Steam is today’s utility player in the industrial energy arena. It cleans and sterilizes, dries and concentrates, separates and evaporates. In cookery, it preserves flavor, texture, and retains nutrients. In chemistry, it fosters reactions by controlling process pressure and temperature. In biotechnology, it’s essential for growing production organisms. Steam ranges in purity from boiler grade for routine tasks, to culinary grade “clean steam” for food and dairy, and graduates up to super pure, pyrogen-free steam for biopharmaceutical use.
Generated steam is ubiquitous because it’s industry-friendly. It’s intrinsically safe, flexible, economical, aseptic, and environmentally benign. Steam can arise from fossil fuels, biomass or biogas. Its high efficiency is due to its ability to return a significant portion of condensate to the generation cycle. With proper maintenance, a steam plant can last for decades.
Magnetrol® has produced a special applications brochure on level instrumentation for the steam generation process. This brochure focuses on the industrial steam boiler. If you are interested in steam applications for power generation, the massive electricity-generating utility boilers are addressed in our Power Gen and Nuclear brochures. This blog post, the first in a three-part series, will discuss three aspects of the steam loop: make-up water treatment, feed water deaeration, and boiler feed water treatment. Future posts in the series will cover the rest of the steam loop as well as flow control applications.
STEAM LOOP: 1. Make-Up Water Treatment
It all begins with water, the essential medium for the steam generation process. Conditioning water properly can increase boiler efficiency and extend the boiler’s operating life. Improper or nonexistent feed water treatment is the major cause of boiler failure.
All natural sources of fresh water require varying degrees of treatment prior to boiler use. Boiler feed water treatment prevents scale and deposits, removes dissolved gases, protects against corrosion, eliminates water and steam carry-over, and optimizes boiler efficiency and minimizes maintenance costs.
Feed water is treated and conditioned in three phases:
1. Raw make-up water is treated before entering the deaerator
2. Treated make-up water and return condensate merge in the deaerator for removal of dissolved gases
3. De-oxygenated and heated feed water enters the boiler where it is further treated with chemicals
Feed water treatment requirements vary greatly. In rare cases, raw make-up water only needs filtering. More commonly, some form of external treatment is needed, as in the typical process units listed below.
Raw water with sediments may require a settling tank with the addition of chemicals to foster precipitation of the suspended matter. Clarified water is then drawn off at a surface outlet.
Continuous Level: Eclipse® 706 Guided Wave Radar (GWR); Echotel® Model 355 Non- Contact Ultrasonic Transmitter
Coarse and Fine Filtration
Suspended solid impurities are reduced or eliminated by passing make-up water through a filter. If the suspended solids are very fine, a flocculation step may be required.
Continuous Level: ECHOTEL Model 355 Non-Contact Ultrasonic Transmitter; Pulsar® R95 Thru-Air Radar Transmitter; Model R82 Thru-air Radar Transmitter
Calcium and magnesium are hard scale forming minerals that build up on boilers and steam-related equipment resulting in costly repairs, increased energy consumption, and plugged equipment. Softening occurs as hardness minerals attach to the softening resin and “exchange” for sodium.
Continuous Level: PULSAR R95 Thru-Air Radar Transmitter; ECLIPSE 706 Guided Wave Radar; Atlas™ or Aurora® Magnetic Level Indicators (MLIs).
Demineralization is typically an ion exchange process whereby minerals or mineral salts are removed from water. Chemical treatment or a feed water evaporator can be used as alternative methods, the latter using extraction steam to remove impurities in raw water.
Point Level: ECHOTEL 961 Ultrasonic Level Switch; Model T20 Single Stage Float Level Switch.
Continuous Level: ECLIPSE 706 GWR; Digital E3 Modulevel® Displacer Transmitter. Point Level: Models T5x and T6x Float-Based Level Switches.
The treated make-up water is routed for storage in a cold-water header tank. From there the water passes on demand through a flash tank for heating and on to the deaerator for degassing.
Continuous Level: Model R82 Non-contact Radar Transmitter; ECLIPSE 706 GWR; PULSAR R95 Thru-Air Radar Transmitter.
STEAM LOOP: 2. Feed Water Deaeration
Because boiler and steam systems are made primarily of steel and the heat transfer medium is water, the potential for corrosion is very high. Dissolved oxygen is the major cause of boiler system corrosion. Oxygen and other gases are removed from both feed water streams—treated make-up water and return condensate feed water—when they merge in the deaerator. Deaerators remove non-condensable gases from feed water streams by steam heating and by aggressively agitating incoming water. The deaerator’s storage section is typically designed to hold enough water for ten minutes of boiler operation at full load.
Level control on a deaerator typically measures the level in the storage tank and modulates a control valve on feed water streams to maintain tank level at the desired set points.
Continuous Level: ECLIPSE 706 Guided Wave Radar; ATLAS or AURORA Magnetic Level Indicators.
Point Level: Series 3 ASME B31.1 External Caged Liquid Level Switch.
STEAM LOOP: 3. Boiler Feed Water Treatment
For higher boiler efficiencies, the feed water is further conditioned prior to and upon entering the boiler. First, the water is preheated by an economizer using the boiler’s hot exhaust gas streams. Next, water treatment inside the boiler reduces foaming, eliminates adherence of suspended matter to boiler internals, further prevents corrosion and scaling by eliminating residual oxygen. Chemical feed systems are employed for both internal boiler treatment and in the make-up water treatment unit.