1080 SERIES
3
SECTION
2
ITEM
1080
DATED
NOVEMBER 2012
SUPERCEDES
MAY 2010
14. Quick-opening and closing valves controlling fluids to
or from an exchanger may cause water hammer, and care
should be taken for proper selection of such equipment.
Water hammer can cause serious damage to heat
exchanger tubes.
OPERATION OF HEAT EXCHANGERS:
1. START-UP: When placing a unit in operation, open the
vent connections and start to circulate the cold medium
only. Be sure the passages in the exchanger are entirely
filled with cold fluid before closing the vents. The hot
medium should then be introduced gradually until all
passages are filled with the liquid or vapor, as the case may
be. Then close the vents and slowly bring the units up to
temperature.
2. BOLTED JOINTS: Heat exchangers are hydrostatically
tested in accordance with code requirements and are
certified as satisfactory by inspection agencies agreed
upon by the manufacturer and the purchaser. However,
normal yielding of gaskets will occur in the interval
between hydrostatic testing in the manufacturer’s shop and
installation at the job site. Therefore, all external bolted
joints should be properly retightened after installation and
again after the exchanger has been heated, to prevent leaks
and blowing out of gaskets.
3. DESIGN AND OPERATING CONDITIONS: Do
not operate equipment under pressure and temperature
conditions in excess of those indicated on the nameplate.
4. SHUTTING DOWN: In shutting down, flow of hot fluid
should be shut off first. If it is necessary to stop circulation
of cooling medium, the circulation of the hot medium
should be stopped also, through bypassing or other means.
When shutting the system, all fluids should be completely
drained to minimize the possibility of freezing and
corrosion. To guard against water hammer, condensate
should be drained from steam heaters and similar units
when starting up, as well as when shutting down.
To minimize water retention after drainage, the tube side of
water-cooled exchangers may require blowing out with air.
5. WATER HAMMER: In the case of steam as the heating
medium, the steam trap should be manually bypassed until
the exchanger is switched to automatic control. Costly
damage can result if care is not exercised in the start-up
of a heat exchanger. Water hammer often results when a
large quantity of steam is allowed to condense rapidly in an
enclosure. Thin-walled tubes are very vulnerable. Copper
tubing is used extensively and is a relatively soft metal.
Water hammer is a type of implosion effect particularly
pronounced when low pressure steam is used, one reason
being the high volume ratio of steam and water at low
pressure. For instance: Volume of 1 lb. of steam at 5 psig
is about 20 cu. ft. Volume of 1 lb. of water (condensate) is
.0168 cu. ft.
This volume ratio of 1200 to 1 gives us some idea of how
the tremendous hammer effect may be produced when there
is enough transfer surface present to remove the latent heat
of vaporization rapidly. Slugs of water are hurled about in
the vacuum created by condensation, and one can visualize
the damage possible to fragile tubes.
When this hammer effect has occurred in the shell of an
exchanger, the damage pattern is quite regular. Tubes are
crushed in on top of the tube bundle, usually at about two-
thirds of the distance from the steam entry nozzle toward
the other end of the tube bundle. So far, there seems to be
no technical explanation for this phenomenon. From study
and examination of damaged exchangers, and investigation
of their operation, we have come to the conclusion the
following is roughly what happens: In a water heater using
steam in the shell, when the demand for hot water ends
the steam control valve closes, but there is a good supply
of steam left in the shell of the exchanger. As this steam
condenses, the pressure drops, often below atmospheric or
even practically to full vacuum. This prevents condensate
from leaving the shell and sometimes even siphons in
condensate from the line beyond the trap. Now, when the
steam valve opens again and admits steam to the shell, the
rapid condensation, as it strikes the cold condensate, causes
streams of water to rise, hitting the top of the shell and
bouncing onto the top tubes. Sometimes the breaks in the
tubes look as though a 4" spike had been driven through the
topside. Other times the tubes may be crushed as if with a
blunt chisel over lengths of a few inches or up to two feet.
MAINTENANCE OF HEAT EXCHANGERS:
1. IMPORTANT: Follow carefully the procedure
recommended for operation. Quick start-up and shut-down
without proper condensate removal is a major cause of heat
exchanger damage.
2. Frequently, and at regular intervals, observe the 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, causing overheating of these
tubes. This overheating may result in severe expansion
strains and leaking tube joints.
3. When removing tube bundles from exchangers for
inspection or cleaning, care should be taken to see that
improper handling does not damage them. Tube bundles
are often of great weight, yet the tubes are small and of
relatively thin metal. The tube bundle should therefore
never be supported on the tubes but should rest on the
parts designed to carry it, i.e., on tube sheets, baffles or
support plates.
Do not handle tube bundles with hooks or other tools which
might damage the tubes. They should be moved about on
cradles or skids. Horizontal tube bundles should be lifted by
means of suitable slings. Baffles can be easily damaged by
dragging a bundle over a rough surface.
4. Provide convenient means as necessary for cleaning heat
exchangers at regular intervals:
(a) Circulating hot wash oil or light distillate through tubes
or shell at high velocity will effectively remove sludge
or other similar soft deposits.
(b) Soft salt deposits may be washed out by circulating hot
fresh water.
(c) Some commercially available cleaning compounds
may be used for removing sludge or coke, provided hot
wash oil or water, as described above, does not give
satisfactory results.
