Precaution during Operation

6.2.1 Flue Gas Temperature

The flue gas temperature rises with an increase in the GT load and decreases with a load decrease.

The flue gas temperature valves in relation to the load values shall be recorded.

These records represent important data for normal operation and for abnormally high or low flue gas temperature situations which the operator shall notice immediately.

Possible causes of abnormal flue gas temperature are as follow:

1) flue gas temperature is abnormally low :

A flue gas temperature abnormally low indicates that there is a low turbine exhaust gas temperature. In this case, the O2 content in the Combustion Turbine exhaust gas is extremely high, and the turbine shall be investigated accordingly.

2) flue gas temperature abnormally high : (1) Abnormally high percentage of excess air

Normally, the O2 content in the Combustion Turbine exhaust gas is at high levels. If the percentage of excess air is extremely high, special attention shall be provided. This occurs frequently, especially at Combustion Turbine excessive partial (low) loads.

(2) Heating surface contaminated with soot, if applicable.

6.2.2 Priming

Generally, the drums water level shall be controlled to the normal water level specified in the steam drum section of this manual.

Sudden fluctuations are not permitted.

Priming (of humid steam) may occur when the HRSG is either subjected to a required evaporation rate at a pressure significantly lower than the normal operating or experiences a very high water level.

HRSG drum capacity is large in comparison with the evaporation rate; however, priming can occur during the sudden load increase/ fluctuations.

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"Manual", in order to reduce the flow of drum water, which requires the drum water level to be maintained at a low level.

Therefore, a minimum water level must be maintained. If required, GT load may have to be reduced.

6.2.3 Superheater

1) Superheater safety relief valve operation

The superheater safety relief valve shall be set to operate at a pressure, which is lower than the drum safety relief valve set point.

Therefore, when the steam rate decreases suddenly, or is reduced to zero, the superheater safety relief valve will operate to maintain flow through the superheater while protecting the superheater from over pressure.

When the steam rate decreases, the operator shall identify immediately the reason, because the superheater safety valve is not designed to maintain the steam pressure within the prescribed limits, for an extended period of time.

2) Superheater damage prevention

The most frequent damage that occur during the HRSG operation are generated by the superheater tubes bulging and warping, because of either improper control during the HRSG startup or shutdown, or HRSG continuous operation. Therefore, the operator shall consider the required precautions.

When the temperature of flue gas passing through the superheater is high, the superheater tube wall temperature is 20~40℃ higher than the steam temperature inside the tube. Also, the boiler water capacity is lower than the boiler evaporation rate. Therefore, if the feed water control system is not properly monitored, fluctuations in the HRSG water level many occur.

Appropriate measures should be taken to prevent the superheater damage, possible to occur during the HRSG operation.

The HRSG operator should monitor the following occurrences in order to prevent the damage:

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(1) Incidents originating inside the superheater tube, (steam/side) scales carried with the HRSG steam moisture have the tendency to adhere to the interior side of the superheater tube and to accumulate, to build an excessively thick deposit.

The heat resistance of this deposit will cause temperature rise of the superheater tube due to scales is it the result of “priming”.

It is very important to always operate the HRSG within "the limit values of feed water and HRSG water" furnished by DHICO.

The causes leading to priming are as follows:

(a) The HRSG water limits, especially pH and dissolved solids content, are excessively high, or oil and grease are mixed in the water (foaming).

(b) Large quantities of chemicals have been injected in the HRSG.

(c) Feed water is extremely contaminated due to a failure of the water treatment system.

(d) Various plant heat exchangers drains or leakage, when connected to the condensate - feed water systems can contaminate the water quality.

(e) HRSG operation at a pressure much lower than the specified pressure.

(f) Items (a) through (d) above refer to the feed water chemical components (high content) action on the HRSG priming.

However, priming can also occur due to HRSG excessive high water level. Corrective action must be taken.

(g) HRSG water can be fed directly to superheater tubes without through of drum internal(separator and dryer) due to drum internals leakage. The result is similar with the priming result.

When intensively primed because above reasons, the superheater steam temperature will drop abruptly.

When the HRSG experiences a slight priming, the steam temperature drop could be so small that it may not be noticed.

(2) Incidents originating outside superheater tubes (gas side).

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cause an excessive temperature rise of the superheater tubes temperature.

However, when the flue gas flow is affected by such deposits, the superheater tubes may be overheated in the areas with an excessive temperature rise of the superheater tubes.

6.2.4 Steam Temperature

The superheater outlet steam temperature is generally little affected by load changes.

1) In the following case, superheater steam temperature rises abnormally and, therefore, careful adjustment is required:

(1) Too high temperature of GT exhaust gas.

2) In the following cases, superheated steam temperature falls abnormally and, therefore, careful adjustment is required:

(1) Too low temperature of GT exhaust gas.

(2) Excessive wet steam at superheater inlet.

(3) Contamination of superheater tubes inside or outside.

(4) Lower than normal steam pressure.

When boiler priming occurs, superheated steam temperature drops sharply and then rises again. The frequency and intensity of such variations of superheated steam temperature increases with the increase in total solid content or alkalinity of HRSG water.

6.2.5 Blowdown Procedure

Blowdown is an important factor in the HRSG water control and has the following two objectives.

1) Prevents the concentration of impurities in the HRSG water.

2) Remove deposits from drum inside.

# Objective No.1

During the HRSG operation, continuous blowdown shall be in operation, in order

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to maintain water quality in each HRSG drum.

# Objective No.2

During the HRSG operation, intermittent blowdown shall be performed by rapidly blowing a certain quantity of HRSG water, under pressure, from the drum, at certain intervals of time.

Normally, the intermittent blowdown shall be performed 1 to 3 times in 24 hours, in order to maintain the water chemistry.

The blowdown shall be preferably performed during the HRSG shutdown to maintain as much heat as possible.

The blowdown valves shall be periodically inspected for leakage.

6.2.6 Access to Stack

The access must be locked for personnel protection purpose considering emergency simple cycle operation.

0 2011.07.16 First Issue BJ Park IS Kim SG Jung REV

NO. DATE DESCRIPTION CHKD. CERT. APPD.

Project Title

CONVERSION OF QURAYYAH OPEN CYCLE POWER