For preliminary engineering the guidelines given below
should be followed. A preliminary network analysis will also be undertaken for
all principal relieving/depressuring cases in order to ensure that the maximum
allowable back pressure at each individual relieving device is not exceeded.
Calculations will be performed in accordance with API RP 520
and 521.
Relief Valve Inlet Lines
The inlet line pressure drop is to be less than 3% of set
pressure calculated at set pressure conditions assuming flow based on installed
relief valve area for conventional and balanced valves. (Note - not for pilots)
However pressure drop in the inlet line for a pilot valve should be calculated
and considered when determining the relief valve capacity. The inlet line shall
not be smaller than the inlet flange of the relief valve.
Relief Valve Outlet Lines
The discharge line shall not be smaller than the outlet
flange of the relief valve. The
computation of the allowable pressure drop using the required relief capacity
is as follows:
Conventional Valves
Size the discharge line from conventional valves to limit
the pressure drop to less than 10% of the set pressure (gauge).
Balanced Valves
Higher pressure drops may be used to affect considerable
cost savings. Limit the pressure drop to
50% of the set pressure (gauge) and to no more than the rating of the internal
bellows.
Where there are a number of valves discharging into the same
manifold, caution should be exercised to ensure that the back pressure in the
manifold allows all relief valves to discharge properly.
The recommended maximum Mach number range is 0.5-0.75
calculated at rated flow rate and downstream end of header conditions. Back pressure based on installed critical
area shall be checked to confirm it is below the system design pressure.
Note that Mach number for vapor lines is calculated from the
following equation:
vs = (
)1/2
)1/2
where vs = sonic
velocity (m/s)
R =
individual gas constant = 8314/M (J/kg K) where
M = gas molecular weight
T = absolute temperature (K)
Flare Headers and Sub-Headers
The equivalent length of headers and sub-headers should be
determined by reference to Crane Technical Paper No. 410. The maximum Mach number range is 0.5-0.75 calculated at
rated flow rate at the downstream end of line conditions.
Flare Stacks
Flare Stack Diameter is generally sized on a velocity basis
although pressure drop should be checked. For stable flare burning in pipe
flares, API RP 521 recommends 0.5 Mach for a peak short term infrequent flow,
with 0.2 Mach maintained for the more normal and possibly more frequent
conditions. Requirements for pipe flares
and other proprietary designs should be discussed with the vendor.
Blowdown Inlet Lines
The maximum velocity should be limited to 200 ft/s.
Blowdown Outlet Lines and High Pressure Vents
Vent stack diameter is generally sized on a velocity basis
although pressure drop should be checked.
Since there is no requirement to maintain a stable flame, the vent tip
can be sized for sonic velocity. This
minimizes the size and provides gas dispersion.
An allowance must be made in the pressure drop calculation for the
pressure discontinuity which occurs at the tip when sonic velocity is
established.
Atmospheric Tank Vents
Non-refrigerated atmospheric tank vents are sized on the
basis of maintaining an operating pressure that the tank can safely
withstand. Flow rates are determined by
consideration of thermal inbreathing and out-breathing, maximum fluid inflow or
outflow, and vapour production resulting from fire exposure. Refer to References API RP- 2000 for design
criteria.
