water seald and drain seal pots maintenance on gas lines

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COM MISSION OF THE EUROPEAN COM MUN ITIES

i n d u s t r i l h e l t h n d s f e t y

Maintenance and repair work

on gas lines and apparatus

Water seals and drain seal pots



COMMISSION OF THE EUROPEAN COMMUNITIES

i n d u s t r i l h e l t h n d s f e t y

Maintenance and repair work

on g s lines nd apparatus

W ater seals and drain seal pots

Steel Indus try S a e l / and h f n l th Com miss ion

Working Party

Safety — Gas Mains

Directorate-General Employment and Social Affairs

8 EUR 6 48 D E E N F R I T N L



Published by the

C O M M I S S I O N OF T H E E U R O P E A N C O M M U N I T I E S

Directorate-General

Scientific and Technical Information and Information Management

Bâtiment Jean Monnet

L U X E M B O U R G

LEGAL NOTICE

Nei ther the Com miss ion of the European C om m uni t ies nor any person act ing on

behal f of the Commission is responsible for the use which might be made of the

fo l lowing in format ion

A bibl iographical s l ip can be found at the end of th is volume

ECSC-EEC-EAEC, Brusse ls -Luxembourg , 1978

Printed in elgium

ISBN 92-825-0625 -8 Cata logue num ber : CD -NQ -78-013-E N-C



C O N T E S T S

1 . W a t e r s e a l s 2

2. Drain Seal Pots 5

3. Operation of Water Seals and Drain Seal Pots 9

Figures 10



-1-

Water seals are shut-off devices in a gas mains systems which

perform an isolating function by means of a seal trap.

Drain seal pots are devices designed to perform two functions

simultaneously in accordance with operating requirements: the removal of

water contained in the gas, or supplies to a water seal, and isolation of

gas in the mains from the surrounding atmosphere. Drain seal pots also

contribute towards reducing dust and other contaminants in the gas as they

become entrained in the condensate and are drained out.

The purpose of this study is to ensure that gas distribution sys

tems in Iron and Steel Works are as safe as possible. The recommendations

which follow take account of the various incidents which may occur in

connection with water seals and drain seal pots (loss of water pressure,

excessive gas pressure, explosion).

It should be noted, however, that isolation systems based on the

use of water seals are, by their very nature, inadequate as a safeguard

against excessive gas pressure and explosion. In the event of such inci

dents a water seal may fail to contain the gas.

Very strict compliance with the following general recommendations

is,

therefore, necessary.

A water seal is not an adequate means of isolating a gas main

system for the purpose of carrying out repairs or other operations which

are without anti-gas precautions or which involve the presence of workmen

inside the pipes. A blank flange or closed spectacle plate valve provides

real protection against explosions.

At all points (in buildings, vessels, trenches etc.) at which gas

might accumulate if the seal were to fail to contain the gas only gas

tight drain seal pots may be installed. Particular attention must be paid

to the points at which the outlets of the vents or gas pipes are located

(proximity of occupied premises, windows, work areas etc.) so that any gas

released could not cause gassing or result is a potentially explosive at

mosphere.



-2-

During winter periods the making of water seals may be dangerous

because of the freezing risk so it is necessary that specific measures be

taken to take account of possible falling in temperature.

1. Water Seals

1.1. Water Seals for Sas Distr ibution Systems and Non-High Top

Pressure Blas t Furnaces

1.1.1 The only ac ce pt ab le typ e of wa ter se al i s th at shown in

Fi gu res 1 and 2. I t i s in th e form of a U-bend o r loop in th e

p i p e .

Al l o the r types mus t be r e j ec ted s ince i t i s no t pos s ib le

to check the con d i t ion o f th e i r in s ide w a l l s . F igu res 1 and 2

i l l u s t r a t e t h e U - b e n d t y p e .

In some in s ta nc es a b u t t er f l y va lve lo ca ted a t the bottom of the

U li m it s th e ef fe ct s of an exp losio n as well as movement of water

con ta ined in the c lo sed wa te r s ea l .

1 .1 .2 The sea l gas re le as e pre ssu re of a water sea l must be a t

le a st th re e t im es th e maximum working pr es su re of th e gas system .

