A trap is used to remove condensate (water) from compressed air, gas and steam lines. Different types of traps are used in the industry to separate moisture such as:
- Inverted bucket type,
- Float and thermostatic
- Fixed Orifice.
The most common type of trap is an inverted bucket trap shown in Figure 14. Steam, air or gas and water enter at the bottom and initially the steam will push the inverted bucket up. When the trap fills with water, the inverted bucket will drop and the water will be blown out into a condensate line. The bucket then rises again and the hole at the top closes. The cycle repeats itself.
The operating principle is the same for both styles. The bucket initially hangs down and holds the discharge valve open. Condensate enters the trap and flows under the bottom edge of the bucket to fill the trap body. Then the condensate will flow out through the open discharge valve at the outlet. Any steam that enters the trap will collect at the top of the inverted bucket giving it buoyancy and causing it to rise, thus closing the discharge valve. Air and CO2 gas will also collect at the top of the inverted bucket and will pass through the vent at the top of the bucket to the upper part of the trap body. Figure 15 illustrates the operation of an inverted bucket steam trap.
The operation of a mechanical steam trap is driven by the difference in density between condensate, gas, steam or air. The denser condensate rests on the bottom of any vessel containing the two fluids. As additional condensate is generated, its level in the vessel will rise. This action is transmitted to a valve via either a “free float” or a float and connecting levers in a mechanical steam trap.
The main problem with these traps is that they are prone to fail in the presence of dirt, line scales or sand. These contaminants in the system may prevent the moisture release valve from closing. Regular inspection and repairs ensure efficient and long term uninterrupted service from traps.
The purpose of the steam trap is to discharge the water of condensation from steam lines, separators and other equipment without permitting steam to escape. In addition, most traps are designed to discharge any air present in the lines or equipment.
Steam traps should be installed in lines wherever condensate must be drained as rapidly as it accumulates, and wherever condensate must be recovered for heating, for hot water needs, or for return to boilers. They are a “must” for steam piping, separators, and all steam heated or steam operated equipment.
There are numerous trap designs, which can be classified into three types, according to their principles of operation. These classes are:
11.1 Mechanical Traps
In the mechanical trap design, use is made of either a ball float or a bucket float to open and close the trap outlet valve, depending upon whether any condensate is present within the trap body.
The ball float trap. As the condensate enters, the float rises and opens the outlet valve. Then as the condensate is discharged, the float sinks and closes the outlet valve. This type also features a bellows controlled air vent located near the top of the trap. If steam surrounds the bellows, then the bellows will expand and close the vent outlet. However, if air (which is cooler than steam) surrounds the bellows, then the bellows will contract and open the vent allowing air to escape.
Float Trap with Thermostatic Air Vent
11.2 Thermostatic Traps
The operation of thermostatic traps depends upon the difference in temperature between the steam and the condensate. They are commonly used on radiators in steam heating systems; hence they are also known as radiator traps. See Figure. 17.
Thermostatic Steam Trap
As the steam in the radiator gives up its heat, it condenses to water. This water must be removed from the radiator as fast as it is formed. The radiator trap is a device that allows the condensed steam or water to be discharged from the radiator, but prevents any steam from discharging. Essentially the trap consists of a corrugated bellows or flat hollow disc to which is attached a valve-shaped plunger. The bellows contains a volatile fluid, which is caused to boil by the heat from the steam that surrounds the bellows.
11.3 Thermodynamic Traps
This type of trap employs the heat energy in the steam and condensate to control its operation. One design of thermodynamic trap is the impulse trap, shown in Figure 18.
This design consists of a piston type valve working within a control cylinder. When cool condensate enters the trap, the pressure of the condensate acting upon the piston disk will lift the valve to the open position thus allowing the condensate to escape through the outlet orifice. A portion of the condensate, however, instead of escaping through the outlet orifice, passes up past the piston disk into the upper part of the control cylinder and then down through a small hole drilled through the centre of the piston valve to the outlet.
If the condensate entering the trap is at steam temperature then the part entering the upper section of the control cylinder will flash into steam as the section is at a lower pressure (outlet pressure). The large volume of steam resulting will plug or choke the small hole through the centre of the valve and pressure will build up above the piston disc thus forcing the valve into the shut position.
11.4 Trap Inspection and Servicing
Frequency of inspection depends on condition of the line. To isolate the trap for inspection, close the inlet and outlet gate valves and open the sediment blow-off or the test valve to relieve the pressure. Inspection or repairs can be made while the trap is in the line or the trap can be removed from the line easily by loosening the unions.
11.5 Water Hammer
Water hammer is the term used to describe a series of shocks produced by a sudden change in velocity of water flowing within a steam pipeline. This sudden change of velocity may be caused by the rapid closing or opening of a valve or by the rapid condensation of a pocket of steam within the pipe.
Another situation which can produce water hammer is the sudden stopping of a motor driven centrifugal pump due to a power interruption or “trip out”. When this happens, the water in the pump discharge line will stop and then reverse direction. Subsequent rapid closing of the check valve at the pump will cause water hammer.
Water hammer will also be produced if steam is admitted to a pipe containing water or condensate. The steam on passing through, above the surface of the water, will raise up behind it a mass of water and thus a pocket of steam will be formed. This steam will rapidly condense due to contact with the water and a vacuum will be formed in the pocket. The water rushing into this vacuum will produce water hammer which will rupture piping or fittings.
It must be stressed therefore, that before admitting steam to any piping system, all water or condensate must be positively removed from all parts of the system. Traps which are fitted to main lines, branch lines, and separators for drainage purposes must be installed with bypass lines around them which may be opened to ensure positive drainage.