Boiler Operating Suggestions – Courtesy of Rentech Boilers
The following is a suggested operating guide for safe and reliable operation of the unit. Routine monitoring of the controls and safety systems by the operator is imperative.
Water Levels
The water level must be continually checked whether the feedwater system is operated automatically or when an operator is in attendance. Proper water level in the steam drum must be maintained at all times. High water level can result in damage to internal separation equipment with a corresponding reduction in separation efficiency. This will result in water carryover and mineral deposition inside the downstream components. If the water level reaches too low a point, the unit is in danger of overheating with possible catastrophic damage. If the level is automatically controlled by the feedwater regulator, it should be adjusted per the manufacturer's recommendations so that the level remains stable near the centerline of the gauge glass.
The water column and water gauge glass should be drained once at the beginning of each shift. This will insure that sludge or sediment will not have a chance to accumulate in the column or gauge glass and can cause an erroneous level indication. The boiler attendant, by observing drained liquid and return of liquid to glass, will be assured of proper actuation of one of the most important safety devices of the unit. Daily testing of level alarms and low water cutoff is also recommended.
Boiler Water Blowdown
Boiler water blowdown is done to remove some of the concentrated water from the pressure vessel while it is under pressure. The removed water containing suspended and dissolved solids is replaced with relatively pure feedwater even though this water is treated prior to use through external processes designed to remove the unwanted substances which contribute to scale and deposit formations. Regardless of their high efficiency, none of these processes in themselves are capable of removing all substances and a small amount of solids will be present in the boiler water. The solids become less soluble in the high temperature of the boiler water and as the water boils off as relatively pure steam, the remaining water becomes thicker with either suspended or dissolved solids.
Internal chemical treatment, based on water analysis, is used primarily to precipitate many of the solids and to maintain them as "sludge" in a fluid form. This sludge along with suspended solids that may be present must be removed by the blowdown process. If the concentration of solids is not lowered through blowdown but rather accumulates, foaming and priming will occur along with scale and other harmful deposits.
The scale forming as salts tend to concentrate and crystallize on the heating surfaces. Scale has a low heat transfer value. It acts as an insulation barrier and retards the heat transfer. This not only results in low operating efficiency and consequently higher fuel consumption, but also presents the possibility of overheating the boiler metal. The result can be tube failures or other pressure vessel metal damage.
There are two principal types of blowdown, they are intermittent and continuous. Intermittent is done manually and is necessary for the operation of the boiler regardless of whether or not continuous blowdown is employed. Continuous blowdown is a continuous and automatic removal of concentrated boiler water.
Intermittent Blowdown
The manual blowdown valve and discharge lines are located at the bottom or low point of the blower. This also provides a means of draining the boiler when it is not under pressure. The intermittent blowdown should be opened fully for a short duration (approximately 10 to 20 seconds), at least once per shift thus insuring proper removal of accumulated solids that have settled out in the mud drum. In cases where the feedwater is exceptionally pure, blowdown may be employed less frequently since less sludge accumulates in the pressure vessel.
Frequent short blows are preferred to infrequent length blows. This is particularly true when the suspended solids content of the water is high. With the use of frequent short blows, a more uniform concentration of the pressure vessel water is maintained. Blowing down is most effective at a time when the generation of steam is at the lowest rate since the feedwater input then is also low, providing a minimum dilution of the boiler water with low concentration feedwater.
It is recommended that the blowdown valve nearest the boiler be opened first and closed last, with blowing down being accomplished by the valve furthest from the boiler. The sequence of operation once established should insure that the valve last opened should be the first closed so that the other valve is saved from throttling service to insure a tight closing. The downstream valve should be cracked slightly to allow the discharge line to warm up, after which this valve opened slowly. Quickly close the downstream valve tightly, then close the valve next to the boiler. The frequency and amount of each blow should be determined by actual water analysis.
The water level should be observed during periods of intermittent blowdown. If the glass cannot be viewed by the person operating the valve, another operator should watch the glass and direct the valve operator. The blow-off valves should never be left open and the operator should never leave until the blowdown operation is completed and the valves closed. Be sure the valves are shut tight. Repair any leaking valves as soon as possible.
Continuous Blowdown
The boiler is equipped with an internal continuous blowdown pipe. The collector pipe is located several inches below the normal water level, at a point where the most concentrated water is found. A manual controlled metering valve is normally utilized to control the flow of concentrated water. Periodic adjustments are made to the valve setting to increase or decrease the amount of blowdown and water testing. Proper monitoring and maintenance of appropriate water conditions in the boiler are mandatory to assure long term integrity of the boiler.
The amount of water lost through a continuous surface blowdown is usually considerably less than the water lost through bottom blow-off in order to remove a given amount of suspended solids. The amount of blowdown depends upon the rate of evaporation and the amount of sludge forming material in the feedwater.
Foaming or priming may occur in the steam drum and cause large quantities of water to pass over into the steam lines. It can be detected by violent fluctuations or sudden dropping of the water level in the gauge glass. This trouble may be caused by dirt or oil in the boiler water, an overdose of treatment chemicals, carrying too high a water level in the steam drum, or excessive rates of change of evaporation. In case of serious trouble, decrease the load on the boiler, then alternately blowdown and feed fresh water several times. If foaming does not stop, cool, empty, and wash out the boiler and refill with fresh, treated water. Any problem in regard to extreme foaming or priming, scale in boiler, corrosion or pitting, should be referred immediately to a company specializing in boiler water chemistry consulting. Do not experiment with "home-made" treatment chemicals.
Saturated Steam Sampling
When a new sample line is being put into service, flow steam and condensate thought it for 24 hours before the samples are collected. Before the sampling period, blow steam though the sample line to remove any material that may have been deposited. Adjust the flow rate to that desired during sampling and flow the sample at this rate for one hour or longer before conductivity measurements are recorded before a sample is collected for other methods of analysis.
When samples of condensate are collected for evaporative analysis, the flasks or other types of containers should be meticulously cleaned prior to use. Borosilicate glass is a satisfactory material for such containers but they should be aged to by allowing them to stand for several days full of distilled water. This aging of borosilicate glass may be hastened by a preliminary treatment with dilute caustic soda (ten grams/liter). Containers fabricated from suitable metals or plastics may also be used.
Samples should be analyzed as soon as possible after collection. After use, the flasks should be rinsed with hydrochloric acid. These flasks should be reserved and used for no other purpose than steam samples. When the condensed sample is to be analyzed by the electrical conductance method, cool it to 25°C. When dissolved gases are to be determined, cool the sample to 20°C or lower.