Blowing Out Steam Lines

8/13/2019 Blowing Out Steam Lines

1/8

BLOWING OUT STEAM LINES

INTRODUCTION

The purpose of blowing the steam lines prior to starting up a new unit is to

remove any foreign matter remaining in the equipment (i.e. super heater) andsteam piping after erection is completed. Considerable damage could result if

such foreign matter was allowed to enter the plant steam system during initial

operation.

On older units the need for steam line blowing should be considered

following major pressure parts repairs, where the possibility of introduction

of foreign material into the system exists.

Since prevention of damage to the plant steam system is the prime concern,

the responsibility for determining the effectiveness of the steam line blowing

operation rests with the plant operator. During the steam line blowing process

the unit should be operated in accordance with the vendor recommended

procedures, with all control systems and protective interlocks functioning.

The principle behind steam blowing lines clean is that the thermal cycling

(heating/cooling) and high velocity gas flowing through the line will "shock"

the pipe and tend to break the mill scale and weld slag away from the pipe

wall. Ideally, steam blowing should be done prior to the system being

insulated to maximum the thermal cycling of the equipment. If the system is

insulated, a greater time between blows may be necessary to allow the system

to cool.

Ideally, to obtain optimum cleaning, the flow conditions in the system during

steam line blowing should equal those during normal operation at maximum

load. Since it is impossible to exactly duplicate these conditions when blowing

through the piping to atmosphere, it is desirable to produce equivalent

conditions by using lower pressure steam with a flow rate such that the

product of steam flow times velocity will equal that under normal full load

conditions. The determination of the total obtainable flow quantity must be

based on flow resistance in the entire system, including the temporary piping.

This determination is normally made by the designer of the blowing system.


2/8

The temporary piping is normally equipped with a shutoff valve that is used

as a blowing valve. With this arrangement, the superheater and the upstream

portion of the temporary piping are maintained at drum pressure at all times

when not actually blowing.

When a boiler and its associated steam piping have been erected, there is a

possibility that debris, such as welding beads and dirt may have been left in

the tubes and piping. In addition, these parts may have an internal coating of

oxide or mill scale, which will flake off during initial temperature changes.

To avoid any hazard of such material causing obstruction or damage to

machinery, it is recommended that arrangements are made to blow the piping

systems to atmosphere before the final connections are made and steam is

passed to the plant. This blowing through is done with steam which is

exhausted through adequate sized, open ended low pressure temporary

piping. The steam blowing operation cleans the system and provided the

steam of requisite purity for the plant.

This low pressure temporary piping must be connected as near as possible to

the terminal point of the section of the system being purged. In some cases an

appropriate valve cover may be removed and the temporary piping led off the

valve chest. The run should be as short as possible consistent with steam

discharging freely to atmosphere with out causing danger to personnel or

damage to structure and equipment.

If a basket type strainer is installed in the steam piping it must be removed

prior to steam blowing and subsequently replaced.

The precautions listed below must be strictly observed.

REQUIREMENT OF TEMPORARY STEAM LINE FOR BLOWING

The diameter of temporary discharge piping shall not be less than the

diameter of the respective line to be steam blown. The spool pieces should

have an opening equal to the diameter of the respective temporary pipeline.The temporary discharge piping and supports shall be capable to withstand

steam blowing parameters.

PROCEDURE FOR BLOWING


3/8

There are two alternative procedures for blowing out the steam lines to the

plant.

One is to raise pressure in the boiler to just below the permissible pressure in

the temporary atmospheric discharge piping, gradually open the boiler stop

valve whilst increasing the firing rate to maintain pressure during the blow,

then gradually close the stop valve whilst reducing the firing rate.

This procedure has the advantage that the blowing period may be as long as

is necessary to clean the steam piping thoroughly.

Usually the waste steam discharged to atmosphere will be dark in colour

when blowing starts, and will become clear as foreign matter is removed from

the system. Several successive bows should be made with an adequate cooling

period between each, as the associated temperature changes will assist inloosening mill scale.

The alternative procedure is the puffing method. The boiler is raised to

around 40kg/cm2g pressure or just below the permissible pressure in the

temporary atmospheric discharge piping. Shut off the firing.

Ensure that the drum water level is at the lowest level.

Open the boiler stop valve quickly; water level in the drum swings upward.

The level may even go beyond visible limit. Permit the boiler pressure to drop

about 10.5 to 14kg/cm2g. The change in steam saturation temperature is kept

less than 42deg.C. Then quickly close the stop valve.

This method has the advantage that the sudden change in steam pressure

produces a sudden change in saturated steam temperature causing a thermal

shock which helps to loosen the adhered scale from the inside of tubes and

piping. The steam expands quickly through the system as the pressure drops.

The scales are than removed by the expanding steam. How ever, all the

pressure parts, including the drum, are subject to thermal shock, and to

prevent excessive stresses and differential expansions, the change insaturation temperature should not exceed 42deg.C.

To ensure thorough cleaning, a number of successive blows must be made

due to the limited blowing period permitted by the allowable pressure

reduction.


4/8


5/8


6/8


7/8

3. Remove all control valves, desuperheater nozzles, flow elements, thermowells, pressure gauges, orifice plates, safety valves and instruments that

may be damaged during the steam blow.

4. Insure all vent pipes have adequate drains and that these drains are routedto a safe location.

5. Install temporary pressure gauges as required to monitor the progress ofthe steam blow and make arrangements for to record pressure readings.

6. Establish adequate radio communication between the control room and theoperator at the shutoff valve. Back this up with a system of visual

communication, such as lights or flags, since audio communication may be

difficult due to high noise levels at the steam shutoff valve.

7. Insure personnel traffic is controlled in area during steam blow. Insure thearea near the vent pipe is clear of all personnel during the steam blow.Protect neighbouring equipment from damage due to the steam vents.

8. Individuals operating the steam blow valves (especially near the vent pipe)should wear double ear protection.

General Cleaning Procedure

1. Discontinue all firing and gradually open the temporary blowoff valve, to

blow through the steam system and out the temporary blowoff piping.

Caution: The first blow should always be done at reduced pressure, in order

to check out the temporary piping system, its supports and anchors.

2. When the drum pressure has dropped to the value corresponding with a

100F (56C) saturated steam temperature decrease, close the temporary

blowoff valve and refire the unit to re-establish blowing conditions.

3. Repeat the above cycle until it is considered that cleaning is satisfactory as

indicated by inspection of impact specimens during the final blows.

Post Cleaning Procedures

1. Drain the temporary and permanent piping.2. Remove temporary spools, blind flanges, temporary vent piping, etc., and

reinstall all components removed prior to steam blow.

Cleanliness Criteria


8/8

The turbo-generator vendor or the primary end user of the steam will

generally specify the criteria to which the system should be cleaned. If steam

is used in a turbo-generator the following criteria is generally used:

Two successive targets must result in mirror finishes that meet the specifiedcriteria.

After a cooling down period of at least 12 hours, two more successive steamblows must result in mirror surfaces that meet the specified criteria.

Criteria

No indentation exceeding 0.8 mm diameter Indentation exceeding 0.4 mm diameter to number not more than 2 per

2500 mm of the target plate surface

Indentations exceeding 0.2 mm to number not more than 10 per 2500 mmof the target plate surface

Indentations less than 0.2 mm to be well dispersed and nowhere present inconcentration

Documents

Blowing Out Steam Lines