Chapter 63 Continuous blowdown/intermittent blowdown

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Define Blowdown

Refers to removal of dissolved and suspended solids from boiler water

Continuous Blowdown

Ongoing process.
Small amount of the concentrated boiler water continuously drained.

Intermittent Blowdown

Performed on an as needed basis.
Larger quantities of water are periodically drained from boiler drain connections(intermittent).

blowdown depends

type of external water treatment (related to pressure boiler).

When a blowdown in a boiler would be less

If a boiler operates above 5000 kPa tend to used demineralized water(very pure).

Which units may required continuous/intermittent blowdown in special circumstances(start up)

Large stations.
steam generators.

When a blowdown in a boiler would be continuous or constant and intermittent once(1) per day

boilers operating below 5000 kpa using sodium zeolite or hot cold lime softening.

Which sections and ASME codes reefers to blowdown

Section VII ASME boiler and pressure vessel.
ASME B31.1 power piping.
CSA B51 code.

explain difference between the power plant terms blowdown blow off

blowdown is process of removal and the water sludge solution itself(continuous blowdown line ).
blow off is the actual equipment(CSA call blow off valves, blow off tank) but is not a rule.

how a steam boiler achieves high concentration of impurities:

When steam drawn off from boiler.
dissolved suspended solids left over from external water treat.
chemical injected into boiler.

Why CBD is necessary

to avoid formation of foaming and impurities carryover.
also formation of sludge.

In a boiler the point of highest dissolved solids concentration in the water is located

dissolved solid will concentrate where steam separates with water.
the highest concentration is a narrow zone just below normal operating level.

how the water with total dissolved solids(TDS) can be removed

At the highest concentration zone a long pipe with holes runs along the whole leg of the steam drum, it is connected by external pipe to a blow off tank or pond.

how TDS level in a boiler water is regulated using a continuous blowdown

the continuous blowdown depends of TDS.
a continuous blow off valve should be adjust for a routine water test.
Valve is a needle one with a valve position indicator(vernier valve).

how many valves a high pressure system needs for a continuous blowdown

More than 1 to control flow because discharge is an atmospheric pressure.

Effects of insufficient continuous blowdown

if valve is not opened enough, TDS will rise and foaming, carryover could occur(typical adjust would be a portion of a turn).

Effects of excessive continuous blowdown

if valve is opened too much, TDS will be low and boiler is meant to operate with high TDS water.
Expensive treated water waste.

What happen with CBD if a boiler used demineralized water

CBD line m or completely maybe almost or completely close under normal operation.

A typical flow rate in the CBD line of industrial power boiler that does not use demineralized water should be approximately

It is typical to have a flow rate of 1% OR MORE of the total steam flow rate

why is necessary use a heat recovery System

Because flow in CBD causes decrease in steam efficiency(water absorbs heat)

Describe a heat recovery system for continuous blowdown

*hot blowdown water flows from boiler through continuous blowdown line to the heat exchanger(water gives up its heat when flow through tubes)
*incoming feed water flows over the tubes in the heat exchanger(absorbs the heat)
*cooled CBD goes to sewer by way of blow down off tank
*feed water goes to deaerator with heat recaptured from CBD to steam drum.

how an intermittent blowdown is also called

bottom blowdown

what does the intermittent blowdown

removes undissolved or suspended solids from the boiler(lowest part of the boiler water space)
greatest concentration of solids is at the bottom of water space

where is located the highest concentration of solids in a water tube boiler

Mud drum

where is located the highest concentration of solids in a fire tube boiler

At the bottom of the shell

where is located the highest concentration of solids in large steam generators

Different locations, bottom of each water wall sections.

Intermittent blow off connection serves also as:

place where the water side of the boiler can be drained

The amount and frequency of intermittent blowdown depends of:

quality of boiler water(good water less interm blowdown)

how utility boilers do a intermittent blowdown

Routine daily task(everyday)
short period

how large steam generators do a intermittent blowdown

initial start up
shutting down
draining the boiler

with respect to circulation, describe the problem associate with the opening of blow-off valves on steam generators operating at very high pressures

blow off valves may NOT be opened under normal working conditions because cause temporary loss of water circulation in waterwall tubing section (TUBES CAN FAILURE FROM OVERHEATING).

