Pressures
vessels shall be designed for maximum operating pressure, expected upset or
settle out conditions, plus a safety factor, which shall be at least the
greater of the following:
·
10% of the maximum operating pressure for
pressures below 1450 psig, but in any case not less than 50 psi.
·
5% of the maximum operating pressure for
pressures above 1450 psig, but in any case not less than 150 psi.
An
appropriate margin shall be applied between design pressure and the flange
rating of associated piping and equipment to avoid over-stressing of equipment
nozzles. A minimum margin of 5% is recommended or 15 psi, whichever is the
greater.
Maximum
operating pressure for individual equipment is discussed below:
Normally
1.05 times of the maximum pressure under normal operation as a minimum will be
considered. The design pressure should
relate to the top of a vessel (top tangent line in vertical drums) and
therefore will not include any liquid head in the vessel.
The design
pressure of the suction side of the compression system is determined as the
maximum settle-out pressure plus a 5% margin. Settle-out conditions will be
based on the suction system at the high pressure trip setting and the discharge
system at relieving conditions.
Square
or rectangular storage tanks operating at atmospheric pressure shall be
designed for the highest density fluid contained and water as a minimum. An
additional design pressure of 0.5 psi minimum will be added where tanks are
blanketed. Tanks shall be designed for 0.03 psi vacuum.
Nomination
of design pressure shall consider the location and elevation of the tank vent
and the potential that liquid overflow from the vent may occur.
NGL
storage spheres will operate based on the vapour pressure at the NGL run-down
temperature. For design pressure, 105% of the maximum operating pressure will
be considered as a minimum.
d. Pumps and Compressors
Equipment
not protected by a PSV and located downstream of a pump or a compressor shall
be designed for the shut off pressure according to the following criteria:
Pump design
pressure will be calculated as:
⁻
the maximum suction pressure based on suction
vessel PSV set pressure and NLL, plus
⁻
Pump shut-in head at the maximum rated speed.
Where vendor
data is unavailable, the shut-in head may be estimated as 1.25 times the
differential pressure developed by the pump at maximum rated speed. Maximum rated speed for variable speed drives
is to be specified as 1.1 times the speed for the normal differential pressure
at maximum rated flow.
When there
is a large difference in delivery and suction pressures consideration shall be
given to whether the delivery pressure can be imposed on the suction line in
which case the suction line shall be rated for the same pressure as the
discharge line. The spec break will be
taken at the first block valve on the suction line.
·
Reciprocating
Pumps
The design
pressure at the discharge of a reciprocating pump, (eg in chemical injection
systems) shall be set at 10% above the maximum expected operating pressure,
overpressure protection being provided by an independent pressure relief
valve. The pump shall be capable of pumping
at the design pressure of the discharge vessel or system.
Compressor
design pressure will be calculated as:
⁻
the maximum suction pressure based on suction
vessel PSV set pressure, plus
⁻
1.15 times the differential pressure developed
by the compressor at maximum continuous speed.
Consideration
may be given to designing the whole compression loop for the full discharge
pressure if this provides a simpler, safer design without excessively
increasing the cost of the system.
The design
pressure of heat exchangers (unless the process system dictates otherwise)
shall be the greater of the following:
·
Maximum operating pressure plus safety margins
as per separators.
·
Pump design pressure as calculated in 5.1.3 or
5.1.5
·
Consideration of ASME/API RP 521 (modified) - 10/13
rule concerning tube rupture in shell and tube heat exchangers. Design pressure
of the lower operating pressure side shall be 77% of the higher operating
pressure side.
The design
pressure of the piping shall not be less than:
·
Set pressure of the relief valve when mounted on
the line plus static head.
·
Set pressure of the relief valve mounted on
equipment plus static head and friction loss.
·
Maximum pressure that an item of equipment can
generate e.g. design pressure of centrifugal pumps, stalling pressure of
reciprocating pumps etc
·
The maximum back-pressure possible (particularly
relevant to vent and drain lines where critical flow is possible)
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