White Paper: Pulp & Paper Mill Boiler Blowdown Energy and Water Savings Opportunity
PART I. GENERAL
1.01 BACKGROUND
A. This paper is meant to give a background into the potential energy and water savings methods associated with steam plant blowdown. While we give a broad overview of legacy and new methods of implementing such methods, the desired outcome would be for Steam Management, Inc. to work with you in a collaborative relationship to find the solutions that best fit your needs.
1.02 INTRODUCTION
A. Performance of the boiler, like efficiency and evaporation ratio, reduces with time due to poor combustion, heat exchange surface fouling, and poor operation and maintenance. Deterioration of fuel quality and water quality can also lead to poor performance. Efficiency testing helps us to find out how far the boiler efficiency drifts away from the optimal efficiency. Any observed abnormal deviations can be investigated to pinpoint problems that require corrective action, hence it is necessary to find out the current level of efficiency for performance evaluation. This is a pre-requisite for energy and water conservation action. The steam plant and piping distribution systems’ energy efficiency depend on the system design, operation, and maintenance.
This paper focuses on the opportunities to capture wasted heat lost in the boiler blowdown system, and provides a real-world example, taken from a successfully executed SMI project. There are two types of boiler blowdown; bottom blowdown, and surface (aka continuous) blowdown. Bottom blowdown is concerned with opening valves on the mud drum to briefly blow out sludge that has accumulated in the boiler water. Bottom blowdown energy losses are typically minimal, as this is typically only done once per shift, for a few minutes at a time. Generally, it is also more difficult to capture heat from bottom blowdown for other processes, due to its intermittent nature.
Heat recovery opportunities on blowdown systems typically center around surface blowdown, as this is a continuous flow from the boiler, and typically ranges from 2-8% of the boiler steam production. We have seen boilers in the field operating as high as 20% on boiler blowdown, although this typically would indicate poor water treatment upstream of the boiler.
1.03 WHY BLOWDOWN THE BOILER
A. As a boiler generates steam, any impurities which are in the boiler feedwater, and which do not boil off with the steam will concentrate in the boiler water.
B. Surface blowdown is water intentionally and continuously wasted from the boiler to avoid concentrations of impurities left behind during the continuing evaporation of steam. The water flows out of the boiler due to the steam pressure within the boiler.
C. As the dissolved solids become more and more concentrated, the steam bubbles tend to become more stable, failing to burst as they reach the water surface of the boiler. The boiler reaches a point (depending on boiler pressure, size, and steam load) where a substantial part of the steam space in the boiler becomes filled with bubbles or froth. This is commonly known as “boiler foaming.” When boiler foam builds up within the steam space, it can leave the boiler and enter the steam system. This is known as “carry-over”, or “priming.”
D. Carry-over is undesirable, not only because the steam is excessively wet as it leaves the boiler, but it also carries high levels of dissolved and perhaps suspended solids. These solids can lead to deposits of crystals in the steam distribution system contaminating control valves, heat exchangers and steam traps.
E. Foaming can be caused by high levels of suspended solids, high alkalinity, or contamination by oils and fats. However, typically a high Total Dissolved Solids (TDS) level is the culprit. Careful control of boiler water TDS level together with attention to these other factors should ensure that the risks of foaming and carryover are minimized.
F. Table 1 provides guidelines by ABMA and ASME for controlling boiler water and steam quality.
PART II. BOILER BLOWDOWN CONTROL
A. Manual Blowdown:
The simplest method of reducing the boiler water contamination is to take a boiler water sample, measure the TDS and if higher than recommended for the particular boiler operating criteria blowdown the water to some point well below the recommended maximum value. This is usually done once each boiler operator shift by the boiler operator. The boiler TDS gradually rises between blowdowns. A typical arrangement would be to open valve at, for example, 3,000 ppm, then close the valve at say minus 20% less or 2,400 ppm. This type of blowdown results in the highest energy and water use since it requires operating the boiler on average well below the recommended TDS level.
