1) For direct discharge of fluids to atmosphere, they should be in the vapor state, below their auto ignition temperature, and should meet one of the following requirements:-
• Flammable vapors of MW less than 28.9 (MW of air).
• Flammable vapors heavier than air with MW less than 70 but with minimum discharge velocity of 500 fps (152 m/s), based on maximum capacity of the relief valve
• Vapors of any MW that are non-flammable, non-toxic and non-condensable 2) In all other cases relieved fluids should be disposed to a flare system.
3) Boilers having more than 47 m² of water heating surface or electric boilers having a power input of more than 500 kW shall have 2 or more PSV.
4) For electric boilers the minimum relieving capacity shall be 1.6 kg/hr/kW input
5) For Boilers, if additional PSV are used the highest pressure setting shall not exceed the MAWP by more than 3%. When multiple PSV are installed, the difference between the highest and lowest set pressure should not be greater than 10% of the highest set pressure.
6) For Pressure vessels, if additional PSV are used the highest pressure setting shall not exceed 105% of MAWP. For multiple PSV, one of which is installed for fire exposure only, this particular valve may be set at a pressure not exceeding 110% of MAWP
7) Fire exposure protection of vessels by water spray @ 0.05 - 0.2 gpm/ft² of total vessel area.
Fire exposure is considered for a wetted area up to 25 ft (7.62 m) from grade. Average or normal liquid level to be considered for vessels, High liquid level for columns, surge drums and KO drums. 50% of storage tank height or 25 ft (7.62 m) from grade, which ever is higher.
8) For flare fire relief load, consider plot area between 2000-5000 ft² (186-465 m²). Use 2500 ft² (232 m²) for a paved drained surface in a plant where NFPA required fire fighting equipment is available.
9) If fire proofing insulation is not provided, environmental factor (F) should be = 1.0
15.2 SAFETY VALVE BACK PRESSURE
1) Super imposed back pressure = pressure at PSV discharge before valve opening 2) Builtup back pressure = pressure at PSV discharge header after valve opening.
3) Conventional PSV shall be used when the superimposed back pressure varies over a range not exceeding 10% of the set pressure (gauge). However the performance of PSV at builtup back pressure to be studied by the vendor before selecting the PSV
4) Balanced type PSV can be used for >10% superimposed back pressure. However when builtup back pressure exceeds 30-50% of set pressure, capacity of PSV for vapors and gases starts to fall below the theoretical capacity. With liquids, the capacity reduction starts at 15% of set pressure. The fall in capacity depends on overpressure, type and make of PSV.
5) Pilot operated relief valves are used when operating pressure is very close to set pressure or when set pressure is below 10-15 psig (0.7 – 1.0 barg)
15.3 SAFETY VALVE RELIEF LOAD CALCULATION
PSV’s relief load can be calculated by considering the following major failure scenarios:
1) Blocked outlet 2) Gas blow by case
3) Inadvertent valve opening 4) Reflux failure
5) Tube rupture 6) Fire case 7) Reverse flow 8) Thermal relief case.
15.3.1 Blocked Outlet
Blocked outlet case can occur when the control valves at the outlet of the vessel close at the same time due to the stoppage of instrument air supply or plant shut down.
The relief quantity shall be the design flow rate to the vessel.
15.3.2 Fire Case
The amount of heat absorbed by a vessel open to fire is markedly affected by the type of fuel feeding the fire.
The following equivalent formulas are used to evaluate the condition where there are prompt firefighting efforts and drainage of flammable materials away from the vessels are available:
18
Where adequate drainage & firefighting equipment do not exist, the following equation should be used:
82 .
34500 F A0
Q= × ×
Where q = Average unit heat absorption (Btu/h/ft² of wetted surface) Q = Total heat absorption (input) to the wetted surface (Btu/h)
F = Environmental factor (Values for various type of installation are given in Table 5 in API 521 page 25)
A = Total wetted surface (ft²)
The discharge area for pressure relief devices on vessels containing super critical fluids, gases or vapors exposed to open fires can be estimated by using the following equation.
1
The relief load can calculated directly, in pounds per hour.
⎟⎟
Where A = Effective discharge area of the valve (in²) A’ = Exposed surface area of the vessel (ft²)
F’ = PSV Factor (= 0.01 min. or 0.045 if value is not known) P1 = Upstream relieving pressure (psia)
P1 = PSV set pressure + allowable over pressure + atmospheric pressure C = Cp/Cv
KD = Coefficient of discharge (=0.975 max) M = Molecular weight of the gas
TW = Vessel wall temperature (R) (for CS vessels = 1100°F)
T1 = Gas temperature (R) at the upstream relieving pressure, determined from the following relation.
n
n T
P P T1=( 1/ )×
Where Pn = Normal operating gas pressure (psia) Tn = Normal operating gas temperature (R) Air Coolers
For air coolers heat absorption equation becomes
0 .
