Document NEMVb1gVzx6jOvm9NQyBMp7Qg

440 CHAPTER 23 1946 Guide Vacuum Pump Controls In the ordinary vacuum system, the vacuum pump is controlled by a vacuum regulator which cuts in when the vacuum drops to the lowest point desired and cuts out when it has been increased to the highest point, these points being varied to suit the particular system or operating conditions. In addition to this vacuum control, a float control is included which will automatically start the pump whenever sufficient condensation ' accumulates in the receiver, regardless of the vacuum on the system. A selector switch is usually provided to allow operation at night as a con densation pump only, also to give manual or continuous operation when desired. There are several variations in the control of the vacuum maintained on the system by the pump. In some sub-atmospheric systems where orifices are used, the vacuum pump control maintains a pressure difference between the supply and the return piping, which is held within relatively close limits. There are other sub-atmospheric systems which utilize special temperature-pressure actuated controls for maintaining the desired conditions in the return lines. Where various zones are connected to the same return main, the return vacuum must-be controlled to meet the requirements of the zone operating at the lowest steam supply pressure. Piston Displacement Vacuum Pumps Piston displacement return vacuum heating pumps may be either elec tric or steam driven. Their piston speed in feet per minute should not exceed 20 times the square root of the number of inches in their stroke. They are usually supplied with an air separating tank, open to atmos phere, placed on the discharge side of the pump and at an elevation sufficiently 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 some times 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 atmos pheric 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 condensation is used for average vacuums and systems. TRAPS Traps are generally classified as to function as (a) separating traps, (b) return, lifting or vacuum traps, and (c) air traps. Separating'traps may be either float operated, thermostatically operated, or float and. thermostatically operated. Return traps for low pressure service are referred to later as alternating receivers in this chapter. Return traps may also operate to receive condensate under a vacuum and return it to atmosphere or a higher pressure. Air traps are generally float operated. Separating traps are used to release water of condensation but to retain steam. The thermostatic, and float and thermostatic types release both condensate and air but retain steam. Separating traps are used for draining condensate from radiators, indirect air heaters, steam piping systems, kitchen equipment, laundry equipment, hospital equipment. Steam Heating Systems and Piping 441 drying equipment and many other kinds of apparatus. Air traps release air but retain water. Devices known as air vents are, in principle, traps which allow the passage of air but prevent the passage of either water or steam. Return traps are used for returning condensate either by. gravity, by steam pressure, or by both, to a boiler or other point of disposal, and for lifting condensate from a lower to a higher elevation, or for handling condensate from a lower to a higher pressure. The fundamental principle upon which the operation of practically all traps depends is that the pressure within the trap at the time of discharge shall be equal to, or slightly in excess of, the pressure against which the trap must discharge, including the friction head, velocity head and static head on the discharge side of the trap. Traps may also be classified according to the principle of operating device which supplies the power to cause them to function as (1) float, (2) bucket, (3) thermostatic, (4) float arid thermostatic, (5) impulse, or. (6) tilting traps. Float Traps. A discharge valve is operated by the rise and fall of a float due to the change of water level in the trap. When the trap is empty the float is in its lowest position, and the discharge valve is closed. A gage glass may be used to indicate the height of water in the 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. The discharge from a float trap is usually continuous since the height of the float, and consequently the area of the outlet, is proportional to the amount of water present: Bucket Traps. Bucket traps are of two types, the upright and inverted, and although they are both of the open float constructiori, their operating principle is entirely different.- In the upright bucket trap, the water of condensation enters the trap and fills the space between the bucket and. the walls of the trap. This causes the bucket to float and forces the valve against-its seat, the valve and its stem usually being fastened to the bucket. When the water rises above the edges of the bucket it flows into it and causes it to sink, thereby withdrawing the valve from its seat. This permits the steam pressure acting on the surface of the water in the bucket tofbrce the water to a discharge opening. When the bucket is emptied it rises and closes the valve and another cycle begins. The discharge from this type, of trap is intermittent. In the inverted bucket trap, steam floats the inverted submerged bucket. and closes the valve. Water entering the trap fills the bucket, which sinks and through compound leverage opens the valve, and the trap dis charges. It is impossible to install a water gage glass on an inverted bucket trap, but if visual inspection is necessary, a gage glass can be placed on the line leading to the trap. No air relief cocks can be used, but they are unnecessary, as the elimination of air is automatically taken care of by air passing through the vent in the top of the inverted bucket regardless of temperature. Thermostatic Traps. Thermostatic traps are of two types, those in which the discharge valve is operated by the relative expansion of metals, and those in which the action of a volatile liquid is utilized for this purpose. - Thermostatic traps of large capacity for draining blast coils or very large: radiators are called blast traps.