SHELL & TUBE HEAT EXCHANGERS Installation, Operation, and Maintenance Instructions
1. Provide sufficient clearance at the channel
or bonnet end of the unit to permit removal of tube bundles from shells. On the floating head end, a space of 3 or 4
feet should be provided to permit the removal of the floating head.
2. Foundations must be adequate so that
exchangers will not settle and cause piping strains. Foundation bolts should be set to allow for
setting inaccuracies. In concrete
footings, pipe sleeves at least one size larger than the bolt diameter slipped
over the bolt and cast in place are best for this purpose, as they allow the
bolt center to be adjusted after the foundation has set.
3. Provide valves and by-passes in the piping
system so that both the shell and tube bundle may be by-passed to permit
cutting out the unit for inspection or
repairs.
4. Provide convenient means for frequent
cleaning of the unit as suggested under "Maintenance."
5. Provide thermometer wells and pressure gage
connections in all piping to and from the unit, as near the unit as possible.
6. Provide necessary air vent cocks so that the
unit can be purged to prevent or relieve vapor or gas binding of either the
tube bundle or the shell.
7. Loosen foundation bolts at one end of the
unit to allow free expansion of shell.
Oval holes in foundation brackets are provided for this purpose.
8. Set exchangers level and square so that pipe
connections may be made without forcing.
9. Inspect all
openings in the heat exchanger for foreign material. Remove all wooden plugs and shipping pads
just before installing. Do not expose
units to the elements with pads or other covers removed from nozzles or other
openings since rain water may enter the unit and cause sever damage due to
freezing and/or corrosion.
10. Be sure entire system
is clean before starting operation to prevent plugging of tubes or shell side
passages with sand or refuse. The use of
strainers or settling tanks in pipelines leading to the heat exchanger is
recommended.
11. Drain connections
should not be piped to a common closed manifold.
12. To guard against
pulsation of the fluids caused by reciprocating pumps, compressors or other
equipment a surge drum should be installed.
OPERATION:
A heat exchanger is a pressure vessel designed for operation at
certain specific limits of pressure and temperature, and the system must be
safeguarded with safety valves and controls so that these design conditions are
not exceeded and that all operating personnel are alerted.
1. When placing a unit in operation, open the
vent connections and start to circulate the cold medium only. Be sure that the passages in the exchanger
are entirely filled with the cold fluid before closing the vents. The hot medium should then be introduced
gradually until all passages are filled with liquid. Then, close vents and slowly bring the unit
up to temperature.
2. Start operation gradually. Do not admit hot fluid to the unit suddenly
when it is empty or cold. Do not shock
unit with cold fluid when it is hot.
3. In shutting down, flow of hot medium should
be shut off first. If it is necessary to
stop circulation of cooling medium, the circulation of hot medium should also
be stopped by by-passing or otherwise.
4. Do not operate equipment under conditions in
excess of those specified on nameplate.
5. In all installations, there should be no
pulsation of fluids since this causes vibration and strain with resulting
leaks.
6. All gasketed joints should be rechecked for
tightness after the unit has been heated to prevent leaks and blowing out
gaskets.
7. Units with packing rings may require
adjustment from time to time to eliminate slight leakage. As joint containing packing rings requires only
a small amount of bolting pressure to seal tight.
Many heat exchangers
handle fluids which are irritating or dangerous to the human system and could
cause problems if bolted and packed joints are not maintained in a leak tight
condition both at operating pressures and temperatures, and also at no flow,
ambient conditions.
If fluid are not
irritating or dangerous a leak will at least cause a slippery situation on the
floor below.
Since one fluid in the
exchanger is at a higher temperature, any leaks might cause burns.
If leakage should appear
at the packed end joint after the cooler is placed in operation, the bolting
should be pulled up only enough to stop it.
This can be accomplished by taking a one-half turn on each successive
bolt starting at one point and continuing around the cooler until all leakage
has been eliminated. Do not tighten this
joint any more than is required to stop the initial leakage.
When the packing has
been repeatedly tightened to the point where there is almost a metal to metal
contact between the bonnet (or channel) and the shell flanges, the two packing
rings should be replaced.
8. Be sure that all parts of the system are
clean and in proper operating condition.
An exchanger cannot perform properly unless all connected equipment is
functioning properly, yet, the exchanger is frequently blamed for non
performance when the actual trouble is elsewhere in the system.
Observe the following precautions to obtain maximum performance:
(A) Exchanger must be full of fluid in both shell
and tube sides.
(B) Provide periodic venting if air tends to
accumulate in system.
(C) Maintain rated flow of both mediums.
(D) Avoid excessive flow of cooling water in
exchangers used as coolers. It is a
frequent cause of tube failure through erosion, and may decrease cooling
efficiency, especially with heavy oils.
(E) Inspect exchanger periodically and clean
thoroughly when necessary, especially inside tubes.
MAINTENANCE:
1. Provide convenient means
for frequent cleaning of heat exchangers as suggested below:
(A) Circulating hot wash oil or light distillate
through tubes or shell at a good velocity will usually effectively remove
sludge or
similar soft deposits.
