Mechanical Traps

Float Traps

There are many designs. All depend on the fact that a float will float in water and sink in steam. When condensate enters the trap the float rises and opens the discharge valve. The steam pressure then blows the condensate out. The float sinks, or fails to rise further, and a position of equilibrium is reached where the condensate is discharged at the same rate as it is flowing into the trap.

Such a trap is shown in Fig 1 below. The float lever A is connected to the valve arm B by means of the toggle link C. The three levers are so arranged as to give a small movement to the valve for a large movement of the float when the trap is nearly empty, whereas the fuller it gets and the higher the float rises the more rapidly does the valve open.

Plain Float Trap

Figure 1 Plain Float Trap

It will be seen that the whole mechanism is accessible without interfering with the assembly of the trap to the piping, and that the valve and its seat can be renewed by simply removing the casing.

Air Discharge from Float Traps

No float trap, unless provided with a special device, will discharge air or other incondensible gas. At the start-up of any plant all the inside of the heating surface is full of air, and this air must be cleared out before full use can be made of the heating surface. Air and/or CO, is present in even the best steam and finds its way eventually into the trap unless means are provided to remove it. When the trap gets full of air the condensate is unable to get in and the trap goes out of action. The trap is blamed and byepassed, when the fault lies with the plant designer who has made inadequate provision for the removal of air.

Open Bucket Traps

Fig 2 below shows an open bucket trap-there are many designs, but all work on the same principle.

Open Bucket Trap

Figure 2 Open Bucket Trap

Condensate enters at A. It fills up the body of the trap. The bucket B, pivoted at C, floats in the rising condensate until the valve D closes on the seating E. The buoyancy of the bucket clearly provides the valve-closing force in this type of trap. When the valve is closed, the bucket can rise no more, so the rising condensate eventually reaches the level of the bucket top and runs over into the bucket. When sufficient water has got into the bucket to overcome its buoyancy and the force of the steam that is holding the valve shut, the bucket drops and opens the valve. The steam pressure blows the condensate out of the bucket up and out of the discharge pipe until the bucket is sufficiently buoyant to float again. The bucket then rises to the position shown in Fig 2 and closes the valve ; the cycle then recommences. The buoyancy of the bucket is so arranged that the bucket never empties completely thus maintaining a water seal between the valve pipe and the bucket so that steam cannot blow through. The action of an open bucket trap must be intermittent. This type of trap is therefore not suitable for application where it is important that there should be a continuous discharge.

The closing force on this trap is the buoyancy of the bucket. The opening force is the weight of the bucket. When the bucket is full, buoyancy disappears and the whole of the weight of the bucket is available to open the valve against the steam pressure holding the valve shut. It is not so easy with this type of trap to arrange for large leverage between bucket movement and valve movement. This means that if the trap is to handle large outputs it must be very large.

The open bucket trap has the following characteristics. There is no possibility of the bucket collapsing. There is less chance of the bucket leaking than there is of a float leaking. Unless the leak in the bucket is quite large the trap will continue to work. If however the leak gets very large the bucket may not rise to close the valve. If a leak is present the trap will operate more frequently than when in normal condition. If, on normal load, the period of the trap action is noted and it is subsequently seen to be working on a shorter cycle, a leaking bucket should be suspected. The frequency of operation should always be observed at some constant pressure. A lower pressure will allow the trap to operate more frequently as the force holding the valve shut is smaller and can consequently be overcome by less load in the bucket. The bucket cannot collapse and there is consequently no limit to the pressure at which an open bucket trap can work except that imposed by valve-area-times-pressure and bucket weight. The bucket trap is more robust than the float trap. The fact that the discharge is intermittent has the advantage that it can be heard operating, but flash steam is more difficult to collect and use.