The minimum tr a p se al i s ob tain ed by lo ca ti n g th e over-f lo w pipe

o f the wa ter s ea l a t a v e r t i c a l d i s t a nc e above the t r a p d ip equ i

va len t to th re e t imes the maximum working pr es su re of the d i s t r i

but ion system i f the over - f low i s s i t u a t ed on the up-s t ream le g of

the water sea l F i g . 1 . ) and four t imes th e working pre ssu re i f i t

i s s i t ua te d on the down-st ream le g P ig . 2 . )

1.1.3 There must be a se pa ra te dr ai n se al pot for each wa ter

sea l dra i n and over - f low pip e .

1 .1 .4 Dur ing the f i l l i n g of a water se a l a cont inu ous f low of

su f f i c i en t ly c l ean wa te r shou ld be ma in ta ined .

1.1.5 The pipework supp lying wa ter to a wa ter se al must be la rg e

enough to provide a su f f i c i en t flow of wa ter to f i l l th e s ea l

ra p id ly . When th e s ea l i s made i . e . when wa ter i s seen to f low

through th e over - f low pip e , the cont inuous w ater f low may be r e s

t r i c t ed , t ak ing in to accoun t the qua l i ty o f wa te r be ing used .



-3-

To this end, a small diameter by-pase is installed parallel to the

water supply pipe, as per Pig. 3i and is opened as soon as the

water seal has been filled. The valve in the main pipe is simulta

neously closed and padlocked.

The water supply pipework must be in form illustrated in Figure 3.

It is made up of the following items which are listed in sequence

with the direction of water flowi-

- a manually operated valve:

- a branch with air vent valve:

- a second manually operated valve which can be padlocked and is

by-passed with a smaller diameter pipe fitted with a shut-off

valve.

When the water seal is opened i.e. when gas is flowing normally

through the main, the three water supply valves are closed and the

air vent valve is open: the vent pipe discharging any gas leakage

to a sufficient height avoiding all risk to personnel.

1.1.6 The water supply should preferably be connected to the

down-stream leg of the water seal.

If,

however, it is connected to the up-stream leg, the water supply

pipework must also include a U-seal such that the gas release

pressure is three times the operating pressure in the gas main

together with an air vent on the down-stream leg of the U-seal so

that the water can be retained in the U-seal if siphonage of the

water in the pipe occurs e.g. as a result of failure in the water

supply.

Fig. 2 illustrates such an arrangement.

1.1.7 If insufficient pressure is available in the water supply

to a full water seal either because of repairs to the water supply

system or because of an accident, action must be taken to establish

an emergency water supply to the seal.

1.1.b The over-flow pipe of the water seal must be of slightly

larger diameter than the water supply pipe. Its diameter will nor

mally be 4 or 5 .



Generally speaking, the over-flow pipe should be positioned on the

opposite leg of the U—seal from the water supply. If necessary,

however, the over-flow and supply pipes may both be placed on the

down-stream leg providing the water supply cannot pass directly

into the over-flow pipe thus giving a false impression that the

seal is made.

1.1.9 In cold weather, the water in the seal must be kept at a

sufficiently high temperature. Braziers must not be used but a

warm air draught is permissible.

The air heater should operate on the principe of electrical heating

elements rather than combustion of any type of fuel whatsoever.

1.2 Water Seals for High Top Pressure Blast Furnaces

To isolate a furnace top from the gas cleaning plant and

gas distribution system, spectacle/plate valves or a water seal

with spectacle/plate valves are used. Basically such a water seal

isn't different from a low pressure water seal because when it is

made,

the gas pressure on the furnace side is greatly reduced.

However, drain seal pots are not used, but siphons are, in order

to achieve gastightness of the over-flow pipes and in emptying the

water seal.

In fact under the high gas pressure of a high top pressure

blast furnace, if drain seal pots were to be used it would be

necessary for their dimensions and water capacity to be much too

great. This is why siphons are used having a head greater than the

maximum furnace gas pressure. Moreover, the water supply is simi

larly installed to give the same pressure head. The three siphons

(water supply, over-flow and drains) have water constantly circu

lating.

A water reservoir with a capacity greater than that of the

water seal ensures a very rapid filling of the seal and it is con

nected in such a way that it constitutes a reservoir for mainte

nance of the siphon water level in case of water supply failure.

Such a water seal is shown in (Fig. ) .



-5-

The water supply must be sufficiently clean and adequate,

also non hazardous means must be provided to prevent the water

from freezing (see

1.1.9·

and

2.1.9·).

