Steam Blows

Steam Systems - Commissioning - Steam Blows
April 25, 2004

Introduction

steamblow.JPG (13185 bytes) 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. superheater) and steam 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.

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.

Note: If the unit is equipped with a main steam stop valve, this may be used as a blowing valve, if the valve design permits this type of service.

An indication of the cleanliness is obtained by impact specimens, installed in the blowoff piping during the final blows. Suitably mounted and supported, polished square bar stock has been effectively used for this purpose. The degree of pitting of the specimen surface following a blow is used to determine the end point of the blowing cycle.

This procedure is general in nature and as such should be adapted to the specific conditions and procedures existing at the site at the time the operation is performed.

Safety

The process or steam blowing imposes abnormal and severe conditions upon the boiler, steam equipment and steam piping. Large, rapid temperature changes occur during each blowing cycle. This cycling of temperature is far more severe than is incurred in normal operation. Thermal stresses may be excessive in the heavy wall portions of the system such as steam drums, headers and piping. It is prudent to consider this fact when performing the blowing procedure and to limit the number of blows to the minimum consistent with cleaning the system.

Since the temporary steam blowing piping may be designed for lower pressure than the boiler, care must be used to prevent over pressuring this piping during the entire steam line blowing operation. The pressure in the temporary piping should be monitored continuously and operators be alert to prevent over pressure.

It is recommended that a means of over pressure protection be provided in the temporary piping, such as, safety valves set at the design pressure of the temporary piping.

Noise levels will be very high during the steam blow. Hearing protection is mandatory for those in the immediate vicinity of the steam outlet. An area at least 100 m from the blow point must be roped off to prevent personnel from approaching too close.

Equipment and Supplies

The following equipment will be required:

Temporary piping
Temporary piping supports, restraints
Silencer
Blow-off valve with quick opening actuator
Targets polished to a mirror finish.

Material: St 37 or ASTM A36 or ASTM A516 Gr 70
Hardness: 140 - 160 HB

Procedure

The unit is started in the normal manner. All normal recommendations and limitations with respect to fuel firing equipment, air handling equipment, drains and vents, etc., should be followed, as if the unit were being started for normal operation. As for any new unit, the steam blowing operation is the first occasion that the unit is fired at any significant rate. Consequently, the startup as well as the steam line blowing must be conducted with great care. The normal startup rate does not apply; the unit must be brought up much slower, while all equipment is checked and expansion movements monitored closely. If the unit is equipped with a fired superheater, the 1000°F (538°C) furnace exit gas temperature limitation must not be exceeded when the unit is fired. The same general precautions taken on any new unit for this period of operation apply equally here.

The drum level will be subject to extreme fluctuations during the blows. As the temporary blowoff valve is opened, the drum level will rise rapidly and may go out of sight in the gauge glass. As the blow progresses the drum water level will re-appear and may drop out of sight. Therefore, it is important that the drum level is established at or slightly above normal operating level before the start of each blow. A small amount of feedwater flow should be established before the start of each blow and the feedwater flow should be increased as soon as the water level drops back in sight, in order to prevent excessive low water level.

When the drum pressure reaches the value calculated to produce the desired blowing flow quantity, the blowing procedure can be started.

It is difficult to avoid carryover from the steam drum to the superheater during the steam line blowing operation. Therefore, boiler water should not be treated with non-volatile chemicals during this process, to avoid deposits of solid materials into the superheater.

Firing of all fuels must be discontinued during all blows.

General Pre-Cleaning Procedures

1. Check all pipe anchors and guides for adequate support of piping, including the temporary vent pipe.

2. Insure that all lines have been inspected and hydrotested.

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 routed to a safe location.

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

6. Establish adequate radio communication between the control room and the operator 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 the area 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 100°F (56°C) 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

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 specified criteria.
After a cooling down period of at least 12 hours, two more successive steam blows 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 mm² of the target plate surface

Indentations exceeding 0.2 mm to number not more than 10 per 2500 mm² of the target plate surface

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


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