List the main components associate with a boilers intermittent blowdown system

*Blow off tank(vessel or drum)
*blow off piping
*blow off valves

explain the need for use of a blow off tank with a high pressure steam boilers

*intermittent blowdown involves hot and high pressure water(it reduce to atmospheric pressure) because of this vapour or high temp. could damage sewer lines if goes directly.

the code that states the rules for installation of blow off tanks for boilers is

CSA code b 51

where the inlet line from boiler is positioned into the blow off tank

under water level so incoming hot water is submerged in the cooler water, also this water agitates water in tank so sludge is kept dispersed

How blow off tank discharges water

if new water enters, tank over flow to sewer carrying sludge along with water.

what kind of boilers required a blow off tank(vessel)

boilers at or above 103 k pa

Remote thermal stations required a blow off tank

NO because boiler below 103 kpa dont need blow of tank and discharge is in a different pound

blow off tank water temperature outlet should not exceed

65 C

B51 code also specified which requirements for a blow off tank

vessel and pipe thicknesses
design pressure
corrosion allowance fr thickness
inspection and cleaning access

which are the parts of a blow off tank(minimum inside diameters)

vent 76 mm.
manhole
pressure gauge connection 13 mm.
cold water connection
line to sewer 50 mm.
drain 38 mm.
antisyphon hole 19 mm. diameter

What is the ASME code for a blow off piping

ASME B31.1(specific materials, pressure ratings, required sizes)

Why is necessary allowance pipe for expansion and contraction

cause pipe experience rapid changes in temp. otherwise pipe will become stressed due to restricted thermal expansion worse if pipe attempt to expand against water tube parts or drums.

What is the ASME code that regulates blow off valves

ASME B 31.1

When a boiler is equipped with a slow and quick opening valve in tandem, the type of valve located nearest the boiler should be the

quick opening valve

how many blow off valves a high pressure boiler must have

2

if a high pressure boiler must have 2 opening valves what kind of valves would be use

1 fast and 1 slow opening valves or
2 slow opening valves

every high pressure boiler should have 2 opening valves, what are the characteristics of a slow opening valve

valve requires at least 5(five) 360 turns of the operating mechanism to change from full close to full open or viceversa.

If a boiler is equipped with 2 slow opening valves maybe of

hard seat
seatless

a slow opening valve could be

Y type
Angle type

why a quick opening valve is located near to the boiler

because acts as a sealing or guard

State the operating sequence for blowing down a boiler with a quick and slow opening valves

sealing or guard valve should be opened first and closed last; then blowing-off valve is opened last and closed first. In this way, the blowing valve will be subjected to wear due to the flow of abrasive water through it when starting to open and just before it closes. On the other hand, the guard valve will not wear as there is no fl ow through it when being opened or closed.

Why a blowing valve is slow opening

prevent water hammer(damage pipes and fittings) when blow off starts or ends

how does a seat less slow opening valve works

uses a cylindrical piston(valve element)
piston has a hard surface and holds abrasive effect
piston descends water in the valve

list 2 additional precautions to observe when blowing down a boiler

i. one person should watch the gauge glass level while another operates the blow-off valves. The valve operator should remain by the valves until the procedure is complete.
ii. When there are several boilers in a battery or header arrangement, the valves and boilers should be visibly numbered or identify, so there is no chance of blowing down the wrong boiler.

when a slow opening valves incorporates a third one (gate valve)

when boilers uses 2 seatless slow opening valves

State the advantage of a seatless type of blow down valve over other types

This type of valve uses a cylindrical piston as the valve element. This piston has a hard surface, ideally suited to withstand the abrasive effect of blowdown water. As the piston descends into the valve body, it displaces water in the valve.

what are the arrangements if a third valve is used in blowdown(seat less valve)

is usually wide open
is closed if boiler is shutdown
is closed if first slow opening valve requires repair or replacement.

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