In both the float and open bucket traps two forces are used; the buoyancy of the buoyant member and the weight of the buoyant member. Ruggedness and reliability demand that the weight he large and the buoyancy small. Now in a float trap it is the buoyancy that opens the valve and the weight that closes it. In an open bucket trap it is the weight that opens the valve and the buoyancy that closes it. This possibly gives the bucket trap an advantage, but just like the float trap there is a pressure above which any bucket trap cannot work. Take Fig 2. If the valve has an area of 0.25 sq. in. and the mechanical advantage of the bucket on the valve is 2 to 1 it means that at a pressure of 8 times the weight of the bucket, the bucket will not be heavy enough to open the valve. At 120 psi.g. there will be a pressure of 30 lb. on the valve. If the mechanical advantage of the bucket is 2, the weight of the bucket necessary to open the valve will be 15 lb. If the bucket weighs only 12 lb. the trap cannot work at 120 psi.

Before a bucket trap is set to work it should be filled with water to ensure that the bucket floats, and to seal the outlet.

Like float traps, open bucket traps can get locked with air unless some means are taken to remove any air or other incondensible gas.

Inverted Bucket Traps

An inverted bucket trap is shown in Fig 3 below.

Inverted Bucket Trap

Figure 3 Inverted Bucket Trap

Condensate enters through the lower connection and through the upturned pipe A that projects into the inside of the inverted bucket. Any air that is trapped inside the bucket B escapes through the small leak hole C. The condensate accumulates inside the trap body and inside the bucket. Provided the condensate is entering fairly fast the leak hole will be inadequate to even out the pressure inside and outside the bucket so that the water level will rise faster outside the bucket than inside. This gives the float buoyancy and keeps the valve shut. As the condensate accumulates, it fills up outside the bucket and, as fast as the leak will allow, the water rises inside the bucket. When sufficient water has risen inside the bucket, its buoyancy disappears and the weight of the bucket opens the valve. The steam pressure then blows the water out from inside the bucket and round outside and through the outlet. As soon as sufficient water has been blown out of the bucket, its buoyancy is restored, it rises and closes the valve, leaving the trap in the position shown in Fig 3. Now it is clear that the bucket would retain its buoyancy unless the steam and/or air inside the bucket could escape. Air escapes through the leak hole and is eventually discharged with the condensate. Steam escapes through the leak and is condensed in the body of the trap. During discharge steam blows straight through the leak hole out of the discharge, but the quantity so lost is so small as to be of little importance.

The valve opening force is the weight of the bucket, as in the open bucket type. The valve closing force is the buoyancy of the bucket.

The inverted bucket trap has several very real advantages. It is very robust, simple and reliable. It will operate under conditions of movement, for example on ship board. It can be made extremely small and light. It will vent air quite sufficiently to ensure that plant never gets air locked, though it may not remove air sufficiently quickly to be satisfactory on plant working intermittently on very short cycles, for example some hemispherical boiling pans.

The inverted bucket trap has some disadvantages. It must always waste a little steam. During discharge the bucket leak is discharging steam to its full capacity. During water accumulation steam must be allowed to condense in the top of the trap in order to allow the bucket to vent air and thus permit it to fill with water. If the bucket vent gets blocked, the trap locks shut.

It is possible, under certain exceptional circumstances, for an inverted bucket trap to lose its water seal. If, for example, it is used for draining a steam pipe carrying highly superheated steam, it is possible, once the system is up to temperature, for the water in the trap to be evaporated, when the trap will blow steam full bore and cannot reseal itself and will have no desire to close. When inverted bucket traps are used in such circumstances it may be desirable to connect them to the plant by a length of unlagged pipe. It also occasionally happens that the water seal is broken by reflux action. This is particularly likely if the trap has to be fitted above the plant it is draining. In most circumstances a non-return valve should be fitted between the trap and the plant it is draining.

Although in theory an inverted bucket trap should always work intermittently except when on full load, in practice a continuous discharge often takes place. There is no harm in this at low pressures, but at high pressures it is undesirable as valve or seat may be scored. The continuous action may be due to the trap or valve parts being incorrectly sized for the pressure and load under which it is working.

An inverted bucket trap must not be completely lagged. The top should be left bare in order to condense the steam that must be leaked off the bucket. The sides and bottom can be lagged.

Before an inverted bucket trap is set to work it should be filled with water to provide a seal.