Yes, Design pressure is a pressure set by the designer at some value higher then the line max. operating pressure.
The Code does not specify a a hard value or percentage above max operating pressure in order to give the engineer maximum flexibility in optimizing the system.
For example a hard requirement of 10% over max operating would not cover all possible fluctuationss in a low pressure system, it would also be very conservitive in very high pressure systems. Also if you system was such that the design pressure at temperature was 5 psi above the flange pressure class rating, you would be forced to go from a 600# to a 900# system. By leaving it up to the designer you can now exercise some engineering judgment on the matter.
The Casti guidebook on B31.3 Proccess Piping has some good direction on this subject.
It should be mentioned that, in B31.3, the establishment of system design pressure must consider the most severe coincident conditions of BOTH pressure and temperature.
Furthermore, B31.3 allows the operation above the design pressure for brief periods of time as long as several other criteria are met.
This code consideration is sometimes used in cases, such as pump dead-head, where a rare event sometimes exceed the design pressure of the system.
An excellent discussion of this is in the "Process Piping" book by Becht.
Consideration of this margin should take into account the type of pressure relief devices in use and the margin that these devices need to prevent them starting to lift during normal operation and to allow them to properly reset following a relieving event.
If you have a high pressure shutdown set to shut things down before lifting relief devices then you will also want some reasonable margin between this device and the max operating pressure.
Typical margins for conventional spring operated relief valves might be 10% (depends on system pressure though) whereas for a pilot operated valve it may be around 3%. I believe there is guidance on such margins in ASME Section VIII Div 1 Appendix M, and in API RP 520 and/or 521.
For B31.3 the design pressure can be found by solving the equation given in SS 345 ss 4.2 a, Design Pressure = Test pressure/1.5
Design pressures per say do not need to include "occasional" pressures or relief valve set pressures. A given design pressure allows continuous[b/] operations of up to and including that pressure, but allow higher "occasional pressures".
"Occasional" pressures can exceed design pressure under certain limitations. SS(302.2.4) Occasional pressures can be up to 33% higher than the design pressure(under certain conditions). Since pressure relief is self-limiting, see the same section, ss(f-1) subject to owners approval and those for self-limiting events ss(f-2) where the 20% limit is given.
Relief valve set pressures may be above design pressure (with a sufficient reduction for operation margins) to allow pipe pressures up to 20%.
Obviously, if transient pressures and allowed time and number of occurances of those events exceed the given limits, then the Design Pressure must be raised until the transient events can be accomodated within the new corresponding limits.
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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)
BTW B31.3 does not specifically have a definition for "maximum allowed operating pressure" at all.
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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)
