Steam Systems

Generating steam is the primary function of most boiler systems. In some industries, where 40-60 percent of all energy is consumed in the generation of steam in fired or wasteheat boilers, efficient operation and regular maintenance can represent a great potential in energy savings.

Industrial Steam Trapping Handbook

For instance, in the absence of an effective maintenance program, it’s common to find 15 to 20 percent of steam traps not working properly. Another energy-waster is to allow steam leaks to persist, reducing steam production by as much as three to five percent. Insulation deterioration can cause another 5 to 10 percent steam loss during rain storms, as the insulation gets wet and loses its effectiveness. Fouled turbines and exchangers can cause as high as a 25 percent efficiency loss.

In a discussion on steam systems, many over-laps may occur when addressing maintenance and operation procedures. For example, Chapter 2, Water Treatment, discusses the addition of amines to keep the pH of the condensate high, avoiding acid attack. In this chapter, that subject is related to corrosion in steam traps, a specific problem area in steam systems.

Besides its use and subsequent availability in numerous industrial processes and also in generating electricity, steam is also employed to drive pumps and compressors as well as providing freeze protection for winter operations. Steam system operation is complex because of its generation, distribution, recovery and use at several different pressure levels.

There are five general “rules” that should be followed for maximum efficiency in steam generation. They are:

1.     Always produce steam at the highest possible temperature and pressure. This is a basic thermodynamic and economic principle.

2.     Always apply steam to process use at the lowest possible pressure and temperature levels.

3.     In fired boilers, only produce steam for valid end uses, such as process steam and reboilers.

4.     Always expand steam from a higher pressure level to a lower pressure level through the most efficient means possible.

5.     Always produce maximum steam from process wasteheat recovery systems.

Proper steam system design will greatly increase operational efficiency. Poorly designed steam traps are the ones mot likely to function improperly or fail completely. Steam Tracing systems (a system designed to monitor steam temperature on a process pipe, for example), frequently evolve in a hap-hazard manner, often to solve a short-term problem, such as a steam trap that doesn’t work. Leaks, freezing, steam system dead-ends and equipment damage can all be consequences of improper design. Heat loss can be avoided by proper insulation design and maintenance.

Because there are so many different potential problem areas to address, it is helpful segregate as many of them as possible in common groups.

General Operational Procedures

1.     Process analyzers and advanced control techniques should be employed to minimize energy consumption of plants. Many plants use feed preheaters to supply heat for operation. Significant energy savings can be associated with system optimization. Specific operating control targets should be employed with energy conservation in mind.

2.     Improperly operated vacuum systems can significantly increase steam usage. Any leaks that develop should be repaired.

3.     Every operating area should have checklists and Standard Operating Instructions (SOIs) to ensure that unneeded steam traps and tracing systems are turned off as they can be a significant source of steam usage. Tracing systems are routinely left on year-round but are only needed during the colder months.

4.     Steam consumption targets and guidelines should be established at all facilities and for all major pieces of equipment. Targets should be routinely adjusted for process feed-rate changes. Target consumption should be plotted relative to load (load curves). The goals should be to operate the plant on these load curves.

5.     Each steam generator should be rated according to its performance characteristics or efficiency. That way, during a period of in-creasing steam demand, the most efficient generators can be loaded first, keeping energy consumption to a minimum while getting the most steam out of the most efficient systems. Also, where options exist and there is flexibility, the most efficient systems should be used first.

6.     Steam systems should be surveyed routinely to identify seldom-used steam lines which could be removed from service. Adjustments to systems should be made as dictated by plant steam requirements. If not automated, these adjustments should be described in a set of clearly stated, written instructions to the operator.

6.     Steam tracing systems should be held to an absolute minimum, as their use can down-grade overall steam distribution efficiency. Alternatives to steam tracing should be investigated, such as electrical heating tapes for remote locations where the monitoring of a steam tracing system would be impractical.

7.     Steam distribution and condensate systems should be designed so that effective corrosion treatment systems can be employed. See Chapter 2, Water Treatment, for information on these treatment systems.

8.     Steam systems should also be designed with adequate metering to be able to keep track of where the steam is going and to routinely get facility-wide and individual process-unit steam balances.

Steam Traps

1.     Every operating area should have a program to routinely check steam traps for proper operation. Testing frequency depends on local experiences but should at least occur yearly.

2.     All traps should be numbered and locations mapped for easier testing and record-keeping. Trap supply and return lines should be noted to simplify isolation and repair.

3.     Maintenance and operational personnel should be adequately trained in trap testing techniques. Where ultrasonic testing is needed, specially trained personnel should be used.

4.     High maintenance priority should be given to the repair or maintenance of failed traps. Attention to such a timely maintenance procedure can reduce failures to three to five percent or less. A failed open trap can mean steam losses of 50-100 lb/hr.