The provisions of paragraph

1.1.9· apply to high top pressure furnace water seals and paragraph

2.1.9·

to water seal siphons.

2. Drain Seal Pots

2.1.

Open Drain Seal Pots

2.1.1. Open drain seals (see Fig. l) should be installed only in

the open air at points where there is no danger of gas accumulating

or infiltrating into occupied premises if the seal fails to contain

the gas.

2.1.2.

There must be a separate seal pot for every condensate

drain pipe, water seal drain and over-flow pipe. There must not be

more than one condensate pipe draining into each drain seal pot.

2.1.3. The dip pipe must be at least 2 inches in diameter. The

seal gas release pressure must not be less than three times the

maximum working pressure of the gas system. The diamter of the

drain seal pot must be at least ten times that of the dip pipe.

2.1.4. The drain seal pots must be continuously fed with running

water through pipes sufficiently large in diameter to prevent any

danger of blockage.

2.1.5·

The gap between the lower end of the dip pipe and the

bottom of the seal pot depend on the frequency of washing out and

the cleanliness of the gas at the point in question. It must be

sufficient to ensure that the accumulated sediment cannot block the

dip pipe.

2.1.6. In order to prevent corrosion:-

- drain pots must stand a few centimeters clear of the ground;

- only non-corroding materials may be used for the dip pipes;

fibre glass

- only steel valves may be used in the drain pipes, over-flow

pipes or water seal drawe-offe in order to avoid electrolytic

oorrosion.



-6-

2.1.7· The valves in the drain pipes and over-flow pipes or

drains of water seals must not be of cast iron as this metal is too

fragile.

They must be placed as close as possible to the main gas line.

Safe and easy means of access must be provided so that they can be

operated quickly and without risk.

Between the gas main and valve in a condensate downcomer a set of

flanges may usefully be fitted to accommodate a blank flange,

especially in the case of pipes more than 3 or 4 inches in diameter.

2.I.Ò. The drain seal pot must be drained by a pipe which must be

at least 4 inches in diameter and as short as possible. The pipe

end must be fitted with a flange to which a blank flange is bolted,

with an elastic ring joint being used. Better still, the drain

pipe should terminate in a flanged valve to which a blank flange

is bolted and rendered water tight by means of an elastic ring

joint.

2.I.9. Measures must be taken to prevent freezing of:

- drain seal pots

- drain pipes

- water supply lines

Such measures must comprise heating of these elements by means of

electric resistors or steam coils and lagging.

If electric resistors are used an ammeter or indicator lamp will

show whether the system is in good working order.

The use of braziers as a means of frost protection must be prohi

bited. The drain seal pot may be heated internally or externally

and the water temperature must be continuously maintained at an

adequate level (Thermostatic control is recommended). Such heating

is not, however, necessary if water is supplied to the pot at an

adequate flow rate and temperature. This may be achieved either by

using clean industrial waste water at this temperature or by em

ploying a special unit to heat the water supply. In such cases the

water is delivered by a dip pipe of non-corroding material, the

distance between the pipe outlet and the bottom of the pot being



-7-

about 1/6 of the trap seal. An air vent comprising a tube at an

acute angle to the direction of water flow is fitted to the dip

pipe level with the over-flow to prevent siphoning of the water in

the pot in the event of an accidental fracture of the water pipe.

Figure 5 illustrates a drain seal pot supplied with heated water

through a dip pipe.

2.1.10. In order to prevent accidental entry of gas into the

water pipe, the latter shall incorporate two manually operated

valves in series with an intermediate air vent valve. These should

be installed immediately up-stream of each pot serving a condensate

pipe or group of seal pots serving a single water seal. The water

supply pipe serving each seal pot must incorporate a U-bend with a

seal trap of at least 50 cm head to prevent entry of gas into the

water supply system.

2.1.11. The pipe by which the water is drained off to the main

drains must also incorporate a U-bend with a seal trap of at least

50 cm head to prevent leakage of gas into the drains or movement

of 'sewage gas' in the opposite direction.

Recommendations 2.1.10. and 2.1.11. may appear too severe. It should,

however, be noted in this connection that there have been many cases of

gas poisoning at points a considerable distance from the areas in which

gas is produced, transported or used, as a result of the gas spreading

through sewers, drains or water lines temporarily not in use.

