Tuesday, March 27, 2012
Posted by Ankit Chugh on 8:45 PM
The temperature of the process liquids being transferred through pipelines often must be maintained to meet the requirements of a process, to prevent thickening and solidification, or simply as an anti-frost measure. This is achieved by the use of jacketed pipes; or by attaching to the product line one or more separate tracer lines carrying a heating medium such as steam or hot water.
The steam usage may be relatively small but the tracing system is often a major part of the steam installation, and the source of many of the problems. Many large users and plant contractors have their own inhouse rules for tracer lines, but the following guide-lines may be useful in other cases.
One or more heat carrying lines of sizes usually from 10 mm (3/8)” up to 25 mm (1") nominal bore are attached to the main product pipe. Transfer of heat to the product line may be done in three ways. – by conduction through direct contact, by convection currents in the air pocket formed inside the insulating jacket, and by radiation. The tracer lines may be of carbon steel or copper, or sometimes stainless steel.
Fig 1.1 - Insulating tracer and product lines
Where the product line is of a particular material to suit the fluid it is carrying, the material for the tracer line must be chosen to avoid electrolytic corrosion at any contact points.
For short runs of tracer, such as around short vertical pipes, or valves and fittings, small bore copper pipes perhaps 6 mm (1/4") bore may be wound around the product lines as in Figure 1.2. The layout should be arranged togive a continuous fall along the tracers as in Figure 1.3 a rather than Figiure 1.3 b.
Fig 1.2 - Typical correct and incorrect arrangement
Figure 1.3 Traces lines around pump casing
The simplest form of tracer is one that is clipped or wired on to the main product line. The maximum heat flow is achieved when the tracer is in tight contact with the product line. The securing clips should be no further apart than 0.3 to 0.45m (12" to 18") on 10 mm (3/8") tracers, 0.45 to 0.6 to 0.9m (24" to 36") on 20 mm (3.4") and larger.
Figure 1.4 Installation of three tracers
The tracer pipes can be literally wired on, but to maintain close contact it is better to use either galvanised or stainless steel bands, about 15mm (1/2") wide and 1.25mm or 0.9mm (18 to 20 SEG) thickness. One very practical method is to use a packing case banding machine. Where tracers are carried around bends particular care should be taken to ensure that good contact is maintained by using three or more bands as in figure 1.4.
Where it is not possible to use bands as at valve bodies, soft annealed stainless steel wire 1.25mm (18SWG) thick is a useful alternative.
Where the temperature difference between the tracer and the product is low, the tracer may be welded to the product line. This can be done either by short run welds as in figure 1.5a or by a continuous weld as in figure 1.5b for maximum heat transfer.
Figure 1.5 Steam tracing elementary diagram
In these cases the tracer is sometimes laid along the top of the pipe rather than at the bottom, which greatly simplifies the welding procedure.
The product being carried in the line can be sensitive to temperature in some cases and it is then important to avoid any local not spots on the pipe.
This is done by introducing a strip of insulating material between the tracer and the product pipe using gals fibre or mineral wool or sometimes packing blocks of inert material as distance pieces.
The insulation must cover both the product line and the tracer but it is important that the air space remains clear. This can be achieved in more than one ways.
1. The product line and tracer can first be wrapped with aluminum, foil, or by galvanized steel sheet, held on by wiring and the insulation is then applied outside this sheet. Alternatively, a small mesh galvanised wire netting can be used in the same way as metal sheet figure 1.7a.
Figure 1.6 Typical trace line and jacket line
2. Sectional insulation, preformed to one or two sizes larger than the insulation is then applied outside this sheet. Alternatively, small mesh galvanized wire netting can be used in the same way as metal sheet figure 1.7b.
Figure 1.7 Flanges for jacketted piping
3. Preformed sectional insulation designed to cover both product line and tracer can be used, as in figure 1.7C.
Preformed sectional insulation is usually preferred to plastic material because being rigid, it retains its thickness and efficiency better. In all cases the insulation should be properly cover all parts otherwise it becomes useless as heat conserving material if mechanical damage is allowed.
The tracing or jacketing of any line normally aims at maintaining the contents of the line at a satisfactory working temperature under all conditions of low ambient temperature with adequate reserve to meet extreme conditions.
Remember that on some exposed sites, with an ambient still air temperature of say-18 deg. C (0 deg. F) the effect of a 24 km/h (15 mph) wind will be to lower the temperature to an equivalent of-38 deg. C (36 deg. F).
