Condensate traps are commonly employed to remove condensed water from steam utilising plant and equipment. In this context they are normally referred to as steam traps. Nigel Egginton, EBE Engineering, explains.
There are many different designs of steam traps to suit a variety of circumstances. The majority of traps involve a self actuating mechanism which detects the presence of condensate in the trap, and when necessary opens to allow the condensate to drain.
Faulty steam traps represent a significant source of wasted energy and condensate as well as replacement and maintenance costs. Dependent upon individual maintenance routines, around 15% to 25% of steam traps could be found to have failed open or partly open within a factory or site at any one time. The consequence of this is that companies are wasting hundreds of thousand of pounds in energy and water, which not only represents a major cost in lost steam but also means additional expenditure in maintaining, storing, inspecting and replacing mechanical steam traps.
Steam traps need to be working at optimum efficiency to minimise their impact on the environment. For example, for each litre of heavy fuel oil burned unnecessarily to compensate for a steam leakage, approximately 3kg of CO2 is emitted to the atmosphere. Over one year this represents sufficient CO2 emissions to equate with the emissions from 25 cars.
Mechanical traps open when condensate is present and close to prevent steam escaping. The mechanical mechanism is activated by various methods, such as internal floats, buckets, bimetallic bellows or discs. On continuous applications, mechanical traps open and close several times a minute, which results in wear and leakage. Mechanical trap manufacturers estimate that the typical lifecycle is about three to five years.
All mechanical devices can and do break down and mechanical steam traps are no exception. Dirt, pressure surges due to sudden steam valve openings and incorrect piping or trap misapplications are some of the main reasons for failure resulting in either the trap leaking or failing closed. Additionally, when such steam traps fail open, and discharge into condensate return systems, they cause pressurisation of the condensate lines, which inhibits trap drainage and often reduces heat output and production.
To summarise, the problems that arise with mechanical traps are: 1. Failure open and partly open thereby wasting steam 2. Parts require maintenance 3. Oversized orifice required due to intermittent condensate discharge 4. Spare parts needed 5. Heat loss from the body of the trap 6. Only fair to medium resistance to wear and tear 7. Only fair to medium resistance to water hammer 8. Require ongoing specialist testing 9. Typically only 1 year guarantee
Manufacturers can ensure that steam trap failure does not occur by installing Venturi Orifice solid state traps. It is not too difficult to understand the scepticism with regard to the solid state Venturi Orifice trap’s performance. It appears too simple and too good to be true. It is relatively simple as it is based upon pure physics, but it is this advanced technology, new engineering concepts and a better understanding of steam systems, that has made the non-mechanical Venturi Orifice trap effective.
Venturi orifice steam traps have no moving parts to fail open thereby wasting steam and energy, or jamming shut risking water hammer and system failure. They comprise of a Venturi and an orifice through which condensate is discharged. Flash steam, produced as the pressure drops during flow through the Venturi, acts to reduce the amount of condensate that escapes through the orifice.
A plate or simple orifice trap has a limited operating range on varying loads. It will work if the loads are relatively constant e.g. distribution systems. The Venturi Orifice trap works by combing Venturi technology with the orifice. The capacity of the Venturi Orifice trap is related to the size of the orifice and also to the back pressure generated inside the Venturi. It is a combination of these two factors that gives the Venturi Orifice trap its overall capacity.
As the condensate passes through the orifice there is a pressure loss. On the upstream side of the orifice (the heat exchanger or steam line side) the condensate has the same pressure and temperature as the steam and therefore contains a lot of energy (it’s hot). As it drops pressure across the orifice, the temperature and pressure of the condensate reduces, resulting in it containing less energy. However, energy cannot disappear. So the difference in energy between the high pressure/temperature upstream side and the low pressure/temperature downstream side (i.e. the condensate return system) is converted into steam.
The higher the pressure difference across a trap (and it is the same for all traps) the more condensate has to be converted into ‘flash’ steam. Venturi Orifice technology uses this flash steam to create a back pressure inside the Venturi.
As the condensate is forced through the orifice of the steam trap by the upstream pressure, the resultant pressure drop generates flash steam. This flash steam is 1,000 times the volume of the condensate, so the sudden expansion results in the condensate being accelerated in the Venturi part of the trap. This sudden acceleration creates an opposite and equal force or back pressure inside the Venturi, which acts to restrict the flow of condensate through the orifice.
Because the amount of flash steam changes, depending upon the operating conditions, the resultant back pressure also changes. This then becomes a self regulating flow of condensate through the trap that gives variable capacity characteristics.
Many companies carry out a steam trap inspection irregularly by which time at least ten percent of their steam traps could have failed open, shut or partly open. Losses can include not only wasted energy but also replacement of damaged equipment and misuse of man hours. Fortunately, installing the low maintenance Venturi orifice steam traps can prevent many of these potential losses.