V)**2)/g
where:
Rho = Density of fluid, (lbm/cubic ft
A = Cross-sectional flow area, (square feet)
V = Velocity of fluid, ft/sec
g = gravitational constant, 32.2 ft-lbm/lbf-sec**2
4.5.3 Expansion loops in flare headers subject to slug loading shall be designed with directional anchors in
all directional changes of pipe run within the loop. The gap between directional anchors and support steel
shall be checked to allow for pipe rotation during thermal movement and to prevent the pipe from binding.
4.6 Vibration Control
4.6.1 The preferred method for controlling vibration in piping systems subject to impulse loading shall be
with piping anchors and guides. Alternate means of vibration control such as rigid struts, and mechanical
and hydraulic snubbers may be considered when anchors and guides do not satisfy required load and
movement requirements.
4.6.2 If spring actuated sway braces are used to control a pipe against sway, the sway braces shall be
installed in the neutral position when the piping system is hot and operating.
4.6.3 Rigid struts shall be capable of handling both tensile and compressive loads. Their use shall not
restrict the piping system’s flexibility.
4.6.4 The use of U-bolts to restrain piping subject to impulse loads shall be avoided. Pipe clamps or hold down pipe clamps utilizing belleville washers with a suitable vibration damping isolating material are preferred. Clamp assemblies shall be designed to permit axial thermal movement of the pipe without
imposing large frictional loads on the piping system.