Steam Traps(MEO CLASS 4)

Almost all mariners are familiar with the problem of steam hammering that occurs in steam lines. One reason for that problem is the generation and accumulation of steam condensate in the steam lines. To drain the condensate from the steam lines, steam traps are used.

Steam trap is nothing but a special type of automatic valve that prevents the passage of steam but allows the condensate to flow through. Generally used in steam lines, they drain the condensate without affecting the steam. This helps in drawing all the energy from steam and thus utilizing its latent heat as the steam is contained in the heating line until it is fully condensed.The three main types of steam traps are:

  • Mechanical
  • Thermostatic
  • Thermodynamic

 

Before we get into the description of the three main types of traps, let’s take a look at the earliest and simplest form of steam trap. The orifice trap consists simply of a circular disc or a small pipe nipple with a small hole drilled at the lowest point of the equipment. The steam condensate will flow through the orifice and the steam gets locked because of its higher volume.

Thus the three main functions of any steam trap are :

  • To prevent steam from escaping
  • To allow the condensate to pass through
  • To discharge air and other gases

    Mechanical Steam Trap

    Mechanical steam traps are installed with ball floats or open floats to control a needle valve which controls the release of condensate. The float moves in accordance with the condensate level. A mechanical linkage attached to the float controls the opening and closing of the float, which solely depends on the level of condensate accumulated in the steam trap.

 Fig. 11.3.1   Float trap with air cock
  • Thermostatic Steam Trap

Thermostatic trap, also known as the temperature trap uses expansion of a bimetallic strip, an oil filled element or a flexible below to activate a valve.

In an oil filled element type steam trap ( as shown in the figure), the element A expands as the temperature of the condensate rises. The expansion of element A leads to the closing of valve D. The temperature at which the valve needs to be actuated is to be set with the help of an adjustment screw E. The pressure of the system in which this type of valves are fitted, keeps on varying and thus there are chances of waterlogging or escaping of the steam.

The bi-metallic strip type steam traps are more efficient than the oil filled steam traps. The bimetallic strip expands and deflects as the temperature of the condensate increases. The deflection of the strip thus closes the valve. This type of steam trap can work on a wide range of pressure without readjustments. It can also work under superheat conditions without water hammering or vibrations.

In the flexible bellow type, the bellows is filled with a mixture that boils at a temperature lower than that at which the steam does. The trap holds back some of the condensate till it fully cools down in order the valve to open, thus self-compensating for operating pressure. This trap cannot resist any kind of water hammer or vibrations and would immediately damage if subjected to superheated steam.

Thermodynamic steam trap

There are two basic categories of thermodynamic steam traps: Thermodynamic Disc and Thermodynamic Impulse. Of the two, disc traps are most commonly used, perhaps because impulse traps can leak pilot steam, and may fail in the presence of even a slight amount of dirt blocking the pilot channel. For these reasons, this article will only cover disc-type traps.

The thermodynamic trap is an extremely robust steam trap with a simple mode of operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap, as depicted in Figure . The only moving part is the disc above the flat face inside the control chamber or cap.

On start-up, incoming pressure raises the disc, and cool condensate plus air is immediately discharged from the inner ring, under the disc, and out through three peripheral outlets (only 2 shown, Figure , i).

Hot condensate flowing through the inlet passage into the chamber under the disc drops in pressure and releases flash steam moving at high velocity. This high velocity creates a low pressure area under the disc, drawing it towards its seat (Figure , ii).

At the same time, the flash steam pressure builds up inside the chamber above the disc, forcing it down against the incoming condensate until it seats on the inner and outer rings. At this point, the flash steam is trapped in the upper chamber, and the pressure above the disc equals the pressure being applied to the underside of the disc from the inner ring. However, the top of the disc is subject to a greater force than the underside, as it has a greater surface area.

Eventually the trapped pressure in the upper chamber falls as the flash steam condenses. The disc is raised by the now higher condensate pressure and the cycle repeats (Figure , iv).

The rate of operation depends on steam temperature and ambient conditions. Most traps will stay closed for between 20 and 40 seconds. If the trap opens too frequently, perhaps due to a cold, wet, and windy location, the rate of opening can be slowed by simply fitting an insulating cover onto the top of the trap.

Advantages of the thermodynamic steam trap

Fig. 11.4.2   - Thermodynamic steam trap
Fig. 11.4.2
Thermodynamic steam trap
  • Thermodynamic traps can operate across their entire working range without any adjustment or change of internals.
  • They are compact, simple, lightweight and have a large condensate capacity for their size.
  • Thermodynamic traps can be used on high pressure and superheated steam and are not affected by water hammer or vibration. The all stainless steel construction offers a high degree of resistance to corrosive condensate.
  • Thermodynamic traps are not damaged by freezing and are unlikely to freeze if installed with the disc in a vertical plane and discharging freely to atmosphere. However, operation in this position may result in wear of the disc edge.
  • As the disc is the only moving part, maintenance can easily be carried out without removing the trap from the line.
  • The audible ‘click’ which occurs as the trap opens and closes makes trap testing very straight forward.

Disadvantages of the thermodynamic steam trap

  • Thermodynamic steam traps will not work positively on very low differential pressures, as the velocity of flow across the underside of the disc is insufficient for lower pressure to occur. They are subjected to a minimum inlet pressure (typically 0.25 bar g) but can withstand a maximum back pressure of 80% of the inlet pressure.
  • Thermodynamic traps can discharge a large amount of air on ‘startup’ if the inlet pressure builds up slowly. However, rapid pressure build-up will cause high velocity air to shut the trap in the same way as steam, and it will ‘air-bind’. In this case a separate thermostatic air vent can be fitted in parallel with the trap. Modern thermodynamic steam traps can have an inbuilt anti-air-binding disc which prevents air pressure building up on top of the disc and allows air to escape, (Figure ).
  • The discharge of the trap can be noisy and this factor may prohibit the use of a thermodynamic trap in some locations, e.g. outside a hospital ward or operating theatre. If this is a problem, it can easily be fitted with a diffuser which considerably reduces the discharge noise.
  • Care should be taken not to oversize a thermodynamic trap as this can increase cycle times and induce wear. Mains drainage applications often only need to be fitted with low capacity versions, providing proper consideration is given to siting the drain pockets correctly.
Fig. 11.4.3  Anti-air-binding disc
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