After erection acid cleaning of water and steam system

Post-erection acid cleaning of water and steam system of utility boilers

shown on the example of a steam boiler type Foster Wheeler Compact OE 700-17,1/568/568/250 no. K-4 in Turów Power Plant and a boiler manufactured by Rafako type OOG 320 no. K-2 in PKN Orlen CHP Plant in Płock.

1.0. Introduction

The boiler structure is mainly made of carbon steel (K18) and alloy steel (16M, 15HM, 110H2M). Only a few elements are made of steel of higher resistance to corrosion (P91). During the boiler erection, the elements of carbon and alloy steel are exposed to corrosion which shall be removed by means of acid cleaning. Careful cleaning of the water and steam cycle internal surfaces allows to shorten the start-up period of the boiler and the entire power unit. In addition, the precise cleaning of the internal surfaces minimises the boiler breakdown potential, especially during the start-up and initial operation.

2.0. Purpose and general principles of acid cleaning process

Post-erection acid cleaning of the power equipment is carried out to clean the internal surfaces from various types of pollutants: process, post-production and post-erection, as well as accidental.

During subsequent operations of acid cleaning process the internal surfaces of the water and steam cycle are degreased, cleaned of post-production scale and atmospheric corrosion products, and in result the clean surfaces are obtained. During flushing (mainly), as well as other acid cleaning operations, the loose mechanical contaminants, which are not soluble in applied baths,  are removed from the system.

The purpose of acid cleaning is to convert the contaminants into compounds which are soluble in applied baths and their removal from the system in a form of the solution. Solid, loose substances which are not soluble in these baths will be removed from the system during the flushing process. In result of passivation, which is the last stage of acid cleaning process, the protective coating is provided to protect the cleaned surfaces against atmospheric corrosion for the time between acid cleaning and steam blowing.

3.0. Selection of acid cleaning method

The most popular method of acid cleaning is a circulation method. It is based on forcing the flow by means of properly selected circulation pumps, to allow the fluid to reach all boiler elements. This method assures the proper flushing of the boiler from solid impurities, which are insoluble in applied baths and acid cleaning of economizer, superheaters, reheaters or steam pipelines.

Another acid cleaning method is the “auto-circulation” method. It is based on forcing the flow by means of inert gas (e.g. nitrogen) only in a boiler evaporator and a drum. For flushing and cleaning of other elements, proper pumps (e.g. circulation pumps) have to be used.

In case of newly built utility boilers usually all elements of water and steam system are subject to acid cleaning. It is a seldom case that water system is cleaned and steam system only blown out.

The flow method was applied by our company during acid cleaning of water and steam system of the steam boiler type OE 700-17,/568/568/250 no. K-4 in Turów Power Plant, which was held between 13 August and 2 September 2003.

Whereas “auto-circulation” method was applied during acid cleaning of boiler type OOG 320 no. K-2 in PKN Orlen CHP Plant in Płock, which was executed between 5 and 14 October 2003.

4.0. Temporary System

 In both cases the temporary system for flushing and acid cleaning of equipment and pipelines was made according to technical designs which met client’s requirements and allowed to clean each boiler element effectively.

Foster Wheeler Compact boiler type OE 700-17,1/568/568/250 has a large capacity of 865.8 m3. The effective flushing of boiler elements was possible by using a large flow rate of over 2150 m3/h which allowed to reach significant linear velocity, even in pipes of large diameters.

The installed temporary system consisted of two auxiliary pumps type OS 250AM/5, capacity 450 m3/h each, and two pumps type 35W50 of capacity 1250 m3/h each. The applied pumps enabled to reach the flow rate over 3000 m3/h.
The diameters of temporary system pipes were adjusted to that flow rate.
The auxiliary valves with electric and manual drives were used to control the flow rate during acid cleaning.

OOG 320 boiler no. K-2, manufactured by RAFAKO and installed at PKN Orlen CHP Plant in Płock has the capacity of 106 m3.  The temporary installation consisted of the nitrogen system for circulation in water walls tubes and two auxiliary pumps type OS 150A/4 to force the flow in boiler superheaters and fuel pipelines (gas and oil).

