Moisture Measurement & Control in SF6 Gas Switchgear

Partial Pressure of water vapour : That part of a total pressure of a gas that is exerted by the water vapour component in the gas. Expressed in units of pressure such as pascals (Pa), millibar (mbar) or millimetres of mercury (mmHg). Saturation water vapour pressure : The maximum pressure of water vapour that can exist in a gas at a given temperature. Expressed in units of pressure as partial water vapour pressure. Parts per million (Volume) : The ratio of the volume of water vapour to the total volume of gas. (Mole fraction times on million). Expressed as ppm (v). Parts per million (Weight of Mass) : The ratio of the weight or mass of water vapour too the total weight or mass of gas. (Specific humidity times one million) Expressed as ppm (w). Dewpoing : The temperature at which dew, or condensation, forms on cooling a gas. This is. in effect, the temperature at which gas becoming saturated in equilibrium with water. Although below freezing point, the correct term for saturation in equilibrium with ice is "Frostpoint", through common practice, the term "Dewpoint", irrespective of temperature, is used. It should be noted that, in the table of hygrometric equivalents on the opposite page, all negative "Dewpoints" are in equilibrium with ice. Dewpoint is expressed in degrees celsius (0C) or degrees fahrenheit (0 F). Absolute Humidity : The mass of water vapour in a unit volume of moist gas at a given temperature and pressure. Expressed as grams per cubic metre (g/m2) or pounds of water per million cubic feet of gas (lb / MMSCF). Relative Humidity : Relative humidity is the ratio of water vapour pressure to the saturation water vapour pressure, at a given temperature and pressure, expressed as a percentage.

 

If the moisture content is known in PPMV, to find the dew point in C the water vapour pressure is calculated by multiplying the ppmv measurement by the system absolute pressure to obtain the partial pressure of the water vapour. Table below shows the relationship between temperature and saturated water vapour pressure. If the water vapour pressure at the corresponding temperature shows in table, the water vapour begins to condense.

 

Table : 1

Pressure of saturated aqueous vapour in mm, Hz for temperature of -59 ⁰C to +109 ⁰C

Temp 0C	0	1	2	3	4	5	6	7	8	9
-50	0.029	0.026	0.023	0.02	0.017	0.015	0.013	0.012	0.01	0.009
-40	0.096	0.086	0.076	0.068	0.06	0.054	0.048	0.042	0.037	0.033
-30	0.288	0.259	0.233	0.209	0.188	0.169	0.151	0.135	0.121	0.108
-20	0.783	0.712	0.646	0.585	0.53	0.48	0.434	0.392	0.354	0.319
-10	1.964	1.798	1.644	1.503	1.373	1.252	1.142	1.041	0.947	0.861
-0	4.58	4.22	3.887	3.578	3.291	3.025	2.778	2.55	2.34	2.144
0	4.58	4.92	5.29	5.680	6.100	6.54	7.01	7.51	8.04	8.61
10	9.21	9.85	10.52	11.24	11.99	12.79	13.64	14.54	15.49	16.49
20	17.55	18.66	19.84	21.09	22.4	23.78	25.24	26.77	28.38	30.08
30	31.86	33.74	35.7	37.78	39.95	42.23	44.62	47.13	49.76	52.51
40	55.4	58.42	61.58	64.89	68.35	71.97	75.75	79.7	83.83	88.14
50	92.6	97.3	102.2	107.3	112.7	118.2	124	130	136.3	142.8
60	149.6	156.6	164	171	179.5	187.8	196.3	205.2	214.4	224
70	233.9	244.2	254	266	277.4	289.3	301.6	314.4	327.6	341.2
80	355.4	370.3	385.2	400.8	417	433.7	451	468.8	487.3	506.3
90	526	546.3	567.2	588.8	611.1	634.1	657.8	682.2	707.4	733.3
100	760	787.5	815.9	845	875.1	906	937.8	970.5	1004	1039

 

What is the dew point of an SF6 system having a moisture content of 300 PPMV at guage pressure 152 kPa (22psig)

Pressure = 300 * 10 ^-4 (22 + 14.7) 51.7 = 0.57mmHg

The 51.7 multiplier is used to convert the pressure from psia to mmHg for use with Table :1

(760 mmHg = 14.7 psi, so 1 psi = 760/14.7 = 51.7 mmHg)

 

The pressure act which the measurment was taken now enters the converssion calculation. For example : if the guage pressure was 600 kPa (87 psig) when the same moisture reading of 300 PPMV was taken, the dew point is now - 12 C. Thus, when converting and comparing PPMV or PPMW and dwe point reading, the pressure at which the reading is recorded is extreamly important

 

Many instruments that measure dew pointoperate at the system pressure. Instruments that measure PPMV are designed for use at pressure slightly higher than atmospheric pressure - gauge pressure slightly above 0 kPa (0 psig) to provide gas flow for sampling. Normaly, the system sampling valve is opened slightly to mentain the gas flow and take a PPMV measurment at a pressure of slightly above atmospheric pressure. This difference in instrument operation shall be taken into account when converting and comparing dew point and PPMV levels

 

Features User selectable units - 0C, 0F, ppm, ppm (w), ppb, g/m3, lbs/MMSCG Various ranges between - 1100C + 200C dewpoints Rechargeable Battery - Over 250 hours of continuous operation on full charge Desiccant dry down assembly for quick measurements 4 - 20mAanalogue output RS485 serial communication Real time graphic logging to PC Advanced DATA logging & PC download Electronic automatic calibration Pressure correction computation Multi - level security to prevent unauthorised tamper True hand held portable device - weighing less than 1.2 kgs Robust ergonomically designed custom housing Fully self - contained User friendly

 

SF6 decomposition products in electrical discharges

Table 2 - Recommended moisture limits and specification from various manufacturers.   Moistures Limit (In PPMV)     Breakers Other Equipments Remarks Manufacturers       A 400 550   B 100   Dead tank puffer - 1   400   Dead tank puffer - 2   810   Live tank puffer     800 Bus C 120 600   D 300 300   E 70   Two pressure type     150 Bus with disconnect switch     500 Bus w/o disconnect switch F 150 1000   SF6 gas manufacturer       A 11* 11* Typical new gas specification Standards       IEC 176 (1971) * (B1) 120* 120* New SF6 gas ASTM D2472 - 90* (B1) 71 71 New SF6 gas