(B) Soft salt deposits may be washed out by
circulating hot fresh water.
(C) Some commercial cleaning compounds such as
“Oakite” or “Dowell” may be effective in removing more stubborn deposits. Use
in accordance with the manufacturer’s instructions.
(D) If none of the above described methods are
effective for the removal of hard scale or coke a mechanical means may be used.
When the heat exchanger
is cleaned, it is important the full characteristics of the fouling material
and the cleaning agent be known and care exercised in handling them according
to instructions.
2. Frequently and at
regular intervals, observe interior and exterior condition of all tubes and
keep them clean. Neglect in keeping all
tubes clean may result in complete stoppage of flow through some tubes, with
consequest overheating of these tubes as compared to surrounding tubes,
resulting in severe expansion strains and leaking tube-to-tube-sheet joints.
When shutting down for
repairs it is imperative that all fluids be drained from the heat exchanger and
that no bolting be loosened until the pressures are down to atmospheric and the
temperature of the parts are down to ambient.
3. Do not attempt to clean
tubes by blowing steam through individual tubes. This overheats the tube and results in severe
expansion strains and leaking tube-to-tube-sheet joints.
4. Do not blow out heat
exchanger with air when fluids normally handled are of an inflammable nature.
5. Do not open heads until
all pressure is off equipment and the unit is drained.
6. Do not handle tube
bundles with hooks or other tools which might damage tubes. Bundles should be moved about on cradles or
skids.
7. Do not tighten bolts
until gasket is positioned properly.
This precaution will eliminate one cause for taking down units because of
leaks.
Since many of the
removable components of the heat
exchanger, particularly in the larger sizes are too heavy for men to handle
care must be used to take this weight with proper rigging to avoid injury.
When a heat exchanger is
dismantled for any cause, it is recommended that new gaskets be used in
re-assemble. This will tend to lessen
the possibility of future leaks because composition gaskets become brittle and
dry out, they do not provide an effective seal when reused. Metal or metal jacketed gaskets when
compressed initially tend to match their gasket contact surfaces. In doing so, the are work hardened to the
point that their reuse provides an imperfect seal and possible damage to the
gasket contact surfaces of the heat exchanger.
8. To tighten a loose tube
joint, use a suitable roller type tube expander. Do not roll tubes that are not leaking as it
needlessly thins the tube wall.
9. Exchangers subject to
fouling or scaling should be cleaned periodically. A light sludge or scale coating on the tube
greatly reduces effectiveness. A marked
increase in pressure drop and/or reduction in performance usually indicates
cleaning is necessary, if the unit has been checked for air or vapor binding and
this has been found not to be the cause.
Since the difficulty of cleaning increases rapidly as the scale thickens
or deposit increases, the intervals between cleanings should not be excessive.
10. To clean or inspect
inside of tubes, remove channel covers (or bonnets). Do not remove channels.
11. To locate leaking
joints between tube and tube sheet or a split tube, proceed as follows:
Channel Type
(a) Remove channel covers
(b) Apply hydraulic pressure in shell
Bonnet Type
(a) Remove bonnets
(b) Bolt test rings in place with gaskets and
packing
(c) Apply hydraulic pressure in shell
Use only cold water for
hydrostatic test. The point where the
water escapes indicates the defective tube or joint.
12. when removing tube
bundles from exchangers for inspection or cleaning, care should be exercised to
avoid damage by improper handling.
Although tube bundles are often of great weight, the tubes are small and
of relatively thin metal. The dead
weight of the bundle, therefore, should never be supported on individual tubes,
but should rest on those parts that are designed to carry it i.e., the tube
sheets, support plates or wood blocks, cut to fit the periphery of the
bundle. In withdrawing tube bundles, it
is recommended that rods or steel cables be passed through two or more of the
tubes and the load taken on the floating tube sheet. Rods should be threaded and provided with
nuts and should pass through a bearing plate at either end of the bundle. A soft wood filler board should be inserted
between bearing plates and tube sheets, in order to prevent damage to tube
ends. A forged steel eye bolt which may
be screwed into either plate is used for pulling and lifting.
When steel cables are
used for lifting, the cable is threaded through one tube and returned through
another. Loops are formed in the ends of
the cable by use of thimbles and wire rope clips. A wood spreader block is inserted between the
cable and the floating tube sheet to prevent crushing on tube end.
If the tube bundle has
been in service for a considerable length of time without being removed it may
be necessary to use a hydraulic jack on the floating tube sheet to get it
started. A good sized steel bearing
plate should be inserted between jack and tube sheet and the tube ends
protected by means of a filler board.
Tube bundles may be
raised horizontally by means of slings.
Baffles can be easily bent & damaged by dragging a bundle over a
rough surface. Diameter of baffles is
practically the same as the inside of the shell and a close fit must be
maintained for the apparatus to function properly. Any damage to baffles should, therefore, be
carefully avoided.
In cleaning a tube
bundle, tubes should not be hammered on with any metallic tool. In case it is necessary to use scrapers, care
should be exercised to see that the scraper is not sharp enough to cut the
metal of the tubes.
No comments:
Post a Comment