2.2.

Airtight Drain Seal Pots

2.2.1. Only airtight drain seal pots may be used in cellars,

trenches,

buildings and at other points where gas might accumulate

or infiltrate into buildings if the seal were to fail to contain

the gas.

2.2.2. These drain seals (see figure 6) are isolated from the sur

rounding atmosphere but have an air vent pipe which, if the gas

pressure becomes too high, will bleed off the gas outside the

premises at a sufficiently safe height. The pot over-flow has a

0 eeal

whose gas release pressure is greater than that of the drain



seal pot itself.

The dimensions of this type of drain seal should be as follows:

- diameter O f dip pipe: 2 minimum

- diameter of drain seal pot: 12.5 times the diameter of the dip

pipe but not less than I50 mm

- diameter of air vent pipe: 3 times the diameter of the dip pipe,

but not less than I50 mm

- gas release pressure of drain seal pot: 3 times the maximum

working pressure of the gas system

- gas release pressure of the over-flow U- seal: 5 times the

maximum operating pressure of the gas system

- maximum diameter of overflow: 2 .

2.2.3· The vent pipe must be of constant cross-section and, if

possible, straight. If, however, the pipe must have a bend in it,

the radius of curvature must be not less than 10 diameters in order

to minimise the amount of water discharged from the U-seal if

excessive pressures develop.

2.2.4. All recommendations given in Section 2.1 Open drain seal

pots also apply to airtight drain seals with the exception of

points 2.1.1.1. and 2.1.3·

?.3. Drain Seals of Sunken DeBign

2.3.I. Drain seals of sunken design have the advantage of being

less susceptible to freezing. Apart from this aspect, the same

requirements apply as to open drain seal

pots.

4.

Improved Seal Pots

The hazards caused by seal pots failing because of pressure

surges,

or explosion, are such that various attempts have been made

to limit the quantity of gas released, and to re—establish the seal

with a sufficient head of water at the end of a pressure surge.

Such improved seal pots are already in use in various

works but there is insufficient experience of them to

be aware

of

any drawbacks or faults.

hey are based on the principles of

limiting to a minimum the volume of water expelled by a pressure



-9-

surge, and maintaining a sufficiently large reservoir of water to

re-establish the water seal at the end of a pressure surge.

The General Commission hopes that steel companies, study

institutes and manufacturers acquire up- to-date information on

such improved seal pots when designing new installations.

In time, the General Commission will not fail to supply

its own evaluations as soon as it is able to.

As far as condensate drains are concerned, mention must be

made of the automatic float drainage system which is in use in

Germany, and which is satisfactory when used on blast furnace gas

and coke ovens gas systems.

3. Operation of Water Seals and Drain Seal Pots

3.1.

Daily inspections must be made to ensure that drain seal water is

flowing freely and that it is at a suitable temperature during

periods of cold weather.

3.2. Valves in water seal drains and over-flow pipes must be capable of

being tightly shut (ball valves are recommended). The open and

closed positions must be readily identifiable.

3.3· When water seals are made, the water drain valves must be shut and

padlocked.

3.4· When water seals are open, their overflow valves must be closed.

3.5. When butterfly valves are installed up-stream or down-stream of a

water seal, it is recommended that they be shut before filling the

seal.



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European Com mu ni t ies — Com miss ion

EUR 6 48 Maintenance and repair work on gas lines and apparatus Water

seals and drain seal pots

Lux em bou rg : Of f ice for Off ic ia l Pub l icat ions of the Europea n Co m m uni t ies

1978 - 22 p. - 14.8 χ 21.0 cm

Industr ia l Heal th and Safety ser ies

DE, EN, FR, IT, NL

ISBN 92-825-0625-8

Ca ta logue nu m be r : CD-NQ -78 -013 -EN-C

BFR

150

DKR

26,20

D M

9,50

FF

21

LIT

4 000

HFL

0,40

UKL

2.50

USD

5

This study, carr ied out by a group of experts wi thin the Steel Industry Safety and

Heal th Commiss ion, i l lustra tes the many condi t ions in which water seals and

drain seal pots in gas pr od uc tio n plant in the steel indu stry blast furn ace an d

coke oven gas) may be found, wi th a v iew to ensur ing maximum safety f rom the

r isks of gassing by or explosions of these gases.



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water seald and drain seal pots maintenance on gas lines

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