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Metering in Power Plant Engineering

Dipl.-Ing. Bernd Freissler gave a talk on the basic principles of metering technology and its applications in power plants at “Chemistry in Steam Circuits”, a seminar run by PowerPlant Chemistry. The seminar was held on 7-8 October 2009 at the Mövenpick Hotel, Stuttgart Airport.

Over and above providing basic information about metering and metering pumps, the ProMinent product manager discussed current trends in metering technology in the early part of his talk.

In the second half of his talk, Bernd Freissler then detailed the ProMinent technologies used in power plants and, with the aid of practical examples, described how the combination of metering systems, measuring and control technology and metering pumps can be used for the efficient and environmental treatment of cooling water, feed water and waste water, as well as for flue gas cleaning.

Metering pumps

The early part of Bernd Freissler’s talk dealt with the actual meaning of metering itself, as he explained that a metering process is effectively the controlled feed of liquids into various processes. Metering the correct dose of a chemical is essential especially in the disinfection of water, as well as in almost all other production processes.

Almost all free-flowing substances can be metered, from sodium hypochlorite for the disinfection of drinking water, to flavourings in foodstuffs and process chemicals in industry. It is possible to use pumps that enhance the individual functions of a metering process, for instance by limiting the volume, by measuring the increasing pressure level and guaranteeing reproducibility by adjusting the preset stroke volume between 0 and 100% or by means of the stroke rate.

Oscillating displacement pumps, that have a rigid compression curve and an oscillating flow rate, with linear dependence of the metering flow, can be used to meet these requirements.

Once he had introduced the different models of pump metering heads and various drives, the product manager for process pumps then moved onto the controls options, ranging from manual stroke length control using actuators with position feedback, to speed control with frequency inverters and stroke rate control with internal pump electronics (microprocessor control).

In terms of process controllers, customers can choose from manual and flow-proportional control, variable-dependent control and process control systems possibly linked by a field bus.

Trends in metering technology

“The current trend is very much towards customer-specific systems and safety,” states Bernd Freissler, “ensuring that we meet all of the aforementioned aspects: customers receive pumps that are readily available and thus guarantee safety for both processes and operators.

One such example is the diaphragm metering pump delta® with its regulated solenoid drive optoDrive®, the integral optoGuard® of which continuously monitors all key hydraulic metering parameters.

The optoGuard automatically detects blocked metering points or broken metering lines, as well as air and gas bubbles trapped in the metering head. These pumps are able to lower operating costs at the same time as conserving the environment as they meter feed chemicals at precisely the quantity required – as little as possible and as much as is needed.”

ProMinent technology – examples from the power plant sector

ProMinent pumps and metering systems can be used wherever there is a need for water treatment in power plants. There are essentially four main applications in power plants: the treatment of cooling water, feed water and waste water and flue gas cleaning.

To protect plants, a variety of chemicals need to be metered in the treatment of cooling water, thereby protecting the entire plant from corrosion, sedimentation and biological growth and resulting in the consumption of cooling water being kept to an absolute minimum.

Feed water also needs extensive treatment to generate steam, either by the metering of corrosion inhibitors or oxygen separators and sodium hydroxide, ammonium hydroxide or hydrazine for pH correction. Softening and reverse osmosis systems can also be used to treat boiler feed water.

ProMinent systems ensure that the operators of coal-fired power plants meet strict environmental regulations by cleaning flue gas by means of a complex process before it is discharged into the atmosphere.

In what is known as the SNCR process, ammonia is injected into the flue gas to reduce levels of nitrogen oxides and the flue gas is desulphurised in the absorber tower by being sprayed with a mixture of water, limestone, burnt lime and lime milk, thereby absorbing much of the sulphur dioxide.

Dust is also removed from the flue gas in electric filters. The efficiency of SO3 separation or of TMT 15 for heavy metal bonding can be increased with metering systems for adipinic acid.

Any waste water formed in flue gas desulphurisation must be treated before it can be discharged; heavily contaminated by the introduction of chemicals, it must be appropriately treated before being discharged into the environment. The main purpose of this treatment is to remove plaster, reduce solids and remove heavy metals from the waste water.

Metering systems can also be used to meter various coagulants, substrates and chemicals to correct the pH value and polymer feed stations can be used to optimise flocculation in heavy metal separation and dehydrate sludge.

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