These traps distinguish between water and steam by the fact that water can be cooler than steam under the same pressure. It follows that they can never discharge condensate immediately it is formed. From this it follows that unless they are fitted to plant where the heated material can absorb some of the sensible heat, heat must be deliberately got rid of in order to cool the condensate to a point where the trap can make its choice.
They have, however, several great advantages. They are very small and light. They remove air with great readiness. They can, with one exception, withstand water hammer without injury. They are open and empty when the plant is shut down and are not therefore liable to damage by frost. They can work at any pressure however high because there is no inherent limitation as in the case of the buoyancy traps. They can work in conditions of movement and vibration that may be unsuitable for the mechanical traps. They are unsuitable for handling very large quantities of condensate, and they cannot discharge condensate at steam temperature.
Fig 1 shows a design of metallic expansion trap.
Figure 1 Metallic Expansion Traps
Condensate enters the tube A and flows out past the valve C. The tube A is made of copper which expands considerably more than the iron body B. When the tube A expands it closes the opening at C by moving towards C. The valve C is pressed towards the tube A by the Spring E. The two nuts D allow adjustment to be made so that the valve shuts tight at any desired temperature. The spring E allows the valve to be pressed back by undue expansion of tube A should the trap be fitted to a line where the temperature is higher than that for which the valve is set, or in case superheated steam reaches the trap.
The trap is very robust and simple, but, unless it is very large, its rate of discharge is small. This trap is a pure thermostat. It opens at a certain (adjustable) temperature ' which must be quite definitely below steam temperature. The discharge is irregular-it is neither truly continuous or properly intermittent. It is wide open at the start up and will therefore clear the system of air rapidly and effectively.
Fig 2 shows a liquid expansion trap. Liquids expand more than solids for a given rise of temperature, so that, by using a liquid, it is possible to get quite a fair valve movement by direct expansion. A liquid expansion trap can therefore be smaller than a metallic expansion trap.
Figure 2 Liquid Expansion Trap
The tube A is filled with a suitable liquid-probably oil of some kind. One end of tube A is brazed into the cap B which carries a screw by which adjustment is made. The other end of the tube is brazed into another cap C which is brazed to the corrugated bellows tube D. The other end of the bellows tube is brazed to the piston E. The bellows tube D acts as a leak-proof gland between the liquid tube A and the piston rod F. The end of the piston rod is screwed into the valve G. When the liquid gets heated its expansion pushes out the piston and closes the valve.
The temperature at which the trap closes can be adjusted by screwing the nut H, after loosening the lock-nut I. This alters the length of the expansion unit and so adjusts the amount by which the piston must move to close the valve. The adjustment nut H is pressed tight against the body of the trap by the heavy spring J. If the trap is overheated, continued expansion after the valve has closed will push the expansion unit back against the spring J and thus protect the trap from damage. Safety precautions of this sort are necessary in both liquid and metallic expansion traps. The expansion force is to all intents and purposes irresistible. If the trap were adjusted to close at saturation temperature and by some chance superheated steam were to reach the trap there would be nowhere for the expansion to go were there no safety spring. Even if superheated steam did not reach the trap there may well be conditions where extra expansion takes place. It takes some little while for the element to get up to temperature. If the trap is not to pass steam while acquiring extra heat, it must be set to close before it has been completely heated up. After the valve has closed the element will still go on absorbing heat and will continue to expand.
This trap is a pure thermostat. It opens and closes at a certain (adjustable) temperature. The discharge temperature must be well below steam temperature. It is wide open when cold, and discharges air freely and efficiently.
It is robust and will withstand water hammer. It can be made suitable for any pressure. It is somewhat sluggish in response and is not made in sizes to handle very large quantities.
Traps of the metallic or liquid expansion types can be fitted so that the expansion element is either on the outlet or the inlet side. If it is fitted with the element on the outlet side a little steam will continually be allowed to pass. This steam will keep the element warm and close the valve. If the element is downstream of the valve the steam pressure is available to help to open the valve ; this may sometimes be advantageous.
Although these traps operate by thermal expansion, they work on a totally different principle to the last two types. Fig 3 shows a balanced pressure expansion trap.
Figure 3 Balanced Pressure Expansion Trap
The expansion element A consists of one or more capsules or bellows. These are filled with a liquid (for example, a mixture of water and alcohol) that boils at a temperature lower than water. When the element is heated, the liquid inside it boils, part of it vaporises and the element expands and so closes the valve B. Although the boiling distends the bellows due to the increase of pressure inside the element, this internal pressure is opposed by the pressure outside. Suppose the trap is used on much higher pressure condensate. The boiling liquid inside it will exert a much higher pressure, but this is opposed by the much higher pressure acting on the outside. There is thus a constant expansion movement at all pressures and the amount of this movement is fixed by the boiling point of the liquid inside the element compared to that of water. At very high pressures the pressure inside the element is very high, but, although the element is very fragile, this does not matter because the element is prevented from bursting by the very high pressure outside.
In the design shown in Fig 3 nuts C and D are provided for adjustment, and spring E makes some provision for preventing damage to the element due to overheating at too low a pressure.
If superheated steam reaches the element the whole balance of forces is upset. The liquid inside boils vigorously and expands greatly with high internal pressure which is not balanced by a high external pressure. Balanced pressure traps must never be used where there is any possibility of superheated steam reaching them. The bellows or capsule is thin and fragile and cannot withstand corrosion, so these traps must not be used where the steam has any corrosive properties. The fragile element will not stand up to the shock of water hammer and must not be used where water hammer is a possibility.
The discharge from these traps is irregular. It is not inherently intermittent, but there is a lag while the liquid is taking in or giving up heat which causes the discharge to be semi-continuous.
The trap will open whenever it is in contact with anything that is cooler than condensate at the pressure at which the plant is working. It will therefore pass air whenever the air has cooled to below steam temperature. Balanced pressure traps are particularly suitable for use as automatic air vents as they are more responsive than liquid expansion traps. Balanced pressure traps are very light, small and cheap. They are not suitable for handling large quantities.