Even 0 deg. C (32 deg. F) in still air can be lowered to an effective-16 deg. C (4 deg, F) with a 30 km/h (20 mph) wind. Such circumstances which must be taken into full consideration when studying the tracer line requirements. Most of the sizing of external tracers is done by rule of thumb.
Rule of thumb practices are generally based on the experiences of a certain company on a particular process and do not necessarily apply elsewhere. There are also widely differing opinions on the layout: some say that multiple tracers should all be below the centre line of the product line whilst other say with equal conviction that it is perfectly satisfactory to space the tracers equally around the line.
Then there are those who will endeavour to size their tracers from 10 mm (3.8"), 15 mm (1/2"), 20 mm (3/4") or 25mm (1") and even larger pipe: whilst another school of thought says that as tracers have only minute contact with the product line it will give much more even distribution of heat if all tracers are from15mm (1/2") pipe in multiples to meet the requirements. This does have the added advantage of needing to hold a stock of only one size of pipe and fittings rather than a variety of sizes.
Type A would suffice for most fuel oil requirements and would also meet the requirement of those lines carrying acid, phenol, water and some other chemicals but in some cases spacer tracing would be employed. The steam pressure is important and must be chosen according to the product temperature required.
For Types A and B (Table 1.1) a steam pressure of 3.5 bar (50 psi) would generally be suitable but for Type C higher pressure may be required.
Table 1.1 - Number of 15mm ½” tracers used with different product line sizes
Ideally jacketed lines should be constructed in no more than 6m, 20 ft lengths and the condensate removed from each section.
Steam should enter at the highest end so that there is a natural fall to the condensate out let.
When it is considered impractical to trap each length, a number of lengths up to a total of 24-30m (80-100 ft) approx may be formed together in moderate climates, but in extremely cold parts of the world 12 m (40 ft) should be maximum.
Always avoid connecting solely through the bottom loop. This can only handle the condensate and baulks the free of steam as.
As a general guide see Table 1.2
Although in most cases 15 mm (1/2") condensate outlet will be adequate, it is usual to make this the same size as the steam connection as it simplifies installation.
Table 1.2 - Steam connection size for jacketed lines
In horizontal runs the steam will generally flow parallel to the product line but as far as possible steam should enter from the high end to allow free flow of the condensate to the low end i.e., it should always be self-draining.
It is generally considered preferable to fit one tracer on the bottom of the line as two tracers at 30 Deg. as three tracers at 45 Deg.
In vertical lines the tracers would be spaced uniformly.
The maximum permissible length of tracer will depend to some extent of the size and initial steam pressure but following, Table 8.3, is a general guide.
Table 1.3 Maximum length of tracer
Bends and low points in the tracer should always be avoided. For example if it is necessary to carry a tracer line round a pipe support or flange this should be done in the horizontal plane.
Where it is essential to maintain the flow of heat to the product the tracer should be taken up to the back of the flange and the coupling should always be on the centre line of the flanged joint.
The same applies to an in line run where the tracer has to be jointed. This can be done in two ways figure.
Typical branch connection
1. Provided A Slitted Spool Up Ward Or Down Ward The Branch On The Main Jacket.
2. Continuous weld Shall Be Done After Testing Or X-ray Of Process Line. If Required.
Each of these is preferable to which could produce a cold spot. Pumps shall be heated by means of externally wrapping steam heating coils around the pump casing and sloping continuously pump must be removable while the tracers supplying suction and discharge piping remain intact.
In general, instruments shall be protected by separate tracers except for pressure gauges, which may be protected by the pipeline of equipment tracer.
Expansion in tracer lines is something which is often over looked. Naturally the steam heated tracer will tend to expand more than the product line. Where the tracer has to pass around flanges the bends are quite adequate to take care of the expansion.
But where this does not occur and there is a long run of uninterrupted tracer, it is essential to provided for expansion which can be done by forming a complete loop.
It is important that the steam supply should always be taken from a source which is continuously available even during a normal shut down period. All distribution or supply lines should be installed at an elevation above the highest point of lines requiring steam tracing, if possible the condensate – collection or return lines should be located at an elevation low enough to permit gravity flow from all connected lines.
Tracer lines and jacketed pipe may have to work at any steam pressure (usually in the range between 0.7 to 17 bar 10 to 250 psi) but always choose the lowest pressure to give the required product temperature. Excessively high pressures cause much waste and should only be used where a high product temperature is essential.
To suit product temperature requirements, it may be necessary to use steam at different pressures. It should be distributed at the highest pressure and reduced to meet the lower pressure requirements. A Spirax Saco Reducing valve can be used for this purpose.