5.0. Scope of Acid Cleaning

The following boiler equipment was subject to flushing, degreasing, acid treatment and passivation processes: feed water pipelines, a boiler economizer with headers, a drum, a boiler evaporator, steam superheaters, steam pipelines, spray pipelines, and in case of OOG 320 boiler also fuel oil, fuel gas and start-up gas systems.

6.0. Description of Acid Cleaning Process

The acid cleaning of systems is performed according to the following technological operations.

6.1. Leak proof test of the system to be cleaned and the temporary system.

After filling the entire system with demineralised water, the leak proof test of the system was performed at the pressure, that could be provided by auxiliary pumps installed in the temporary system.

6.2. Pre-flushing of Acid Cleaning System

The flushing was carried out with demineralised water circulation by means of auxiliary pumps, with capacity that assured the maximum water flow rates in the systems being flushed . Flushing was continued until the clear, solids free flushing water was achieved. During flushing the measurements of suspended solids and total iron concentration were taken.

  • Flushing commencement
  • Flushing completion
  • Measurement of suspended matter quantity

6.3. Degreasing

Degreasing process was performed by means of water solution of IMPUREX B-63 at temperature 60 - 70°C. During the process, the temperature, p, m, pH values, suspended matter, PO4-3, total iron concentration and COD were measured.

6.4. Flushing after Degreasing

The flushing , which removed the remains of degreasing solution from the system, was performed by means of circulation with auxiliary pumps of capacity, which assured the maximum water flow rate in the systems being flushed. The flushing was continued until clear, washing water without suspended solids and with neutral pH value was reached, while controlling the values of “p“, “m“, pH, suspended matter, PO4-3 concentration and specific conductivity.

6.5. Flow Test

Flow testing of boiler tubes i.e. boiler economizer, waterwalls, superheater and reheater is performed to check if the boiler elements are not clogged. The flow through the pipes was checked by means of thermo-visual camera, while heating the system up to 40÷50°C.
Hanger tubes of the 2nd pass
Wall superheater
of OOG-320 boiler no. K-2 in PKN Orlen CHP Plant in Płock.

6.6. Acid Treatment

The acid treatment converts corrosion products and partially also silica into water soluble compounds and removes it from the system in a form of the solution. Acid treatment was performed with a water solution inhibited with hydrofluoric acid. During acid treatment the measurement of temperature, acid and iron concentration (Fetot and Fe+2) was performed.
Drum interior after degreasing
Drum interior after acid cleaning and passivation
of boiler type OE 700-17,1/568/568/250 no. K-4 in Turów Power Plant

6.7. Flushing after Acid Treatment

The flushing was continued until acid reaction disappeared, and it was finished when the flushing water was clear, free of solids, of the specific conductivity

6.8. Passivation

The surfaces after acid treatment were then passivated in order to protect them against atmospheric corrosion until the boiler firing up.

  • Endoscope images of OOG-320 K2 boiler in PKN Orlen CHP Plant in Płock after acid cleaning process
  • cleaned weld seam (downcomer tube)
  • waterwall tube (inlet to bottom header)

6.9. Neutralization of Post-Acid Cleaning Effluents

Pursuant to agreements with environmental and chemical departments of power plants, all waste water after acid cleaning was neutralised and drained to the settling ponds in Turów Power Plant and to the waste water treatment plant in PKN Orlen Płock.

7.0. Effects of Acid Cleaning

The evaluation of process correctness was made according to Polish Standard PN-90/M-34315 “Steam and water boilers. The principles of inspection after erection”, item 2.3.“The quality of the cleaned surfaces is deemed satisfactory if oil, rust, scale and protective lining are removed from the internal surfaces.”

The evaluation of the cleaning effects was made based on the following:

  • inspection of: drum,
  • washout holes of downcomers,
  • waterwall headers,
  • inlet and outlet headers of RH I and RH II, lower headers of separators, main feed water control valve,
  •  concentration of acid and iron at each stage reached the balance, i.e. their values were constant,
  • testing samples (sections),
  • endoscope tests of water wall tubes cleanness.