B. Continuous Blowdown:
The continuous method is an improvement to the manual blowdown where a blowdown throttling valve is used to control the TDS level. Continuous blowdown valves are special valves that have stages to reduce the problem of erosion that results in damage and subsequent failure to shut off. The valves are also marked with reference values, to document the operating points of the valve and aid in adjustments. The continuous blow down valve position is usually set manually at a predetermined open position to maintain a maximum TDS level. This position is periodically adjusted by the boiler operator based on historical data and by periodically sampling boiler water and measuring TDS to ensure the maximum TDS level does not get exceeded. This is an improvement over the manual blowdown method, but changes in boiler demand due to process or heating load swing levels are not accounted for. This method also results in higher than necessary energy losses since operators are required to overshoot the blowdown flow rate to ensure acceptable TDS levels.
C. Closed Loop Digitally Controlled Blowdown:
This method continuously measures the boiler water conductivity, which corresponds to dissolved solids, compares it with a set point, and modulates a blowdown control valve if the TDS level is too high. Different types are available, and selection depends on the boiler type, boiler pressure, and blowdown flow requirements. The benefits of the closed loop system are labor savings of automation, tighter control of boiler TDS, and energy and water savings.
PART III. POTENTIAL WATER AND ENERGY RECOVERY
A. Since boiler blowdown water is at a high temperature and pressure, it represents a considerable loss of energy and water. The degree of loss is determined by the operating pressure, and blowdown flow requirements.
B. The plants’ supply water quality will determine the degree of treatment and cost associated with makeup water to replace losses from the steam system. Table 1 provides guidelines by ABMA and ASME for controlling boiler water and steam quality. Quality ranges from unsoftened city supplied or private well systems, to softened water (which is a recommended minimal source for steam boilers, to RO or DI make up water). As the quality of the makeup water decreases, the amount of blowdown required increases to maintain the proper boiler water TDS.
C. Code requirements, environmental constraints, and sewer material limits typically require the blowdown water to be cooled before it can be discharged to the sewer system. A typical acceptable maximum is 140°F. This is typically done by discharging the blowdown to a flash tank with the flashed steam being vented to atmosphere, reducing the blowdown to 212°F. The remaining liquid is cooled with plant water before being discharged to drain.
D. The hot high-pressure blowdown water will flash steam when entering a low-pressure vessel for heat recovery. This low-pressure steam can be recovered and used for heating the deaerator. Deaerators are used for two main reasons, one, to remove oxygen from the boiler feedwater, and two, to help preheat the water before being sent to the boiler. The recovered flash steam from the blowdown will offset the deaerator steam consumption. The remaining hot condensate from the vessel can then be sent through a plate type heat exchanger and used to pre-heat the boiler makeup water. Thereby, saving the cooling water required to cool the condensate to below the Code requirement and the energy to pre-heat the makeup water.
PART IV. EXAMPLE
A. This example project is for a Specialty Paper Company Steam Plant with the following pre retrofit Operating Data:
1. 2 Boilers each 30,000 pounds per hours steam capacity
2. Boiler operating pressure 150 PSIG
3. Boiler water TDS manually controlled with TDS range of (1,800 to 2,200 ppm)
4. Feedwater TDS (100 ppm)
5. Average steam rate 15,290 pounds per hour
6. Makeup water temperature 50°F
7. Deaerator pressure 30 PSIG
8. Annual operating hours 8,760
9. Fuel cost: #6 Oil $1.33 per THERM
10. Steam plant annual fuel use 900,000 gallons
11. Boiler combustion efficiency 80.5%
12. Water cost $5.00 per 1,000 gallons
B. This example is to retrofit the steam plant with an automatically controlled closed loop digital blowdown control and blowdown heat and water recovery system. Estimated annual savings:
1. Total Annual Cost Savings: $43,750
2. Total Annual Energy Savings: 2,951 MMBTU.
3. Total Annual Fuel Oil Savings: 19,367 gallons.
4. Total Annual Water Savings: 899,205 gallons.
C. Click Here to View the Calculations.
If your facility is looking to complete an analysis or would like to discuss our solution based services, contact us today at:
Chase Bean, PE, PMP, CEM
Director of Engineering
cbean@steammgt.com