Mechanical steam traps

The groupings then may be described as mechanical, which will include both ball float and inverted bucket steam traps thermostatic, which will include both balanced pressure and bimetallic elements and thermodynamic or disc pattern traps (Figure 22.13). Each type of trap has its own characteristics, and these will make one pattern of trap more suitable for use on a given application than another. In practice, it is usual to find that the applications in any given plant fall into a small number of categories, and it often is possible to standardize on a quite small number of trap types. 

Steam traps are installed in condensate, mechanical return systems and are a frequently overlooked item for reducing operating costs. Large industrial process plants typically have many hundreds of steam traps installed to recover low-energy condensate and remove (potentially corrosive) air and carbon dioxide. 

Steam traps are automatic mechanisms that remove low heat-content air and condensate from the steam delivery system. The lack of steam traps or use of traps that fail to function properly leads to a gradual decline in heat-transfer efficiency, waterlogged heat exchangers, and water hammer (which may in turn result in ruptured pipes). When adequate maintenance of steam traps is neglected, this ultimately leads to a serious overall loss of operating efficiency. There are various types of steam traps, each designed for a specific function. Some common variations are discussed in the following sections. 

Even where effective external mechanical deaeration and/or internal oxygen scavenging is provided, oxygen still may present a serious risk as a consequence of air ingress into leaking pipes, steam traps, and air vents. 

Apart from the oxygen corrosion that results in HW and LP steam heating systems where water losses occur as a result of leaking pump mechanical seals, excess BD, faulty steam traps, and other sources, a subsequent effect is the development of fouling. This effect stems from the production of corrosion debris and (high iron content) sludge that eventually settle out in the boiler. This corrosion debris, sludge, and other foulants must be periodically removed from the boiler by BD, which merely adds to the water loss, and the cycle perpetuates. 

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

After modification works in steam-water circuit, such as diversion of steam trap drains to dump condenser, connection of LPFT recirculation line upstream of gland steam condenser, provision of spray cooling arrangement for exhaust hood of main condenser, revision of operating level of LPH - 1 and rectification of mechanical electrical deficiencies in TG system, TG was rolled continuously for 100 h at 3000 rpm in March 1997. Salient featmes of TG rolling trials were as follows ... 

A steam trap is essentially a control valve with a simple mechanism for separating steam and condensate. It is designed in such a way that it opens to allow... 

Many evaporator calandrias condense steam at subatmospheric pressures. Consequently, condensate must be removed by pumping, either with mechanical pumps or pumping steam traps. Large condensate loads are usually best handled when liquid levels are controlled and condensate removed with the appropriate method. 

For a given operating condition, the density of condensate is considerably different from that of steam or air. The principle is used by mechanical-type traps. This density difference always exists, except at the critical pressure however, the densities of steam and air are too close for distinction. It is also important to note that any flash steam produced in the line from the calandria is regarded as live steam. The trap will dose even though condensate is foilowing the pocket of flash steam. 

The energy in steam can easily be transformed into mechanical or heat energy upon condensation. A steam-generation system is designed to safely return cooled condensate to the boiler. A device called a steam trap is used to collect and transfer this material. Low points in the steam system are used to capture cooled condensate before it can damage the piping or equipment. 

The inverted-bucket trap, which is shown in Figure 22.1, is a mechanically actuated steam trap that uses an upside-down, or inverted, bucket as a float. The bucket is connected to the outiet valve through a mechanical linkage. The bucket sinks when condensate fills the steam trap, which opens the outlet valve and drains the bucket. It floats when steam enters the trap and closes the valve. 

When properly selected, installed, and maintained, steam traps are relatively trouble-free and highly efficient. The critical factors that affect efficiency include capacity and pressure ratings, steam quality, mechanical damage, and calibration. 

Inverted-bucket and float-type steam traps are highly susceptible to mechanical damage. If the level arms or mechanical linkages are damaged or distorted, the trap carmot operate properly. Regular inspection and maintenance of these types of traps are essential. 

There are two problems with the use of the control scheme shown in Fig. 13.1. First, steam traps are mechanically unreliable. Second, as valve A closes, the pressure in the lower half of the channel head may briefly fall below the frequently unstable pressure in the condensate... 

The steam trap neither aids nor retards condensate drainage, unless it is mechanically malfunctioning, meaning it is sticking either open or closed. Also, replacing the steam trap with a condensate drum and LRC also neither retards nor aids drainage. Condensate drainage is a function of system hydraulics, as discussed in subsequent sections. 

The metals industry faces a similar phenomenon. For example, a high percentage of used aluminum products are recycled by being added to a vat of molten aluminum. If water is present in the recycled material (for example, an aluminum can may still contain liquid) a violent explosion can occur. This event is sometimes referred to as a steam trap explosion. The Aluminum Association has outlined the following mechanism for such an explosion ... 

A represents mechanical pump or steam ejector B, booster pump D, cryo, turbomolecular, sorption, ion, or trapped diffusion pumps. 

In a i-l. three-necked flask are mixed 150 g. (r.63 moles) of /3-hydroxyethyl methyl sulfide (p. 54) (Note i) and 200 g. of dry chloroform (Note 2). The flask is placed on a steam bath and is fitted with a dropping funnel, a mechanical stirrer, and a condenser. The condenser is fitted with a trap to remove the vapors of hydrogen chloride and sulfur dioxide (page 2). A solution of 204 g. (1.7 moles) (Note 3) of thionyl chloride in 200 g. (135 cc.) of dry chloroform is added dropwise to the /3-hydroxyethyl methyl sulfide over a period of about two hours (Note 4). The reaction mixture is stirred vigorously during this addition and for about four hours after the addition is complete. The chloroform is distilled on the steam bath and the residue is distilled under reduced pressure. The yield is 135-153 g- (75 5 per cent of the theoretical amount) of a product boiling at 55-s6°/3o mm- (Note 5). 

The condensate return (returned, condensate or simply condensate) tank is positioned at the lowest point in a condensate return (CR) system and collects condensate from the various steam heating and process applications. If traps or condensate pumps are installed to aid the return of the condensate (as is usually the case), this is called a mechanical CR system. Nevertheless, some parts of the system may return condensate by gravity either to a condensate tank or directly to a FW tank. 

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