Relief Valve Standard API Effective Orifice Area

Relief Valve Standard API Effective Orifice Area

API has generated the standard API Effective Orifice Area with corresponding designation letter (following table). Besides, standard effective orifice area, it also has a series of corresponding inlet size, outlet size combination for various pressure classes of flanged relief valves. All these information are tabulated in API Std 526 Flanged Steel Pressure Relief Valves.

API Relief Valve Standard Effective Discharge Orifice Areas
Designation	in2	cm2
D	0.110	0.71
E	0.196	1.26
F	0.307	1.98
G	0.503	3.24
H	0.785	5.06
J	1.287	8.30
K	1.838	11.85
L	2.853	18.40
M	3.600	23.23
N	4.340	28.00
P	6.380	41.16
Q	11.050	71.29
R	16.000	103.22
T	26.000	167.74

One shall remember above effective areas are valid only when used with the sizing equations contained in API RP 520 Part I - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery.

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HC Gas Heat capacity ratio (Approx.)

Heat capacity ratio (or Isentropic exponent, k) is the ratio of specific heat capacity at constant pressure (C_P) to specific heat capacity at constant volume (C_V). It is commonly used in critical pressure ratio estimation, compression head estimation, etc. Following is a approximate k factor of Hydrocarbon gases. It may be used when only HC gas molecular weight (MW) and temperature (T) is available.

Source : GPSA

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Typical Efficiency for Compressor

Compressor efficiency is very much subject to compressor type, design, etc. It possibly range from 50% to 90%. Each compressor from different manufacturer may have different efficiency. Following are some typical compressor efficiency may be used during design phase. It shall be decided by compressor manufacturer.

Centrifugal : 75-80%
Note : nowadays several manufacturers may have some compressor efficiency upto 84%.

Centrifugal : 75-85%

Reciprocating :
- 65% with Compression ratio of 1.5
- 75% with Compression ratio of 2.0
- 80-85% with Compression ratio of 3-6

Rotary : 70%

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Why CO2 with present of water is corrosive ?

Carbon dioxide (chemical formula: CO₂ ) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and present in oil and gas fluid. CO₂ by itself is not corrosive, however, with the present of free water (H₂O), it becomes corrosive and poses a thread to many oil and gas facilities.

CO₂ with present of free water would lead to generation of Carbonic acid (H₂CO₃).

CO₂ + H₂O ==> H₂CO₃ ==> H⁺ + HCO₃^2-

When Carbonic Acid contact with steel (Fe), reaction occur.

2Fe + H₂CO₃ ==> Fe₂CO₃ + H₂

With minimum level of Oxygen would aggregate the corrosion :
Corrosion without Acid Carbonic :

4Fe + 3O₂ + 6H₂O ==> 4Fe(OH)₃

and Corrosion with Acid Carbonic :

Fe + 2H₂CO₃ ==> Fe(HCO₃)₂ + H₂

2Fe(HCO₃)₂ + 1/2 O₂ ==> Fe₂O₃ + 4CO₂ + 2H₂O

Carbonic acid is weak acid would reduce the pH of fluid and this further aggregate the corrosion.

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