21000 F A1
Q= × ×
Total bare tube area to be considered instead of finned area, since fins are burned out in the first few minutes of fire.
Water spray nozzles are some times mounted below the tubes in case of fire. API 520 recommends a minimum of 0.05 - 0.2 gpm/ft² water spray rate.
15.3.3 Inadvertent Valve Opening
1
Y = Downstream pressure (bara) X = Ratio of pressure drop P1 = Upstream pressure (bara)
γ1= Upstream gas density (kg/m³) 15.3.4 Thermal Expansion
As per API RP 521, the relieving capacity requirements for hydraulic expansion cases will be very small as relieving fluid will be liquid. Therefore ¾” X 1” nominal pipe size relief valve is commonly used
Cold fluid blocked in and continuous heat input from hot fluid. Two conditions can occur:
Cold fluid remains liquid and expands c
Q B
WL = × / Cold fluid vaporizes
L
Where WL, Wv = weight of liquid or vapor relieved (lb/h) Q = Normal exchanger duty (Btu/h)
L = Latent heat of vaporization at relieving conditions (Btu/lb) T1 = Hot side inlet temperature (°F)
Tbp = Cold side boiling temperature at relieving pressure (°F)
Tav = Average of inlet and outlet temperature of cold side during normal operation (°F)
c = Specific heat of cold medium (Btu/lb °F ) B = Coefficient of expansion of cold medium (1/°F)
Fluid °API Expansion Coef. Fluid °API Expansion Coef.
Oil 3 to 34.9 0.0004 Oil 79 to 88.9 0.0008 Oil 35 to 50.9 0.0005 Oil 89 to 93.9 0.00085 Oil 51 to 63.9 0.0006 Oil =>94 0.0009 Oil 64 to 78.9 0.0007 Water 0.0001
15.3.5 Tube failure and leakage
Tube failure is considered a viable contingency when the design pressure of the low pressure side is less than 2/3 rd (=67%) of design pressure of high pressure side.
However, if the high pressure side of the exchanger operates at 1000 psig (69 barg) or more and contains a vapor or liquid that can flash or result in vaporization of liquid on the low pressure side, complete tube failure should be considered, regardless of the pressure differential.
Install a PSV, if the piping and downstream equipment on the low pressure side do not have the capacity to handle material leaked from high pressure side without exceeding 110% of the equipment design pressure.
A tube failure is considered to be a sharp break in one tube. The high pressure fluid flows through both openings, which is equal to twice the cross section area of a single tube.
Following equation can be used to calculate the flow from high to low pressure sides.
ρ
d = Tube inside diameter (mm) C = Orifice coefficient
∆P= Differential pressure (Operating pressure of high pressure side (bara) - Relieving pressure of PSV (1.1 times set pressure of PSV) (bara)
ρ = Density of high pressure fluid (kg/m³)
For a discharge coef. of 0.7, Cp/Cv=1.33 and twice the cross sectional area of one tube
Where qL, qv = Quantity of liquid (gpm) or vapour (lb/h) d = tube inside diameter (inch)
M = MW
P1 = normal high pressure side (psig) Sg = specific gravity
P2 = 1.1 times low pressure design pressure (psig) Z = compressibility factor
T = vapor temperature (°R) at operating conditions
If the calculated discharge exceeds the normal total flow in the high pressure side, the latter flow should be used.
Possible flash of liquid to vapor shall be taken into account due to both pressure reduction and mixing of a volatile fluid with a hot fluid.
Valves on low pressure side provided only for isolation may be assumed fully open, control valves in a position equivalent to the minimum normal flow unless the valve could automatically
close due to the emergency situation.
15.4 SETTLE-OUT PRESSURE
In high pressure oil & gas handling facilities the compressor suction KO drum has to be designed based on settle-out pressure. In case of compressor trip the suction and discharge shutdown valves will close and the gas from high pressure discharge side will flow through the recycle control valve to the suction side. A new settle-out condition is reached.
The settle-out pressure is calculated by the following formula:
2
Where Ps = Settle-out pressure (barg)
P1 = Suction side pressure before settle-out (barg) P2 = Discharge side pressure before settle-out (barg)
V1 = Suction side volume (including KO drum, piping, etc) (m³) V2 = Discharge side volume (including KO drum, piping, etc) (m³)