Sizing and Selection of Air Vents

Air vents allow gases to automatically vent from liquids under pressure. The following steps will guide your selection of the appropriate vent for your steam system.

1) Estimate the amount of air to be vented in CFM at standard atmospheric pressure of 14.7 psia and 70 degrees F. (Cubic meters per minute at 1 bar and 21 degrees C)
2) Indicate the maximum operating pressure of your system.
3) Indicate the material of construction desired.
4) Indicate minimum specific gravity of the liquid.
5) Use the designated capacity to find the orifice size using the table on this page.
The table is expressed as imperial values. To view this table in SI metric units, click here.
6) Find vent or vents with the correct orifice size to handle required capacity, verifying that vent will operate at the required maximum operating pressure of the system at the required minimum specific gravity. Also, verify that selected vent is constructed of suitable materials for your application. To do this, you will need to download our Air Vents literature. pdf (438k)

Discharge of Air Through an Orifice in Standard Cubic Feet per minute at a standard atmospheric pressure of 14.7 psia and 70°F.
Pressure psig	Orifice diameter, inch
Pressure psig	1/16	5/64	3/32	#38	7/64	1/8	9/64	5/32	3/16	7/32	1/4	9/32	5/16	11/32	3/8	7/16	1/2	9/16	5/8	3/4	7/8	1-1/16
5	0.65	1.01	1.45	1.55	1.98	2.58	3.27	4.03	5.81	7.90	10.3	13.1	16.1	19.5	23.2	31.6	41.3	52.3	64.5	92.9	126	186
6	0.71	1.10	1.59	1.70	2.16	2.82	3.57	4.41	6.35	8.64	11.3	14.3	17.6	21.3	25.4	34.5	45.1	57.1	70.5	102	138	204
7	0.76	1.19	1.71	1.83	2.33	3.04	3.85	4.75	6.84	9.31	12.2	15.4	19.0	23.0	27.3	37.2	48.6	61.5	76.0	109	149	220
6	0.86	1.34	1.93	2.07	2.63	3.43	4.34	5.36	7.72	10.5	13.7	17.4	21.4	25.9	30.9	42.0	54.9	69.4	85.7	123	168	248
12	0.98	1.54	2.21	2.37	3.01	3.93	4.98	6.14	8.85	12.0	15.7	19.9	24.6	29.7	35.4	48.2	62.9	79.6	98.3	142	193	284
15	1.09	1.71	2.46	2.63	3.34	4.37	5.53	6.82	9.83	13.4	17.5	22.1	27.3	33.0	39.3	53.5	69.9	88.4	109	157	214	316
20	1.25	1.95	2.81	3.01	3.82	4.99	6.32	7.8	11.2	15.3	20.0	25.3	31.2	37.7	44.9	61.1	79.8	101	125	180	245	361
25	1.38	2.16	3.11	3.33	4.23	5.53	6.99	8.63	12.4	16.9	22.1	28.0	34.5	41.8	49.7	67.7	88.4	112	138	199	271	399
30	1.54	2.40	3.46	3.70	4.71	6.15	7.78	9.61	13.8	18.8	24.6	31.1	38.4	46.5	55.3	75.3	98.4	125	154	221	301	444
35	1.73	2.71	3.90	4.17	5.31	6.93	8.77	10.8	15.6	21.2	27.7	35.1	43.3	52.4	62.4	84.9	111	140	173	250	340	501
40	1.93	3.01	4.34	4.64	5.9	7.71	9.75	12.0	17.3	23.6	30.8	39.0	48.2	58.3	69.4	94.4	123	156	193	277	378	557
45	2.12	3.31	4.77	5.10	6.49	8.47	10.7	13.2	19.1	26.0	33.9	42.9	53.0	64.1	76.3	104	136	172	212	305	415	612
50	2.31	3.61	5.2	5.56	7.07	9.24	11.7	14.4	20.8	28.3	37.0	46.8	57.7	69.9	83.1	113	148	187	231	333	453	667
60	2.69	4.20	6.05	6.48	8.23	10.8	13.6	16.8	24.2	32.9	43.0	54.4	67.2	81.3	96.8	132	172	218	269	387	527	777
70	3.06	4.79	6.90	7.38	9.39	12.3	15.5	19.2	27.6	37.5	49.0	62.1	76.6	92.7	110	150	196	248	306	441	601	886
80	3.44	5.37	7.74	8.28	10.5	13.8	17.4	21.5	31.0	42.1	55.0	69.7	86.0	104	124	169	220	279	344	495	674	994
90	3.81	5.96	8.58	9.2	11.7	15.3	19.3	23.8	34.3	46.7	61.0	77.2	95.3	115	137	187	244	309	381	549	747	1102
100	4.19	6.54	9.42	10.1	12.8	16.7	21.2	26.2	37.7	51.3	67.0	84.8	105	127	151	205	268	339	419	603	820	1210
110	4.56	7.12	10.3	11.0	14.0	18.2	23.1	28.5	41.0	55.8	72.9	92.3	114	138	164	223	292	369	456	656	893	1317
125	5.11	7.99	11.5	12.3	15.7	20.5	25.9	32.0	46.0	62.7	81.8	104	128	155	184	251	327	414	511	737	1003	1478
150	6.04	9.44	13.6	14.6	18.5	24.2	30.6	37.8	54.4	74.0	96.7	122	151	183	217	296	387	489	604	870	1184	1746
200	7.89	12.3	17.8	19.0	24.2	31.6	40.0	49.3	71.0	96.7	126	160	197	239	284	387	505	639	789	1136	1547	2281
250	9.74	15.2	21.9	23.5	29.8	39.0	49.3	60.9	87.6	119	156	197	243	295	351	477	623	789	974	1402	1909	2814
300	11.6	18.1	26.1	27.9	35.5	46.3	58.6	72.4	104	142	185	235	290	350	417	568	741	938	1158	1668	2271	3348
400	15.3	23.9	34.4	36.8	46.8	61.1	77.3	95.5	137	187	244	309	382	462	550	748	978	1327	1527	2199	2994	4414
500	19.0	29.6	42.7	45.7	58.1	75.8	96.0	119	171	232	303	384	474	574	683	929	1214	1536	1896	2731	3717	5480
600	22.6	35.4	51.0	54.6	69.4	90.6	115	142	204	277	362	459	566	685	815	1110	1450	1835	2265	3261	4439	6546
750	28.2	44.0	63.4	67.9	86.3	113	143	176	254	345	451	571	704	852	1014	1381	1803	2283	2818	4058	5523	8144
1000	37.4	58.4	84.1	90.1	115	150	189	234	337	458	598	757	935	1131	1346	1832	2393	3029	3740	5385	7329	10807

Example: Find a model number that will vent 52 CFM (1.47 cmm) of air (including a safety factor of 1.5 to 2.0 CFM/.04-.05 cmm) from a liquid with a specific gravity of 0.93 at 250 psi (17.24 bar). Using the table above, follow the 250 psi (17,24 bar) line across to the number 60.9(1.72). Orifice size is 5/32". Now check the vent table for the 5/32" orifice lines to locate a vent for 250 psi (17.24 bar) or higher with 0.90 gravity liquid.* The Model 3-AV is the correct vent to use in this case.

* Since specific gravity in this case falls between 0.95 and 0.90, use 0.90 gravity data.