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524 CHAPTER 21 VALVE ACCESS PLUG 1956 Guide Steam Heating Systems 525 Fig. 21. Single Port Float Trap Fig. 22. Multiport Float Trap high to allow gravity flow of the condensate to the boiler. If the boiler pressure is too high for such gravity feed, then an additional steam pump for feeding the boiler is desirable. The extra pump is sometimes avoided by using a closed separating tank, with a. float controlled vent. In both arrangements, the air taken from the system must be discharged against the full discharge pressure of the vacuum pump. In the case of high or medium pressure boilers, it is better to use the atmospheric separator and the second pump. In figuring the required displacement for such pumps, a value of from 6 to 10 times the volumetric flow of condensate is used for average vacuums and systems. STEAM TRAPS Steam traps, as the name implies, are automatic devices used to trap or hold steam in an apparatus or piping system until it has given up its latent heat, and to allow condensate, and air to pass as soon as it accumu lates. In general, traps consist of a vessel in which to accumulate the condensate, an orifice through whioh the condensate is discharged, a valve to close the orifice port, mechanisms to operate the valve, and inlet and outlet openings for the entrance and discharge, of the condensate from the trap vessel. Steam traps are classified according to the type of operating device by which they function. The traps which are available on the market today may be classified as (1) float, (2) thermostatic, .(3) float and thermostatic, (4) upright bucket, (5) inverted bucket, (6) flash, (7) impulse, (8) tilting, (9) lifting, and (10) boiler return trap or alternating receiver. Float Traps. Float traps operate by the rise and fall of a float due to a change of condensate level in the trap. When the trap is empty, the float is in its lowest position and the discharge valve is closed. As condensate accumulates in the trap Fig. 25. Ttpical Float and Thermostatic Trap Fig. 26. Upright Bucket Trap chamber, the float rises and gradually opens the valve, and the pressure of the steam pushes the condensate out ofthe valve. The discharge from a float trap is generally continuous, since the opening of the valve is proportioned to the flow of condensate through the trap. A gage glass may be used to indicate the height of condensate in the trap chamber. Unless float traps are well made and proportioned, there is danger of considerable steam leakage through the discharge valve due to unequal expansion of the valve and seat, and the sticking of moving parts. Float traps are made in sizes from \ to 3 in., and for pressures varying from vacuum conditions to 200 psig. Float'traps are used for draining condensate from steam separators, steam headers, blast coils, heating systems, steam water heaters, laundry equipment, sterilizers, and other equipment. When used for draining low pressure systems, float traps should be equipped with a thermostatic air vent (See Float and Thermo static Traps). Figs 21 and 22 illustrate types of float traps which are in use at the present time. Thermostatic Traps. Thermostatic traps function by means of elements which expand and contract under the influence of heat and cold. Early types of thermo static traps employed carbon posts and bi-metallic elements for expansion. In general, the modem type of thermostatic trap consists of thin corrugated metal bellows or discs enclosing a hollow chamber. The chamber is either filled with a liquid, or a small amount of a volatile liquid, such as alcohol, is introduced. The liquid expands or becomes a gas when steam comes in contact with the expansive ele ment. The pressure created in either case expands the element and closes the valve of the trap against the escape of steam. When condensate or air comes in contact with the element, it cools and contracts, opening the valve and allowing them to escape. The discharge from this type of trap is intermittent. Thermostatic traps find their use generally for the draining of condensate from radiators, convectors, pipe coils, drips,- unit heaters, water heaters, cooking kettles, and other equipment. Except for radiators and convectors, it is recommended that a strainer be installed' on the inlet connection to the trap to prevent dirt, pipe scale, and other foreign sub stance from entering the trap. A cooling leg of a length of pipe should also be pro vided ahead of the trap on unit heaters and similar apparatus to cool the condensate in order to help in the trap action. Thermostatic traps are made in sizes from i OUTLET Fig- 27- Inverted Bucket Trap with Central Guide