DUCK NOTES #01
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These notes answer the "guaranteed failed" questions in oral examination for Transport Canada Ship Safety engineering licenses.
“An applicant is considered to have failed an engineering knowledge examination if applicant incorrectly answers a question on any of the following matters: “
1.
confirming the water-level in a boiler by using
the water gauge and column test method,
2.
the precautions to be taken when blowing down a
boiler,
3.
the danger involved in relighting the fire in an
oil-fired furnace where un-burnt gases may have accumulated;
4.
the precautions to be taken to prevent the
contents of a boiler from backing up into another boiler through blow-down or
scum valves;
5.
the precautions to be taken when connecting one
boiler to another; and
6.
the definition, causes, prevention and effects of
water hammer.
Q. Testing gauge glass and water column:
A. (See diagram) To determine whether the pipes and terminal cocks are clear a blowing procedure sometimes referred to as cross blowing is adopted. First do the column.
(1) Cocks A, C and D open, cocks B and E closed, this checks that A, pipe X and the column are clear.
(2) Then with cocks E, B and D open, cocks A and C closed, checks that E and pipe Y are clear.
Next, blow the water gauge glass with A and E open.
(1)
Close steam (B) and water cocks (C)
then open the drain (D). Nothing should blow out of the gauge if the steam
and water cocks are not leaking.
(2)
Open and close water cock (C) to
check that the water cock connection to the boiler is clear.
(3)
Open and close steam cock (B) to
check that the steam cock connection to the boiler is clear.
(4)
Close the drain (D).
(5)
Open the water cock (C). Water
should then gradually rise up to the top of the gauge glass.
(6)
Open the steam cock (B) and the
water in the glass should fall to the level of the water in the boiler.
- If when
(5) is reached the water cock is opened and water does not flow up the gauge
glass, the water level in the boiler is below the water cock connection to the
boiler and it is unsafe to put feed water into the boiler.
- If when the water cock has been opened the water flows to the top of the
gauge glass and then when the steam cock is opened the water flows down and
out of the glass, the water level is between the water cock connection to the
boiler and the bottom of the gauge glass. In this case it is safe to put feed
water into the boiler.
- If after (5) when the glass is full of water, the steam cock is opened and
the water in the glass does not descend in the glass, the water level is above
the steam cock connection to the boiler and there is a danger of priming the
boiler if any additional feed is put into it.
Q. Precautions to be taken when blowing down a
boiler.
A. A modern boiler should never be blown down while the boiler
is steaming at high rates. To blow down...
Cut the
fires and close the steam and feeds stops. Allow the boiler to cool off. At
25psi, blow down to sea, the shipside valve should be open first, then the
blow down valve. This will maintain control to the operator should a pipe
burst.
Care must be taken that no cold sea water enters the boiler due to residual
steam cooling to create a vacuum. A non return valve would prevent this from
happening but prudence would dictate shutting off blow down valve when getting
close to no pressure.
Open boiler vent to allow as much natural cooling as possible at atmospheric
pressure. Remove belly plug to drain the remainder contents in the bilge.
One door can then be open, top or bottom, but not both and with care.
Loosening dogs, but not undoing then to determine condition on other side…
steam, hot water. Once determine door can be open being careful of remaining
hot water.
Q. What are the dangers in relighting a boiler.
A. Obvious precaution should be taken before lighting. These
include, making sure all doors are closed and gas passages are open, and there
is water in the boiler. Un-burnt fuel and vapors exist in the boiler, these
may lead to an explosive atmosphere. To prevent this, carry out a visual
inspection of the furnace, followed by purging by forced draught with all air
registers open for several minutes.
Q. Precaution to take to prevent backing one
boiler into another.
A. Non return / check valves, manual monitoring, flow meters /
alarm. Isolating boiler operation and blow downs.
Q. Precautions to take when connecting one boiler
to another.
A. The objective is to minimize dangerous thermal shock. And
stresses. The connecting steam pipe must have drain open, and be gradually
preheated by cracking the steam valve slightly. Once pressure is about 90% of
one another, the steam stop valve can then be open.
Q. Water hammer. What is it, how can it be prevented.
Life threatening water
hammer in a steam system is generally "condensation induced water hammer".
This occurs when steam becomes totally enveloped in sub-cooled condensate
which has not been drained from the system. The steam, once surrounded, can
"implosively" condense leaving a vacuum that condensate, pushed by system
pressure, rushes in to fill. The resulting collision of condensate with
itself or a system component can generate over-pressures due to the impact of
well over 1000 psi--the magnitude depends on how fast the condensate gets
moving as it accelerates into the void. The surest way to guard against water
hammer in a steam system is to make sure condensate is not able to collect and
cool in a steam system.
Use these helpful hints from www.kirsner.org to understand this important safety phenomena
