ANSI FCI 87-1-2017 pdf free download.Classification and Operating Principles of Steam Traps
1.0 SCOPE
This standard is for the purpose of establishing and illustrating various classifications of Steam Traps in accordance with their basic principles of operation. This standard does not attempt to define details of conception or construction.
2.0 DEFINITIONS
2.1 Steam trap – An integral, self actuated valve which automatically vents air in the steam system and drains condensate from a steam containing enclosure while remaining tight to live steam. Most steam traps will also pass non-condensible gases while remaining tight to live steam. Note: Some designs will allow a minimal steam flow at a controlled or adjusted rate using a separate secondary orifice. 2.2 Production tests – Tests carried out by the manufacturer to confirm that the steam trap functions correctly. These tests may be witnessed by the purchaser or his representative. In this case, these tests are referred to as witness tests. 2.3 Performance characteristics – Carried out to determine the operational characteristics of a particular design of steam trap.
3.0 OPERATING PRINCIPLES
3.1 Factors Affecting Condensate Flow Through a Steam Trap One of the most common components to all steam traps is the discharge orifice. In some trap types, the valve seat and the discharge orifice are the same; in others, the discharge orifice may be smaller than the valve seat. 3.2 In considering steam trap capacities, therefore, it is important to explain what happens when fluid flows through an orifice. While in steam trapping we are concerned with the discharge of cold water, air and other gases,condensate and air mixtures, and hot (flashing) condensate, the main function of the trap is to discharge condensate, usually in the flashing state. The water discharge capacity of an orifice depends on the following factors:
1. The area and shape of the orifice and the coefficient of discharge C D
2. Lift of valve from orifice
3. The pressure drop across the orifice
4. Density of the water
5. The temperature of the water.
6. The physical changes that take place when water flows through the orifice.
3.2.2 Valve Lift
The formula in paragraph 3.2.1 assumes a full open orifice. In a steam trap this is not necessarily true. The fact that, in some stream traps, the valve does not lift fully to the equivalent free area of the orifice does not imply poor design. It, however, makes comparison of steam trap capacities on orifice diameter alone unreliable.
3.2.3 Pressure
Before and After the Orifice (Pressure Drop) The theoretical flow in 3.2.1 is based on the difference in pressure immediately before and immediately after the orifice, which is also called pressure drop. In a steam trap, the body and the mechanism of the trap offer resistance to flow, and the pipe connections on the discharge cause similar interference. So, in practice, the pressures at the orifice proper are rarely, if ever, known and are variable, with a corresponding effect on discharge capacity.
3.2.4 Density of Water
The density of water as compared to steam is another factor to consider. At typical atmospheric conditions, the density of water is 62.3 lbs/ft 3 and for steam it is .037 lbs/ft 3 . Therefore, 1 lb of steam at typical atmospheric conditions occupies a volume of approximately 1700 times that of water. 3.2.5 Temperature and Phase Changes Relative to Discharge Capacity Discharge capacity of condensate through a steam trap is most seriously affected by the physical changes that take place.
When discharging to atmospheric pressure, the physical state of water normally does not change as it flows through an orifice or a trap, provided the temperature is below 212°F at the inlet. Condensate above 212°F at the inlet, when reaching atmospheric pressure, cannot retain all of its heat and some of the heat causes “flash” steam to be generated. The result is a considerable increase in the specific volume and a corresponding reduction in the density of the mixture of steam and water flowing through the orifice. The theoretical effect of “flash” steam generation can be seen in Table 1, showing the volume of 1 lb. of condensate following release to atmosphere from various upstream pressures and temperatures.