Note : It may be necessary to steam trace the valve body to stop the water freezing in the diaphragm chamber.
A number of tracers can be supplied from one local distribution header. This header should be adequately sized to meet the maximum load and drained at its low point by a steam trap as. All branches should be taken off the top of this header, one branch to each tracer line. These branches should be fitted with isolating valves.
The size of the header will, of course, depend upon the steam pressure and the total load on the tracers but the following, Table – 1.4, is suggested as a general guide :
Table 1.4 Suggested header size of steam supply header
No more than 15 tracers shall be served by one distribution header.
This in general should always follow the rule. One tracer-one trap. No two tracers can have exactly the same duty so group trapping two or more tracers to one trap can considerably impair the efficiency of heat transfer.
Even with multiple tracers on a single product line each tracer should be separately trapped.
When branch tracers are taken to serve valves then each should be separately trapped.
Pipes delivered to the site may contain mill scale, paint, preserving oils etc. and during storage and erection will collect dirt, sand, weld splatter and other debris, so that on completion the average tracer line contains considerable amount of ‘muck’.
Hydraulic testing will convert this ‘muck’ into a mobile sludge which is not adequately washed out by simply draining down after testing.
It is most important that the lines are properly cleaned by blowing through with steam to an open end before diverting to the steam traps.
Unless this is done the traps will almost certainly fail to operate correctly and more time will be spent cleaning them out when the plant is commissioned. Almost any type of steam trap could be used to drain tracer lines, but some lend themselves to this application better than others. The trap should be physically small and light in weight, and as they are often fitted in exposed positions they should be resistant to frost. The temperature at which the condensate is discharged by the trap is perhaps the most important consideration, in selection between types.
Thermodynamic traps are the simplest and most robust of all traps. They meet all the above criteria and they discharge condensate at a temperature close to that of steam. Thus they are especially suitable on those critical tracing applications where the holding back of condensate in the tracer line until it has sub-cooled would be unacceptable. Tracers or jackets on lines carrying sulphur or bitumen typify these applications where the tracer must be at steam temperature along its whole length.
It must be remembered that every time a thermodynamic trap opens, it discharges condensate at the maximum rate corresponding to the differential pressure applied. The instantaneous release rates of the steam flashing off the condensate can be appreciable, and care is needed to ensure that condensate return lines are adequately sized if high back pressures are to be avoided. Thus, the use of swept connections from trap discharges into common headers of generous size will help avoid problems.
Just as the distribution of steam is from a common header, it often is convenient to connect a number of traps to a common condensate header and this simplifies maintenance. As noted, the discharge should preferably enter the header through swept connections and the headers be adequately sized as suggested in the Table 1.5.
Table 1.5 Suggested header size for steam condensate lines
No more than 15 tracers shall be served by one condensate collection line. These sizes may be increased where high pressures and traps discharging condensate at near steam temperature are used or decreased with low pressures and traps discharging cooler condensate.
1. Weld S. S. Flange to S. S. Pipe (Weld – A)
2. String S. S Pipe to the C. S. Jacket spool
3. Make Weld (B)
4. Sliding C.S. Jacket Spool, carry out NDT of weld (A) & (B) (If required as per specs).
5. Make Weld (c)
6. Weld C.S. Mitre to the Jacket Spool.
7. Prepare the Spool (D) with jacket (E), as per steps 1-2-4, without Branch.
8. String the spool (F) to the S. S. Pipe (D) and weld the S.S Branch.
9. Carry out NDT of Weld (G), sliding the Jacket Spool (F).
10. String the Jacket Spools (H) & (I) and weld them to the Spool (F).
11. Make Weld (L) and carry out NDT (if required as per specs).
12. Weld the C.S. Splitted Spools.
13. Weld the C.S. Splitted Spools.
14. Carry out Hydrostatic Test of the Jacket.
• NDT indicates Non-Destructive Testing of a weld joint.
• Above sequence of fabrication should be used only as a guide. It should be modified to suit the shape of a jacketed line.
• In the above sample, the jacket elbow is a 3-piece mitre. Normally, jacketed elbow are as follows :
Process Line : Long Radius (R=1.5D) E1bows
Jacket : Short Radius (R=D) Elbow/Mitre
Design and engineering of lines like tracer, core and jacket are different not only for routing but care has to taken from the construction and maintenance point of view. These lines always work in different environments ( i.e. at low or elevated temperature ) so jacketed and traced lines require special consideration from the piping engineer.
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