5.     All traps in closed systems should have atmospheric vents so that trap operation can be visually checked. If trap headers are not equipped with these, they should be modified.

6.     Proper trap design should be selected for each specific application. Inverted bucket traps may be preferred over thermostatic and thermodynamic-type traps for certain applications.

7.     It is important to be able to observe the discharge from traps through the header. Although several different techniques can be used, the most foolproof method for testing traps is observation. Ultrasonic, acoustical and pyrometric test methods often suggest erroneous conclusions.

8.     Traps should be properly sized for the expected condensate load. Improper sizing can cause steam losses, freezing and mechanical failures.

9.     Condensate collection systems should be properly designed to minimize frozen and/or premature trap failures. Condensate piping should be sized to accommodate 10 percent of the traps failing to open.

Insulation

1.     Systems should be regularly surveyed to re-place or repair missing and deteriorated insulation. This is especially important after insulation has been removed to repair steam leaks.

2.     An overall survey of steam lines should be conducted every five years (or one fifth of the facility per year) to identify areas where insulation or weatherproofing has deteriorated. Typical culprits include prolonged exposure to moisture, chemicals or hydrocarbons. Instruments to measure the effectiveness of insulation include thermographic (heat image) devices. This instrument gives an indication of surface temperatures by displaying various colors. It is ideal for large areas. Others include portable infrared pyrometers, or heat guns, that measure surface heat by infrared wave emitted from the surface and contact-type pyrometers and surface crayons, which must be in contact with the surface to measure heat.

3.     Following any maintenance work, areas where work has been performed should be inspected to see where insulation should be repaired or replaced. Removable insulation blankets should have been reinstalled on all equip- ment. The last step in any maintenance work should be the repair, replacement or reinstallation of insulation. System components often overlooked and left uninsulated include valves, turbines, pumps and flanges.

4.     Optimal insulation thickness should be applied to any new piping systems.

5.     During steam line surveys, insulation should be visually inspected for the following defects:

·         Physical damage

·         Cracks in vapor barriers

·         Broken bands or wires

·         Broken or damaged

·         weather-tight jointseals

·         Damaged covers and

·         weatherproofing

Leaks

1.     All steam leaks should be repaired as quickly as possible. Leaks are one of the most visible forms of energy waste. The table in Figure 27 shows steam loss at pounds per hour, for a given sized hole, at a given pressure. Steam leaks can also suggest management indifference to efficient operation and pose significant safety hazards. Steam leaks don’t get smaller, neither does the cost of fixing them.

2.     Standard procedures should dictate that proper gaskets and packing are used in steam system flanges and valves.

3.     An on-stream, leak-repair specialist should be employed to repair leaks when the steam system cannot be taken down.

4.     All steam systems should be designed for minimum leakage. For example, flanges and threaded piping should be minimized.

Pressure

1.     There are large incentives to use steam at its lowest possible pressure for heating, primarily to reduce energy consumption. Process or equipment changes will often allow the use of lower steam pressure. These considerations are part of the plant initial design phase and any changes recommended should undergo an economic analysis to justify process or equipment changes.

2.     The utilization of steam at all pressure levels should be maximized. High pressure steam should not be reduced in pressure through control valves and low pressure steam should not be vented. Typically, there are large in centives to eliminate steam venting and pressure letdown. A significant reduction in fuel cost is perhaps the largest incentive. Instrumentation should be designed to continuously monitor steam pressure letdown and venting. In short, all steam systems should be balanced.

3.     Reboilers and steam preheaters should use only the lowest steam pressure possible. This can often be done by using extended tube surfaces, nucleate boiling tubes and lower tower pressures.

Tabel : Estimate of steam losses

Special Notes on Turbines

1.     Steam turbines should always be operated at the lowest back pressure possible. In topping turbines, high back pressure can be caused by inadequate piping or high steam consumption from declining turbine efficiency. A high pressure drop between the turbine exhaust and the steam header could mean the piping is restrictive. In condensing turbines, high back pressure can be caused by vacuum system problems.

2.     Condensing turbines are not very efficient as they tend to lose energy and utilize only 15 to 20 percent of the available steam thermal energy. At some point, consideration should be given to replacing these turbines with top-ping turbines, electric motors or direct-drive gas turbines.

3.     Low turbine efficiency is often the result of blade fouling. Fouling is usually a result of water that has not been treated properly. See Chapter 2, Water Treatment, for further recommendations. Water-washing turbines on-stream will often restore their efficiency. Improperly treated feedwater can also cause permanent long-term damage to boiler waterwall surfaces and superheater tubes.

Posted in: Chemical Engineering Processing

Share |