Boiler & Steam Generator Scale

ENGINEERING ENCYCLOPEDIA Saudi Aramco DeskTop Standards

BOILER AND STEAM GENERATOR SCALES

NOTE: THE SOURCE OF THE TECHNICAL MATERIAL IN THIS VOLUME IS THE PROFESSIONAL ENGINEERING DEVELOPMENT PROGRAM (PEDP) OF ENGINEERING SERVICES. WARNING. THE MATERIALS CONTAINED IN THIS MANUAL WERE DEVELOPED FOR THE SAUDI ARABIAN OIL COMPANY (SAUDI ARAMCO) AND ARE INTENDED FOR THE EXCLUSIVE USE OF SAUDI ARAMCO EMPLOYEES. ANY MATERIAL CONTAINED IN THIS MANUAL WHICH IS NOT ALREADY IN THE PUBLIC DOMAIN MAY NOT BE COPIED, REPRODUCED, SOLD, GIVEN, OR DISCLOSED TO THIRD PARTIES, OR OTHERWISE USED, IN WHOLE OR IN PART, WITHOUT THE PRIOR WRITTEN PERMISSION OF VICE PRESIDENT, ENGINEERING SERVICES, SAUDI ARAMCO.

ENGINEERING ENCYCLOPEDIA Saudi Aramco DeskTop Standards

CONTENTS

PAGES

TYPES OF SCALES ..............................................................................................1 Introduction ................................................................................................. 1 Carbonates................................................................................................... 9 Silica..........................................................................................................12 Sulfates...................................................................................................... 14 Iron Scales................................................................................................. 15 Causes of Boiler Scales and Deposits........................................................18 PREVENTING DAMAGE BY SCALE...............................................................19 Softening ................................................................................................... 19 Oxygen Removal.......................................................................................20 Effects of Dissolved Oxygen.........................................................20 Mechanical Deaeration .................................................................. 21 Chemical Oxygen Removal...........................................................25 Polymers, Chelants, and Inhibitors............................................................33 Polymers ........................................................................................33 Chelants .........................................................................................34 Inhibitors........................................................................................35 Blowdown ................................................................................................. 36 Cleaning Boilers........................................................................................38 Alkaline Cleaning ..........................................................................39 Acid Cleaning ................................................................................40 Chelant Cleaning............................................................................41 GLOSSARY ......................................................................................................... 42

BOILER AND STEAM GENERATOR SCALES

ENGINEERING ENCYCLOPEDIA Saudi Aramco DeskTop Standards

TYPES OF SCALES Introduction The use of boilers in the petroleum industry has diminished over the years, but many are still used. Boilers are no longer used on drilling rigs, but they are used in gas processing and refining. Steam generators are used for thermal stimulation in the production of shallow, heavy oil deposits. The composition of boiler feed water and quality in relation to boiler operation have been studied for 150 years. Much of the knowledge we apply to oil field water treating was developed in connection with the improvement of boiler safety. The purpose of boiler water treatment is to prevent the formation of scales and deposits, prevent corrosion in boiler and steam systems, and to maintain steam purity (Figure 1). To avoid:

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Scale Deposition Corrosion Steam Contamination Overheating Embrittlement

FIGURE 1. Objectives of Boiler Feed Water Treatment Boiler water treatment can be divided into two classes: external and internal. External treatment is that which is applied to the water before it enters the boiler and consists of suspended solids removal, dissolved gas removal, and softening. Internal treatment is that which is added to the boiler itself such as chelants, polymers, buffers, caustic, and inhibitors. The purity of boiler water, thus, the extent of treatment required, increases as the working pressure increases. It is convenient to classify boilers as low pressure (200500