Design parameters - onshore pipeline
Design Parameters
This section reviews the relationship between the
following pressures - MAOP, MOP, design pressure, surge pressure and test
pressures - which are to be considered during design. It determines the design
parameters and provides definitions and guidance on the parameters identified.
Design Pressure (DP)
Sometimes known as internal design pressure, this is
the pressure that is used in all of the stress calculations in the codes and represents
the maximum internal pressure that the pipeline should be capable of receiving
during its lifetime at the lowest part of the pipeline system. The design pressure
needs to be considered closely with the MAOP, MOP and surge pressure (see below
for definitions).
Design pressures should not be set unduly high otherwise
excessively thick pipe will be required.
Design pressures for onshore gas pipelines have been
limited in the past by practice or statute. The design codes vary in their
maximum pressure, if stated, but as pressures rise above 100 barg, additional
safety studies and protection features are justified. The next logical step after
100 barg occurs at 149 barg, which is the flange rating of ANSI Class 900
equipment, above which any equipment would need to be ANSI Class 1500 which is
more expensive.
Note:
ANSI flange pressure ratings are reduced when product
temperatures exceed certain temperatures, hence product temperature must be
considered as well as pressure. There is no particular upper limit to design
pressure, the main problem occurring due to the wall thickness required when
using design factors of 0.6 or 0.5. These design factors may be required
depending on the location and features encountered along the pipeline route.
Design pressures for liquid pipelines are not so limited
as that of gas, but are often determined by the additional cost of a thicker
pipeline versus the cost of additional booster stations. It is uncommon to see
design pressures for liquid pipelines at more than 150 barg.
Maximum Allowable Operating Pressure (MAOP)
The MAOP is never set higher than the design pressure.
It is sometimes equal to the design pressure, but is often set marginally (5%
or less) below the design pressure for gas pipelines due to the general lack of
surge pressure, but often up to 10% or more below the design pressure for
liquid pipelines due to surge problems. It is a pressure that the operating and
control system for the pipeline should limit the pressure in the pipeline to as
an maximum using control valves, alarms and other features such as variable
speed pump sets. The variance between the MAOP and the design pressure allows
shutdown alarms and other protective devices to be set to ensure that the
pipeline does not exceed the design pressure.
The MAOP is linked to the test pressure in different
ways. ASME B 31.8 sets the maximum value of the MAOP by referring to it as a
fraction of the test pressure, the fraction depending on the classification,
design factor and type of pressure test undertaken (hydrostatic or gas). BS
8010 states that the test pressure shall not be less than the MAOP plus any
allowance for surge or abnormal operation, i.e. the MAOP cannot be higher than
the test pressure minus the surge allowance or the Design Pressure, whichever is lower.
Maximum Operating Pressure (MOP)
The MOP is defined as the normal maximum operating
pressure. It is set below or equal to the MAOP. It is defined in order to allow
a ‘normal’ set point to be set on the pipeline, above which a warning alarm might
be set, but which would not shut down the pipeline until the pressure exceeds
the MAOP. The design codes use the MOP to define the minimum test pressure, but
care is needed using this value as the test pressure as there is then no
difference between the MOP and MAOP. There is no allowance within the design codes
for raising the MOP above the MAOP without re-testing the pipeline to a higher
pressure (hence raising the MAOP).
Surge Pressure
Surge pressure is the term used to describe the
increase in pressure caused when a flowing fluid is brought to a halt by closure
of a valve or shutdown of a pump or compressor. For the purposes of pipeline
design this term also includes any other pressure variation which could be in addition
to the MAOP. The surge pressure can be calculated and depends on a number of
factors, including density of fluid, velocity of fluid, length of pipeline,
speed of closure or shutdown, pressure of fluid and sonic velocity of the
fluid. The design codes all limit the surge pressure added to the MAOP to a
maximum of 110% of the design pressure. For pipelines where this is a regular occurrence,
or where the pressure may exceed 110% of the design pressure, surge alleviating
devices must be fitted to prevent this happening or the design pressure raised
to accommodate the surge pressure within 110% of its value. Gas pipelines do
not suffer unduly from surge due to the large degree of compressibility within
the fluid and, although calculated, does not normally cause any particular
problems. However, this can become a major factor for liquid pipelines which
have a relatively high fluid velocity (>2.5 m/sec) or are subject to sudden
closure of valves or pumps.
Test Pressure
Test pressure is set by the design codes to prove that
the pipeline is sufficiently strong and free from material or construction
defects. It is intended that the test pressure will be in excess of the MOP as
a minimum and in excess of the design pressure as a maximum. Care must be taken
when calculating the test pressure so as not to exceed the specified minimum
yield strength of the linepipe at the lowest point when being hydrostatically
tested due to the hydrostatic head.
Pipelines in mountainous or hilly regions are often sectioned
into different lengths for testing due to this problem.
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