It is understood that this valve is placed between the ordinary throttle valve and the pump.
_The Bordo blow-off valve_ is shown in Figs. 638 and 639; it consists of a brass or iron body which resembles the shell of a plug-cock, but with this difference, it has a sharper taper than the regular plug-cock; in this device the plug is usually made of brass—tinned on the outside. In process of making and while hot a sheath of babbitt metal or its equivalent is cast upon the plug; the metals amalgamate and practically become one casting.
The parts of valve are as follows, 1, the body, 2, the plug, 3, the packing and lifting gland, 4, the lifting cam, 5, lock-nut, 6, two brass rings of equal size, with a special gasket between them—all as shown in the engravings.
_The valve is operated_ with a wrench on the square of the plug. The lifting gland when adjusted is permanently held by a lock-nut. By releasing the lock-nut with the wrench and turning the gland to the left, the plug is lifted so that it will turn easily. When the lock-nut is moved up, the lifting cam (which couples the packing gland to the plug) can be pulled out; the gland is then free to be removed for repacking. In use the best method of handling is to open and close the valve slowly—never with a jerk.
_The tendency toward higher pressure_ steam boiler installation has made apparent the need of a blow-off, like this one described, made of strong material and correct in principle; in fact the high steam pressures now carried have almost made a revolution in boiler appliances.
The Fig. 640 represents two valves applied to the end of a blow-off pipe. The valve next to the boiler is open at all times excepting when the operating valve, next to the sewer is to be attended to for repairs, etc.
_The table below is_ intended to correspond with the letters to be seen in the illustration, Fig. 640.
==================================================================== Size | Diam.|Centre|Centre|Centre|Centre| Diam. | No. |Diam.|Thick. in | of | to | to | to | to |of Bolt| of | of | of Inches|Flange| Face |Bottom| Top |Bottom|Circle |Bolts|Bolts|Flange +------+------+------+------+------+ | | +------ | A | B | C | D | E | | | | I ======+======+======+======+======+======+=======+=====+=====+====== 1 |4-1/2 |2-1/2 |4-1/2 |4-1/2 |2-1/8 |3-1/4 | 4 | 1/2 |11/16 ------+------+------+------+------+------+-------+-----+-----+------ 1-1/4| 5 |2-3/4 |5-1/8 |5-1/4 |2-1/2 |3-3/4 | 4 | 1/2 | 3/4 ------+------+------+------+------+------+-------+-----+-----+------ 1-1/2| 6 |3-1/4 |5-7/8 | 6 |2-3/4 |4-1/2 | 4 | 5/8 |13/16 ------+------+------+------+------+------+-------+-----+-----+------ 2 |6-1/2 |3-3/4 | 7 |7-1/4 |3-3/8 | 5 | 4 | 5/8 | 7/8 ------+------+------+------+------+------+-------+-----+-----+------ 2-1/2|7-1/2 |4-1/4 |8-1/4 |8-5/8 | 4 |5-7/8 | 4 | 3/4 | 1 ------+------+------+------+------+------+-------+-----+-----+------ 3 |8-1/4 | 5 |9-1/2 | 10 |4-5/8 |6-5/8 | 8 | 5/8 | 1-1/8 ------+------+------+------+------+------+-------+-----+-----+------