How AFT Impulse Can Help Engineers To Comply With ASME Codes

Engineers are often responsible to design systems to comply with codes and standards. In recent years codes that apply to waterhammer and surge pressures have become more prominent. Increasingly AFT Impulse is being used to help engineers comply with such codes. Once AFT Impulse has calculated the maximum pressure due to a transient event, what shall the designer do with this value? The answer to this question depends on the code that is being used as the piping design basis. In this article we’ll be discussing how two important piping design codes – B31.4 and B31.3 – deal with surge pressure (transient pressure) and how the results from AFT Impulse shall be used in order to comply with their requirements. Codes

1. ASME Code for pressure piping B31.4. Pressure Transportation Systems for Liquid Hydrocarbons and Other Liquids.

401.2 Pressure 401.2.2 Internal Design Pressure. 

The piping component at any point in the piping system shall be designed for an internal design pressure which shall not be less than the maximum steady state operating pressure at that point, or less than the static head pressure at that point with the line in a static condition. The maximum steady state operating pressure shall be the sum of the static head pressure, pressure required to overcome friction losses, and any required back pressure. Credit may be given for hydrostatic external pressure, in the appropriate manner, in modifying the internal design pressure for use in calculations involving the pressure design of piping components (see para.404.1.3). Pressure rise above maximum steady state operating pressure due to surges and other variations from normal operations is allowed in accordance with para. 402.2.4.

402.2.4 Ratings – Allowance for Variations From Normal Operations. 

Surge pressures in a liquid pipeline are produced by a change in the velocity of the moving stream that results from shutting down of a pump station or pumping unit, closing of a valve, or blockage of the moving stream. Surge pressure attenuates (decreases in intensity) as it moves away from its point of origin. Surge calculations shall be made, and adequate controls and protective equipment shall be provided, so that the level of pressure rise due to surges and other variations from normal operations shall not exceed the internal design pressure at any point in the piping system and equipment by more than 10%. From B31.4 extracted paragraphs the following conclusions can be drawn:

1. B31.4 refers directly to the maximum value of the overpressure, establishing a limit of 10% above the Steady State operating pressure.
2. As the reference pressure B31.4 uses the static state pressure, in other words the designer shall use the Max. Static pressure from AFT Impulse instead of the Max. Stagnation pressure.
3. Operating pressure shall be the reference for design instead of the design pressure calculated for the pipe.
4. ASME Code for pressure piping B31.3. Process Piping

301.2.2 Required Pressure Containment or Relief

1. (a) Provision shall be made to safely contain or relieve

(see para. 322.6.3) any pressure to which the piping may be subjected. Piping not protected by a pressure relieving device, or that can be isolated from a pressure relieving device, shall be designed for at least the highest pressure that can be developed. (b) Sources of pressure to be considered include ambient influences, pressure oscillations and surges, improper operation, decomposition of unstable fluids, static head, and failure of control devices. (c) The allowances of para. 302.2.4(f) are permitted, provided that the other requirements of para. 302.2.4 are also met.

302.2.4 Allowances for Pressure and Temperature Variations.

Occasional variations of pressure and/or temperature may occur in a piping system. Such variations shall be considered in selecting design pressure (para. 301.2) and design temperature (para. 301.3). The most severe coincident pressure and temperature shall determine the design conditions unless all of the following Criteria are met: (a) The piping system shall have no pressure containing components of cast iron or other nonductile metal. (b) Nominal pressure stresses shall not exceed the yield strength at temperature (see para. 302.3 of this Code and Sy data in BPV Code, Section II, Part D, Table Y-1). (c) Combined longitudinal stresses shall not exceed the limits established in para. 302.3.6. (d) The total number of pressure-temperature variations above the design conditions shall not exceed 1000 during the life of the piping system. (e) In no case shall the increased pressure exceed the test pressure used under para. 345 for the piping system. (f) Occasional variations above design conditions shall remain within one of the following limits for pressure design. (1) Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than (a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr, or (b) 20% for no more than 50 hr at any one time and no more than 500 hr/yr.   From B31.3 extracted paragraphs the following conclusions can be drawn:

1. Surge pressure is included in the group of loads called occasional.
2. The piping design pressure shall take into account the maximum pressure that may occur in the system including surge pressure due to a transient event.
3. The maximum surge pressure shall not exceed in any case the test pressure calculated for the pipe.
4. The maximum stress produce the loads created by the surge pressure shall not exceed: 1.33 S_h (S_h=allowable stress for the operating temperature).

In order to meet the 3^rd requirement the AFT Impulse user shall calculate the forces originated on  the bends of the system  and feed this data to a pipe stress program (see Trey- Jim Wilcox article: Evaluating Dynamic Loads in Piping Systems Caused by Waterhammer; http://www.aft.com/training/white-papers).

Conclusion:

The allowable pressure due to a transient event (surge allowable) varies from code to code. B31.4 refers directly to the maximum pressure value establishing as a limit 10% above the operating pressure. B31.3 not only establishes a surge pressure allowable (Test pressure) but also deals with the mechanical effect in the pipe, limiting the limiting piping  stresses to a maximum of 33% above S_h (S_h=allowable stress for the operating temperature).